TY - JOUR A1 - Thapa, Samudrajit A1 - Lomholt, Michael Andersen A1 - Krog, Jens A1 - Cherstvy, Andrey G. A1 - Metzler, Ralf T1 - Bayesian analysis of single-particle tracking data using the nested-sampling algorithm: maximum-likelihood model selection applied to stochastic-diffusivity data JF - Physical chemistry, chemical physics : PCCP ; a journal of European Chemical Societies N2 - We employ Bayesian statistics using the nested-sampling algorithm to compare and rank multiple models of ergodic diffusion (including anomalous diffusion) as well as to assess their optimal parameters for in silico-generated and real time-series. We focus on the recently-introduced model of Brownian motion with "diffusing diffusivity'-giving rise to widely-observed non-Gaussian displacement statistics-and its comparison to Brownian and fractional Brownian motion, also for the time-series with some measurement noise. We conduct this model-assessment analysis using Bayesian statistics and the nested-sampling algorithm on the level of individual particle trajectories. We evaluate relative model probabilities and compute best-parameter sets for each diffusion model, comparing the estimated parameters to the true ones. We test the performance of the nested-sampling algorithm and its predictive power both for computer-generated (idealised) trajectories as well as for real single-particle-tracking trajectories. Our approach delivers new important insight into the objective selection of the most suitable stochastic model for a given time-series. We also present first model-ranking results in application to experimental data of tracer diffusion in polymer-based hydrogels. Y1 - 2018 U6 - https://doi.org/10.1039/c8cp04043e SN - 1463-9076 SN - 1463-9084 VL - 20 IS - 46 SP - 29018 EP - 29037 PB - Royal Society of Chemistry CY - Cambridge ER - TY - JOUR A1 - Abdoul-Carime, Hassan A1 - Bald, Ilko A1 - Illenberger, Eugen A1 - Kopyra, Janina T1 - Selective Synthesis of Ethylene and Acetylene from Dimethyl Sulfide Cold Films Controlled by Slow Electrons JF - The journal of physical chemistry : C, Nanomaterials and interfaces N2 - One of the major challenges in chemical synthesis is to trigger and control a specific reaction route leading to a specific final product, while side products are avoided. Methodologies based on resonant processes at the molecular level, for example, photochemistry, offer the possibility of inducing selective reactions. Electrons at energies below the molecular ionization potential (<10 eV) are known to dissociate molecules via resonant processes with higher cross sections and specificity than photons. Here we show that even subexcitation electrons with energies as low as 1 eV produce ethylene and acetylene from dimethyl sulfide in competing reactions. However, the production of ethylene can specifically be targeted by controlling the energy of electrons (similar to 3 to 4 eV). Finally, pure ethylene is selectively desorbed by heating the substrate from 90 to 105 K. Beyond the synthesis of these versatile hydrocarbons for various industrial applications from a biogenic sulfur compound, our findings demonstrate the feasibility of electron induced selective chemistry applicable on the nanoscale. Y1 - 2018 U6 - https://doi.org/10.1021/acs.jpcc.8b07377 SN - 1932-7447 VL - 122 IS - 42 SP - 24137 EP - 24142 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Evsevleev, Sergei A1 - Mishurova, Tatiana A1 - Cabeza, Sandra A1 - Koos, R. A1 - Sevostianov, Igor A1 - Garcés, Gonzales A1 - Requena, Guillermo A1 - Fernandez, R. A1 - Bruno, Giovanni T1 - The role of intermetallics in stress partitioning and damage evolution of AlSil2CuMgNi alloy JF - Materials Science and Engineering: A-Structural materials: properties, microstructure and processing N2 - Load partitioning between phases in a cast AlSi12CuMgNi alloy was investigated by in-situ compression test during neutron diffraction experiments. Computed tomography (CT) was used to determine volume fractions of eutectic Si and intermetallic (IM) phases, and to assess internal damage after ex-situ compression tests. The CT reconstructed volumes showed the interconnectivity of IM phases, which build a 3D network together with eutectic Si. Large stresses were found in IMs, revealing their significant role as a reinforcement for the alloy. An existing micromechanical model based on Maxwell scheme was extended to the present case, assuming the alloy as a three-phase composite (Al matrix, eutectic Si, IM phases). The model agrees well with the experimental data. Moreover, it allows predicting the principal stresses in each phase, while experiments can only determine stress differences between the axial and radial sample directions. Finally, we showed that the addition of alloying elements not only allowed developing a 3D interconnected network, but also improved the strength of the Al matrix, and the ability of the alloy constituents to bear mechanical load. KW - Aluminum alloys KW - Neutron diffraction KW - Micromechanical modeling KW - Internal stress KW - Damage KW - Computed tomography Y1 - 2018 U6 - https://doi.org/10.1016/j.msea.2018.08.070 SN - 0921-5093 SN - 1873-4936 VL - 736 SP - 453 EP - 464 PB - Elsevier CY - Lausanne ER - TY - JOUR A1 - Thiede, Tobias A1 - Cabeza, Sandra A1 - Mishurova, Tatiana A1 - Nadammal, Naresh A1 - Kromm, Arne A1 - Bode, Johannes A1 - Haberland, Christoph A1 - Bruno, Giovanni T1 - Residual Stress in Selective Laser Melted Inconel 718 BT - Influence of the Removal from Base Plate and Deposition Hatch Length JF - Materials performance and characterization N2 - The residual stress distribution in IN718 elongated prisms produced by selective laser melting was studied by means of neutron (bulk) and laboratory X-ray (surface) diffraction. Two deposition hatch lengths were considered. A horizontal plane near the top surface (perpendicular to the building direction) and a vertical plane near the lateral surface (parallel to the building direction) were investigated. Samples both in as-built (AB) condition and removed from the base plate (RE) were characterized. While surface stress fields seem constant for the AB condition, X-ray diffraction shows stress gradients along the hatch direction in the RE condition. The stress profiles correlate with the distortion maps obtained by tactile probe measurements. Neutron diffraction shows bulk stress gradients for all principal components along the main sample directions. We correlate the observed stress patterns with the hatch length, i.e., with its effect on temperature gradients and heat flow. The bulk stress gradients partially disappear after removal from the base plate. KW - residual stress KW - IN718 KW - neutron diffraction KW - laboratory x-ray diffraction KW - additive manufacturing KW - selective laser melting KW - coordinate measurement machine Y1 - 2018 U6 - https://doi.org/10.1520/MPC20170119 SN - 2379-1365 SN - 2165-3992 VL - 7 IS - 4 SP - 717 EP - 735 PB - American Society for Testing and Materials CY - West Conshohocken ER - TY - JOUR A1 - Mishurova, Tatiana A1 - Artzt, Katia A1 - Haubrich, Jan A1 - Requena, Guillermo A1 - Bruno, Giovanni T1 - New aspects about the search for the most relevant parameters optimizing SLM materials JF - Additive manufacturing N2 - While the volumetric energy density is commonly used to qualify a process parameter set, and to quantify its influence on the microstructure and performance of additively manufactured (AM) materials and components, it has been already shown that this description is by no means exhaustive. In this work, new aspects of the optimization of the selective laser melting process are investigated for AM Ti-6Al-4V. We focus on the amount of near-surface residual stress (RS), often blamed for the failure of components, and on the porosity characteristics (amount and spatial distribution). First, using synchrotron x-ray diffraction we show that higher RS in the subsurface region is generated if a lower energy density is used. Second, we show that laser de-focusing and sample positioning inside the build chamber also play an eminent role, and we quantify this influence. In parallel, using X-ray Computed Tomography, we observe that porosity is mainly concentrated in the contour region, except in the case where the laser speed is small. The low values of porosity (less than 1%) do not influence RS. KW - SLM KW - Ti-6Al-4V KW - X-ray synchrotron diffraction KW - Computed tomography KW - Residual stress Y1 - 2018 U6 - https://doi.org/10.1016/j.addma.2018.11.023 SN - 2214-8604 SN - 2214-7810 VL - 25 SP - 325 EP - 334 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Stegemann, Robert A1 - Cabeza, Sandra A1 - Pelkner, Matthias A1 - Lyamkin, Viktor A1 - Pittner, Andreas A1 - Werner, Daniel A1 - Wimpory, Robert A1 - Boin, Mirko A1 - Kreutzbruck, Marc A1 - Bruno, Giovanni T1 - Influence of the microstructure on magnetic stray fields of low-carbon steel welds JF - Journal of Nondestructive Evaluation N2 - This study examines the relationship between the magnetic mesostructure with the microstructure of low carbon steel tungsten inert gas welds. Optical microscopy revealed variation in the microstructure of the parent material, in the heat affected and fusion zones, correlating with distinctive changes in the local magnetic stray fields measured with high spatial resolution giant magneto resistance sensors. In the vicinity of the heat affected zone high residual stresses were found using neutron diffraction. Notably, the gradients of von Mises stress and triaxial magnetic stray field modulus follow the same tendency transverse to the weld. In contrast, micro-X-ray fluorescence characterization indicated that local changes in element composition had no independent effect on magnetic stray fields. KW - TIG-welding KW - GMR sensors KW - Magnetic stray field KW - Neutron diffraction KW - Residual stress KW - Microstructure KW - Low carbon steel Y1 - 2018 U6 - https://doi.org/10.1007/s10921-018-0522-0 SN - 0195-9298 SN - 1573-4862 VL - 37 IS - 3 PB - Springer CY - New York ER - TY - JOUR A1 - Fernandez, R. A1 - Cabeza, Sandra A1 - Mishurova, Tatiana A1 - Fernandez-Castrillo, P. A1 - Gonzalez-Doncel, Gaspar A1 - Bruno, Giovanni T1 - Residual stress and yield strength evolution with annealing treatments in an age-hardenable aluminum alloy matrix composite JF - Materials Science and Engineering: A N2 - We investigated the possibility of minimizing tensile matrix residual stresses in age hardenable aluminum alloy metal matrix composites without detrimentally affect their mechanical properties (such as yield strength). Specifically, we performed thermal treatments at different temperatures and times in an age-hardenable aluminum matrix composite 2014Al-15vol%Al2O3. Using X-ray synchrotron radiation diffraction and mechanical tests, we show that below a certain treatment temperature (250 degrees C) it is possible to identify an appropriate thermal treatment capable of relaxing residual stress in this composite while even increasing its yield strength, with respect to the as processed conditions. KW - Residual stress KW - Yield strength KW - Annealing treatment KW - Metal matrix composite KW - Diffraction Y1 - 2018 U6 - https://doi.org/10.1016/j.msea.2018.06.031 SN - 0921-5093 SN - 1873-4936 VL - 731 SP - 344 EP - 350 PB - Elsevier CY - Lausanne ER - TY - JOUR A1 - Buljak, Vladimir A1 - Bruno, Giovanni T1 - Numerical modeling of thermally induced microcracking in porous ceramics BT - an approach using cohesive elements JF - Journal of the European Ceramic Society N2 - A numerical framework is developed to study the hysteresis of elastic properties of porous ceramics as a function of temperature. The developed numerical model is capable of employing experimentally measured crystallographic orientation distribution and coefficient of thermal expansion values. For realistic modeling of the microstructure, Voronoi polygons are used to generate polycrystalline grains. Some grains are considered as voids, to simulate the material porosity. To model intercrystalline cracking, cohesive elements are inserted along grain boundaries. Crack healing (recovery of the initial properties) upon closure is taken into account with special cohesive elements implemented in the commercial code ABAQUS. The numerical model can be used to estimate fracture properties governing the cohesive behavior through inverse analysis procedure. The model is applied to a porous cordierite ceramic. The obtained fracture properties are further used to successfully simulate general non-linear macroscopic stress-strain curves of cordierite, thereby validating the model. KW - analysis KW - Cohesive finite elements KW - Interfacial strength Y1 - 2018 U6 - https://doi.org/10.1016/j.jeurceramsoc.2018.03.041 SN - 0955-2219 SN - 1873-619X VL - 38 IS - 11 SP - 4099 EP - 4108 PB - Elsevier CY - Oxford ER - TY - JOUR A1 - Sevostianov, Igor A1 - Bruno, Giovanni T1 - Maxwell scheme for internal stresses in multiphase composites JF - Mechanics of Materials N2 - The paper focuses on the reformulation of classic Maxwell's (1873) homogenization method for calculation of the residual stresses in matrix composites. For this goal, we equate the far fields produced by a set of inhomogeneities subjected to known eigenstrains and by a fictitious domain with unknown eigenstrain. The effect of interaction between the inhomogeneities is reduced to the calculation of the additional field acting on an inhomogeneity due to the eigenstrains in its neighbors. An explicit formula for residual stresses is derived for the general case of a multiphase composite. The method is illustrated by several examples. The results are compared with available experimental data as well as with predictions provided by the non-interaction approximation (Eshelby solution). It is shown that accounting for interaction can explain many experimentally observed phenomena and is required for adequate quantitative analytical modeling of the residual stresses in matrix composites. KW - Residual stress KW - Multiphase composites KW - Interaction KW - Micromechanical schemes KW - Anisotropy Y1 - 2018 U6 - https://doi.org/10.1016/j.mechmat.2018.12.005 SN - 0167-6636 SN - 1872-7743 VL - 129 SP - 320 EP - 331 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Bruno, Giovanni A1 - Kachanov, Mark A1 - Sevostianov, Igor A1 - Shyam, Amit T1 - Micromechanical modeling of non-linear stress-strain behavior of polycrystalline microcracked materials under tension JF - Acta materialia N2 - The stress-strain behavior of microcracked polycrystalline materials (such as ceramics or rocks) under conditions of tensile, displacement-controlled, loading is discussed. Micromechanical explanation and modeling of the basic features, such as non-linearity and hysteresis in stress-strain curves, is developed, with stable microcrack propagation and "roughness" of intergranular cracks playing critical roles. Experiments involving complex loading histories were done on large- and medium grain size beta-eucryptite ceramic. The model is shown to reproduce the basic features of the observed stress-strain curves. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. KW - Nonlinearity KW - Stress-strain relations KW - Hysteresis KW - Tension KW - Ceramics KW - Rocks KW - Polycrystals Y1 - 2018 U6 - https://doi.org/10.1016/j.actamat.2018.10.024 SN - 1359-6454 SN - 1873-2453 VL - 164 SP - 50 EP - 59 PB - Elsevier CY - Oxford ER - TY - JOUR A1 - Nellesen, Jens A1 - Laquai, R. A1 - Müller, B. R. A1 - Kupsch, Andreas A1 - Hentschel, M. P. A1 - Anar, N. B. A1 - Soppa, E. A1 - Tillmann, W. A1 - Bruno, Giovanni T1 - In situ analysis of damage evolution in an Al/ Al2O3 MMC under tensile load by synchrotron X-ray refraction imaging JF - Journal of materials science N2 - The in situ analysis of the damage evolution in a metal matrix composite (MMC) using synchrotron X-ray refraction radiography (SXRR) is presented. The investigated material is an Al alloy (6061)/10 vol MMC after T6 heat treatment. In an interrupted tensile test the gauge section of dog bone-shaped specimens is imaged in different states of tensile loading. On the basis of the SXRR images, the relative change of the specific surface (proportional to the amount of damage) in the course of tensile loading was analyzed. It could be shown that the damage can be detected by SXRR already at a stage of tensile loading, in which no observation of damage is possible with radiographic absorption-based imaging methods. Moreover, the quantitative analysis of the SXRR images reveals that the amount of damage increases homogeneously by an average of 25% with respect to the initial state. To corroborate the experimental findings, the damage distribution was imaged in 3D after the final tensile loading by synchrotron X-ray refraction computed tomography (SXRCT) and absorption-based synchrotron X-ray computed tomography (SXCT). It could be evidenced that defects and damages cause pronounced indications in the SXRCT images. Y1 - 2018 U6 - https://doi.org/10.1007/s10853-017-1957-x SN - 0022-2461 SN - 1573-4803 VL - 53 IS - 8 SP - 6021 EP - 6032 PB - Springer CY - New York ER - TY - JOUR A1 - Mishurova, Tatiana A1 - Cabeza, Sandra A1 - Thiede, Tobias A1 - Nadammal, Naresh A1 - Kromm, Arne A1 - Klaus, Manuela A1 - Genzel, Christoph A1 - Haberland, Christoph A1 - Bruno, Giovanni T1 - The Influence of the Support Structure on Residual Stress and Distortion in SLM Inconel 718 Parts JF - Metallurgical and Materials Transactions A N2 - The effect of support structure and of removal from the base plate on the residual stress state in selective laser melted IN718 parts was studied by means of synchrotron X-ray diffraction. The residual stresses in subsurface region of two elongated prisms in as-built condition and after removal from the base plate were determined. One sample was directly built on a base plate and another one on a support structure. Also, the distortion on the top surface due to stress release was measured by contact profilometry. High tensile residual stress values were found, with pronounced stress gradient along the hatching direction. In the sample on support, stress redistribution took place after removal from the base plate, as opposed to simple stress relaxation for the sample without support. The sample on support structure showed larger distortion compared to sample without support. We conclude that the use of a support decreases stress values but stress-relieving heat treatments are still needed. Y1 - 2018 U6 - https://doi.org/10.1007/s11661-018-4653-9 SN - 1073-5623 SN - 1543-1940 VL - 49A IS - 7 SP - 3038 EP - 3046 PB - Springer CY - New York ER - TY - JOUR A1 - Cabeza, Sandra A1 - Müller, Bernd R. A1 - Pereyra, Ricio A1 - Fernandez, Ricardo A1 - Gonzalez-Doncel, Gaspar A1 - Bruno, Giovanni T1 - Evidence of damage evolution during creep of Al-Mg alloy using synchrotron X-ray refraction JF - Journal of applied crystallography N2 - In order to provide further evidence of damage mechanisms predicted by the recent solid-state transformation creep (SSTC) model, direct observation of damage accumulation during creep of Al-3.85Mg was made using synchrotron X-ray refraction. X-ray refraction techniques detect the internal specific surface (i.e. surface per unit volume) on a length scale comparable to the specimen size, but with microscopic sensitivity. A significant rise in the internal specific surface with increasing creep time was observed, providing evidence for the creation of a fine grain substructure, as predicted by the SSTC model. This substructure was also observed by scanning electron microscopy. KW - aluminium alloys KW - creep KW - damage KW - synchrotron X-ray refraction KW - electron microscopy KW - subgrain structure Y1 - 2018 U6 - https://doi.org/10.1107/S1600576718001449 SN - 1600-5767 VL - 51 SP - 420 EP - 427 PB - International Union of Crystallography CY - Chester ER - TY - JOUR A1 - Fernandez Serrano, Ricardo A1 - Bruno, Giovanni A1 - Gonzalez-Doncel, Gaspar T1 - Fractal nature of aluminum alloys substructures under creep and its implications JF - Journal of applied physics N2 - The present work offers an explanation for the variation of the power-law stress exponent, n, with the stress sigma normalized to the shear modulus G in aluminum alloys. The approach is based on the assumption that the dislocation structure generated with deformation has a fractal nature. It fully explains the evolution of n with sigma/G even beyond the so-called power law breakdown region. Creep data from commercially pure Al99.8%, Al-3.85%Mg, and ingot AA6061 alloy tested at different temperatures and stresses are used to validate the proposed ideas. Finally, it is also shown that the fractal description of the dislocation structure agrees well with current knowledge. Published by AIP Publishing. Y1 - 2018 U6 - https://doi.org/10.1063/1.5012035 SN - 0021-8979 SN - 1089-7550 VL - 123 IS - 14 PB - American Institute of Physics CY - Melville ER - TY - JOUR A1 - Zhang, Heshou A1 - Yan, Huirong T1 - Polarization of submillimetre lines from interstellar medium JF - Monthly notices of the Royal Astronomical Society N2 - Magnetic fields play important roles in many astrophysical processes. However, there is no universal diagnostic for the magnetic fields in the interstellar medium (ISM) and each magnetic tracer has its limitation. Any new detection method is thus valuable. Theoretical studies have shown that submillimetre fine-structure lines are polarized due to atomic alignment by ultraviolet photon-excitation, which opens up a new avenue to probe interstellar magnetic fields. We will, for the first time, perform synthetic observations on the simulated three-dimensional ISM to demonstrate the measurability of the polarization of submillimetre atomic lines. The maximum polarization for different absorption and emission lines expected from various sources, including star-forming regions are provided. Our results demonstrate that the polarization of submillimetre atomic lines is a powerful magnetic tracer and add great value to the observational studies of the submilimetre astronomy. KW - polarization KW - turbulence KW - H II regions KW - ISM: magnetic fields KW - photodissociation region (PDR) KW - submillimetre: ISM Y1 - 2017 U6 - https://doi.org/10.1093/mnras/stx3164 SN - 0035-8711 SN - 1365-2966 VL - 475 IS - 2 SP - 2415 EP - 2420 PB - Oxford University Press CY - Oxford ER - TY - JOUR A1 - Henkel, Carsten A1 - Klimchitskaya, G. L. A1 - Mostepanenko, V. M. T1 - Influence of the chemical potential on the Casimir-Polder interaction between an atom and gapped graphene or a graphene-coated substrate JF - Physical review : A, Atomic, molecular, and optical physics N2 - We present a formalism based on first principles of quantum electrodynamics at nonzero temperature which permits us to calculate the Casimir-Polder interaction between an atom and a graphene sheet with arbitrary mass gap and chemical potential, including graphene-coated substrates. The free energy and force of the Casimir-Polder interaction are expressed via the polarization tensor of graphene in (2 + 1)-dimensional space-time in the framework of the Dirac model. The obtained expressions are used to investigate the influence of the chemical potential of graphene on the Casimir-Polder interaction. Computations are performed for an atom of metastable helium interacting with either a freestanding graphene sheet or a graphene-coated substrate made of amorphous silica. It is shown that the impacts of the nonzero chemical potential and the mass gap on the Casimir-Polder interaction are in opposite directions, by increasing and decreasing the magnitudes of the free energy and force, respectively. It turns out, however, that the temperature-dependent part of the Casimir-Polder interaction is decreased by a nonzero chemical potential, whereas the mass gap increases it compared to the case of undoped, gapless graphene. The physical explanation for these effects is provided. Numerical computations of the Casimir-Polder interaction are performed at various temperatures and atom-graphene separations. Y1 - 2018 U6 - https://doi.org/10.1103/PhysRevA.97.032504 SN - 2469-9926 SN - 2469-9934 VL - 97 IS - 3 PB - American Physical Society CY - College Park ER - TY - JOUR A1 - Bouchoule, Isabelle A1 - Schemmer, Max A1 - Henkel, Carsten T1 - Cooling phonon modes of a Bose condensate with uniform few body losses JF - Scipost Physics N2 - We present a general analysis of the cooling produced by losses on condensates or quasi-condensates. We study how the occupations of the collective phonon modes evolve in time, assuming that the loss process is slow enough so that each mode adiabatically follows the decrease of the mean density. The theory is valid for any loss process whose rate is proportional to the jth power of the density, but otherwise spatially uniform. We cover both homogeneous gases and systems confined in a smooth potential. For a low-dimensional gas, we can take into account the modified equation of state due to the broadening of the cloud width along the tightly confined directions, which occurs for large interactions. We find that at large times, the temperature decreases proportionally to the energy scale mc(2), where m is the mass of the particles and c the sound velocity. We compute the asymptotic ratio of these two quantities for different limiting cases: a homogeneous gas in any dimension and a one-dimensional gas in a harmonic trap. Y1 - 2018 U6 - https://doi.org/10.21468/SciPostPhys.5.5.043 SN - 2542-4653 VL - 5 IS - 5 PB - Scipost foundation CY - Amsterdam ER - TY - JOUR A1 - Weilbacher, Peter Michael A1 - Monreal-Ibero, Ana A1 - Verhamme, Anne A1 - Sandin, Christer A1 - Steinmetz, Matthias A1 - Kollatschny, Wolfram A1 - Krajnovic, Davor A1 - Kamann, Sebastian A1 - Roth, Martin M. A1 - Erroz-Ferrer, Santiago A1 - Marino, Raffaella Anna A1 - Maseda, Michael V. A1 - Wendt, Martin A1 - Bacon, Roland A1 - Dreizler, Stefan A1 - Richard, Johan A1 - Wisotzki, Lutz T1 - Lyman-continuum leakage as dominant source of diffuse ionized gas in the Antennae galaxy JF - Astronomy and astrophysics : an international weekly journal N2 - The Antennae galaxy (NGC 4038/39) is the closest major interacting galaxy system and is therefore often studied as a merger prototype. We present the first comprehensive integral field spectroscopic dataset of this system, observed with the MUSE instrument at the ESO VLT. We cover the two regions in this system which exhibit recent star formation: the central galaxy interaction and a region near the tip of the southern tidal tail. In these fields, we detect H II regions and diffuse ionized gas to unprecedented depth. About 15% of the ionized gas was undetected by previous observing campaigns. This newly detected faint ionized gas is visible everywhere around the central merger, and shows filamentary structure. We estimate diffuse gas fractions of about 60% in the central field and 10% in the southern region. We are able to show that the southern region contains a significantly different population of H II regions, showing fainter luminosities. By comparing H II region luminosities with the HST catalog of young star clusters in the central field, we estimate that there is enough Lyman-continuum leakage in the merger to explain the amount of diffuse ionized gas that we detect. We compare the Lyman-continuum escape fraction of each H II region against emission line ratios that are sensitive to the ionization parameter. While we find no systematic trend between these properties, the most extreme line ratios seem to be strong indicators of density bounded ionization. Extrapolating the Lyman-continuum escape fractions to the southern region, we conclude that simply from the comparison of the young stellar populations to the ionized gas there is no need to invoke other ionization mechanisms than Lyman-continuum leaking H II regions for the diffuse ionized gas in the Antennae. KW - galaxies: interactions KW - galaxies: individual: NGC 4038, NGC 4039 KW - galaxies: ISM KW - ISM: structure KW - H II regions Y1 - 2018 U6 - https://doi.org/10.1051/0004-6361/201731669 SN - 1432-0746 VL - 611 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Kretschmer, Marlene A1 - Cohen, Judah A1 - Matthias, Vivien A1 - Runge, Jakob A1 - Coumou, Dim T1 - The different stratospheric influence on cold-extremes in Eurasia and North America JF - npj Climate and Atmospheric Science N2 - The stratospheric polar vortex can influence the tropospheric circulation and thereby winter weather in the mid-latitudes. Weak vortex states, often associated with sudden stratospheric warmings (SSW), have been shown to increase the risk of cold-spells especially over Eurasia, but its role for North American winters is less clear. Using cluster analysis, we show that there are two dominant patterns of increased polar cap heights in the lower stratosphere. Both patterns represent a weak polar vortex but they are associated with different wave mechanisms and different regional tropospheric impacts. The first pattern is zonally symmetric and associated with absorbed upward-propagating wave activity, leading to a negative phase of the North Atlantic Oscillation (NAO) and cold-air outbreaks over northern Eurasia. This coupling mechanism is well-documented in the literature and is consistent with the downward migration of the northern annular mode (NAM). The second pattern is zonally asymmetric and linked to downward reflected planetary waves over Canada followed by a negative phase of the Western Pacific Oscillation (WPO) and cold-spells in Central Canada and the Great Lakes region. Causal effect network (CEN) analyses confirm the atmospheric pathways associated with this asymmetric pattern. Moreover, our findings suggest the reflective mechanism to be sensitive to the exact region of upward wave-activity fluxes and to be state-dependent on the strength of the vortex. Identifying the causal pathways that operate on weekly to monthly timescales can pave the way for improved sub-seasonal to seasonal forecasting of cold spells in the mid-latitudes. Y1 - 2018 U6 - https://doi.org/10.1038/s41612-018-0054-4 SN - 2397-3722 VL - 1 PB - Nature Publ. Group CY - London ER - TY - JOUR A1 - Sposini, Vittoria A1 - Chechkin, Aleksei V. A1 - Seno, Flavio A1 - Pagnini, Gianni A1 - Metzler, Ralf T1 - Random diffusivity from stochastic equations BT - comparison of two models for Brownian yet non-Gaussian diffusion JF - New Journal of Physics N2 - A considerable number of systems have recently been reported in which Brownian yet non-Gaussian dynamics was observed. These are processes characterised by a linear growth in time of the mean squared displacement, yet the probability density function of the particle displacement is distinctly non-Gaussian, and often of exponential(Laplace) shape. This apparently ubiquitous behaviour observed in very different physical systems has been interpreted as resulting from diffusion in inhomogeneous environments and mathematically represented through a variable, stochastic diffusion coefficient. Indeed different models describing a fluctuating diffusivity have been studied. Here we present a new view of the stochastic basis describing time dependent random diffusivities within a broad spectrum of distributions. Concretely, our study is based on the very generic class of the generalised Gamma distribution. Two models for the particle spreading in such random diffusivity settings are studied. The first belongs to the class of generalised grey Brownian motion while the second follows from the idea of diffusing diffusivities. The two processes exhibit significant characteristics which reproduce experimental results from different biological and physical systems. We promote these two physical models for the description of stochastic particle motion in complex environments. Y1 - 2018 U6 - https://doi.org/10.1088/1367-2630/aab696 SN - 1367-2630 SP - 1 EP - 33 PB - Deutsche Physikalische Gesellschaft / Institute of Physics CY - Bad Honnef und London ER - TY - JOUR A1 - Ślęzak, Jakub A1 - Metzler, Ralf A1 - Magdziarz, Marcin T1 - Superstatistical generalised Langevin equation BT - non-Gaussian viscoelastic anomalous diffusion JF - New Journal of Physics N2 - Recent advances in single particle tracking and supercomputing techniques demonstrate the emergence of normal or anomalous, viscoelastic diffusion in conjunction with non-Gaussian distributions in soft, biological, and active matter systems. We here formulate a stochastic model based on a generalised Langevin equation in which non-Gaussian shapes of the probability density function and normal or anomalous diffusion have a common origin, namely a random parametrisation of the stochastic force. We perform a detailed analysis demonstrating how various types of parameter distributions for the memory kernel result in exponential, power law, or power-log law tails of the memory functions. The studied system is also shown to exhibit a further unusual property: the velocity has a Gaussian one point probability density but non-Gaussian joint distributions. This behaviour is reflected in the relaxation from a Gaussian to a non-Gaussian distribution observed for the position variable. We show that our theoretical results are in excellent agreement with stochastic simulations. KW - anomalous diffusion KW - generalised langevin equation KW - superstatistics KW - non-Gaussian diffusion Y1 - 2018 U6 - https://doi.org/10.1088/1367-2630/aaa3d4 SN - 1367-2630 VL - 20 IS - 023026 SP - 1 EP - 25 PB - Deutsche Physikalische Gesellschaft / Institute of Physics CY - Bad Honnef und London ER - TY - JOUR A1 - Goldobin, Denis S. A1 - Tyulkina, Irina V. A1 - Klimenko, Lyudmila S. A1 - Pikovskij, Arkadij T1 - Collective mode reductions for populations of coupled noisy oscillators JF - Chaos : an interdisciplinary journal of nonlinear science N2 - We analyze the accuracy of different low-dimensional reductions of the collective dynamics in large populations of coupled phase oscillators with intrinsic noise. Three approximations are considered: (i) the Ott-Antonsen ansatz, (ii) the Gaussian ansatz, and (iii) a two-cumulant truncation of the circular cumulant representation of the original system’s dynamics. For the latter, we suggest a closure, which makes the truncation, for small noise, a rigorous first-order correction to the Ott-Antonsen ansatz, and simultaneously is a generalization of the Gaussian ansatz. The Kuramoto model with intrinsic noise and the population of identical noisy active rotators in excitable states with the Kuramoto-type coupling are considered as examples to test the validity of these approximations. For all considered cases, the Gaussian ansatz is found to be more accurate than the Ott-Antonsen one for high-synchrony states only. The two-cumulant approximation is always superior to both other approximations. Synchrony of large ensembles of coupled elements can be characterised by the order parameters—the mean fields. Quite often, the evolution of these collective variables is surprisingly simple, which makes a description with only a few order parameters feasible. Thus, one tries to construct accurate closed low-dimensional mathematical models for the dynamics of the first few order parameters. These models represent useful tools for gaining insight into the underlaying mechanisms of some more sophisticated collective phenomena: for example, one describes coupled populations by virtue of coupled equations for the relevant order parameters. A regular approach to the construction of closed low-dimensional systems is also beneficial for dealing with phenomena, which are beyond the applicability scope of these models; for instance, with such an approach, one can determine constraints on clustering in populations. There are two prominent types of situations, where the low-dimensional models can be constructed: (i) for a certain class of ideal paradigmatic systems of coupled phase oscillators, the Ott-Antonsen ansatz yields an exact equation for the main order parameter and (ii) the Gaussian approximation for the probability density of the phases, also yielding a low-dimensional closure, is frequently quite accurate. In this paper, we compare applications of these two model reductions for situations, where neither of them is perfectly accurate. Furthermore, we construct a new reduction approach which practically works as a first-order correction to the best of the two basic approximations. Y1 - 2018 U6 - https://doi.org/10.1063/1.5053576 SN - 1054-1500 SN - 1089-7682 VL - 28 IS - 10 PB - American Institute of Physics CY - Melville ER - TY - JOUR A1 - Rosenblum, Michael A1 - Pikovskij, Arkadij T1 - Efficient determination of synchronization domains from observations of asynchronous dynamics JF - Chaos : an interdisciplinary journal of nonlinear science N2 - We develop an approach for a fast experimental inference of synchronization properties of an oscillator. While the standard technique for determination of synchronization domains implies that the oscillator under study is forced with many different frequencies and amplitudes, our approach requires only several observations of a driven system. Reconstructing the phase dynamics from data, we successfully determine synchronization domains of noisy and chaotic oscillators. Our technique is especially important for experiments with living systems where an external action can be harmful and shall be minimized. Published by AIP Publishing. Y1 - 2018 U6 - https://doi.org/10.1063/1.5037012 SN - 1054-1500 SN - 1089-7682 VL - 28 IS - 10 PB - American Institute of Physics CY - Melville ER - TY - JOUR A1 - Zheng, Chunming A1 - Pikovskij, Arkadij T1 - Delay-induced stochastic bursting in excitable noisy systems JF - Physical review : E, Statistical, nonlinear and soft matter physics N2 - We show that a combined action of noise and delayed feedback on an excitable theta-neuron leads to rather coherent stochastic bursting. An idealized point process, valid if the characteristic timescales in the problem are well separated, is used to describe statistical properties such as the power spectral density and the interspike interval distribution. We show how the main parameters of the point process, the spontaneous excitation rate, and the probability to induce a spike during the delay action can be calculated from the solutions of a stationary and a forced Fokker-Planck equation. Y1 - 2018 U6 - https://doi.org/10.1103/PhysRevE.98.042148 SN - 2470-0045 SN - 2470-0053 VL - 98 IS - 4 PB - American Physical Society CY - College Park ER - TY - JOUR A1 - Peter, Franziska A1 - Pikovskij, Arkadij T1 - Transition to collective oscillations in finite Kuramoto ensembles JF - Physical review : E, Statistical, nonlinear and soft matter physics N2 - We present an alternative approach to finite-size effects around the synchronization transition in the standard Kuramoto model. Our main focus lies on the conditions under which a collective oscillatory mode is well defined. For this purpose, the minimal value of the amplitude of the complex Kuramoto order parameter appears as a proper indicator. The dependence of this minimum on coupling strength varies due to sampling variations and correlates with the sample kurtosis of the natural frequency distribution. The skewness of the frequency sample determines the frequency of the resulting collective mode. The effects of kurtosis and skewness hold in the thermodynamic limit of infinite ensembles. We prove this by integrating a self-consistency equation for the complex Kuramoto order parameter for two families of distributions with controlled kurtosis and skewness, respectively. Y1 - 2018 U6 - https://doi.org/10.1103/PhysRevE.97.032310 SN - 2470-0045 SN - 2470-0053 VL - 97 IS - 3 PB - American Physical Society CY - College Park ER - TY - JOUR A1 - Pikovskij, Arkadij T1 - Reconstruction of a random phase dynamics network from observations JF - Physics letters : A N2 - We consider networks of coupled phase oscillators of different complexity: Kuramoto–Daido-type networks, generalized Winfree networks, and hypernetworks with triple interactions. For these setups an inverse problem of reconstruction of the network connections and of the coupling function from the observations of the phase dynamics is addressed. We show how a reconstruction based on the minimization of the squared error can be implemented in all these cases. Examples include random networks with full disorder both in the connections and in the coupling functions, as well as networks where the coupling functions are taken from experimental data of electrochemical oscillators. The method can be directly applied to asynchronous dynamics of units, while in the case of synchrony, additional phase resettings are necessary for reconstruction. KW - Phase dynamics KW - Network reconstruction Y1 - 2017 U6 - https://doi.org/10.1016/j.physleta.2017.11.012 SN - 0375-9601 SN - 1873-2429 VL - 382 IS - 4 SP - 147 EP - 152 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Grebenkov, Denis S. A1 - Metzler, Ralf A1 - Oshanin, Gleb T1 - Towards a full quantitative description of single-molecule reaction kinetics in biological cells JF - Physical chemistry, chemical physics : a journal of European Chemical Societies N2 - The first-passage time (FPT), i.e., the moment when a stochastic process reaches a given threshold value for the first time, is a fundamental mathematical concept with immediate applications. In particular, it quantifies the statistics of instances when biomolecules in a biological cell reach their specific binding sites and trigger cellular regulation. Typically, the first-passage properties are given in terms of mean first-passage times. However, modern experiments now monitor single-molecular binding-processes in living cells and thus provide access to the full statistics of the underlying first-passage events, in particular, inherent cell-to-cell fluctuations. We here present a robust explicit approach for obtaining the distribution of FPTs to a small partially reactive target in cylindrical-annulus domains, which represent typical bacterial and neuronal cell shapes. We investigate various asymptotic behaviours of this FPT distribution and show that it is typically very broad in many biological situations, thus, the mean FPT can differ from the most probable FPT by orders of magnitude. The most probable FPT is shown to strongly depend only on the starting position within the geometry and to be almost independent of the target size and reactivity. These findings demonstrate the dramatic relevance of knowing the full distribution of FPTs and thus open new perspectives for a more reliable description of many intracellular processes initiated by the arrival of one or few biomolecules to a small, spatially localised region inside the cell. Y1 - 2018 U6 - https://doi.org/10.1039/c8cp02043d SN - 1463-9076 SN - 1463-9084 VL - 20 IS - 24 SP - 16393 EP - 16401 PB - Royal Society of Chemistry CY - Cambridge ER - TY - JOUR A1 - Lopez, Carlos G. A1 - Manova, Anna A1 - Hoppe, Corinna A1 - Dreja, Michael A1 - Schmiedel, Peter A1 - Job, Mareile A1 - Richtering, Walter A1 - Böker, Alexander A1 - Tsarkova, Larisa A. T1 - Combined UV-Vis-absorbance and reflectance spectroscopy study of dye transfer kinetics in aqueous mixtures of surfactants JF - Colloids and surfaces : an international journal devoted to the principles and applications of colloid and interface science ; A, Physicochemical and engineering aspects N2 - We report an analytical approach to study the competitive processes of solubilisation in micelles and of adsorption onto hydrophobic surfaces of poorly soluble hydrophobic dyes. The method is demonstrated on model systems containing two sources of Disperse Red 60: a bulk powder and a donor red textile, with molecularly dissolved dye stabilised in an aqueous environment by mixed micelles of anionic and non-ionic surfactants. The process of dye transfer between a donor textile (red polyester), surfactant micelles and an acceptor textile (white polyamide) was quantified by a combination of colorimetric analyses. UV-Vis absorbance was used to follow the extraction of the dye and to evaluate the solubilisation capacity of the micellar solution. A calibration curve for textile reflectance versus the adsorbed dye was generated to quantify the mass of dye transferred onto the acceptor textile. A combination of both techniques allowed us to compare the amount of dye desorbed from the donor textile and adsorbed onto the acceptor textile as a function of time for systems undergoing exhaustion-solubilisation mechanisms and only solubilisation mechanism. Up to similar or equal to 10 min of the washing process, the released dye is predominantly solubilised in surfactant micelles. At later times, the adsorption of the dye on the hydrophobic surface is energetically favoured. The shift of the desorption equilibrium in the presence of the acceptor textile results in similar or equal to 30% increase in the release of the dye. The reported methodology provides direct comparative analysis between the solubilisation capacity of amphiphilic stabilisers and the tendency of the dye to adsorb on solid substrates, important for designing novel concepts of disperse dye solubilisation and dye transfer inhibition during textile washing. KW - Dye transfer KW - Disperse dyes KW - Surfactants KW - Detergent KW - Colorimetric analysis KW - Washing fastness Y1 - 2018 U6 - https://doi.org/10.1016/j.colsurfa.2018.04.024 SN - 0927-7757 SN - 1873-4359 VL - 550 SP - 74 EP - 81 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Gruner, David A1 - Hainich, Rainer A1 - Sander, Andreas Alexander Christoph A1 - Shenar, Tomer A1 - Todt, Helge Tobias A1 - Oskinova, Lida A1 - Ramachandran, Varsha A1 - Ayres, T. A1 - Hamann, Wolf-Rainer T1 - The extreme O-type spectroscopic binary HD 93129A A quantitative, multiwavelength analysis JF - Astronomy and astrophysics : an international weekly journal N2 - Context. HD 93129A was classified as the earliest O-type star in the Galaxy (O2 If*) and is considered as the prototype of its spectral class. However, interferometry shows that this object is a binary system, while recent observations even suggest a triple configuration. None of the previous spectral analyses of this object accounted for its multiplicity. With new high-resolution UV and optical spectra, we have the possibility to reanalyze this key object, taking its binary nature into account for the first time. Aims. We aim to derive the fundamental parameters and the evolutionary status of HD 93129A, identifying the contributions of both components to the composite spectrum Results. Despite the similar spectral types of the two components, we are able to find signatures from each of the components in the combined spectrum, which allows us to estimate the parameters of both stars. We derive log(L/L-circle dot) = 6.15, T-eff = 52 kK, and log (M)over dot = -4.7[M-circle dot yr(-1)] for the primary Aa, and log(L/L-circle dot) = 5.58, T-eff = 45 kK, and log (M)over dot = -5.8 [M(circle dot)yr(-1)] for the secondary Ab. Conclusions. Even when accounting for the binary nature, the primary of HD 93129A is found to be one of the hottest and most luminous O stars in our Galaxy. Based on the theoretical decomposition of the spectra, we assign spectral types O2 If* and O3 III(f*) to components Aa and Ab, respectively. While we achieve a good fit for a wide spectral range, specific spectral features are not fully reproduced. The data are not sufficient to identify contributions from a hypothetical third component in the system. KW - stars: individual: HD 93129A KW - stars: atmospheres KW - stars: fundamental parameters KW - stars: early-typeP Y1 - 2018 U6 - https://doi.org/10.1051/0004-6361/201833178 SN - 1432-0746 VL - 621 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Leto, Paolo A1 - Trigilio, C. A1 - Oskinova, Lida A1 - Ignace, R. A1 - Buemi, C. S. A1 - Umana, G. A1 - Ingallinera, A. A1 - Leone, Francesco A1 - Phillips, N. M. A1 - Agliozzo, Claudia A1 - Todt, Helge Tobias A1 - Cerrigone, L. T1 - A combined multiwavelength VLA/ALMA/Chandra study unveils the complex magnetosphere of the B-type star HR5907 JF - Monthly notices of the Royal Astronomical Society N2 - We present new radio/millimeter measurements of the hot magnetic star HR5907 obtained with the VLA and ALMA interferometers. We find that HR5907 is the most radio luminous early type star in the cm-mm band among those presently known. Its multi-wavelength radio light curves are strongly variable with an amplitude that increases with radio frequency. The radio emission can be explained by the populations of the non-thermal electrons accelerated in the current sheets on the outer border of the magnetosphere of this fast-rotating magnetic star. We classify HR5907 as another member of the growing class of strongly magnetic fast-rotating hot stars where the gyro-synchrotron emission mechanism efficiently operates in their magnetospheres. The new radio observations of HR5907 are combined with archival X-ray data to study the physical condition of its magnetosphere. The X-ray spectra of HR5907 show tentative evidence for the presence of non-thermal spectral component. We suggest that non-thermal X-rays originate a stellar X-ray aurora due to streams of non-thermal electrons impacting on the stellar surface. Taking advantage of the relation between the spectral indices of the X-ray power-law spectrum and the non-thermal electron energy distributions, we perform 3-D modelling of the radio emission for HR5907. The wavelength-dependent radio light curves probe magnetospheric layers at different heights above the stellar surface. A detailed comparison between simulated and observed radio light curves leads us to conclude that the stellar magnetic field of HR 5907 is likely non-dipolar, providing further indirect evidence of the complex magnetic field topology of HR5907. KW - stars: chemically peculiar KW - stars: early-type KW - stars: individual: HR 5907 KW - stars: magnetic field KW - radio continuum: stars KW - X-rays: stars Y1 - 2018 U6 - https://doi.org/10.1093/mnras/sty244 SN - 0035-8711 SN - 1365-2966 VL - 476 IS - 1 SP - 562 EP - 579 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Dereudre, David A1 - Houdebert, Pierre T1 - Sharp phase transition for the continuum Widom-Rowlinson model JF - Annales de l'Institut Henri Poincaré. B, Probability and statistics N2 - The Widom-Rowlinson model (or the Area-interaction model) is a Gibbs point process in R-d with the formal Hamiltonian defined as the volume of Ux epsilon omega B1(x), where. is a locally finite configuration of points and B-1(x) denotes the unit closed ball centred at x. The model is also tuned by two other parameters: the activity z > 0 related to the intensity of the process and the inverse temperature beta >= 0 related to the strength of the interaction. In the present paper we investigate the phase transition of the model in the point of view of percolation theory and the liquid-gas transition. First, considering the graph connecting points with distance smaller than 2r > 0, we show that for any beta >= 0, there exists 0 <(similar to a)(zc) (beta, r) < +infinity such that an exponential decay of connectivity at distance n occurs in the subcritical phase (i.e. z <(similar to a)(zc) (beta, r)) and a linear lower bound of the connection at infinity holds in the supercritical case (i.e. z >(similar to a)(zc) (beta, r)). These results are in the spirit of recent works using the theory of randomised tree algorithms (Probab. Theory Related Fields 173 (2019) 479-490, Ann. of Math. 189 (2019) 75-99, Duminil-Copin, Raoufi and Tassion (2018)). Secondly we study a standard liquid-gas phase transition related to the uniqueness/non-uniqueness of Gibbs states depending on the parameters z, beta. Old results (Phys. Rev. Lett. 27 (1971) 1040-1041, J. Chem. Phys. 52 (1970) 1670-1684) claim that a non-uniqueness regime occurs for z = beta large enough and it is conjectured that the uniqueness should hold outside such an half line ( z = beta >= beta(c) > 0). We solve partially this conjecture in any dimension by showing that for beta large enough the non-uniqueness holds if and only if z = beta. We show also that this critical value z = beta corresponds to the percolation threshold (similar to a)(zc) (beta, r) = beta for beta large enough, providing a straight connection between these two notions of phase transition. KW - Gibbs point process KW - DLR equations KW - Boolean model KW - Continuum KW - percolation KW - Random cluster model KW - Fortuin-Kasteleyn representation KW - Randomised tree algorithm KW - OSSS inequality Y1 - 2018 U6 - https://doi.org/10.1214/20-AIHP1082 SN - 0246-0203 SN - 1778-7017 VL - 57 IS - 1 SP - 387 EP - 407 PB - Association des Publications de l'Institut Henri Poincaré CY - Bethesda, Md. ER - TY - JOUR A1 - Chechkin, Aleksei V. A1 - Sokolov, Igor M. T1 - Random search with resetting BT - a unified renewal approach JF - Physical review letters N2 - We provide a unified renewal approach to the problem of random search for several targets under resetting. This framework does not rely on specific properties of the search process and resetting procedure, allows for simpler derivation of known results, and leads to new ones. Concentrating on minimizing the mean hitting time, we show that resetting at a constant pace is the best possible option if resetting helps at all, and derive the equation for the optimal resetting pace. No resetting may be a better strategy if without resetting the probability of not finding a target decays with time to zero exponentially or faster. We also calculate splitting probabilities between the targets, and define the limits in which these can be manipulated by changing the resetting procedure. We moreover show that the number of moments of the hitting time distribution under resetting is not less than the sum of the numbers of moments of the resetting time distribution and the hitting time distribution without resetting. Y1 - 2018 U6 - https://doi.org/10.1103/PhysRevLett.121.050601 SN - 0031-9007 SN - 1079-7114 VL - 121 IS - 5 PB - American Physical Society CY - College Park ER - TY - JOUR A1 - Reese, Ronja A1 - Winkelmann, Ricarda A1 - Gudmundsson, Gudmundur Hilmar T1 - Grounding-line flux formula applied as a flux condition in numerical simulations fails for buttressed Antarctic ice streams JF - The Cryosphere : TC ; an interactive open access journal of the European Geosciences Union N2 - Currently, several large-scale ice-flow models impose a condition on ice flux across grounding lines using an analytically motivated parameterisation of grounding-line flux. It has been suggested that employing this analytical expression alleviates the need for highly resolved computational domains around grounding lines of marine ice sheets. While the analytical flux formula is expected to be accurate in an unbuttressed flow-line setting, its validity has hitherto not been assessed for complex and realistic geometries such as those of the Antarctic Ice Sheet. Here the accuracy of this analytical flux formula is tested against an optimised ice flow model that uses a highly resolved computational mesh around the Antarctic grounding lines. We find that when applied to the Antarctic Ice Sheet the analytical expression provides inaccurate estimates of ice fluxes for almost all grounding lines. Furthermore, in many instances direct application of the analytical formula gives rise to unphysical complex-valued ice fluxes. We conclude that grounding lines of the Antarctic Ice Sheet are, in general, too highly buttressed for the analytical parameterisation to be of practical value for the calculation of grounding-line fluxes. Y1 - 2018 U6 - https://doi.org/10.5194/tc-12-3229-2018 SN - 1994-0416 SN - 1994-0424 VL - 12 IS - 10 SP - 3229 EP - 3242 PB - Copernicus CY - Göttingen ER - TY - JOUR A1 - Reese, Ronja A1 - Albrecht, Torsten A1 - Mengel, Matthias A1 - Asay-Davis, Xylar A1 - Winkelmann, Ricarda T1 - Antarctic sub-shelf melt rates via PICO JF - The Cryosphere : TC ; an interactive open access journal of the European Geosciences Union N2 - Ocean-induced melting below ice shelves is one of the dominant drivers for mass loss from the Antarctic Ice Sheet at present. An appropriate representation of sub-shelf melt rates is therefore essential for model simulations of marine-based ice sheet evolution. Continental-scale ice sheet models often rely on simple melt-parameterizations, in particular for long-term simulations, when fully coupled ice-ocean interaction becomes computationally too expensive. Such parameterizations can account for the influence of the local depth of the ice-shelf draft or its slope on melting. However, they do not capture the effect of ocean circulation underneath the ice shelf. Here we present the Potsdam Ice-shelf Cavity mOdel (PICO), which simulates the vertical overturning circulation in ice-shelf cavities and thus enables the computation of sub-shelf melt rates consistent with this circulation. PICO is based on an ocean box model that coarsely resolves ice shelf cavities and uses a boundary layer melt formulation. We implement it as a module of the Parallel Ice Sheet Model (PISM) and evaluate its performance under present-day conditions of the Southern Ocean. We identify a set of parameters that yield two-dimensional melt rate fields that qualitatively reproduce the typical pattern of comparably high melting near the grounding line and lower melting or refreezing towards the calving front. PICO captures the wide range of melt rates observed for Antarctic ice shelves, with an average of about 0.1 ma(-1) for cold sub-shelf cavities, for example, underneath Ross or Ronne ice shelves, to 16 ma(-1) for warm cavities such as in the Amundsen Sea region. This makes PICO a computationally feasible and more physical alternative to melt parameterizations purely based on ice draft geometry. Y1 - 2018 U6 - https://doi.org/10.5194/tc-12-1969-2018 SN - 1994-0416 SN - 1994-0424 VL - 12 IS - 6 SP - 1969 EP - 1985 PB - Copernicus CY - Göttingen ER - TY - JOUR A1 - Heinrich, Benjamin W. A1 - Ehlert, Christopher A1 - Hatter, Nino A1 - Braun, Lukas A1 - Lotze, Christian A1 - Saalfrank, Peter A1 - Franke, Katharina J. T1 - Control of oxidation and spin state in a single-molecule junction JF - ACS nano N2 - The oxidation and spin state of a metal-organic molecule determine its chemical reactivity and magnetic properties. Here, we demonstrate the reversible control of the oxidation and spin state in a single Fe porphyrin molecule in the force field of the tip of a scanning electron tunneling microscope. Within the regimes of half-integer and integer spin state, we can further track the evolution of the magnetocrystalline anisotropy. Our experimental results are corroborated by density functional theory and wave function theory. This combined analysis allows us to draw a complete picture of the molecular states over a large range of intramolecular deformations. KW - porphyrin KW - oxidation state KW - spin state KW - scanning tunneling microscopy KW - scanning tunneling spectroscopy KW - density functional theory Y1 - 2018 U6 - https://doi.org/10.1021/acsnano.8b00312 SN - 1936-0851 SN - 1936-086X VL - 12 IS - 4 SP - 3172 EP - 3177 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Raman Venkatesan, Thulasinath A1 - Gulyakova, Anna A. A1 - Frübing, Peter A1 - Gerhard, Reimund T1 - Relaxation processes and structural transitions in poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) relaxor-ferroelectric terpolymers as seen in dielectric spectroscopy JF - IEEE transactions on dielectrics and electrical insulation N2 - Dielectric relaxation processes and structural transitions in Poly(vinylidenefluoride-trifluoroethylene-chlorofluoroethylene) (P(VDF-TrFE-CFE)) terpolymer films with two different monomer compositions were investigated in comparison with Poly(vinylidenefluoride-trifluoroethylene) (P(VDF-TrFE)) copolymer films as reference material. Differential Scanning Calorimetry was employed to assess annealing effects on phase transitions and crystalline structure, while relaxation processes were investigated by means of Dielectric Relaxation Spectroscopy, the results of which indicate the existence of two separate dispersion regions, denoted as processes A and B, respectively. Process A appears at a certain temperature independent of frequency, but is strongly influenced by the crystallisation temperature and the CFE content, while peak B shows typical features of a relaxation process and is less influenced by crystallisation temperature and CFE content. Furthermore, peak B is related to the glass transition which is more pronounced in the terpolymer than in P(VDF-TrFE). A closer analysis indicates that the addition of CFE and thermal annealing gradually shift the ferro-to-paraelectric transition in P(VDF-TrFE) to lower temperatures, while the phase transition is transformed more and more into a relaxation. KW - P(VDF-TrFE-CFE) terpolymer KW - relaxor-ferroelectric polymer KW - dielectric relaxation spectroscopy KW - Curie transition KW - differential scanning calorimetry Y1 - 2018 U6 - https://doi.org/10.1109/TDEI.2018.007440 SN - 1070-9878 SN - 1558-4135 VL - 25 IS - 6 SP - 2229 EP - 2235 PB - Institut of Electrical and Electronics Engineers CY - Piscataway ER - TY - JOUR A1 - Qiu, Xunlin A1 - Groth, Frederick A1 - Wirges, Werner A1 - Gerhard, Reimund T1 - Cellular polypropylene foam films as DC voltage insulation and as piezoelectrets BT - a comparison JF - IEEE transactions on dielectrics and electrical insulation N2 - Polymer foams are in industrial use for several decades. More recently, non-polar polymer foams were found to be piezoelectric (so-called piezoelectrets) after internal electrical charging of the cavities. So far, few studies have been carried out on the electrical-insulation properties of polymer foams. Here, we compare the piezoelectric and the DC-voltage electrical-insulation properties of cellular polypropylene (PP) foams. Their cavity microstructure can be adjusted via inflation in high-pressure nitrogen gas in combination with a subsequent thermal treatment. While inflation is effective for improving the piezoelectricity, it is detrimental for the electrical-insulation properties. The original cellular PP foam shows a breakdown strength of approximately 230 MV/m, within the same range as that of solid PP. The breakdown strength decreases with increasing degree of inflation, and the dependence on the foam thickness follows an inverse power law with an exponent of 1.2. Nevertheless, up to a thickness of 140 mu m (3.5 times the original thickness), the breakdown strength of cellular-foam PP films is at least 7 times that of an air gap with the same thickness. In addition, the influence of high temperatures and high humidities on the piezoelectricity and the breakdown strength of cellular PP was studied. It was found that the piezoelectric d(33) coefficient decays rapidly already at 70 degrees C, while the breakdown strength slightly increases during storage at 70 or 90 degrees C. Under a relative humidity of 95%, the breakdown strength increases with storage time, while the piezoelectric d(33) coefficient slightly decreases. KW - Cellular polypropylene (PP) KW - polymer-foam films KW - ferro- and piezoelectrets KW - charge storage KW - electrical breakdown KW - dielectric barrier discharges Y1 - 2018 U6 - https://doi.org/10.1109/TDEI.2018.007192 SN - 1070-9878 SN - 1558-4135 VL - 25 IS - 3 SP - 829 EP - 834 PB - Institut of Electr. and Electronics Engineers CY - Piscataway ER - TY - JOUR A1 - Rychkov, Dmitry A1 - Gerhard, Reimund A1 - Kuznetsov, Alexey A1 - Rychkov, Andrey T1 - Influence of charge density on the trap energy spectrum in fluoroethylenepropylene copolymer films with chemically modified surfaces JF - IEEE transactions on dielectrics and electrical insulation N2 - Tetrafluoroethylene-hexafluoropropylene copolymer (FEP) films were treated with titanium-tetrachloride vapor by means of molecular-layer deposition. The treatment leads to considerable improvements of the electret-charge stability on positively charged films. A slight improvement is also observed for negatively charged films. In line with our previous findings, we attribute the improvement in electret properties to the formation of deeper traps on the chemically modified polymer surfaces. Here, we investigate the influence of the charge density on electret stability of FEP films with modified surfaces. Trap-energy spectra obtained from thermally-stimulated-discharge measurements indicate that the charge stability on modified FEP films depends on how the surface traps are populated and on the availability of additional deeper traps. KW - Electrets KW - chemical modification KW - surface treatment KW - space-charge stability KW - charge-carrier transport KW - polytetrafluoroethylene (PTFE) KW - fluoroethylenepropylene (FEP) copolymer Y1 - 2018 U6 - https://doi.org/10.1109/TDEI.2018.007437 SN - 1070-9878 SN - 1558-4135 VL - 25 IS - 3 SP - 840 EP - 844 PB - Inst. of Electr. and Electronics Engineers CY - Piscataway ER - TY - JOUR A1 - Gidion, Gunnar A1 - Gerhard, Reimund T1 - The Bow on a String BT - Bow Vibrations Detected with Ferroelectret Sensors JF - Acta Acustica united with Acustica N2 - The interaction between a bowed string instrument and its player is conveyed by the bow, the vibrational properties of which can be measured either separately on the bow or during the bowing procedure. Here, two piezoelectric film sensors, made of a ferroelectret material, are installed on a violin bow, one sensor at the tip and one at the frog. With these sensors, a violin is played under normal conditions, and the signals are analysed. The features in the resulting spectrograms are identified as string harmonics and longitudinal bow-hair resonances. The bow-hair sections on both sides of the bow-string contact exhibit separate resonances which are observed as absorption dips in the spectra. Owing to the sensor positions at the bow-hair terminations, it can be inferred that the two bow-hair sections act as mutual vibration absorbers. From a regression of the observed resonances, the longitudinal bow-hair velocity can be obtained. With additional film sensors under the violin bridge, body vibrations were also detected providing further details of the coupling mechanisms. Y1 - 2018 U6 - https://doi.org/10.3813/AAA.919174 SN - 1610-1928 SN - 1861-9959 VL - 104 IS - 2 SP - 315 EP - 322 PB - Hirzel Verlag CY - Stuttgart ER - TY - JOUR A1 - Shenar, Tomer A1 - Hainich, Rainer A1 - Todt, Helge Tobias A1 - Moffat, Anthony F. J. A1 - Sander, Andreas Alexander Christoph A1 - Oskinova, Lida A1 - Ramachandran, Varsha A1 - Munoz, M. A1 - Pablo, H. A1 - Sana, Hugues A1 - Hamann, Wolf-Rainer T1 - The shortest-period Wolf-Rayet binary in the small magellanic cloud BT - Part of a high-order multiple system Spectral and orbital analysis of SMC AB 6 JF - Astronomy and astrophysics : an international weekly journal N2 - Context. SMC AB6 is the shortest-period (P = 6.5 d) Wolf-Rayet (WR) binary in the Small Magellanic Cloud. This binary is therefore a key system in the study of binary interaction and formation of WR stars at low metallicity. The WR component in AB6 was previously found to be very luminous (log L = 6.3 [L-circle dot]) compared to its reported orbital mass (approximate to 8 M-circle dot), placing it significantly above the Eddington limit. Aims. Through spectroscopy and orbital analysis of newly acquired optical data taken with the Ultraviolet and Visual Echelle Spectrograph (UVES), we aim to understand the peculiar results reported for this system and explore its evolutionary history. Methods. We measured radial velocities via cross-correlation and performed a spectral analysis using the Potsdam Wolf-Rayet model atmosphere code. The evolution of the system was analyzed using the Binary Population and Spectral Synthesis evolution code. Results. AB6 contains at least four stars. The 6.5 d period WR binary comprises the WR primary (WN3:h, star A) and a rather rapidly rotating (v(eq) = 265 km s(-1)) early O-type companion (O5.5 V, star B). Static N III and N IV emission lines and absorption signatures in He lines suggest the presence of an early-type emission line star (O5.5 I(f), star C). Finally, narrow absorption lines portraying a long-term radial velocity variation show the existence of a fourth star (O7.5 V, star D). Star D appears to form a second 140 d period binary together with a fifth stellar member, which is a B-type dwarf or a black hole. It is not clear that these additional components are bound to the WR binary. We derive a mass ratio of M-O/M-WR = 2.2 +/- 0.1. The WR star is found to be less luminous than previously thought (log L = 5.9 [L-circle dot]) and, adopting M-O = 41 M-circle dot for star B, more massive (M-WR = 18 M-circle dot). Correspondingly, the WR star does not exceed the Eddington limit. We derive the initial masses of M-i,M-WR = 60 M-circle dot and M-i,M-O = 40 M-circle dot and an age of 3.9 Myr for the system. The WR binary likely experienced nonconservative mass transfer in the past supported by the relatively rapid rotation of star B. Conclusions. Our study shows that AB6 is a multiple - probably quintuple - system. This finding resolves the previously reported puzzle of the WR primary exceeding the Eddington limit and suggests that the WR star exchanged mass with its companion in the past. KW - stars: massive KW - binaries: spectroscopic KW - stars: Wolf-Rayet KW - Magellanic Clouds KW - stars: individual: SMC AB 6 KW - stars: atmospheres Y1 - 2018 U6 - https://doi.org/10.1051/0004-6361/201833006 SN - 1432-0746 SN - 0004-6361 VL - 616 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Hainich, Rainer A1 - Oskinova, Lida A1 - Shenar, Tomer A1 - Marchant Campos, Pablo A1 - Eldridge, J. J. A1 - Sander, Andreas Alexander Christoph A1 - Hamann, Wolf-Rainer A1 - Langer, Norbert A1 - Todt, Helge Tobias T1 - Observational properties of massive black hole binary progenitors JF - Astronomy and astrophysics : an international weekly journal N2 - Context: The first directly detected gravitational waves (GW 150914) were emitted by two coalescing black holes (BHs) with masses of ≈ 36 M⊙ and ≈ 29 M⊙. Several scenarios have been proposed to put this detection into an astrophysical context. The evolution of an isolated massive binary system is among commonly considered models. Aims: Various groups have performed detailed binary-evolution calculations that lead to BH merger events. However, the question remains open as to whether binary systems with the predicted properties really exist. The aim of this paper is to help observers to close this gap by providing spectral characteristics of massive binary BH progenitors during a phase where at least one of the companions is still non-degenerate. Methods: Stellar evolution models predict fundamental stellar parameters. Using these as input for our stellar atmosphere code (Potsdam Wolf-Rayet), we compute a set of models for selected evolutionary stages of massive merging BH progenitors at different metallicities. Results: The synthetic spectra obtained from our atmosphere calculations reveal that progenitors of massive BH merger events start their lives as O2-3V stars that evolve to early-type blue supergiants before they undergo core-collapse during the Wolf-Rayet phase. When the primary has collapsed, the remaining system will appear as a wind-fed high-mass X-ray binary. Based on our atmosphere models, we provide feedback parameters, broad band magnitudes, and spectral templates that should help to identify such binaries in the future. Conclusions: While the predicted parameter space for massive BH binary progenitors is partly realized in nature, none of the known massive binaries match our synthetic spectra of massive BH binary progenitors exactly. Comparisons of empirically determined mass-loss rates with those assumed by evolution calculations reveal significant differences. The consideration of the empirical mass-loss rates in evolution calculations will possibly entail a shift of the maximum in the predicted binary-BH merger rate to higher metallicities, that is, more candidates should be expected in our cosmic neighborhood than previously assumed. KW - gravitational waves KW - binaries: close KW - stars: early-type KW - stars: atmospheres KW - stars: winds KW - outflows KW - stars: mass-loss Y1 - 2018 U6 - https://doi.org/10.1051/0004-6361/201731449 SN - 1432-0746 VL - 609 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Sander, Andreas Alexander Christoph A1 - Fürst, F. A1 - Kretschmar, P. A1 - Oskinova, Lida A1 - Todt, Helge Tobias A1 - Hainich, Rainer A1 - Shenar, Tomer A1 - Hamann, Wolf-Rainer T1 - Coupling hydrodynamics with comoving frame radiative transfer BT - Stellar wind stratification in the high-mass X-ray binary Vela X-1 JF - Astronomy and astrophysics : an international weekly journal N2 - Aims. To gain a realistic picture of the donor star in Vela X-1, we constructed a hydrodynamically consistent atmosphere model describing the wind stratification while properly reproducing the observed donor spectrum. To investigate how X-ray illumination affects the stellar wind, we calculated additional models for different X-ray luminosity regimes. Methods. We used the recently updated version of the Potsdam Wolf-Rayet code to consistently solve the hydrodynamic equation together with the statistical equations and the radiative transfer. Results. The wind flow in Vela X-1 is driven by ions from various elements, with Fe III and S III leading in the outer wind. The model-predicted mass-loss rate is in line with earlier empirical studies. The mass-loss rate is almost unaffected by the presence of the accreting NS in the wind. The terminal wind velocity is confirmed at u(infinity) approximate to 600 km s(-1). On the other hand, the wind velocity in the inner region where the NS is located is only approximate to 100 km s(-1), which is not expected on the basis of a standard beta-velocity law. In models with an enhanced level of X-rays, the velocity field in the outer wind can be altered. If the X-ray flux is too high, the acceleration breaks down because the ionization increases. Conclusions. Accounting for radiation hydrodynamics, our Vela X-1 donor atmosphere model reveals a low wind speed at the NS location, and it provides quantitative information on wind driving in this important HMXB. KW - stars: mass-loss KW - stars: winds, outflows KW - stars: early-type KW - stars: atmospheres KW - stars: massive KW - X-rays: binaries Y1 - 2018 U6 - https://doi.org/10.1051/0004-6361/201731575 SN - 1432-0746 VL - 610 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Oskinova, Lida A1 - Bulik, Tomasz A1 - Gomez-Moran, Ada Nebot T1 - Infrared outbursts as potential tracers of common-envelope events in high-mass X-ray binary formation JF - Astronomy and astrophysics : an international weekly journal N2 - Context. Classic massive binary evolutionary scenarios predict that a transitional common-envelope (CE) phase could be preceded as well as succeeded by the evolutionary stage when a binary consists of a compact object and a massive star, that is, a high-mass X-ray binary (HMXB). The observational manifestations of common envelope are poorly constrained. We speculate that its ejection might be observed in some cases as a transient event at mid-infrared (IR) wavelengths. Aims. We estimate the expected numbers of CE ejection events and HMXBs per star formation unit rate, and compare these theoretical estimates with observations. Methods. We compiled a list of 85 mid-IR transients of uncertain nature detected by the Spitzer Infrared Intensive Transients Survey and searched for their associations with X-ray, optical, and UV sources. Results. Confirming our theoretical estimates, we find that only one potential HMXB may be plausibly associated with an IR-transient and tentatively propose that X-ray source NGC4490-X40 could be a precursor to the SPIRITS 16az event. Among other interesting sources, we suggest that the supernova remnant candidate [BWL2012] 063 might be associated with SPIRITS 16ajc. We also find that two SPIRITS events are likely associated with novae, and seven have potential optical counterparts. Conclusions. The massive binary evolutionary scenarios that involve CE events do not contradict currently available observations of IR transients and HMXBs in star-forming galaxies. KW - X-rays: binaries KW - stars: massive KW - infrared: general KW - infrared: galaxies Y1 - 2018 U6 - https://doi.org/10.1051/0004-6361/201832925 SN - 0004-6361 SN - 1432-0746 VL - 613 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Torrejon, J. M. A1 - Reig, Pablo A1 - Fürst, F. A1 - Martinez-Chicharro, M. A1 - Postnov, K. A1 - Oskinova, Lida T1 - NuSTAR rules out a cyclotron line in the accreting magnetar candidate 4U2206+54 JF - Monthly notices of the Royal Astronomical Society N2 - Based on our new NuSTAR X-ray telescope data, we rule out any cyclotron line up to 60 keV in the spectra of the high-mass X-ray binary 4U2206+54. In particular, we do not find any evidence of the previously claimed line around 30 keV, independently of the source flux, along the spin pulse. The spin period has increased significantly, since the last observation, up to 5750 +/- 10 s, confirming the rapid spin-down rate (nu)over dot = -1.8 x 10(-14) Hz s(-1). This behaviour might be explained by the presence of a strongly magnetized neutron star (B-s > several times 10(13) G) accreting from the slow wind of its main-sequence O9.5 companion. KW - Stars: individual: 4U2206+54, BD+53 2790 KW - X-rays: binaries Y1 - 2018 U6 - https://doi.org/10.1093/mnras/sty1628 SN - 0035-8711 SN - 1365-2966 VL - 479 IS - 3 SP - 3366 EP - 3372 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Jay, Raphael M. A1 - Norell, Jesper A1 - Eckert, Sebastian A1 - Hantschmann, Markus A1 - Beye, Martin A1 - Kennedy, Brian A1 - Quevedo, Wilson A1 - Schlotter, William F. A1 - Dakovski, Georgi L. A1 - Minitti, Michael P. A1 - Hoffmann, Matthias C. A1 - Mitra, Ankush A1 - Moeller, Stefan P. A1 - Nordlund, Dennis A1 - Zhang, Wenkai A1 - Liang, Huiyang W. A1 - Kunnus, Kristian A1 - Kubicek, Katharina A1 - Techert, Simone A. A1 - Lundberg, Marcus A1 - Wernet, Philippe A1 - Gaffney, Kelly A1 - Odelius, Michael A1 - Föhlisch, Alexander T1 - Disentangling Transient Charge Density and Metal-Ligand Covalency in Photoexcited Ferricyanide with Femtosecond Resonant Inelastic Soft X-ray Scattering JF - The journal of physical chemistry letters N2 - Soft X-ray spectroscopies are ideal probes of the local valence electronic structure of photocatalytically active metal sites. Here, we apply the selectivity of time resolved resonant inelastic X-ray scattering at the iron L-edge to the transient charge distribution of an optically excited charge-transfer state in aqueous ferricyanide. Through comparison to steady-state spectra and quantum chemical calculations, the coupled effects of valence-shell closing and ligand-hole creation are experimentally and theoretically disentangled and described in terms of orbital occupancy, metal-ligand covalency, and ligand field splitting, thereby extending established steady-state concepts to the excited-state domain. pi-Back-donation is found to be mainly determined by the metal site occupation, whereas the ligand hole instead influences sigma-donation. Our results demonstrate how ultrafast resonant inelastic X-ray scattering can help characterize local charge distributions around catalytic metal centers in short-lived charge-transfer excited states, as a step toward future rationalization and tailoring of photocatalytic capabilities of transition-metal complexes. Y1 - 2018 U6 - https://doi.org/10.1021/acs.jpclett.8b01429 SN - 1948-7185 VL - 9 IS - 12 SP - 3538 EP - 3543 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Jay, Raphael Martin A1 - Eckert, Sebastian A1 - Fondell, Mattis A1 - Miedema, Piter S. A1 - Norell, Jesper A1 - Pietzsch, Annette A1 - Quevedo, Wilson A1 - Niskanen, Johannes A1 - Kunnus, Kristjan A1 - Föhlisch, Alexander T1 - The nature of frontier orbitals under systematic ligand exchange in (pseudo-)octahedral Fe(II) complexes JF - Physical chemistry, chemical physics : a journal of European Chemical Societies N2 - Understanding and controlling properties of transition metal complexes is a crucial step towards tailoring materials for sustainable energy applications. In a systematic approach, we use resonant inelastic X-ray scattering to study the influence of ligand substitution on the valence electronic structure around an aqueous iron(II) center. Exchanging cyanide with 2-2′-bipyridine ligands reshapes frontier orbitals in a way that reduces metal 3d charge delocalization onto the ligands. This net decrease of metal–ligand covalency results in lower metal-centered excited state energies in agreement with previously reported excited state dynamics. Furthermore, traces of solvent-effects were found indicating a varying interaction strength of the solvent with ligands of different character. Our results demonstrate how ligand exchange can be exploited to shape frontier orbitals of transition metal complexes in solution-phase chemistry; insights upon which future efforts can built when tailoring the functionality of photoactive systems for light-harvesting applications. Y1 - 2018 U6 - https://doi.org/10.1039/c8cp04341h SN - 1463-9076 SN - 1463-9084 VL - 20 IS - 44 SP - 27745 EP - 27751 PB - Royal Society of Chemistry CY - Cambridge ER - TY - JOUR A1 - Eckert, Sebastian A1 - Vaz da Cruz, Vinicius A1 - Ertan, Emelie A1 - Ignatova, Nina A1 - Polyutov, Sergey A1 - Couto, Rafael C. A1 - Fondell, Mattis A1 - Dantz, Marcus A1 - Kennedy, Brian A1 - Schmitt, Thorsten A1 - Pietzsch, Annette A1 - Odelius, Michael A1 - Föhlisch, Alexander T1 - One-dimensional cuts through multidimensional potential-energy surfaces by tunable x rays JF - Physical review : A, Atomic, molecular, and optical physics N2 - The concept of the potential-energy surface (PES) and directional reaction coordinates is the backbone of our description of chemical reaction mechanisms. Although the eigenenergies of the nuclear Hamiltonian uniquely link a PES to its spectrum, this information is in general experimentally inaccessible in large polyatomic systems. This is due to (near) degenerate rovibrational levels across the parameter space of all degrees of freedom, which effectively forms a pseudospectrum given by the centers of gravity of groups of close-lying vibrational levels. We show here that resonant inelastic x-ray scattering (RIXS) constitutes an ideal probe for revealing one-dimensional cuts through the ground-state PES of molecular systems, even far away from the equilibrium geometry, where the independent-mode picture is broken. We strictly link the center of gravity of close-lying vibrational peaks in RIXS to a pseudospectrum which is shown to coincide with the eigenvalues of an effective one-dimensional Hamiltonian along the propagation coordinate of the core-excited wave packet. This concept, combined with directional and site selectivity of the core-excited states, allows us to experimentally extract cuts through the ground-state PES along three complementary directions for the showcase H2O molecule. Y1 - 2018 U6 - https://doi.org/10.1103/PhysRevA.97.053410 SN - 2469-9926 SN - 2469-9934 VL - 97 IS - 5 PB - American Physical Society CY - College Park ER - TY - JOUR A1 - Jelken, Joachim A1 - Pandiyarajan, Chinnayan Kannan A1 - Genzer, Jan A1 - Lomadze, Nino A1 - Santer, Svetlana T1 - Fabrication of flexible hydrogel sheets featuring periodically spaced circular holes with continuously adjustable size in realtime JF - ACS applied materials & interfaces N2 - We report on the formation of stimuli-responsive structured hydrogel thin films whose pattern geometry can be adjusted on demand and tuned reversibly by varying solvent quality or by changing temperature. The hydrogel films, similar to 100 nm in thickness, were prepared by depositing layers of random copolymers comprising N-isopropylacrylamide and ultraviolet (UV)-active methacryloyloxybenzophenone units onto solid substrates. A two-beam interference pattern technique was used to cross-link the selected areas of the film; any unreacted material was extracted using ethanol after UV light-assisted cross-linking. In this way, we produced nanoholes, perfectly ordered structures with a narrow size distribution, negligible tortuosity, adjustable periodicity, and a high density. The diameter of the circular holes ranged from a few micrometers down to several tens of nanometers; the hole periodicity could be adjusted readily by changing the optical period of the UV interference pattern. The holes were reversibly closed and opened by swelling/deswelling the polymer networks in the presence of ethanol and water, respectively, at various temperatures. The reversible regulation of the hole diameter can be repeated many times within a few seconds. The hydrogel sheet with circular holes periodically arranged may also be transferred onto different substrates and be employed as tunable templates for the deposition of desired substances. KW - photosensitive polymers KW - PNIPAm KW - hydrogels KW - UV cross-linking KW - stimuli-responsive structured polymer films KW - azobenzene-containing molecules Y1 - 2018 U6 - https://doi.org/10.1021/acsami.8b09580 SN - 1944-8244 VL - 10 IS - 36 SP - 30844 EP - 30851 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Loebner, Sarah A1 - Lomadze, Nino A1 - Kopyshev, Alexey A1 - Koch, Markus A1 - Guskova, Olga A1 - Saphiannikova, Marina A1 - Santer, Svetlana T1 - Light-Induced Deformation of Azobenzene-Containing Colloidal Spheres BT - Calculation and Measurement of Opto-Mechanical Stresses JF - The journal of physical chemistry : B, Condensed matter, materials, surfaces, interfaces & biophysical chemistry N2 - We report on light-induced deformation of colloidal spheres consisting of azobenzene-containing polymers. The colloids of the size between 60 nm and 2 mu m in diameter were drop casted on a glass surface and irradiated with linearly polarized light. It was found that colloidal particles can be deformed up to ca. 6 times of their initial diameter. The maximum degree of deformation depends on the irradiation wavelength and intensity, as well as on colloidal particles size. On the basis of recently proposed theory by Toshchevikov et al. [J. Phys. Chem. Lett. 2017, 8, 1094], we calculated the optomechanical stresses (ca. 100 MPa) needed for such giant deformations and compared them with the experimental results. Y1 - 2018 U6 - https://doi.org/10.1021/acs.jpcb.7b11644 SN - 1520-6106 VL - 122 IS - 6 SP - 2001 EP - 2009 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Niebuur, Bart-Jan A1 - Puchmayr, Jonas A1 - Herold, Christian A1 - Kreuzer, Lucas A1 - Hildebrand, Viet A1 - Müller-Buschbaum, Peter A1 - Laschewsky, Andre A1 - Papadakis, Christine M. T1 - Polysulfobetaines in aqueous solution and in thin film geometry JF - Materials N2 - Polysulfobetaines in aqueous solution show upper critical solution temperature (UCST) behavior. We investigate here the representative of this class of materials, poly (N,N-dimethyl-N-(3-methacrylamidopropyl) ammonio propane sulfonate) (PSPP), with respect to: (i) the dynamics in aqueous solution above the cloud point as function of NaBr concentration; and (ii) the swelling behavior of thin films in water vapor as function of the initial film thickness. For PSPP solutions with a concentration of 5 wt.%, the temperature dependence of the intensity autocorrelation functions is measured with dynamic light scattering as function of molar mass and NaBr concentration (0-8 mM). We found a scaling of behavior for the scattered intensity and dynamic correlation length. The resulting spinodal temperatures showed a maximum at a certain (small) NaBr concentration, which is similar to the behavior of the cloud points measured previously by turbidimetry. The critical exponent of susceptibility depends on NaBr concentration, with a minimum value where the spinodal temperature is maximum and a trend towards the mean-field value of unity with increasing NaBr concentration. In contrast, the critical exponent of the correlation length does not depend on NaBr concentration and is lower than the value of 0.5 predicted by mean-field theory. For PSPP thin films, the swelling behavior was found to depend on film thickness. A film thickness of about 100 nm turns out to be the optimum thickness needed to obtain fast hydration with H2O. KW - polyzwitterions KW - polysulfobetaines KW - dynamic light scattering KW - phase behavior Y1 - 2018 U6 - https://doi.org/10.3390/ma11050850 SN - 1996-1944 VL - 11 IS - 5 PB - MDPI CY - Basel ER - TY - JOUR A1 - Wisotzki, Lutz A1 - Bacon, R. A1 - Brinchmann, J. A1 - Cantalupo, S. A1 - Richter, Philipp A1 - Schaye, J. A1 - Schmidt, Kasper Borello A1 - Urrutia, Tanya A1 - Weilbacher, Peter Michael A1 - Akhlaghi, M. A1 - Bouche, N. A1 - Contini, T. A1 - Guiderdoni, B. A1 - Herenz, E. C. A1 - Inami, H. A1 - Kerutt, Josephine Victoria A1 - Leclercq, F. A1 - Marino, R. A. A1 - Maseda, M. A1 - Monreal-Ibero, A. A1 - Nanayakkara, T. A1 - Richard, J. A1 - Saust, R. A1 - Steinmetz, Matthias A1 - Wendt, Martin T1 - Nearly all the sky is covered by Lyman-alpha emission around high-redshift galaxies JF - Nature : the international weekly journal of science N2 - Galaxies are surrounded by large reservoirs of gas, mostly hydrogen, that are fed by inflows from the intergalactic medium and by outflows from galactic winds. Absorption-line measurements along the lines of sight to bright and rare background quasars indicate that this circumgalactic medium extends far beyond the starlight seen in galaxies, but very little is known about its spatial distribution. The Lyman-alpha transition of atomic hydrogen at a wavelength of 121.6 nanometres is an important tracer of warm (about 104 kelvin) gas in and around galaxies, especially at cosmological redshifts greater than about 1.6 at which the spectral line becomes observable from the ground. Tracing cosmic hydrogen through its Lyman-a emission has been a long-standing goal of observational astrophysics(1-3), but the extremely low surface brightness of the spatially extended emission is a formidable obstacle. A new window into circumgalactic environments was recently opened by the discovery of ubiquitous extended Lyman-alpha emission from hydrogen around high-redshift galaxies(4,5). Such measurements were previously limited to especially favourable systems(6-8) or to the use of massive statistical averaging(9,10) because of the faintness of this emission. Here we report observations of low-surface-brightness Lyman-alpha emission surrounding faint galaxies at redshifts between 3 and 6. We find that the projected sky coverage approaches 100 per cent. The corresponding rate of incidence (the mean number of Lyman-alpha emitters penetrated by any arbitrary line of sight) is well above unity and similar to the incidence rate of high-column-density absorbers frequently detected in the spectra of distant quasars(11-14). This similarity suggests that most circumgalactic atomic hydrogen at these redshifts has now been detected in emission. Y1 - 2018 U6 - https://doi.org/10.1038/s41586-018-0564-6 SN - 0028-0836 SN - 1476-4687 VL - 562 IS - 7726 SP - 229 EP - 232 PB - Nature Publ. Group CY - London ER - TY - JOUR A1 - Willner, Sven N. A1 - Otto, Christian A1 - Levermann, Anders T1 - Global economic response to river floods JF - Nature climate change N2 - Increasing Earth’s surface air temperature yields an intensification of its hydrological cycle. As a consequence, the risk of river floods will increase regionally within the next two decades due to the atmospheric warming caused by past anthropogenic greenhouse gas emissions. The direct economic losses caused by these floods can yield regionally heterogeneous losses and gains by propagation within the global trade and supply network. Here we show that, in the absence of large-scale structural adaptation, the total economic losses due to fluvial floods will increase in the next 20 years globally by 17% despite partial compensation through market adjustment within the global trade network. China will suffer the strongest direct losses, with an increase of 82%. The United States is mostly affected indirectly through its trade relations. By contrast to the United States, recent intensification of the trade relations with China leaves the European Union better prepared for the import of production losses in the future. Y1 - 2018 U6 - https://doi.org/10.1038/s41558-018-0173-2 SN - 1758-678X SN - 1758-6798 VL - 8 IS - 7 SP - 594 EP - 598 PB - Nature Publ. Group CY - London ER - TY - JOUR A1 - Willner, Sven N. A1 - Levermann, Anders A1 - Zhao, Fang A1 - Frieler, Katja T1 - Adaptation required to preserve future high-end river flood risk at present levels JF - Science Advances N2 - Earth’s surface temperature will continue to rise for another 20 to 30 years even with the strongest carbon emission reduction currently considered. The associated changes in rainfall patterns can result in an increased flood risk worldwide. We compute the required increase in flood protection to keep high-end fluvial flood risk at present levels. The analysis is carried out worldwide for subnational administrative units. Most of the United States, Central Europe, and Northeast and West Africa, as well as large parts of India and Indonesia, require the strongest adaptation effort. More than half of the United States needs to at least double their protection within the next two decades. Thus, the need for adaptation to increased river flood is a global problem affecting industrialized regions as much as developing countries. Y1 - 2018 U6 - https://doi.org/10.1126/sciadv.aao1914 SN - 2375-2548 VL - 4 IS - 1 PB - American Assoc. for the Advancement of Science CY - Washington ER - TY - JOUR A1 - Seyrich, Maximilian A1 - Alirezaeizanjani, Zahra A1 - Beta, Carsten A1 - Stark, Holger T1 - Statistical parameter inference of bacterial swimming strategies JF - New journal of physics : the open-access journal for physics N2 - We provide a detailed stochastic description of the swimming motion of an E. coli bacterium in two dimension, where we resolve tumble events in time. For this purpose, we set up two Langevin equations for the orientation angle and speed dynamics. Calculating moments, distribution and autocorrelation functions from both Langevin equations and matching them to the same quantities determined from data recorded in experiments, we infer the swimming parameters of E. coli. They are the tumble rate lambda, the tumble time r(-1), the swimming speed v(0), the strength of speed fluctuations sigma, the relative height of speed jumps eta, the thermal value for the rotational diffusion coefficient D-0, and the enhanced rotational diffusivity during tumbling D-T. Conditioning the observables on the swimming direction relative to the gradient of a chemoattractant, we infer the chemotaxis strategies of E. coli. We confirm the classical strategy of a lower tumble rate for swimming up the gradient but also a smaller mean tumble angle (angle bias). The latter is realized by shorter tumbles as well as a slower diffusive reorientation. We also find that speed fluctuations are increased by about 30% when swimming up the gradient compared to the reversed direction. KW - E.coli KW - run and tumble KW - chemotaxis KW - stochastic processes KW - bacterial swimming strategies KW - parameter inference Y1 - 2018 U6 - https://doi.org/10.1088/1367-2630/aae72c SN - 1367-2630 VL - 20 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Grines, Evgeny A1 - Osipov, Grigory V. A1 - Pikovskij, Arkadij T1 - Describing dynamics of driven multistable oscillators with phase transfer curves JF - Chaos : an interdisciplinary journal of nonlinear science N2 - Phase response curve is an important tool in the studies of stable self-sustained oscillations; it describes a phase shift under action of an external perturbation. We consider multistable oscillators with several stable limit cycles. Under a perturbation, transitions from one oscillating mode to another one may occur. We define phase transfer curves to describe the phase shifts at such transitions. This allows for a construction of one-dimensional maps that characterize periodically kicked multistable oscillators. We show that these maps are good approximations of the full dynamics for large periods of forcing. Published by AIP Publishing. Y1 - 2018 U6 - https://doi.org/10.1063/1.5037290 SN - 1054-1500 SN - 1089-7682 VL - 28 IS - 10 PB - American Institute of Physics CY - Melville ER - TY - JOUR A1 - Smirnov, Lev A. A1 - Osipov, Grigory V. A1 - Pikovskij, Arkadij T1 - Solitary synchronization waves in distributed oscillator populations JF - Physical review : E, Statistical, nonlinear and soft matter physics N2 - We demonstrate the existence of solitary waves of synchrony in one-dimensional arrays of oscillator populations with Laplacian coupling. Characterizing each community with its complex order parameter, we obtain lattice equations similar to those of the discrete nonlinear Schrodinger system. Close to full synchrony, we find solitary waves for the order parameter perturbatively, starting from the known phase compactons and kovatons; these solutions are extended numerically to the full domain of possible synchrony levels. For nonidentical oscillators, the existence of dissipative solitons is shown. Y1 - 2018 U6 - https://doi.org/10.1103/PhysRevE.98.062222 SN - 2470-0045 SN - 2470-0053 VL - 98 IS - 6 SP - 062222-1 EP - 062222-7 PB - American Physical Society CY - College Park ER - TY - JOUR A1 - Bolotov, Maxim I. A1 - Smirnov, Lev A. A1 - Osipov, Grigory V. A1 - Pikovskij, Arkadij T1 - Simple and complex chimera states in a nonlinearly coupled oscillatory medium JF - Chaos : an interdisciplinary journal of nonlinear science N2 - We consider chimera states in a one-dimensional medium of nonlinear nonlocally coupled phase oscillators. In terms of a local coarse-grained complex order parameter, the problem of finding stationary rotating nonhomogeneous solutions reduces to a third-order ordinary differential equation. This allows finding chimera-type and other inhomogeneous states as periodic orbits of this equation. Stability calculations reveal that only some of these states are stable. We demonstrate that an oscillatory instability leads to a breathing chimera, for which the synchronous domain splits into subdomains with different mean frequencies. Further development of instability leads to turbulent chimeras. Published by AIP Publishing. Y1 - 2018 U6 - https://doi.org/10.1063/1.5011678 SN - 1054-1500 SN - 1089-7682 VL - 28 IS - 4 PB - American Institute of Physics CY - Melville ER - TY - JOUR A1 - Ozturk, Ugur A1 - Marwan, Norbert A1 - Korup, Oliver A1 - Saito, H. A1 - Agarwa, Ankit A1 - Grossman, M. J. A1 - Zaiki, M. A1 - Kurths, Jürgen T1 - Complex networks for tracking extreme rainfall during typhoons JF - Chaos : an interdisciplinary journal of nonlinear science N2 - Reconciling the paths of extreme rainfall with those of typhoons remains difficult despite advanced forecasting techniques. We use complex networks defined by a nonlinear synchronization measure termed event synchronization to track extreme rainfall over the Japanese islands. Directed networks objectively record patterns of heavy rain brought by frontal storms and typhoons but mask out contributions of local convective storms. We propose a radial rank method to show that paths of extreme rainfall in the typhoon season (August-November, ASON) follow the overall southwest-northeast motion of typhoons and mean rainfall gradient of Japan. The associated eye-of-the-typhoon tracks deviate notably and may thus distort estimates of heavy typhoon rainfall. We mainly found that the lower spread of rainfall tracks in ASON may enable better hindcasting than for westerly-fed frontal storms in June and July. Y1 - 2018 U6 - https://doi.org/10.1063/1.5004480 SN - 1054-1500 SN - 1089-7682 VL - 28 IS - 7 PB - American Institute of Physics CY - Melville ER - TY - JOUR A1 - Agarwal, Ankit A1 - Maheswaran, Rathinasamy A1 - Marwan, Norbert A1 - Caesar, Levke A1 - Kurths, Jürgen T1 - Wavelet-based multiscale similarity measure for complex networks JF - The European physical journal : B, Condensed matter and complex systems N2 - In recent years, complex network analysis facilitated the identification of universal and unexpected patterns in complex climate systems. However, the analysis and representation of a multiscale complex relationship that exists in the global climate system are limited. A logical first step in addressing this issue is to construct multiple networks over different timescales. Therefore, we propose to apply the wavelet multiscale correlation (WMC) similarity measure, which is a combination of two state-of-the-art methods, viz. wavelet and Pearson’s correlation, for investigating multiscale processes through complex networks. Firstly we decompose the data over different timescales using the wavelet approach and subsequently construct a corresponding network by Pearson’s correlation. The proposed approach is illustrated and tested on two synthetics and one real-world example. The first synthetic case study shows the efficacy of the proposed approach to unravel scale-specific connections, which are often undiscovered at a single scale. The second synthetic case study illustrates that by dividing and constructing a separate network for each time window we can detect significant changes in the signal structure. The real-world example investigates the behavior of the global sea surface temperature (SST) network at different timescales. Intriguingly, we notice that spatial dependent structure in SST evolves temporally. Overall, the proposed measure has an immense potential to provide essential insights on understanding and extending complex multivariate process studies at multiple scales. KW - Statistical and Nonlinear Physics Y1 - 2018 U6 - https://doi.org/10.1140/epjb/e2018-90460-6 SN - 1434-6028 SN - 1434-6036 VL - 91 IS - 11 PB - Springer CY - New York ER - TY - JOUR A1 - Armin, Ardalan A1 - Chen, Zhiming A1 - Jin, Yaocheng A1 - Zhang, Kai A1 - Huang, Fei A1 - Shoaee, Safa T1 - A Shockley-Type polymer BT - Fullerene solar cell JF - Advanced energy materials N2 - Charge extraction rate in solar cells made of blends of electron donating/accepting organic semiconductors is typically slow due to their low charge carrier mobility. This sets a limit on the active layer thickness and has hindered the industrialization of organic solar cells (OSCs). Herein, charge transport and recombination properties of an efficient polymer (NT812):fullerene blend are investigated. This system delivers power conversion efficiency of >9% even when the junction thickness is as large as 800 nm. Experimental results indicate that this material system exhibits exceptionally low bimolecular recombination constant, 800 times smaller than the diffusion-controlled electron and hole encounter rate. Comparing theoretical results based on a recently introduced modified Shockley model for fill factor, and experiments, clarifies that charge collection is nearly ideal in these solar cells even when the thickness is several hundreds of nanometer. This is the first realization of high-efficiency Shockley-type organic solar cells with junction thicknesses suitable for scaling up. KW - charge transport KW - non-Langevin recombination KW - organic solar cells KW - thick junctions Y1 - 2018 U6 - https://doi.org/10.1002/aenm.201701450 SN - 1614-6832 SN - 1614-6840 VL - 8 IS - 7 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Abeysekara, A. U. A1 - Archer, A. A1 - Benbow, Wystan A1 - Bird, Ralph A1 - Brose, Robert A1 - Buchovecky, M. A1 - Buckley, J. H. A1 - Bugaev, V. A1 - Chromey, A. J. A1 - Connolly, M. P. A1 - Cui, Wei A1 - Daniel, M. K. A1 - Falcone, A. A1 - Feng, Qi A1 - Finley, John P. A1 - Fortson, L. A1 - Furniss, Amy A1 - Huetten, M. A1 - Hanna, David A1 - Hervet, O. A1 - Holder, J. A1 - Hughes, G. A1 - Humensky, T. B. A1 - Johnson, Caitlin A. A1 - Kaaret, Philip A1 - Kar, P. A1 - Kertzman, M. A1 - Kieda, David A1 - Krause, M. A1 - Krennrich, F. A1 - Kumar, S. A1 - Lang, M. J. A1 - Lin, T. T. Y. A1 - McArthur, S. A1 - Moriarty, P. A1 - Mukherjee, Reshmi A1 - Ong, R. A. A1 - Otte, Adam Nepomuk A1 - Park, Nahee A1 - Petrashyk, A. A1 - Pohl, Martin A1 - Pueschel, Elisa A1 - Quinn, J. A1 - Ragan, K. A1 - Reynolds, P. T. A1 - Richards, Gregory T. A1 - Roache, E. A1 - Rulten, C. A1 - Sadeh, I. A1 - Santander, Marcos A1 - Sembroski, G. H. A1 - Shahinyan, Karlen A1 - Sushch, I. A1 - Tyler, J. A1 - Wakely, S. P. A1 - Weinstein, A. A1 - Wells, R. M. A1 - Wilcox, P. A1 - Wilhelm, Alina A1 - Williams, D. A. A1 - Williamson, T. J. A1 - Zitzer, B. A1 - Abdollahi, S. A1 - Ajello, Marco A1 - Baldini, Luca A1 - Barbiellini, G. A1 - Bastieri, Denis A1 - Bellazzini, Ronaldo A1 - Berenji, B. A1 - Bissaldi, Elisabetta A1 - Blandford, R. D. A1 - Bonino, R. A1 - Bottacini, E. A1 - Brandt, Terri J. A1 - Bruel, P. A1 - Buehler, R. A1 - Cameron, R. A. A1 - Caputo, R. A1 - Caraveo, P. A. A1 - Castro, D. A1 - Cavazzuti, E. A1 - Charles, Eric A1 - Chiaro, G. A1 - Ciprini, S. A1 - Cohen-Tanugi, Johann A1 - Costantin, D. A1 - Cutini, S. A1 - de Palma, F. A1 - Di Lalla, N. A1 - Di Mauro, M. A1 - Di Venere, L. A1 - Dominguez, A. A1 - Favuzzi, C. A1 - Fegan, S. J. A1 - Franckowiak, Anna A1 - Fukazawa, Yasushi A1 - Funk, Stefan A1 - Fusco, Piergiorgio A1 - Gargano, Fabio A1 - Gasparrini, Dario A1 - Giglietto, Nicola A1 - Giordano, F. A1 - Giroletti, Marcello A1 - Green, D. A1 - Grenier, I. A. A1 - Guillemot, L. A1 - Guiriec, Sylvain A1 - Hays, Elizabeth A1 - Hewitt, John W. A1 - Horan, D. A1 - Johannesson, G. A1 - Kensei, S. A1 - Kuss, M. A1 - Larsson, Stefan A1 - Latronico, L. A1 - Lemoine-Goumard, Marianne A1 - Li, J. A1 - Longo, Francesco A1 - Loparco, Francesco A1 - Lovellette, M. N. A1 - Lubrano, Pasquale A1 - Magill, Jeffrey D. A1 - Maldera, Simone A1 - Mazziotta, Mario Nicola A1 - McEnery, J. E. A1 - Michelson, P. F. A1 - Mitthumsiri, W. A1 - Mizuno, Tsunefumi A1 - Monzani, Maria Elena A1 - Morselli, Aldo A1 - Moskalenko, Igor V. A1 - Negro, M. A1 - Nuss, E. A1 - Ojha, R. A1 - Omodei, Nicola A1 - Orienti, M. A1 - Orlando, E. A1 - Palatiello, M. A1 - Paliya, Vaidehi S. A1 - Paneque, D. A1 - Perkins, Jeremy S. A1 - Persic, M. A1 - Pesce-Rollins, Melissa A1 - Petrosian, Vahe' A1 - Piron, F. A1 - Porter, Troy A. A1 - Principe, G. A1 - Raino, S. A1 - Rando, Riccardo A1 - Rani, B. A1 - Razzano, Massimilano A1 - Razzaque, Soebur A1 - Reimer, A. A1 - Reimer, Olaf A1 - Reposeur, T. A1 - Sgro, C. A1 - Siskind, E. J. A1 - Spandre, Gloria A1 - Spinelli, P. A1 - Suson, D. J. A1 - Tajima, Hiroyasu A1 - Thayer, J. B. A1 - Thompson, David J. A1 - Torres, Diego F. A1 - Tosti, Gino A1 - Troja, Eleonora A1 - Valverde, J. A1 - Vianello, Giacomo A1 - Vogel, M. A1 - Wood, K. A1 - Yassine, M. A1 - Alfaro, R. A1 - Alvarez, C. A1 - Alvarez, J. D. A1 - Arceo, R. A1 - Arteaga-Velazquez, J. C. A1 - Rojas, D. Avila A1 - Ayala Solares, H. A. A1 - Becerril, A. A1 - Belmont-Moreno, E. A1 - BenZvi, S. Y. A1 - Bernal, A. A1 - Braun, J. A1 - Brisbois, C. A1 - Caballero-Mora, K. S. A1 - Capistran, T. A1 - Carraminana, A. A1 - Casanova, Sabrina A1 - Castillo, M. A1 - Cotti, U. A1 - Cotzomi, J. A1 - Coutino de Leon, S. A1 - De Leon, C. A1 - De la Fuente, E. A1 - Dichiara, S. A1 - Dingus, B. L. A1 - DuVernois, M. A. A1 - Diaz-Velez, J. C. A1 - Engel, K. A1 - Enriquez-Rivera, O. A1 - Fiorino, D. W. A1 - Fleischhack, H. A1 - Fraija, N. A1 - Garcia-Gonzalez, J. A. A1 - Garfias, F. A1 - Gonzalez Munoz, A. A1 - Gonzalez, M. M. A1 - Goodman, J. A. A1 - Hampel-Arias, Z. A1 - Harding, J. P. A1 - Hernandez, S. A1 - Hernandez-Almada, A. A1 - Hona, B. A1 - Hueyotl-Zahuantitla, F. A1 - Hui, C. M. A1 - Huntemeyer, P. A1 - Iriarte, A. A1 - Jardin-Blicq, A. A1 - Joshi, V. A1 - Kaufmann, S. A1 - Lara, A. A1 - Lauer, R. J. A1 - Lee, W. H. A1 - Lennarz, D. A1 - Leon Vargas, H. A1 - Linnemann, J. T. A1 - Longinotti, A. L. A1 - Luis-Raya, G. A1 - Luna-Garcia, R. A1 - Lopez-Coto, R. A1 - Malone, K. A1 - Marinelli, S. S. A1 - Martinez, O. A1 - Martinez-Castellanos, I. A1 - Martinez-Castro, J. A1 - Martinez-Huerta, H. A1 - Matthews, J. A. A1 - Miranda-Romagnoli, P. A1 - Moreno, E. A1 - Mostafa, M. A1 - Nayerhoda, A. A1 - Nellen, L. A1 - Newbold, M. A1 - Nisa, M. U. A1 - Noriega-Papaqui, R. A1 - Pelayo, R. A1 - Pretz, J. A1 - Perez-Perez, E. G. A1 - Ren, Z. A1 - Rho, C. D. A1 - Riviere, C. A1 - Rosa-Gonzalez, D. A1 - Rosenberg, M. A1 - Ruiz-Velasco, E. A1 - Salazar, H. A1 - Greus, F. Salesa A1 - Sandoval, A. A1 - Schneider, M. A1 - Arroyo, M. Seglar A1 - Sinnis, G. A1 - Smith, A. J. A1 - Springer, R. W. A1 - Surajbali, P. A1 - Taboada, Ignacio A1 - Tibolla, O. A1 - Tollefson, K. A1 - Torres, I. A1 - Ukwatta, Tilan N. A1 - Villasenor, L. A1 - Weisgarber, T. A1 - Westerhoff, Stefan A1 - Wisher, I. G. A1 - Wood, J. A1 - Yapici, Tolga A1 - Yodh, G. A1 - Zepeda, A. A1 - Zhou, H. T1 - VERITAS and Fermi-LAT Observations of TeV Gamma-Ray Sources Discovered by HAWC in the 2HWC Catalog JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - The High Altitude Water Cherenkov (HAWC) collaboration recently published their 2HWC catalog, listing 39 very high energy (VHE; >100 GeV) gamma-ray sources based on 507 days of observation. Among these, 19 sources are not associated with previously known teraelectronvolt (TeV) gamma-ray sources. We have studied 14 of these sources without known counterparts with VERITAS and Fermi-LAT. VERITAS detected weak gamma-ray emission in the 1 TeV-30 TeV band in the region of DA 495, a pulsar wind nebula coinciding with 2HWC J1953+294, confirming the discovery of the source by HAWC. We did not find any counterpart for the selected 14 new HAWC sources from our analysis of Fermi-LAT data for energies higher than 10 GeV. During the search, we detected gigaelectronvolt (GeV) gamma-ray emission coincident with a known TeV pulsar wind nebula, SNR G54.1+0.3 (VER J1930+188), and a 2HWC source, 2HWC J1930+188. The fluxes for isolated, steady sources in the 2HWC catalog are generally in good agreement with those measured by imaging atmospheric Cherenkov telescopes. However, the VERITAS fluxes for SNR G54.1+0.3, DA 495, and TeV J2032+4130 are lower than those measured by HAWC, and several new HAWC sources are not detected by VERITAS. This is likely due to a change in spectral shape, source extension, or the influence of diffuse emission in the source region. KW - gamma rays: general Y1 - 2018 U6 - https://doi.org/10.3847/1538-4357/aade4e SN - 0004-637X SN - 1538-4357 VL - 866 IS - 1 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Archer, A. A1 - Benbow, Wystan A1 - Bird, Ralph A1 - Brose, Robert A1 - Buchovecky, M. A1 - Bugaev, V A1 - Cui, Wei A1 - Danie, M. K. A1 - Falcone, A. A1 - Feng, Qi A1 - Finley, John P. A1 - Flinders, A. A1 - Fortson, L. A1 - Furniss, Amy A1 - Gillanders, Gerard H. A1 - Huttens, M. A1 - Hanna, David A1 - Hervet, O. A1 - Holder, J. A1 - Hughes, G. A1 - Humensky, T. B. A1 - Johnson, Caitlin A. A1 - Kaaret, Philip A1 - Kar, P. A1 - Kelley-Hoskins, N. A1 - Kieda, David A1 - Krause, Maria A1 - Krennrich, F. A1 - Kumar, S. A1 - Lang, M. J. A1 - Lin, T. T. Y. A1 - McArthur, S. A1 - Moriarty, P. A1 - Mukherjee, Reshmi A1 - Nieto, Daniel A1 - Ong, R. A. A1 - Otte, A. N. A1 - Park, Nahee A1 - Petrashyk, A. A1 - Pohl, Martin A1 - Popkow, Alexis A1 - Pueschel, Elisa A1 - Quinn, J. A1 - Ragan, K. A1 - Reynold, P. T. A1 - Richards, Gregory T. A1 - Roache, E. A1 - Rulten, C. A1 - Sadeh, I A1 - Sembroski, G. H. A1 - Shahinyan, Karlen A1 - Tyler, J. A1 - Wakely, S. P. A1 - Weiner, O. M. A1 - Weinstein, A. A1 - Wells, R. M. A1 - Wilcox, P. A1 - Wilhelm, Alina A1 - Williams, David A. A1 - Brisken, W. F. A1 - Pontrelli, P. T1 - HESS J1943+213 BT - An extreme blazar shining through the galactic plane JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - HESS J1943+213 is a very high energy (VHE; > 100 GeV) gamma-ray source in the direction of the Galactic plane. Studies exploring the classification of the source are converging toward its identification as an extreme synchrotron BL Lac object. Here we present 38 hr of VERITAS observations of HESS J1943+213 taken over 2 yr. The source is detected with a significance of similar to 20 standard deviations, showing a remarkably stable flux and spectrum in VHE gamma-rays. Multifrequency Very Long Baseline Array (VLBA) observations of the source confirm the extended, jet-like structure previously found in the 1.6 GHz band with the European VLBI Network and detect this component in the 4.6 and 7.3 GHz bands. The radio spectral indices of the core and the jet and the level of polarization derived from the VLBA observations are in a range typical for blazars. Data from VERITAS, Fermi-LAT, Swift-XRT, the FLWO 48 ' telescope, and archival infrared and hard X-ray observations are used to construct and model the spectral energy distribution (SED) of the source with a synchrotron self-Compton model. The well-measured gamma-ray peak of the SED with VERITAS and Fermi-LAT provides constraining upper limits on the source redshift. Possible contribution of secondary gamma-rays from ultra-high-energy cosmic-ray-initiated electromagnetic cascades to the gamma-ray emission is explored, finding that only a segment of the VHE spectrum can be accommodated with this process. A variability search is performed across X-ray and gamma-ray bands. No statistically significant flux or spectral variability is detected. KW - astroparticle physics KW - BL Lacertae objects: individual (HESS J1943+213, VER J1943+213) KW - galaxies: active KW - galaxies: jets KW - galaxies: nuclei KW - gamma rays: galaxies Y1 - 2018 U6 - https://doi.org/10.3847/1538-4357/aacbd0 SN - 0004-637X SN - 1538-4357 VL - 862 IS - 1 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Abeysekara, A. U. A1 - Archer, A. A1 - Benbow, Wystan A1 - Bird, Ralph A1 - Brill, A. A1 - Brose, Robert A1 - Buckley, J. H. A1 - Christiansen, Jessie L. A1 - Chromey, A. J. A1 - Daniel, M. K. A1 - Falcone, A. A1 - Feng, Qi A1 - Finley, John P. A1 - Fortson, L. A1 - Furniss, Amy A1 - Gillanders, Gerard H. A1 - Gueta, O. A1 - Hanna, David A1 - Hervet, O. A1 - Holder, J. A1 - Hughes, G. A1 - Humensky, T. B. A1 - Johnson, Caitlin A. A1 - Kaaret, Philip A1 - Kar, P. A1 - Kelley-Hoskins, N. A1 - Kertzman, M. A1 - Kieda, David A1 - Krause, Maria A1 - Krennrich, F. A1 - Lang, M. J. A1 - Moriarty, P. A1 - Mukherjee, Reshmi A1 - Ong, R. A. A1 - Otte, A. N. A1 - Park, N. A1 - Petrashyk, A. A1 - Pohl, Martin A1 - Pueschel, Elisa A1 - Quinn, J. A1 - Ragan, K. A1 - Reynolds, P. T. A1 - Richards, Gregory T. A1 - Roache, E. A1 - Rulten, C. A1 - Sadeh, I. A1 - Santander, Marcos A1 - Scott, S. S. A1 - Sembroski, G. H. A1 - Shahinyan, Karlen A1 - Tyler, J. A1 - Wakely, S. P. A1 - Weinstein, A. A1 - Wells, R. M. A1 - Wilcox, P. A1 - Wilhelm, Alina A1 - Williams, D. A. A1 - Williamson, T. J. A1 - Zitzer, B. A1 - Kaur, A. T1 - VERITAS Observations of the BL Lac Object TXS 0506+056 JF - The astrophysical journal : an international review of spectroscopy and astronomical physics ; Part 2, Letters N2 - On 2017 September 22, the IceCube Neutrino Observatory reported the detection of the high-energy neutrino event IC 170922A, of potential astrophysical origin. It was soon determined that the neutrino direction was consistent with the location of the gamma-ray blazar TXS 0506+056. (3FGL J0509.4+ 0541), which was in an elevated gamma-ray emission state as measured by the Fermi satellite. Very Energetic Radiation Imaging Telescope Array System (VERITAS) observations of the neutrino/blazar region started on 2017 September 23 in response to the neutrino alert and continued through 2018 February 6. While no significant very-high-energy (VHE; E > 100 GeV) emission was observed from the blazar by VERITAS in the two-week period immediately following the IceCube alert, TXS 0506+ 056 was detected by VERITAS with a significance of 5.8 standard deviations (sigma) in the full 35 hr data set. The average photon flux of the source during this period was (8.9 +/- 1.6). x. 10(-12) cm(-2) s(-1), or 1.6% of the Crab Nebula flux, above an energy threshold of 110 GeV, with a soft spectral index of 4.8. +/-. 1.3. KW - astroparticle physics KW - BL Lacertae objects: individual (TXS 0506+056, VER J0509+057) KW - gamma rays: galaxies KW - quasars: general KW - neutrinos Y1 - 2018 U6 - https://doi.org/10.3847/2041-8213/aad053 SN - 2041-8205 SN - 2041-8213 VL - 861 IS - 2 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Abeysekara, A. U. A1 - Archer, A. A1 - Aune, Taylor A1 - Benbow, Wystan A1 - Bird, Ralph A1 - Brose, Robert A1 - Buchovecky, M. A1 - Bugaev, V. A1 - Cui, Wei A1 - Daniel, M. K. A1 - Falcone, A. A1 - Feng, Qi A1 - Finley, John P. A1 - Fleischhack, H. A1 - Flinders, A. A1 - Fortson, L. A1 - Furniss, Amy A1 - Gotthelf, Eric V. A1 - Grube, J. A1 - Hanna, David A1 - Hervet, O. A1 - Holder, J. A1 - Huang, K. A1 - Hughes, G. A1 - Humensky, T. B. A1 - Huetten, M. A1 - Johnson, Caitlin A. A1 - Kaaret, Philip A1 - Kar, P. A1 - Kelley-Hoskins, N. A1 - Kertzman, M. A1 - Kieda, David A1 - Krause, Maria A1 - Kumar, S. A1 - Lang, M. J. A1 - Lin, T. T. Y. A1 - Maier, Gernot A1 - McArthur, S. A1 - Moriarty, P. A1 - Mukherjee, Reshmi A1 - Ong, R. A. A1 - Otte, Adam Nepomuk A1 - Pandel, Dirk A1 - Park, Nahee A1 - Petrashyk, A. A1 - Pohl, Martin A1 - Popkow, Alexis A1 - Pueschel, Elisa A1 - Quinn, J. A1 - Ragan, K. A1 - Reynolds, P. T. A1 - Richards, Gregory T. A1 - Roache, E. A1 - Rousselle, J. A1 - Rulten, C. A1 - Sadeh, I. A1 - Santander, M. A1 - Sembroski, G. H. A1 - Shahinyan, Karlen A1 - Tyler, J. A1 - Vassiliev, V. V. A1 - Wakely, S. P. A1 - Ward, J. E. A1 - Weinstein, A. A1 - Wells, R. M. A1 - Wilcox, P. A1 - Wilhelm, Alina A1 - Williams, David A. A1 - Zitzer, B. T1 - A Very High Energy gamma-Ray Survey toward the Cygnus Region of the Galaxy JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - We present results from deep observations toward the Cygnus region using 300 hr of very high energy (VHE)gamma-ray data taken with the VERITAS Cerenkov telescope array and over 7 yr of high-energy.-ray data taken with the Fermi satellite at an energy above 1 GeV. As the brightest region of diffuse gamma-ray emission in the northern sky, the Cygnus region provides a promising area to probe the origins of cosmic rays. We report the identification of a potential Fermi-LAT counterpart to VER J2031+415 (TeV J2032+4130) and resolve the extended VHE source VER J2019+368 into two source candidates (VER J2018+367* and VER J2020+368*) and characterize their energy spectra. The Fermi-LAT morphology of 3FGL J2021.0+4031e (the Gamma Cygni supernova remnant) was examined, and a region of enhanced emission coincident with VER J2019+407 was identified and jointly fit with the VERITAS data. By modeling 3FGL J2015.6+3709 as two sources, one located at the location of the pulsar wind nebula CTB 87 and one at the quasar QSO J2015+371, a continuous spectrum from 1 GeV to 10 TeV was extracted for VER J2016+371 (CTB 87). An additional 71 locations coincident with Fermi-LAT sources and other potential objects of interest were tested for VHE gamma-ray emission, with no emission detected and upper limits on the differential flux placed at an average of 2.3% of the Crab Nebula flux. We interpret these observations in a multiwavelength context and present the most detailed gamma-ray view of the region to date. KW - acceleration of particles KW - cosmic rays KW - gamma rays: general KW - ISM: supernova remnants Y1 - 2018 U6 - https://doi.org/10.3847/1538-4357/aac4a2 SN - 0004-637X SN - 1538-4357 VL - 861 IS - 2 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Abeysekara, A. U. A1 - Benbow, Wystan A1 - Bird, Ralph A1 - Brantseg, T. A1 - Brose, Robert A1 - Buchovecky, M. A1 - Buckley, J. H. A1 - Bugaev, V. A1 - Connolly, M. P. A1 - Cui, Wei A1 - Daniel, M. K. A1 - Falcone, A. A1 - Feng, Qi A1 - Finley, John P. A1 - Fortson, L. A1 - Furniss, Amy A1 - Gillanders, Gerard H. A1 - Gunawardhana, Isuru A1 - Huetten, M. A1 - Hanna, David A1 - Hervet, O. A1 - Holder, J. A1 - Hughes, G. A1 - Humensky, T. B. A1 - Johnson, Caitlin A. A1 - Kaaret, Philip A1 - Kar, P. A1 - Kertzman, M. A1 - Krennrich, F. A1 - Lang, M. J. A1 - Lin, T. T. Y. A1 - McArthur, S. A1 - Moriarty, P. A1 - Mukherjee, Reshmi A1 - Ong, R. A. A1 - Otte, Adam Nepomuk A1 - Park, N. A1 - Petrashyk, A. A1 - Pohl, Martin A1 - Pueschel, Elisa A1 - Quinn, J. A1 - Ragan, K. A1 - Reynolds, P. T. A1 - Richards, Gregory T. A1 - Roache, E. A1 - Rulten, C. A1 - Sadeh, I. A1 - Santander, M. A1 - Sembroski, G. H. A1 - Shahinyan, Karlen A1 - Wakely, S. P. A1 - Weinstein, A. A1 - Wells, R. M. A1 - Wilcox, P. A1 - Williams, D. A. A1 - Zitzer, B. A1 - Jorstad, Svetlana G. A1 - Marscher, Alan P. A1 - Lister, Matthew L. A1 - Kovalev, Yuri Y. A1 - Pushkarev, A. B. A1 - Savolainen, Tuomas A1 - Agudo, I. A1 - Molina, S. N. A1 - Gomez, J. L. A1 - Larionov, Valeri M. A1 - Borman, G. A. A1 - Mokrushina, A. A. A1 - Tornikoski, Merja A1 - Lahteenmaki, A. A1 - Chamani, W. A1 - Enestam, S. A1 - Kiehlmann, S. A1 - Hovatta, Talvikki A1 - Smith, P. S. A1 - Pontrelli, P. T1 - Multiwavelength Observations of the Blazar BL Lacertae BT - a new Fast TeV Gamma-Ray Flare JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - Combined with measurements made by very-long-baseline interferometry, the observations of fast TeV gamma-ray flares probe the structure and emission mechanism of blazar jets. However, only a handful of such flares have been detected to date, and only within the last few years have these flares been observed from lower-frequency-peaked BL. Lac objects and flat-spectrum radio quasars. We report on a fast TeV gamma-ray flare from the blazar BL. Lacertae observed by the Very Energetic Radiation Imaging Telescope Array System (VERITAS). with a rise time of similar to 2.3 hr and a decay time of similar to 36 min. The peak flux above 200 GeV is (4.2 +/- 0.6) x 10(-6) photon m(-2) s(-1) measured with a 4-minute-binned light curve, corresponding to similar to 180% of the flux that is observed from the Crab Nebula above the same energy threshold. Variability contemporaneous with the TeV gamma-ray flare was observed in GeV gamma-ray, X-ray, and optical flux, as well as in optical and radio polarization. Additionally, a possible moving emission feature with superluminal apparent velocity was identified in Very Long Baseline Array observations at 43 GHz, potentially passing the radio core of the jet around the time of the gamma-ray flare. We discuss the constraints on the size, Lorentz factor, and location of the emitting region of the flare, and the interpretations with several theoretical models that invoke relativistic plasma passing stationary shocks. KW - BL Lacertae objects: individual (BL Lacertae = VER J2202+422) KW - galaxies: active Y1 - 2018 U6 - https://doi.org/10.3847/1538-4357/aab35c SN - 0004-637X SN - 1538-4357 VL - 856 IS - 2 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Abeysekara, A. U. A1 - Archer, A. A1 - Benbow, Wystan A1 - Bird, Ralph A1 - Brose, Robert A1 - Buchovecky, M. A1 - Bugaev, V. A1 - Connolly, M. P. A1 - Cui, Wei A1 - Errando, Manel A1 - Falcone, A. A1 - Feng, Qi A1 - Finley, John P. A1 - Flinders, A. A1 - Fortson, L. A1 - Furniss, Amy A1 - Gillanders, Gerard H. A1 - Huetten, M. A1 - Hanna, David A1 - Hervet, O. A1 - Holder, J. A1 - Hughes, G. A1 - Humensky, T. B. A1 - Johnson, Caitlin A. A1 - Kaaret, Philip A1 - Kar, P. A1 - Kelley-Hoskins, N. A1 - Kertzman, M. A1 - Kieda, David A1 - Krause, Maria A1 - Krennrich, F. A1 - Lang, M. J. A1 - Lin, T. T. Y. A1 - Maier, Gernot A1 - McArthur, S. A1 - Moriarty, P. A1 - Mukherjee, Reshmi A1 - Ong, R. A. A1 - Park, N. A1 - Perkins, Jeremy S. A1 - Petrashyk, A. A1 - Pohl, Martin A1 - Popkow, Alexis A1 - Pueschel, Elisa A1 - Quinn, J. A1 - Ragan, K. A1 - Reynolds, P. T. A1 - Richards, Gregory T. A1 - Roache, E. A1 - Rulten, C. A1 - Sadeh, I. A1 - Santander, M. A1 - Sembroski, G. H. A1 - Shahinyan, Karlen A1 - Tyler, J. A1 - Wakely, S. P. A1 - Weiner, O. M. A1 - Weinstein, A. A1 - Wells, R. M. A1 - Wilcox, P. A1 - Wilhelm, Alina A1 - Williams, David A. A1 - Zitzer, B. A1 - Vurm, Indrek A1 - Beloborodov, Andrei T1 - A Strong Limit on the Very-high-energy Emission from GRB 150323A JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - On 2015 March 23, the Very Energetic Radiation Imaging Telescope Array System (VERITAS) responded to a Swift-Burst Alert Telescope (BAT) detection of a gamma-ray burst, with observations beginning 270 s after the onset of BAT emission, and only 135 s after the main BAT emission peak. No statistically significant signal is detected above 140 GeV. The VERITAS upper limit on the fluence in a 40-minute integration corresponds to about 1% of the prompt fluence. Our limit is particularly significant because the very-high-energy (VHE) observation started only similar to 2 minutes after the prompt emission peaked, and Fermi-Large Area Telescope observations of numerous other bursts have revealed that the high-energy emission is typically delayed relative to the prompt radiation and lasts significantly longer. Also, the proximity of GRB 150323A (z = 0.593) limits the attenuation by the extragalactic background light to similar to 50% at 100-200 GeV. We conclude that GRB 150323A had an intrinsically very weak high-energy afterglow, or that the GeV spectrum had a turnover below similar to 100 GeV. If the GRB exploded into the stellar wind of a massive progenitor, the VHE non-detection constrains the wind density parameter to be A greater than or similar to 3 x 10(11) g . cm(-1), consistent with a standard Wolf-Rayet progenitor. Alternatively, the VHE emission from the blast wave would be weak in a very tenuous medium such as the interstellar medium, which therefore cannot be ruled out as the environment of GRB 150323A. KW - gamma rays: general KW - gamma-ray burst: general KW - gamma-ray burst: individual (GRB 150323A) Y1 - 2018 U6 - https://doi.org/10.3847/1538-4357/aab371 SN - 0004-637X SN - 1538-4357 VL - 857 IS - 1 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Petruk, Oleh A1 - Kuzyo, T. A1 - Orlando, S. A1 - Pohl, Martin A1 - Miceli, M. A1 - Bocchino, F. A1 - Beshley, V. A1 - Brose, Robert T1 - Post-adiabatic supernova remnants in an interstellar magnetic field BT - oblique shocks and non-uniform environment JF - Monthly notices of the Royal Astronomical Society N2 - We present very-high-resolution 1D MHD simulations of the late-stage supernova remnants (SNRs). In the post-adiabatic stage, the magnetic field has an important and significant dynamical effect on the shock dynamics, the flow structure, and hence the acceleration and emission of cosmic rays. We find that the tangential component of the magnetic field provides pressure support that to a fair degree prevents the collapse of the radiative shell and thus limits the total compression ratio of the partially or fully radiative forward shock. A consequence is that the spectra of cosmic rays would not be as hard as in hydrodynamic simulations. We also investigated the effect on the flow profiles of the magnetic-field inclination and a large-scale gradient in the gas density and/or the magnetic field. A positive density gradient shortens the evolutionary stages, whereas a shock obliquity lowers the shock compression. The compression of the tangential component of the magnetic field leads to its dominance in the downstream region of post-adiabatic shocks for a wide range of orientation of the upstream field, which may explain why one preferentially observes tangential radio polarization in old SNRs. As most cosmic rays are produced at late stages of SNR evolution, the post-adiabatic phase and the influence of the magnetic field during it are most important for modeling the cosmic-ray acceleration at old SNRs and the gamma-ray emission from late-stage SNRs interacting with clouds. KW - shock waves KW - ISM: magnetic fields KW - ISM: supernova remnants Y1 - 2018 U6 - https://doi.org/10.1093/mnras/sty1750 SN - 0035-8711 SN - 1365-2966 VL - 479 IS - 3 SP - 4253 EP - 4270 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Macias, Oscar A1 - Gordon, Chris A1 - Crocker, Roland M. A1 - Coleman, Brendan A1 - Paterson, Dylan A1 - Horiuchi, Shunsaku A1 - Pohl, Martin T1 - Galactic bulge preferred over dark matter for the Galactic centre gamma-ray excess JF - Nature Astronomy N2 - An anomalous gamma-ray excess emission has been found in the Fermi Large Area Telescope data1 covering the centre of the Galaxy2,3. Several theories have been proposed for this ‘Galactic centre excess’. They include self-annihilation of dark-matter particles4, an unresolved population of millisecond pulsars5, an unresolved population of young pulsars6, or a series of burst events7. Here, we report on an analysis that exploits hydrodynamical modelling to register the position of interstellar gas associated with diffuse Galactic gamma-ray emission. We find evidence that the Galactic centre excess gamma rays are statistically better described by the stellar over-density in the Galactic bulge and the nuclear stellar bulge, rather than a spherical excess. Given its non-spherical nature, we argue that the Galactic centre excess is not a dark-matter phenomenon but rather associated with the stellar population of the Galactic bulge and the nuclear bulge. Y1 - 2018 U6 - https://doi.org/10.1038/s41550-018-0414-3 SN - 2397-3366 VL - 2 IS - 5 SP - 387 EP - 392 PB - Nature Publ. Group CY - London ER - TY - JOUR A1 - Denker, Carsten A1 - Kuckein, Christoph A1 - Verma, Meetu A1 - Manrique Gonzalez, Sergio Javier Gonzalez A1 - Diercke, Andrea A1 - Enke, Harry A1 - Klar, Jochen A1 - Balthasar, Horst A1 - Louis, Rohan E. A1 - Dineva, Ekaterina Ivanova T1 - High-cadence Imaging and Imaging Spectroscopy at the GREGOR Solar Telescope-A Collaborative Research Environment for High-resolution Solar Physics JF - The astrophysical journal : an international review of spectroscopy and astronomical physics ; Supplement series N2 - In high-resolution solar physics, the volume and complexity of photometric, spectroscopic, and polarimetric ground-based data significantly increased in the last decade, reaching data acquisition rates of terabytes per hour. This is driven by the desire to capture fast processes on the Sun and the necessity for short exposure times "freezing" the atmospheric seeing, thus enabling ex post facto image restoration. Consequently, large-format and high-cadence detectors are nowadays used in solar observations to facilitate image restoration. Based on our experience during the "early science" phase with the 1.5 m GREGOR solar telescope (2014–2015) and the subsequent transition to routine observations in 2016, we describe data collection and data management tailored toward image restoration and imaging spectroscopy. We outline our approaches regarding data processing, analysis, and archiving for two of GREGOR's post-focus instruments (see http://gregor.aip.de), i.e., the GREGOR Fabry–Pérot Interferometer (GFPI) and the newly installed High-Resolution Fast Imager (HiFI). The heterogeneous and complex nature of multidimensional data arising from high-resolution solar observations provides an intriguing but also a challenging example for "big data" in astronomy. The big data challenge has two aspects: (1) establishing a workflow for publishing the data for the whole community and beyond and (2) creating a collaborative research environment (CRE), where computationally intense data and postprocessing tools are colocated and collaborative work is enabled for scientists of multiple institutes. This requires either collaboration with a data center or frameworks and databases capable of dealing with huge data sets based on virtual observatory (VO) and other community standards and procedures. KW - astronomical databases KW - methods: data analysis KW - Sun: chromosphere KW - Sun: photosphere KW - techniques: image processing KW - techniques: spectroscopic Y1 - 2018 U6 - https://doi.org/10.3847/1538-4365/aab773 SN - 0067-0049 SN - 1538-4365 VL - 236 IS - 1 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Vafin, Sergei A1 - Rafighi, Iman A1 - Pohl, Martin A1 - Niemiec, Jacek T1 - The Electrostatic Instability for Realistic Pair Distributions in Blazar/EBL Cascades JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - This work revisits the electrostatic instability for blazar-induced pair beams propagating through the intergalactic medium (IGM) using linear analysis and PIC simulations. We study the impact of the realistic distribution function of pairs resulting from the interaction of high-energy gamma-rays with the extragalactic background light. We present analytical and numerical calculations of the linear growth rate of the instability for the arbitrary orientation of wave vectors. Our results explicitly demonstrate that the finite angular spread of the beam dramatically affects the growth rate of the waves, leading to the fastest growth for wave vectors quasi-parallel to the beam direction and a growth rate at oblique directions that is only a factor of 2-4 smaller compared to the maximum. To study the nonlinear beam relaxation, we performed PIC simulations that take into account a realistic wide-energy distribution of beam particles. The parameters of the simulated beam-plasma system provide an adequate physical picture that can be extrapolated to realistic blazar-induced pairs. In our simulations, the beam looses only 1% of its energy, and we analytically estimate that the beam would lose its total energy over about 100 simulation times. An analytical scaling is then used to extrapolate the parameters of realistic blazar-induced pair beams. We find that they can dissipate their energy slightly faster by the electrostatic instability than through inverse-Compton scattering. The uncertainties arising from, e.g., details of the primary gamma-ray spectrum are too large to make firm statements for individual blazars, and an analysis based on their specific properties is required. KW - gamma rays: general KW - instabilities KW - magnetic fields KW - relativistic processes KW - waves Y1 - 2018 U6 - https://doi.org/10.3847/1538-4357/aab552 SN - 0004-637X SN - 1538-4357 VL - 857 IS - 1 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Archer, A. A1 - Benbow, W. A1 - Bird, R. A1 - Brose, Robert A1 - Buchovecky, M. A1 - Buckley, J. H. A1 - Bugaev, V. A1 - Connolly, M. P. A1 - Cui, W. A1 - Daniel, M. K. A1 - Feng, Q. A1 - Finley, J. P. A1 - Fortson, L. A1 - Furniss, A. A1 - Gillanders, G. A1 - Huetten, M. A1 - Hanna, D. A1 - Hervet, O. A1 - Holder, J. A1 - Hughes, G. A1 - Humensky, T. B. A1 - Johnson, C. A. A1 - Kaaret, P. A1 - Kar, P. A1 - Kelley-Hoskins, N. A1 - Kertzman, M. A1 - Kieda, D. A1 - Krause, M. A1 - Krennrich, F. A1 - Kumar, S. A1 - Lang, M. J. A1 - Lin, T. T. Y. A1 - Maier, G. A1 - McArthur, S. A1 - Moriarty, P. A1 - Mukherjee, R. A1 - Ong, R. A. A1 - Otte, A. N. A1 - Petrashyk, A. A1 - Pohl, M. A1 - Pueschel, Elisa A1 - Quinn, J. A1 - Ragan, K. A1 - Reynolds, P. T. A1 - Richards, G. T. A1 - Roache, E. A1 - Rulten, C. A1 - Sadeh, I. A1 - Santander, M. A1 - Sembroski, G. H. A1 - Staszak, D. A1 - Sushch, I. A1 - Wakely, S. P. A1 - Wells, R. M. A1 - Wilcox, P. A1 - Wilhelm, Alina A1 - Williams, D. A. A1 - Williamson, T. J. A1 - Zitzer, B. T1 - Measurement of cosmic-ray electrons at TeV energies by VERITAS JF - Physical review : D, Particles, fields, gravitation, and cosmology N2 - Cosmic-ray electrons and positrons (CREs) at GeV-TeV energies are a unique probe of our local Galactic neighborhood. CREs lose energy rapidly via synchrotron radiation and inverse-Compton scattering processes while propagating within the Galaxy, and these losses limit their propagation distance. For electrons with TeV energies, the limit is on the order of a kiloparsec. Within that distance, there are only a few known astrophysical objects capable of accelerating electrons to such high energies. It is also possible that the CREs are the products of the annihilation or decay of heavy dark matter (DM) particles. VERITAS, an array of imaging air Cherenkov telescopes in southern Arizona, is primarily utilized for gamma-ray astronomy but also simultaneously collects CREs during all observations. We describe our methods of identifying CREs in VERITAS data and present an energy spectrum, extending from 300 GeV to 5 TeV, obtained from approximately 300 hours of observations. A single power-law fit is ruled out in VERITAS data. We find that the spectrum of CREs is consistent with a broken power law, with a break energy at 710 +/- 40(stat) +/- 140(syst) GeV. Y1 - 2018 U6 - https://doi.org/10.1103/PhysRevD.98.062004 SN - 2470-0010 SN - 2470-0029 VL - 98 IS - 6 PB - American Physical Society CY - College Park ER - TY - JOUR A1 - Archer, A. A1 - Benbow, W. A1 - Bird, R. A1 - Brose, Robert A1 - Buchovecky, M. A1 - Bugaev, V. A1 - Connolly, M. P. A1 - Cui, W. A1 - Daniel, M. K. A1 - Falcone, A. A1 - Feng, Q. A1 - Finley, J. P. A1 - Fleischhack, H. A1 - Fortson, L. A1 - Furniss, A. A1 - Hanna, D. A1 - Hervet, O. A1 - Holder, J. A1 - Hughes, G. A1 - Humensky, T. B. A1 - Hutten, M. A1 - Johnson, C. A. A1 - Kaaret, P. A1 - Kelley-Hoskins, N. A1 - Kieda, D. A1 - Krause, M. A1 - Krennrich, F. A1 - Kumar, S. A1 - Lang, M. J. A1 - Maier, G. A1 - McArthur, S. A1 - Moriarty, P. A1 - Mukherjee, R. A1 - Nieto, D. A1 - Ong, R. A. A1 - Otte, A. N. A1 - Park, N. A1 - Petrashyk, A. A1 - Pohl, Martin A1 - Popkow, A. A1 - Pueschel, Elisa A1 - Quinn, J. A1 - Ragan, K. A1 - Reynolds, P. T. A1 - Richards, G. T. A1 - Roache, E. A1 - Rulten, C. A1 - Sadeh, I. A1 - Tyler, J. A1 - Wakely, S. P. A1 - Weiner, O. M. A1 - Wilcox, P. A1 - Wilhelm, Alina A1 - Williams, D. A. A1 - Wissel, S. A. A1 - Zitzer, B. T1 - Measurement of the iron spectrum in cosmic rays by VERITAS JF - Physical review : D, Particles, fields, gravitation, and cosmology N2 - We present a new measurement of the energy spectrum of iron nuclei in cosmic rays from 20 TeV to 500 TeV; The measurement makes use of a template-based analysis method, which, for the first time, is applied to the energy reconstruction of iron-induced air showers recorded by the VERITAS array of imaging atmospheric Cherenkov telescopes. The event selection makes use of the direct Cherenkov light which is emitted by charged particles before the first interaction, as well as other parameters related to the shape of the recorded air shower images. The measured spectrum is well described by a power law dF/dE = f(0) center dot (E/E-0)(-gamma) over the full energy range, with gamma = 2.82 +/- 0.30(stat)(-0.27)(+0.24)(syst) and f(0) = (4.82 +/- 0.98(stat)(-2.70)(+2.12)(syst)) x 10(-7) m(-2) s(-1) TeV-1 sr(-1) at E-0 = 50 TeV, with no indication of a cutoff or spectral break. The measured differential flux is compatible with previous results, with improved statistical uncertainty at the highest energies. Y1 - 2018 U6 - https://doi.org/10.1103/PhysRevD.98.022009 SN - 2470-0010 SN - 2470-0029 VL - 98 IS - 2 PB - American Physical Society CY - College Park ER - TY - JOUR A1 - Chen, Ye A1 - Loisch, Gregor A1 - Gross, Matthias A1 - Jao, Chun-Sung A1 - Krasilnikov, Mikhail A1 - Oppelt, Anne A1 - Osterhoff, Jens A1 - Pohl, Martin A1 - Qian, Houjun A1 - Stephan, Frank A1 - Vafin, Sergei T1 - Generation of quasi continuous-wave electron beams in an L-band normal conducting pulsed RF injector for laboratory astrophysics experiments JF - Nuclear instruments & methods in physics research : a journal on accelerators, instrumentation and techniques applied to research in nuclear and atomic physics, materials science and related fields in physics ; A, Accelerators, spectrometers, detectors and associated equipment N2 - We report on an approach to produce quasi continuous-wave (cw) electron beams with an average beam current of milliamperes and a mean beam energy of a few MeV in a pulsed RF injector. Potential applications are in the planned laboratory astrophysics programs at DESY. The beam generation is based on field emission from a specially designed metallic field emitter. A quasi cw beam profile is formed over subsequent RF cycles at the resonance frequency of the gun cavity. This is realized by debunching in a cut disk structure accelerating cavity (booster) downstream of the gun. The peak and average beam currents can be tuned in beam dynamics simulations by adjusting operation conditions of the booster cavity. Optimization of the transverse beam size at specific positions (e.g., entrance of the plasma experiment) is performed by applying magnetic focusing fields provided by solenoids along the beam line. In this paper, the design of a microtip field emitter is introduced and characterized in electromagnetic field simulations in the gun cavity. A series of particle tracking simulations are conducted for multi-parametric optimization of the parameters of the produced quasi cw electron beams. The obtained results will be presented and discussed. In addition, measurements of the parasitic field emission (PFE) current (dark current) in the PITZ gun will be exemplarily shown to distinguish its order of magnitude from the produced beam current by the designed field emitter. KW - Cw electron beam KW - RF gun KW - Booster cavity KW - Laboratory astrophysics KW - Field emission KW - Beam dynamics Y1 - 2018 U6 - https://doi.org/10.1016/j.nima.2018.06.063 SN - 0168-9002 SN - 1872-9576 VL - 903 SP - 119 EP - 125 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Steffen, Will A1 - Röckstrom, Johan A1 - Richardson, Katherine A1 - Lenton, Timothy M. A1 - Folke, Carl A1 - Liverman, Diana A1 - Summerhayes, Colin P. A1 - Barnosky, Anthony D. A1 - Cornell, Sarah E. A1 - Crucifix, Michel A1 - Donges, Jonathan A1 - Fetzer, Ingo A1 - Lade, Steven J. A1 - Scheffer, Marten A1 - Winkelmann, Ricarda A1 - Schellnhuber, Hans Joachim T1 - Trajectories of the Earth System in the Anthropocene JF - Proceedings of the National Academy of Sciences of the United States of America N2 - We explore the risk that self-reinforcing feedbacks could push the Earth System toward a planetary threshold that, if crossed, could prevent stabilization of the climate at intermediate temperature rises and cause continued warming on a "Hothouse Earth" pathway even as human emissions are reduced. Crossing the threshold would lead to a much higher global average temperature than any interglacial in the past 1.2 million years and to sea levels significantly higher than at any time in the Holocene. We examine the evidence that such a threshold might exist and where it might be. If the threshold is crossed, the resulting trajectory would likely cause serious disruptions to ecosystems, society, and economies. Collective human action is required to steer the Earth System away from a potential threshold and stabilize it in a habitable interglacial-like state. Such action entails stewardship of the entire Earth System-biosphere, climate, and societies-and could include decarbonization of the global economy, enhancement of biosphere carbon sinks, behavioral changes, technological innovations, new governance arrangements, and transformed social values. KW - Earth System trajectories KW - climate change KW - Anthropocene KW - biosphere feedbacks KW - tipping elements Y1 - 2018 U6 - https://doi.org/10.1073/pnas.1810141115 SN - 0027-8424 VL - 115 IS - 33 SP - 8252 EP - 8259 PB - National Acad. of Sciences CY - Washington ER - TY - JOUR A1 - Gorenflot, Julien A1 - Paulke, Andreas A1 - Piersimoni, Fortunato A1 - Wolf, Jannic A1 - Kan, Zhipeng A1 - Cruciani, Federico A1 - El Labban, Abdulrahman A1 - Neher, Dieter A1 - Beaujuge, Pierre M. A1 - Laquai, Frederic T1 - From recombination dynamics to device performance BT - quantifying the efficiency of exciton dissociation, charge separation, and extraction in bulk heterojunction solar cells with Fluorine-Substituted polymer donors JF - dvanced energy materials N2 - An original set of experimental and modeling tools is used to quantify the yield of each of the physical processes leading to photocurrent generation in organic bulk heterojunction solar cells, enabling evaluation of materials and processing condition beyond the trivial comparison of device performances. Transient absorption spectroscopy, “the” technique to monitor all intermediate states over the entire relevant timescale, is combined with time-delayed collection field experiments, transfer matrix simulations, spectral deconvolution, and parametrization of the charge carrier recombination by a two-pool model, allowing quantification of densities of excitons and charges and extrapolation of their kinetics to device-relevant conditions. Photon absorption, charge transfer, charge separation, and charge extraction are all quantified for two recently developed wide-bandgap donor polymers: poly(4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b′]dithiophene-3,4-difluorothiophene) (PBDT[2F]T) and its nonfluorinated counterpart poly(4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b′]dithiophene-3,4-thiophene) (PBDT[2H]T) combined with PC71BM in bulk heterojunctions. The product of these yields is shown to agree well with the devices' external quantum efficiency. This methodology elucidates in the specific case studied here the origin of improved photocurrents obtained when using PBDT[2F]T instead of PBDT[2H]T as well as upon using solvent additives. Furthermore, a higher charge transfer (CT)-state energy is shown to lead to significantly lower energy losses (resulting in higher VOC) during charge generation compared to P3HT:PCBM. KW - bulk heterojunction KW - charge generation yield KW - charge recombination yield KW - polymer solar cells KW - transient absorption spectroscopy Y1 - 2018 U6 - https://doi.org/10.1002/aenm.201701678 SN - 1614-6832 SN - 1614-6840 VL - 8 IS - 4 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Shivhare, Rishi A1 - Erdmann, Tim A1 - Hoermann, Ulrich A1 - Collado-Fregoso, Elisa A1 - Zeiske, Stefan A1 - Benduhn, Johannes A1 - Ullbrich, Sascha A1 - Huebner, Rene A1 - Hambsch, Mike A1 - Kiriy, Anton A1 - Voit, Brigitte A1 - Neher, Dieter A1 - Vandewal, Koen A1 - Mannsfeld, Stefan C. B. T1 - Alkyl Branching Position in Diketopyrrolopyrrole Polymers BT - Interplay between Fibrillar Morphology and Crystallinity and Their Effect on Photogeneration and Recombination in Bulk-Heterojunction Solar Cells JF - Chemistry of materials : a publication of the American Chemical Society N2 - Diketopyrrolopyrrole (DPP)-based donor acceptor copolymers have gained a significant amount of research interest in the organic electronics community because of their high charge carrier mobilities in organic field-effect transistors (OFETs) and their ability to harvest near-infrared (NIR) photons in solar cells. In this study, we have synthesized four DPP based donor-acceptor copolymers with variations in the donor unit and the branching point of the solubilizing alkyl chains (at the second or sixth carbon position). Grazing incidence wide-angle X-ray scattering (GIWAXS) results suggest that moving the branching point further away from the polymer backbone increases the tendency for aggregation and yields polymer phases with a higher degree of crystallinity (DoC). The polymers were blended with PC70BM and used as active layers in solar cells. A careful analysis of the energetics of the neat polymer and blend films reveals that the charge-transfer state energy (E-CT) of the blend films lies exceptionally close to the singlet energy of the donor (E-D*), indicating near zero electron transfer losses. The difference between the optical gap and open-circuit voltage (V-OC) is therefore determined to be due to rather high nonradiative 418 +/- 13 mV) and unavoidable radiative voltage losses (approximate to 255 +/- 8 mV). Even though the four materials have similar optical gaps, the short-circuit current density (J(SC)) covers a vast span from 7 to 18 mA cm(-2) for the best performing system. Using photoluminescence (PL) quenching and transient charge extraction techniques, we quantify geminate and nongeminate losses and find that fewer excitons reach the donor-acceptor interface in polymers with further away branching points due to larger aggregate sizes. In these material systems, the photogeneration is therefore mainly limited by exciton harvesting efficiency. Y1 - 2018 U6 - https://doi.org/10.1021/acs.chemmater.8b02739 SN - 0897-4756 SN - 1520-5002 VL - 30 IS - 19 SP - 6801 EP - 6809 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Braunger, Steffen A1 - Mundt, Laura E. A1 - Wolff, Christian Michael A1 - Mews, Mathias A1 - Rehermann, Carolin A1 - Jost, Marko A1 - Tejada, Alvaro A1 - Eisenhauer, David A1 - Becker, Christiane A1 - Andres Guerra, Jorge A1 - Unger, Eva A1 - Korte, Lars A1 - Neher, Dieter A1 - Schubert, Martin C. A1 - Rech, Bernd A1 - Albrecht, Steve T1 - Cs(x)FA(1-x)Pb(l(1-y)Br(y))(3) Perovskite Compositions BT - the Appearance of Wrinkled Morphology and its Impact on Solar Cell Performance JF - The journal of physical chemistry : C, Nanomaterials and interfaces N2 - We report on the formation of wrinkle-patterned surface morphologies in cesium formamidinium-based Cs(x)FA(1-y)Pb(I1-yBry)(3) perovskite compositions with x = 0-0.3 and y = 0-0.3 under various spin-coating conditions. By varying the Cs and Br contents, the perovskite precursor solution concentration and the spin-coating procedure, the occurrence and characteristics of the wrinkle-shaped morphology can be tailored systematically. Cs(0.17)FA(0.83)Pb(I0.83Br0.17)(3) perovskite layers were analyzed regarding their surface roughness, microscopic structure, local and overall composition, and optoelectronic properties. Application of these films in p-i-n perovskite solar cells (PSCs) with indium-doped tin oxide/NiOx/perovskite/C-60/bathocuproine/Cu architecture resulted in up to 15.3 and 17.0% power conversion efficiency for the flat and wrinkled morphology, respectively. Interestingly, we find slightly red-shifted photoluminescence (PL) peaks for wrinkled areas and we are able to directly correlate surface topography with PL peak mapping. This is attributed to differences in the local grain size, whereas there is no indication for compositional demixing in the films. We show that the perovskite composition, crystallization kinetics, and layer thickness strongly influence the formation of wrinkles which is proposed to be related to the release of compressive strain during perovskite crystallization. Our work helps us to better understand film formation and to further improve the efficiency of PSCs with widely used mixed-perovskite compositions. Y1 - 2018 U6 - https://doi.org/10.1021/acs.jpcc.8b06459 SN - 1932-7447 SN - 1932-7455 VL - 122 IS - 30 SP - 17123 EP - 17135 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - González-Galán, Ana A1 - Oskinova, Lida A1 - Popov, Sergei B. A1 - Haberl, F. A1 - Kühnel, M. A1 - Gallagher, John S. A1 - Schurch, Matthew A1 - Guerrero, Martín A. T1 - A multiwavelength study of SXP 1062, the long-period X-ray pulsar associated with a supernova remnant JF - Monthly notices of the Royal Astronomical Society N2 - SXP 1062 is a Be X-ray binary (BeXB) located in the Small Magellanic Cloud. It hosts a long-period X-ray pulsar and is likely associated with the supernova remnant MCSNR J0127−7332. In this work we present a multiwavelength view on SXP 1062 in different luminosity regimes. We consider monitoring campaigns in optical (OGLE survey) and X-ray (Swift telescope). During these campaigns a tight coincidence of X-ray and optical outbursts is observed. We interpret this as typical Type I outbursts as often detected in BeXBs at periastron passage of the neutron star (NS). To study different X-ray luminosity regimes in depth, during the source quiescence we observed it with XMM–Newton while Chandra observations followed an X-ray outburst. Nearly simultaneously with Chandra observations in X-rays, in optical the RSS/SALT telescope obtained spectra of SXP 1062. On the basis of our multiwavelength campaign we propose a simple scenario where the disc of the Be star is observed face-on, while the orbit of the NS is inclined with respect to the disc. According to the model of quasi-spherical settling accretion our estimation of the magnetic field of the pulsar in SXP 1062 does not require an extremely strong magnetic field at the present time. KW - stars: neutron KW - pulsars: individual: SXP 1062 KW - galaxies: individual: Small Magellanic Cloud KW - X-rays: binaries Y1 - 2017 U6 - https://doi.org/10.1093/mnras/stx3127 SN - 0035-8711 SN - 1365-2966 VL - 475 IS - 2 SP - 2809 EP - 2821 PB - Oxford University Press CY - Oxford ER - TY - JOUR A1 - Molina-Garcia, Daniel A1 - Sandev, Trifce A1 - Safdari, Hadiseh A1 - Pagnini, Gianni A1 - Chechkin, Aleksei V. A1 - Metzler, Ralf T1 - Crossover from anomalous to normal diffusion BT - truncated power-law noise correlations and applications to dynamics in lipid bilayers JF - New Journal of Physics N2 - Abstract The emerging diffusive dynamics in many complex systems show a characteristic crossover behaviour from anomalous to normal diffusion which is otherwise fitted by two independent power-laws. A prominent example for a subdiffusive–diffusive crossover are viscoelastic systems such as lipid bilayer membranes, while superdiffusive–diffusive crossovers occur in systems of actively moving biological cells. We here consider the general dynamics of a stochastic particle driven by so-called tempered fractional Gaussian noise, that is noise with Gaussian amplitude and power-law correlations, which are cut off at some mesoscopic time scale. Concretely we consider such noise with built-in exponential or power-law tempering, driving an overdamped Langevin equation (fractional Brownian motion) and fractional Langevin equation motion. We derive explicit expressions for the mean squared displacement and correlation functions, including different shapes of the crossover behaviour depending on the concrete tempering, and discuss the physical meaning of the tempering. In the case of power-law tempering we also find a crossover behaviour from faster to slower superdiffusion and slower to faster subdiffusion. As a direct application of our model we demonstrate that the obtained dynamics quantitatively describes the subdiffusion–diffusion and subdiffusion–subdiffusion crossover in lipid bilayer systems. We also show that a model of tempered fractional Brownian motion recently proposed by Sabzikar and Meerschaert leads to physically very different behaviour with a seemingly paradoxical ballistic long time scaling. KW - anomalous diffusion KW - truncated power-law correlated noise KW - lipid bilayer membrane dynamics Y1 - 2018 U6 - https://doi.org/10.1088/1367-2630/aae4b2 SN - 1367-2630 VL - 20 PB - IOP Publishing Ltd CY - London und Bad Honnef ER - TY - JOUR A1 - Sandev, Trifce A1 - Tomovski, Zivorad A1 - Dubbeldam, Johan L. A. A1 - Chechkin, Aleksei V. T1 - Generalized diffusion-wave equation with memory kernel JF - Journal of physics : A, Mathematical and theoretical N2 - We study generalized diffusion-wave equation in which the second order time derivative is replaced by an integro-differential operator. It yields time fractional and distributed order time fractional diffusion-wave equations as particular cases. We consider different memory kernels of the integro-differential operator, derive corresponding fundamental solutions, specify the conditions of their non-negativity and calculate the mean squared displacement for all cases. In particular, we introduce and study generalized diffusion-wave equations with a regularized Prabhakar derivative of single and distributed orders. The equations considered can be used for modeling the broad spectrum of anomalous diffusion processes and various transitions between different diffusion regimes. KW - diffusion-wave equation KW - Mittag-Leffler function KW - anomalous diffusion Y1 - 2018 U6 - https://doi.org/10.1088/1751-8121/aaefa3 SN - 1751-8113 SN - 1751-8121 VL - 52 IS - 1 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Sandev, Trifce A1 - Metzler, Ralf A1 - Chechkin, Aleksei V. T1 - From continuous time random walks to the generalized diffusion equation JF - Fractional calculus and applied analysis : an international journal for theory and applications N2 - We obtain a generalized diffusion equation in modified or Riemann-Liouville form from continuous time random walk theory. The waiting time probability density function and mean squared displacement for different forms of the equation are explicitly calculated. We show examples of generalized diffusion equations in normal or Caputo form that encode the same probability distribution functions as those obtained from the generalized diffusion equation in modified form. The obtained equations are general and many known fractional diffusion equations are included as special cases. KW - continuous time random walk (CTRW) KW - generalized diffusion equation KW - Mittag-Leffler functions KW - anomalous diffusion Y1 - 2018 U6 - https://doi.org/10.1515/fca-2018-0002 SN - 1311-0454 SN - 1314-2224 VL - 21 IS - 1 SP - 10 EP - 28 PB - De Gruyter CY - Berlin ER - TY - JOUR A1 - Janowski, Marcin Andrzej A1 - Zoschke, Reimo A1 - Scharff, Lars B. A1 - Jaime, Silvia Martinez A1 - Ferrari, Camilla A1 - Proost, Sebastian A1 - Xiong, Jonathan Ng Wei A1 - Omranian, Nooshin A1 - Musialak-Lange, Magdalena A1 - Nikoloski, Zoran A1 - Graf, Alexander A1 - Schoettler, Mark Aurel A1 - Sampathkumar, Arun A1 - Vaid, Neha A1 - Mutwil, Marek T1 - AtRsgA from Arabidopsis thaliana is important for maturation of the small subunit of the chloroplast ribosome JF - The plant journal N2 - Plastid ribosomes are very similar in structure and function to the ribosomes of their bacterial ancestors. Since ribosome biogenesis is not thermodynamically favorable under biological conditions it requires the activity of many assembly factors. Here we have characterized a homolog of bacterial RsgA in Arabidopsis thaliana and show that it can complement the bacterial homolog. Functional characterization of a strong mutant in Arabidopsis revealed that the protein is essential for plant viability, while a weak mutant produced dwarf, chlorotic plants that incorporated immature pre-16S ribosomal RNA into translating ribosomes. Physiological analysis of the mutant plants revealed smaller, but more numerous, chloroplasts in the mesophyll cells, reduction of chlorophyll a and b, depletion of proplastids from the rib meristem and decreased photosynthetic electron transport rate and efficiency. Comparative RNA sequencing and proteomic analysis of the weak mutant and wild-type plants revealed that various biotic stress-related, transcriptional regulation and post-transcriptional modification pathways were repressed in the mutant. Intriguingly, while nuclear- and chloroplast-encoded photosynthesis-related proteins were less abundant in the mutant, the corresponding transcripts were increased, suggesting an elaborate compensatory mechanism, potentially via differentially active retrograde signaling pathways. To conclude, this study reveals a chloroplast ribosome assembly factor and outlines the transcriptomic and proteomic responses of the compensatory mechanism activated during decreased chloroplast function. Significance Statement AtRsgA is an assembly factor necessary for maturation of the small subunit of the chloroplast ribosome. Depletion of AtRsgA leads to dwarfed, chlorotic plants, a decrease of mature 16S rRNA and smaller, but more numerous, chloroplasts. Large-scale transcriptomic and proteomic analysis revealed that chloroplast-encoded and -targeted proteins were less abundant, while the corresponding transcripts were increased in the mutant. We analyze the transcriptional responses of several retrograde signaling pathways to suggest the mechanism underlying this compensatory response. KW - ribosome assembly KW - chloroplast ribosome KW - assembly factor KW - 30S subunit KW - RsgA KW - Arabidopsis thaliana Y1 - 2018 U6 - https://doi.org/10.1111/tpj.14040 SN - 0960-7412 SN - 1365-313X VL - 96 IS - 2 SP - 404 EP - 420 PB - Wiley CY - Hoboken ER - TY - JOUR A1 - Shprits, Yuri Y. A1 - Menietti, J. D. A1 - Drozdov, Alexander A1 - Horne, Richard B. A1 - Woodfield, Emma E. A1 - Groene, J. B. A1 - de Soria-Santacruz, M. A1 - Averkamp, T. F. A1 - Garrett, H. A1 - Paranicas, C. A1 - Gurnett, Don A. T1 - Strong whistler mode waves observed in the vicinity of Jupiter’s moons JF - Nature Communications N2 - Understanding of wave environments is critical for the understanding of how particles are accelerated and lost in space. This study shows that in the vicinity of Europa and Ganymede, that respectively have induced and internal magnetic fields, chorus wave power is significantly increased. The observed enhancements are persistent and exceed median values of wave activity by up to 6 orders of magnitude for Ganymede. Produced waves may have a pronounced effect on the acceleration and loss of particles in the Jovian magnetosphere and other astrophysical objects. The generated waves are capable of significantly modifying the energetic particle environment, accelerating particles to very high energies, or producing depletions in phase space density. Observations of Jupiter’s magnetosphere provide a unique opportunity to observe how objects with an internal magnetic field can interact with particles trapped in magnetic fields of larger scale objects. Y1 - 2018 U6 - https://doi.org/10.1038/s41467-018-05431-x SN - 2041-1723 VL - 9 PB - Nature Publ. Group CY - London ER - TY - JOUR A1 - Hörmann, Ulrich A1 - Zeiske, Stefan A1 - Piersimoni, Fortunato A1 - Hoffmann, Lukas A1 - Schlesinger, Raphael A1 - Koch, Norbert A1 - Riedl, Thomas A1 - Andrienko, Denis A1 - Neher, Dieter T1 - Stark effect of hybrid charge transfer states at planar ZnO/organic interfaces JF - Physical review : B, Condensed matter and materials physics N2 - We investigate the bias dependence of the hybrid charge transfer state emission at planar heterojunctions between the metal oxide acceptor ZnO and three donor molecules. The electroluminescence peak energy linearly increases with the applied bias, saturating at high fields. Variation of the organic layer thickness and deliberate change of the ZnO conductivity through controlled photodoping allow us to confirm that this bias-induced spectral shift relates to the internal electric field in the organic layer rather than the filling of states at the hybrid interface. We show that existing continuum models overestimate the hole delocalization and propose a simple electrostatic model in which the linear and quadratic Stark effects are explained by the electrostatic interaction of a strongly polarizable molecular cation with its mirror image. Y1 - 2018 U6 - https://doi.org/10.1103/PhysRevB.98.155312 SN - 2469-9950 SN - 2469-9969 VL - 98 IS - 15 PB - American Physical Society CY - College Park ER - TY - JOUR A1 - Ni, Binbin A1 - Cao, Xing A1 - Shprits, Yuri Y. A1 - Summers, Danny A1 - Gu, Xudong A1 - Fu, Song A1 - Lou, Yuequn T1 - Hot Plasma Effects on the Cyclotron-Resonant Pitch-Angle Scattering Rates of Radiation Belt Electrons Due to EMIC Waves JF - Geophysical research letters N2 - To investigate the hot plasma effects on the cyclotron-resonant interactions between electromagnetic ion cyclotron (EMIC) waves and radiation belt electrons in a realistic magnetospheric environment, calculations of the wave-induced bounce-averaged pitch angle diffusion coefficients are performed using both the cold and hot plasma dispersion relations. The results demonstrate that the hot plasma effects have a pronounced influence on the electron pitch angle scattering rates due to all three EMIC emission bands (H+, He+, and O+) when the hot plasma dispersion relation deviates significantly from the cold plasma approximation. For a given wave spectrum, the modification of the dispersion relation by hot anisotropic protons can strongly increase the minimum resonant energy for electrons interacting with O+ band EMIC waves, while the minimum resonant energies for H+ and He+ bands are not greatly affected. For H+ band EMIC waves, inclusion of hot protons tends to weaken the pitch angle scattering efficiency of >5MeV electrons. The most crucial differences introduced by the hot plasma effects occur for >3MeV electron scattering rates by He+ band EMIC waves. Mainly due to the changes of resonant frequency and wave group velocity when the hot protons are included, the difference in scattering rates can be up to an order of magnitude, showing a strong dependence on both electron energy and equatorial pitch angle. Our study confirms the importance of including hot plasma effects in modeling the scattering of ultra-relativistic radiation belt electrons by EMIC waves. Y1 - 2018 U6 - https://doi.org/10.1002/2017GL076028 SN - 0094-8276 SN - 1944-8007 VL - 45 IS - 1 SP - 21 EP - 30 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Karamzadeh Toularoud, Nasim A1 - Heimann, Sebastian A1 - Dahm, Torsten A1 - Krüger, Frank T1 - Application based seismological array design by seismicity scenario modelling JF - Geophysical journal international N2 - The design of an array configuration is an important task in array seismology during experiment planning. Often the array response function (ARF), which depends on the relative position of array stations and frequency content of the incoming signals, is used as the array design criterion. In practice, additional constraints and parameters have to be taken into account, for example, land ownership, site-specific noise levels or characteristics of the seismic sources under investigation. In this study, a flexible array design framework is introduced that implements a customizable scenario modelling and optimization scheme by making use of synthetic seismograms. Using synthetic seismograms to evaluate array performance makes it possible to consider additional constraints. We suggest to use synthetic array beamforming as an array design criterion instead of the ARF. The objective function of the optimization scheme is defined according to the monitoring goals, and may consist of a number of subfunctions. The array design framework is exemplified by designing a seven-station small-scale array to monitor earthquake swarm activity in Northwest Bohemia/Vogtland in central Europe. Two subfunctions are introduced to verify the accuracy of horizontal slowness estimation; one to suppress aliasing effects due to possible secondary lobes of synthetic array beamforming calculated in horizontal slowness space and the other to reduce the event’s mislocation caused by miscalculation of the horizontal slowness vector. Subsequently, a weighting technique is applied to combine the subfunctions into one single scalar objective function to use in the optimization process. KW - Array Seismology KW - Array design KW - Seismicity modelling Y1 - 2018 SN - 0956-540X SN - 1365-246X VL - 216 IS - 3 SP - 1711 EP - 1727 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Kurpiers, Jona A1 - Ferron, Thomas A1 - Roland, Steffen A1 - Jakoby, Marius A1 - Thiede, Tobias A1 - Jaiser, Frank A1 - Albrecht, Steve A1 - Janietz, Silvia A1 - Collins, Brian A. A1 - Howard, Ian A. A1 - Neher, Dieter T1 - Probing the pathways of free charge generation in organic bulk heterojunction solar cells JF - Nature Communications N2 - The fact that organic solar cells perform efficiently despite the low dielectric constant of most photoactive blends initiated a long-standing debate regarding the dominant pathways of free charge formation. Here, we address this issue through the accurate measurement of the activation energy for free charge photogeneration over a wide range of photon energy, using the method of time-delayed collection field. For our prototypical low bandgap polymer:fullerene blends, we find that neither the temperature nor the field dependence of free charge generation depend on the excitation energy, ruling out an appreciable contribution to free charge generation though hot carrier pathways. On the other hand, activation energies are on the order of the room temperature thermal energy for all studied blends. We conclude that charge generation in such devices proceeds through thermalized charge transfer states, and that thermal energy is sufficient to separate most of these states into free charges. Y1 - 2018 U6 - https://doi.org/10.1038/s41467-018-04386-3 SN - 2041-1723 VL - 9 PB - Nature Publ. Group CY - London ER - TY - JOUR A1 - Sini, Gjergji A1 - Schubert, Marcel A1 - Risko, Chad A1 - Roland, Steffen A1 - Lee, Olivia P. A1 - Chen, Zhihua A1 - Richter, Thomas V. A1 - Dolfen, Daniel A1 - Coropceanu, Veaceslav A1 - Ludwigs, Sabine A1 - Scherf, Ullrich A1 - Facchetti, Antonio A1 - Frechet, Jean M. J. A1 - Neher, Dieter T1 - On the Molecular Origin of Charge Separation at the Donor-Acceptor Interface JF - Advanced energy materials N2 - Fullerene-based acceptors have dominated organic solar cells for almost two decades. It is only within the last few years that alternative acceptors rival their dominance, introducing much more flexibility in the optoelectronic properties of these material blends. However, a fundamental physical understanding of the processes that drive charge separation at organic heterojunctions is still missing, but urgently needed to direct further material improvements. Here a combined experimental and theoretical approach is used to understand the intimate mechanisms by which molecular structure contributes to exciton dissociation, charge separation, and charge recombination at the donor-acceptor (D-A) interface. Model systems comprised of polythiophene-based donor and rylene diimide-based acceptor polymers are used and a detailed density functional theory (DFT) investigation is performed. The results point to the roles that geometric deformations and direct-contact intermolecular polarization play in establishing a driving force ( energy gradient) for the optoelectronic processes taking place at the interface. A substantial impact for this driving force is found to stem from polymer deformations at the interface, a finding that can clearly lead to new design approaches in the development of the next generation of conjugated polymers and small molecules. KW - donor-acceptor interfaces KW - energy gradients KW - geometrical deformations KW - nonfullerene acceptors KW - organic photovoltaics KW - photocurrent generation KW - polymer solar cells Y1 - 2018 U6 - https://doi.org/10.1002/aenm.201702232 SN - 1614-6832 SN - 1614-6840 VL - 8 IS - 12 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Ran, Niva A. A1 - Love, John A. A1 - Heiber, Michael C. A1 - Jiao, Xuechen A1 - Hughes, Michael P. A1 - Karki, Akchheta A1 - Wang, Ming A1 - Brus, Viktor V. A1 - Wang, Hengbin A1 - Neher, Dieter A1 - Ade, Harald A1 - Bazan, Guillermo C. A1 - Thuc-Quyen Nguyen, T1 - Charge generation and recombination in an organic solar cell with low energetic offsets JF - dvanced energy materials N2 - Organic bulk heterojunction (BHJ) solar cells require energetic offsets between the donor and acceptor to obtain high short-circuit currents (J(SC)) and fill factors (FF). However, it is necessary to reduce the energetic offsets to achieve high open-circuit voltages (V-OC). Recently, reports have highlighted BHJ blends that are pushing at the accepted limits of energetic offsets necessary for high efficiency. Unfortunately, most of these BHJs have modest FF values. How the energetic offset impacts the solar cell characteristics thus remains poorly understood. Here, a comprehensive characterization of the losses in a polymer:fullerene BHJ blend, PIPCP:phenyl-C61-butyric acid methyl ester (PC61BM), that achieves a high V-OC (0.9 V) with very low energy losses (E-loss = 0.52 eV) from the energy of absorbed photons, a respectable J(SC) (13 mA cm(-2)), but a limited FF (54%) is reported. Despite the low energetic offset, the system does not suffer from field-dependent generation and instead it is characterized by very fast nongeminate recombination and the presence of shallow traps. The charge-carrier losses are attributed to suboptimal morphology due to high miscibility between PIPCP and PC61BM. These results hold promise that given the appropriate morphology, the J(SC), V-OC, and FF can all be improved, even with very low energetic offsets. KW - energetic offset KW - fill factor KW - morphology KW - organic solar cells KW - recombination Y1 - 2018 U6 - https://doi.org/10.1002/aenm.201701073 SN - 1614-6832 SN - 1614-6840 VL - 8 IS - 5 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Alqahtani, Obaid A1 - Babics, Maxime A1 - Gorenflot, Julien A1 - Savikhin, Victoria A1 - Ferron, Thomas A1 - Balawi, Ahmed H. A1 - Paulke, Andreas A1 - Kan, Zhipeng A1 - Pope, Michael A1 - Clulow, Andrew J. A1 - Wolf, Jannic A1 - Burn, Paul L. A1 - Gentle, Ian R. A1 - Neher, Dieter A1 - Toney, Michael F. A1 - Laquai, Frederic A1 - Beaujuge, Pierre M. A1 - Collins, Brian A. T1 - Mixed Domains Enhance Charge Generation and Extraction in Bulk-Heterojunction Solar Cells with Small-Molecule Donors JF - Advanced energy materials N2 - The interplay between nanomorphology and efficiency of polymer-fullerene bulk-heterojunction (BHJ) solar cells has been the subject of intense research, but the generality of these concepts for small-molecule (SM) BHJs remains unclear. Here, the relation between performance; charge generation, recombination, and extraction dynamics; and nanomorphology achievable with two SM donors benzo[1,2-b:4,5-b]dithiophene-pyrido[3,4-b]-pyrazine BDT(PPTh2)(2), namely SM1 and SM2, differing by their side-chains, are examined as a function of solution additive composition. The results show that the additive 1,8-diiodooctane acts as a plasticizer in the blends, increases domain size, and promotes ordering/crystallinity. Surprisingly, the system with high domain purity (SM1) exhibits both poor exciton harvesting and severe charge trapping, alleviated only slightly with increased crystallinity. In contrast, the system consisting of mixed domains and lower crystallinity (SM2) shows both excellent exciton harvesting and low charge recombination losses. Importantly, the onset of large, pure crystallites in the latter (SM2) system reduces efficiency, pointing to possible differences in the ideal morphologies for SM-based BHJ solar cells compared with polymer-fullerene devices. In polymer-based systems, tie chains between pure polymer crystals establish a continuous charge transport network, whereas SM-based active layers may in some cases require mixed domains that enable both aggregation and charge percolation to the electrodes. KW - charge transport KW - domain purity KW - microscopy KW - mixed domains KW - organic solar cells KW - photovoltaic devices KW - resonant X-ray scattering KW - small molecules KW - transient spectroscopy Y1 - 2018 U6 - https://doi.org/10.1002/aenm.201702941 SN - 1614-6832 SN - 1614-6840 VL - 8 IS - 19 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Woodfield, Emma E. A1 - Horne, Richard B. A1 - Glauert, S. A. A1 - Menietti, J. D. A1 - Shprits, Yuri Y. A1 - Kurth, William S. T1 - Formation of electron radiation belts at Saturn by Z-mode wave acceleration JF - Nature Communications N2 - At Saturn electrons are trapped in the planet’s magnetic field and accelerated to relativistic energies to form the radiation belts, but how this dramatic increase in electron energy occurs is still unknown. Until now the mechanism of radial diffusion has been assumed but we show here that in-situ acceleration through wave particle interactions, which initial studies dismissed as ineffectual at Saturn, is in fact a vital part of the energetic particle dynamics there. We present evidence from numerical simulations based on Cassini spacecraft data that a particular plasma wave, known as Z-mode, accelerates electrons to MeV energies inside 4 RS (1 RS = 60,330 km) through a Doppler shifted cyclotron resonant interaction. Our results show that the Z-mode waves observed are not oblique as previously assumed and are much better accelerators than O-mode waves, resulting in an electron energy spectrum that closely approaches observed values without any transport effects included. Y1 - 2018 U6 - https://doi.org/10.1038/s41467-018-07549-4 SN - 2041-1723 VL - 9 PB - Nature Publ. Group CY - London ER - TY - JOUR A1 - Giangrisostomi, Erika A1 - Ovsyannikov, Ruslan A1 - Sorgenfrei, Nomi A1 - Zhang, Teng A1 - Lindblad, Andreas A1 - Sassa, Yasmine A1 - Cappel, Ute B. A1 - Leitner, Torsten A1 - Mitzner, Rolf A1 - Svensson, Svante A1 - Martensson, Nils A1 - Föhlisch, Alexander T1 - Low Dose Photoelectron Spectroscopy at BESSY II BT - electronic structure of matter in its native state JF - Journal of electron spectroscopy and related phenomena : the international journal on theoretical and experimental aspects of electron spectroscopy N2 - The implementation of a high-transmission, angular-resolved time-of-Right electron spectrometer with a 1.25 MHz pulse selector at the PM4 soft X-ray dipole beamline of the synchrotron BESSY II creates unique capabilities to inquire electronic structure via photoelectron spectroscopy with a minimum of radiation dose. Solid-state samples can be prepared and characterized with standard UHV techniques and rapidly transferred from various preparation chambers to a 4-axis temperature-controlled measurement stage. A synchronized MHz laser system enables excited-state characterization and dynamical studies starting from the picosecond timescale. This article introduces the principal characteristics of the PM4 beamline and LowDosePES end-station. Recent results from graphene, an organic hole transport material for solar cells and the transition metal dichalcogenide MoS2 are presented to demonstrate the instrument performances. Y1 - 2018 U6 - https://doi.org/10.1016/j.elspec.2017.05.011 SN - 0368-2048 SN - 1873-2526 VL - 224 SP - 68 EP - 78 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Pontius, Niko A1 - Beye, Martin A1 - Trabant, Christoph A1 - Mitzner, Rolf A1 - Sorgenfrei, Nomi A1 - Kachel, Torsten A1 - Woestmann, Michael A1 - Roling, Sebastian A1 - Zacharias, Helmut A1 - Ivanov, Rosen A1 - Treusch, Rolf A1 - Buchholz, Marcel A1 - Metcalf, Pete A1 - Schuessler-Langeheine, Christian A1 - Föhlisch, Alexander T1 - Probing the non-equilibrium transient state in magnetite by a jitter-free two-color X-ray pump and X-ray probe experiment JF - Structural dynamics N2 - We present a general experimental concept for jitter-free pump and probe experiments at free electron lasers. By generating pump and probe pulse from one and the same X-ray pulse using an optical split-and-delay unit, we obtain a temporal resolution that is limited only by the X-ray pulse lengths. In a two-color X-ray pump and X-ray probe experiment with sub 70 fs temporal resolution, we selectively probe the response of orbital and charge degree of freedom in the prototypical functional oxide magnetite after photoexcitation. We find electronic order to be quenched on a time scale of (30 +/- 30) fs and hence most likely faster than what is to be expected for any lattice dynamics. Our experimental result hints to the formation of a short lived transient state with decoupled electronic and lattice degree of freedom in magnetite. The excitation and relaxation mechanism for X-ray pumping is discussed within a simple model leading to the conclusion that within the first 10 fs the original photoexcitation decays into low-energy electronic excitations comparable to what is achieved by optical pump pulse excitation. Our findings show on which time scales dynamical decoupling of degrees of freedom in functional oxides can be expected and how to probe this selectively with soft X-ray pulses. Results can be expected to provide crucial information for theories for ultrafast behavior of materials and help to develop concepts for novel switching devices. (C) 2018 Author(s). Y1 - 2018 U6 - https://doi.org/10.1063/1.5042847 SN - 2329-7778 VL - 5 IS - 5 PB - American Institute of Physics CY - Melville ER - TY - JOUR A1 - Kim, Kyung-Chan A1 - Shprits, Yuri Y. T1 - Survey of the Favorable Conditions for Magnetosonic Wave Excitation JF - Journal of geophysical research : Space physics N2 - The ratio of the proton ring velocity (VR) to the local Alfven speed (VA), in addition to proton ring distributions, plays a key factor in the excitation of magnetosonic waves at frequencies between the proton cyclotron frequency fcp and the lower hybrid resonance frequency fLHR in the Earth's magnetosphere. Here we investigate whether there is a statistically significant relationship between occurrences of proton rings and magnetosonic waves both outside and inside the plasmapause using particle and wave data from Van Allen Probe-A during the time period of October 2012 to December 2015. We also perform a statistical survey of the ratio of the ring energy (ER, corresponding to VR) to the Alfven energy (EA, corresponding to VA) to determine the favorable conditions under which magnetosonic waves in each of two frequency bands (fcp < f ≤ 0.5 fLHR and 0.5 fLHR < f < fLHR) can be excited. The results show that the magnetosonic waves in both frequency bands occur around the postnoon (12–18 magnetic local time, MLT) sector outside the plasmapause when ER is comparable to or lower than EA, and those in lower-frequency bands (fcp < f ≤ 0.5 fLHR) occur around the postnoon sector inside the plasmapause when ER/EA > ~9. However, there is one discrepancy between occurrences of proton rings and magnetosonic waves in low-frequency bands around the prenoon sector (6–12 MLT) outside the plasmapause, which suggests either that the waves may have propagated during active time from the postnoon sector after being excited during quiet time, or they may have locally excited in the prenoon sector during active time. Y1 - 2018 U6 - https://doi.org/10.1002/2017JA024865 SN - 2169-9380 SN - 2169-9402 VL - 123 IS - 1 SP - 400 EP - 413 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Li, Tian-yi A1 - Benduhn, Johannes A1 - Li, Yue A1 - Jaiser, Frank A1 - Spoltore, Donato A1 - Zeika, Olaf A1 - Ma, Zaifei A1 - Neher, Dieter A1 - Vandewal, Koen A1 - Leo, Karl T1 - Boron dipyrromethene (BODIPY) with meso-perfluorinated alkyl substituents as near infrared donors in organic solar cells JF - Journal of materials chemistry : A, Materials for energy and sustainability N2 - Three furan-fused BODIPYs were synthesized with perfluorinated methyl, ethyl and n-propyl groups on the meso-carbon. They were obtained with high yields by reacting the furan-fused 2-carboxylpyrrole in corresponding perfluorinated acid and anhydride. With the increase in perfluorinated alkyl chain length, the molecular packing in the single crystal is influenced, showing increasing stacking distance and decreasing slope angle. All the BODIPYs were characterized as intense absorbers in near infrared region in solid state, peaking at similar to 800 nm with absorption coefficient of over 280 000 cm(-1). Facilitated by high thermal stability, the furan-fused BODIPYs were employed in vacuum-deposited organic solar cells as electron donors. All devices exhibit PCE over 6.0% with the EQE maximum reaching 70% at similar to 790 nm. The chemical modification of the BODIPY donors have certain influence on the active layer morphology, and the highest PCE of 6.4% was obtained with a notably high jsc of 13.6 mA cm(-2). Sensitive EQE and electroluminance studies indicated that the energy losses generated by the formation of a charge transfer state and the radiative recombination at the donor-acceptor interface were comparable in the range of 0.14-0.19 V, while non-radiative recombination energy loss of 0.38 V was the main energy loss route resulting in the moderate V-oc of 0.76 V. Y1 - 2018 U6 - https://doi.org/10.1039/c8ta06261g SN - 2050-7488 SN - 2050-7496 VL - 6 IS - 38 SP - 18583 EP - 18591 PB - Royal Society of Chemistry CY - Cambridge ER - TY - JOUR A1 - Liebig, Ferenc A1 - Sarhan, Radwan Mohamed A1 - Prietzel, Claudia Christina A1 - Thünemann, Andreas F. A1 - Bargheer, Matias A1 - Koetz, Joachim T1 - Undulated Gold Nanoplatelet Superstructures BT - In Situ Growth of Hemispherical Gold Nanoparticles onto the Surface of Gold Nanotriangles JF - Langmuir N2 - Negatively charged flat gold nanotriangles, formed in a vesicular template phase and separated by an AOT-micelle-based depletion flocculation, were reloaded by adding a cationic polyelectrolyte, that is, a hyperbranched polyethylenimine (PEI). Heating the system to 100 degrees C in the presence of a gold chloride solution, the reduction process leads to the formation of gold nanoparticles inside the polymer shell surrounding the nanoplatelets. The gold nanoparticle formation is investigated by UV-vis spectroscopy, small-angle X-ray scattering, and dynamic light scattering measurements in combination with transmission electron microscopy. Spontaneously formed gold clusters in the hyperbranched PEI shell with an absorption maximum at 350 nm grow on the surface of the nanotriangles as hemispherical particles with diameters of similar to 6 nm. High-resolution micrographs show that the hemispherical gold particles are crystallized onto the {111} facets on the bottom and top of the platelet as well as on the edges without a grain boundary. Undulated gold nanoplatelet superstructures with special properties become available, which show a significantly modified performance in SERS-detected photocatalysis regarding both reactivity and enhancement factor. Y1 - 2018 U6 - https://doi.org/10.1021/acs.langmuir.7b02898 SN - 0743-7463 VL - 34 IS - 15 SP - 4584 EP - 4594 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Liebig, Ferenc A1 - Henning, Ricky A1 - Sarhan, Radwan Mohamed A1 - Prietzel, Claudia Christina A1 - Bargheer, Matias A1 - Koetz, Joachim T1 - A new route to gold nanoflowers JF - Nanotechnology N2 - Catanionic vesicles spontaneously formed by mixing the anionic surfactant bis(2-ethylhexyl)sulfosuccinate sodium salt with the cationic surfactant cetyltrimethylammonium bromide were used as a reducing medium to produce gold clusters, which are embedded and well-ordered into the template phase. The gold clusters can be used as seeds in the growth process that follows by adding ascorbic acid as a mild reducing component. When the ascorbic acid was added very slowly in an ice bath round-edged gold nanoflowers were produced. When the same experiments were performed at room temperature in the presence of Ag+ ions, sharp-edged nanoflowers could be synthesized. The mechanism of nanoparticle formation can be understood to be a non-diffusion-limited Ostwald ripening process of preordered gold nanoparticles embedded in catanionic vesicle fragments. Surface-enhanced Raman scattering experiments show an excellent enhancement factor of 1.7 . 10(5) for the nanoflowers deposited on a silicon wafer. KW - catanionic vesicles KW - gold cluster KW - gold nanoflowers KW - crystal growth KW - HRTEM KW - SEM Y1 - 2018 U6 - https://doi.org/10.1088/1361-6528/aaaffd SN - 0957-4484 SN - 1361-6528 VL - 29 IS - 18 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Stete, Felix A1 - Schossau, Phillip A1 - Bargheer, Matias A1 - Koopman, Wouter-Willem Adriaan T1 - Size-Dependent coupling of Hybrid Core-Shell Nanorods BT - Toward Single-Emitter Strong-Coupling JF - The journal of physical chemistry : C, Nanomaterials and interfaces N2 - Owing to their ability of concentrating electromagnetic fields to subwavelength mode volumes, plasmonic nanoparticles foster extremely high light-matter coupling strengths reaching far into the strong-coupling regime of light matter interaction. In this article, we present an experimental investigation on the dependence of coupling strength on the geometrical size of the nanoparticle. The coupling strength for differently sized hybrid plasmon-core exciton-shell nanorods was extracted from the typical resonance anticrossing of these systems, obtained by controlled modification of the environment permittivity using layer-by-layer deposition of polyelectrolytes. The observed size dependence of the coupling strength can be explained by a simple model approximating the electromagnetic mode volume by the geometrical volume of the particle. On the basis of this model, the coupling strength for particles of arbitrary size can be predicted, including the particle size necessary to support single-emitter strong coupling. Y1 - 2018 U6 - https://doi.org/10.1021/acs.jpcc.8b04204 SN - 1932-7447 VL - 122 IS - 31 SP - 17976 EP - 17982 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Davidzon, Iary A1 - Ilbert, Olivier A1 - Faisst, Andreas L. A1 - Sparre, Martin A1 - Capak, Peter L. T1 - An Alternate Approach to Measure Specific Star Formation Rates at 2 < z < 7 JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - We trace the specific star formation rate (sSFR) of massive star-forming galaxies (greater than or similar to 10(10)M(circle dot)) from z similar to 2 to 7. Our method is substantially different from previous analyses, as it does not rely on direct estimates of star formation rate, but on the differential evolution of the galaxy stellar mass function (SMF). We show the reliability of this approach by means of semianalytical and hydrodynamical cosmological simulations. We then apply it to real data, using the SMFs derived in the COSMOS and CANDELS fields. We find that the sSFR is proportional to (1 + z)(1.1) (+/-) (0.2) at z > 2, in agreement with other observations but in tension with the steeper evolution predicted by simulations from z similar to 4 to 2. We investigate the impact of several sources of observational bias, which, however, cannot account for this discrepancy. Although the SMF of high-redshift galaxies is still affected by significant errors, we show that future large-area surveys will substantially reduce them, making our method an effective tool to probe the massive end of the main sequence of star-forming galaxies. KW - galaxies: evolution KW - galaxies: high-redshift KW - galaxies: star formation Y1 - 2018 U6 - https://doi.org/10.3847/1538-4357/aaa19e SN - 0004-637X SN - 1538-4357 VL - 852 IS - 2 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Yazmaciyan, Aren A1 - Stolterfoht, Martin A1 - Burn, Paul L. A1 - Lin, Qianqian A1 - Meredith, Paul A1 - Armin, Ardalan T1 - Recombination losses above and below the transport percolation threshold in bulk heterojunction organic solar cells JF - Advanced energy materials N2 - Achieving the highest power conversion efficiencies in bulk heterojunction organic solar cells requires a morphology that delivers electron and hole percolation pathways for optimized transport, plus sufficient donor:acceptor contact area for near unity charge transfer state formation. This is a significant structural challenge, particularly in semiconducting polymer:fullerene systems. This balancing act in the model high efficiency PTB7:PC70BM blend is studied by tuning the donor:acceptor ratio, with a view to understanding the recombination loss mechanisms above and below the fullerene transport percolation threshold. The internal quantum efficiency is found to be strongly correlated to the slower carrier mobility in agreement with other recent studies. Furthermore, second-order recombination losses dominate the shape of the current density-voltage curve in efficient blend combinations, where the fullerene phase is percolated. However, below the charge transport percolation threshold, there is an electric-field dependence of first-order losses, which includes electric-field-dependent photogeneration. In the intermediate regime, the fill factor appears to be limited by both first- and second-order losses. These findings provide additional basic understanding of the interplay between the bulk heterojunction morphology and the order of recombination in organic solar cells. They also shed light on the limitations of widely used transport models below the percolation threshold. KW - bulk heterojunctions KW - charge transport KW - organic solar cells KW - percolation threshold KW - recombination losses Y1 - 2018 U6 - https://doi.org/10.1002/aenm.201703339 SN - 1614-6832 SN - 1614-6840 VL - 8 IS - 18 PB - Wiley-VCH CY - Weinheim ER -