TY  - JOUR
A1  - Köhler, Raphael H.
A1  - Handorf, Dörthe
A1  - Jaiser, Ralf
A1  - Dethloff, Klaus
A1  - Zängl, Günther
A1  - Majewski, Detlev
A1  - Rex, Markus
T1  - Improved circulation in the Northern hemisphere by adjusting gravity wave drag parameterizations in seasonal experiments with ICON-NWP
JF  - Earth and Space Science : ESS
N2  - The stratosphere is one of the main potential sources for subseasonal to seasonal predictability in midlatitudes in winter. The ability of an atmospheric model to realistically simulate the stratospheric dynamics is essential in order to move forward in the field of seasonal predictions in midlatitudes. Earlier studies with the ICOsahedral Nonhydrostatic atmospheric model (ICON) point out that stratospheric westerlies in ICON are underestimated. This is the first extensive study on the evaluation of Northern Hemisphere stratospheric winter circulation with ICON in numerical weather prediction (NWP) mode. Seasonal experiments with the default setup are able to reproduce the basic climatology of the stratospheric polar vortex. However, westerlies are too weak and major stratospheric warmings too frequent in ICON. Both a reduction of the nonorographic, and a reduction of the orographic gravity wave and wake drag lead to a strengthening of the stratospheric vortex and a bias reduction, in particular in January. However, the effect of the nonorographic gravity wave drag scheme on the stratosphere is stronger. Stratosphere-troposphere coupling is intensified and more realistic due to a reduced gravity wave drag. Furthermore, an adjustment of the subgrid-scale orographic drag parameterization leads to a significant error reduction in the mean sea level pressure. As a result of these findings, we present our current suggested improved setup for seasonal experiments with ICON-NWP. <br /> Plain Language Summary Although seasonal forecasts for midlatitudes have the potential to be highly beneficial to the public sector, they are still characterized by a large amount of uncertainty. Exact simulations of the circulation in the stratosphere can help to improve tropospheric predictability on seasonal time scales. For this reason, we investigate how well the new German atmospheric model is able to simulate the stratospheric circulation. The model reproduces the basic behavior of the Northern Hemisphere stratospheric polar vortex, but the westerly circulation in winter is underestimated. The stratospheric circulation is influenced by gravity waves that exert drag on the flow. These processes are only partly physically represented in the model, but are very important and are hence parameterized. By adjusting the parameterizations for the gravity wave drag, the stratospheric polar vortex is strengthened, thereby yielding a more realistic stratospheric circulation. In addition, the altered parameterizations improve the simulated surface pressure pattern. Based upon this, we present our current suggested improved model setup for seasonal experiments.
Y1  - 2021
U6  - https://doi.org/10.1029/2021EA001676
SN  - 2333-5084
VL  - 8
IS  - 3
PB  - American Geophysical Union
CY  - Malden, Mass.
ER  - 
TY  - JOUR
A1  - Lever, Fabiano
A1  - Mayer, Dennis
A1  - Metje, Jan
A1  - Alisauskas, Skirmantas
A1  - Calegari, Francesca
A1  - Düsterer, Stefan
A1  - Feifel, Raimund
A1  - Niebuhr, Mario
A1  - Manschwetus, Bastian
A1  - Kuhlmann, Marion
A1  - Mazza, Tommaso
A1  - Robinson, Matthew Scott
A1  - Squibb, Richard J.
A1  - Trabattoni, Andrea
A1  - Wallner, Måns
A1  - Wolf, Thomas J. A.
A1  - Gühr, Markus
T1  - Core-level spectroscopy of 2-thiouracil at the sulfur L1 and L2,3 edges utilizing a SASE free-electron-laser
JF  - Molecules
N2  - In this paper, we report X-ray absorption and core-level electron spectra of the nucleobase derivative 2-thiouracil at the sulfur L1- and L2,3-edges. We used soft X-rays from the free-electron laser FLASH2 for the excitation of isolated molecules and dispersed the outgoing electrons with a magnetic bottle spectrometer. We identified photoelectrons from the 2p core orbital, accompanied by an electron correlation satellite, as well as resonant and non-resonant Coster–Kronig and Auger–Meitner emission at the L1- and L2,3-edges, respectively. We used the electron yield to construct X-ray absorption spectra at the two edges. The experimental data obtained are put in the context of the literature currently available on sulfur core-level and 2-thiouracil spectroscopy.
KW  - X-ray
KW  - photoelectron
KW  - sulfur
KW  - thiouracil
KW  - nucleobases
KW  - Coster–Kronig
KW  - Auger–Meitner
KW  - NEXAFS
KW  - FLASH
Y1  - 2021
U6  - https://doi.org/10.3390/molecules26216469
SN  - 1420-3049
VL  - 26
IS  - 21
PB  - MDPI
CY  - Basel
ER  - 
TY  - JOUR
A1  - Mardoukhi, Yousof
A1  - Chechkin, Aleksei V.
A1  - Metzler, Ralf
T1  - Spurious ergodicity breaking in normal and fractional Ornstein–Uhlenbeck process
JF  - New Journal of Physics
N2  - The Ornstein–Uhlenbeck process is a stationary and ergodic Gaussian process, that is fully determined by its covariance function and mean. We show here that the generic definitions of the ensemble- and time-averaged mean squared displacements fail to capture these properties consistently, leading to a spurious ergodicity breaking. We propose to remedy this failure by redefining the mean squared displacements such that they reflect unambiguously the statistical properties of any stochastic process. In particular we study the effect of the initial condition in the Ornstein–Uhlenbeck process and its fractional extension. For the fractional Ornstein–Uhlenbeck process representing typical experimental situations in crowded environments such as living biological cells, we show that the stationarity of the process delicately depends on the initial condition.
KW  - Ornstein–Uhlenbeck process
KW  - stationary stochastic process
KW  - ensemble and time averaged mean squared displacement
Y1  - 2020
U6  - https://doi.org/10.1088/1367-2630/ab950b
SN  - 1367-2630
VL  - 22
PB  - IOP
CY  - London
ER  - 
TY  - JOUR
A1  - Marschall, Raphael
A1  - Skorov, Yuri
A1  - Zakharov, Vladimir
A1  - Rezac, Ladislav
A1  - Gerig, Selina-Barbara
A1  - Christou, Chariton
A1  - Dadzie, S. Kokou
A1  - Migliorini, Alessandra
A1  - Rinaldi, Giovanna
A1  - Agarwal, Jessica
A1  - Vincent, Jean-Baptiste
A1  - Kappel, David
T1  - Cometary comae-surface links the physics of gas and dust from the surface to a spacecraft
JF  - Space science reviews
N2  - A comet is a highly dynamic object, undergoing a permanent state of change. These changes have to be carefully classified and considered according to their intrinsic temporal and spatial scales. The Rosetta mission has, through its contiguous in-situ and remote sensing coverage of comet 67P/Churyumov-Gerasimenko (hereafter 67P) over the time span of August 2014 to September 2016, monitored the emergence, culmination, and winding down of the gas and dust comae. This provided an unprecedented data set and has spurred a large effort to connect in-situ and remote sensing measurements to the surface. In this review, we address our current understanding of cometary activity and the challenges involved when linking comae data to the surface. We give the current state of research by describing what we know about the physical processes involved from the surface to a few tens of kilometres above it with respect to the gas and dust emission from cometary nuclei. Further, we describe how complex multidimensional cometary gas and dust models have developed from the Halley encounter of 1986 to today. This includes the study of inhomogeneous outgassing and determination of the gas and dust production rates. Additionally, the different approaches used and results obtained to link coma data to the surface will be discussed. We discuss forward and inversion models and we describe the limitations of the respective approaches. The current literature suggests that there does not seem to be a single uniform process behind cometary activity. Rather, activity seems to be the consequence of a variety of erosion processes, including the sublimation of both water ice and more volatile material, but possibly also more exotic processes such as fracture and cliff erosion under thermal and mechanical stress, sub-surface heat storage, and a complex interplay of these processes. Seasons and the nucleus shape are key factors for the distribution and temporal evolution of activity and imply that the heliocentric evolution of activity can be highly individual for every comet, and generalisations can be misleading.
KW  - comets
KW  - coma
KW  - gas
KW  - dust
KW  - dynamics
KW  - modelling
KW  - inversion
Y1  - 2020
U6  - https://doi.org/10.1007/s11214-020-00744-0
SN  - 0038-6308
SN  - 1572-9672
VL  - 216
IS  - 8
PB  - Springer
CY  - Dordrecht
ER  - 
TY  - JOUR
A1  - Haubitz, Toni
A1  - Fudickar, Werner
A1  - Linker, Torsten
A1  - Kumke, Michael Uwe
T1  - pH-sensitive fluorescence switching of pyridylanthracenes
BT  - the effect of the isomeric pattern
JF  - The journal of physical chemistry : A, Molecules, spectroscopy, kinetics, environment & general theory
N2  - 9,10-substituted anthracenes are known for their useful optical properties like fluorescence, which makes them frequently used probes in sensing applications. In this article, we investigate the fundamental photophysical properties of three pyridyl-substituted variants. The nitrogen atoms in the pyridinium six-membered rings are located in the ortho-, meta-, and para-positions in relation to the anthracene core. Absorption, fluorescence, and transient absorption measurements were carried out and were complemented by theoretical calculations. We monitored the photophysics of the anthracene derivatives in chloroform and water investigating the protonated as well as their nonprotonated forms. We found that the optical properties of the nonprotonated forms are strongly determined by the anthracene chromophore, with only small differences to other 9,10-substituted anthracenes, for example diphenyl anthracene. In contrast, protonation leads to a strong decrease in fluorescence intensity and lifetime. Transient absorption measurements and theoretical calculations revealed the formation of a charge-transfer state in the protonated chromophores, where electron density is shifted from the anthracene moiety toward the protonated pyridyl substituents. While the para- and ortho-derivatives' charge transfer is still moderately fluorescent, the meta-derivative is affected much stronger and shows nearly no fluorescence. This nitrogen-atom-position-dependent sensitivity to hydronium activity makes a combination of these fluorophores very attractive for pH-sensing applications covering a broadened pH range.
KW  - Absorption
KW  - Aromatic compounds
KW  - Fluorescence
KW  - Hydrocarbons
KW  - Reaction mechanisms
Y1  - 2020
U6  - https://doi.org/10.1021/acs.jpca.0c09911
SN  - 1089-5639
SN  - 1520-5215
VL  - 124
IS  - 52
SP  - 11017
EP  - 11024
PB  - American Chemical Society
CY  - Washington
ER  - 
TY  - JOUR
A1  - Malass, Ihsane
A1  - Tarkhanov, Nikolaj Nikolaevič
T1  - A perturbation of the de Rham complex
T1  - Возмущение комплекса де Рама
JF  - Journal of Siberian Federal University : Mathematics & Physics
JF  - Žurnal Sibirskogo Federalʹnogo Universiteta : Matematika i fizika
N2  - We consider a perturbation of the de Rham complex on a compact manifold with boundary. This perturbation goes beyond the framework of complexes, and so cohomology does not apply to it. On the other hand, its curvature is "small", hence there is a natural way to introduce an Euler characteristic and develop a Lefschetz theory for the perturbation. This work is intended as an attempt to develop a cohomology theory for arbitrary sequences of linear mappings.
N2  - Рассмотрим возмущение комплекса де Рама на компактном многообразии с краем. Это возмущение выходит за рамки комплексов, и поэтому когомологии к нему не относятся. С другой стороны, его кривизна "мала", поэтому существует естественный способ ввести характеристику Эйлера и разработать теорию Лефшеца для возмущения. Данная работа предназначена для попытки разработать теорию когомологий для произвольных последовательностей линейных отображений.
KW  - de Rham complex
KW  - cohomology
KW  - Hodge theory
KW  - Neumann problem
KW  - комплекс де Рама
KW  - когомологии
KW  - теория Ходжа
KW  - проблема Неймана
Y1  - 2020
U6  - https://doi.org/10.17516/1997-1397-2020-13-5-519-532
SN  - 1997-1397
SN  - 2313-6022
VL  - 13
IS  - 5
SP  - 519
EP  - 532
PB  - Siberian Federal University
CY  - Krasnojarsk
ER  - 
TY  - JOUR
A1  - Abdalla, Hassan E.
A1  - Adam, Remi
A1  - Aharonian, Felix A.
A1  - Benkhali, Faical Ait
A1  - Angüner, Ekrem Oǧuzhan
A1  - Arakawa, Masanori
A1  - Arcaro, C
A1  - Armand, Catherine
A1  - Armstrong, T.
A1  - Egberts, Kathrin
T1  - Very high energy γ-ray emission from two blazars of unknown redshift and upper limits on their distance
JF  - Monthly Notices of the Royal Astronomical Society
N2  - We report on the detection of very high energy (VHE; E > 100 GeV) gamma-ray emission from the BL Lac objects KUV 00311-1938 and PKS 1440-389 with the High Energy Stereoscopic System (H.E.S.S.). H.E.S.S. observations were accompanied or preceded by multiwavelength observations with Fermi/LAT, XRT and UVOT onboard the Swift satellite, and ATOM. Based on an extrapolation of the Fermi/LAT spectrum towards the VHE gamma-ray regime, we deduce a 95 per cent confidence level upper limit on the unknown redshift of KUV 00311-1938 of z < 0.98 and of PKS 1440-389 of z < 0.53. When combined with previous spectroscopy results, the redshift of KUV 00311-1938 is constrained to 0.51 <= z < 0.98 and of PKS 1440-389 to 0.14 (sic) z < 0.53.
KW  - BL Lacertae objects: individual
KW  - galaxies: high-redshift
KW  - gamma-rays: general
KW  - Resolved and unresolved sources as a function of wavelength
Y1  - 2020
VL  - 494
IS  - 4
PB  - Wiley-Blackwell
CY  - Oxford
ER  - 
TY  - JOUR
A1  - Schirdewahn, Daniel
T1  - Stability of a parametric harmonic oscillator with dichotomic noise
JF  - Chaos : an interdisciplinary journal of nonlinear science
N2  - The harmonic oscillator is a powerful model that can appear as a limit case when examining a nonlinear system. A well known fact is that, without driving, the inclusion of a friction term makes the origin of the phase space-which is a fixed point of the system-linearly stable. In this work, we include a telegraph process as perturbation of the oscillator's frequency, for example, to describe the motion of a particle with fluctuating charge gyrating in an external magnetic field. Increasing intensity of this colored noise is capable of changing the quality of the fixed point. To characterize the stability of the system, we use a stability measure that describes the growth of the displacement of the system's phase space position and express it in a closed form. We expand the respective exponent for light friction and low noise intensity and compare both the exact analytic solution and the expansion to numerical values. Our findings allow stability predictions for several physical systems.
Y1  - 2020
U6  - https://doi.org/10.1063/5.0012946
SN  - 1054-1500
SN  - 1089-7682
VL  - 30
IS  - 9
PB  - American Institute of Physics
CY  - Melville
ER  - 
TY  - JOUR
A1  - Rosenblum, Michael
T1  - Controlling collective synchrony in oscillatory ensembles by precisely timed pulses
JF  - Chaos : an interdisciplinary journal of nonlinear science
N2  - We present an efficient technique for control of synchrony in a globally coupled ensemble by pulsatile action. We assume that we can observe the collective oscillation and can stimulate all elements of the ensemble simultaneously. We pay special attention to the minimization of intervention into the system. The key idea is to stimulate only at the most sensitive phase. To find this phase, we implement an adaptive feedback control. Estimating the instantaneous phase of the collective mode on the fly, we achieve efficient suppression using a few pulses per oscillatory cycle. We discuss the possible relevance of the results for neuroscience, namely, for the development of advanced algorithms for deep brain stimulation, a medical technique used to treat Parkinson's disease.
Y1  - 2020
U6  - https://doi.org/10.1063/5.0019823
SN  - 1054-1500
SN  - 1089-7682
VL  - 30
IS  - 9
PB  - American Institute of Physics
CY  - Melville
ER  - 
TY  - JOUR
A1  - Xu, Pengbo
A1  - Zhou, Tian
A1  - Metzler, Ralf
A1  - Deng, Weihua
T1  - Lévy walk dynamics in an external harmonic potential
JF  - Physical review : E, Statistical, nonlinear, and soft matter physics
N2  - Levy walks (LWs) are spatiotemporally coupled random-walk processes describing superdiffusive heat conduction in solids, propagation of light in disordered optical materials, motion of molecular motors in living cells, or motion of animals, humans, robots, and viruses. We here investigate a key feature of LWs-their response to an external harmonic potential. In this generic setting for confined motion we demonstrate that LWs equilibrate exponentially and may assume a bimodal stationary distribution. We also show that the stationary distribution has a horizontal slope next to a reflecting boundary placed at the origin, in contrast to correlated superdiffusive processes. Our results generalize LWs to confining forces and settle some longstanding puzzles around LWs.
Y1  - 2020
U6  - https://doi.org/10.1103/PhysRevE.101.062127
SN  - 2470-0045
SN  - 2470-0053
SN  - 1550-2376
SN  - 1063-651X
SN  - 1539-3755
VL  - 101
IS  - 6
PB  - American Physical Society
CY  - College Park
ER  - 
TY  - JOUR
A1  - Quade, Markus
A1  - Isele, Thomas
A1  - Abel, Markus
T1  - Machine learning control
BT  - explainable and analyzable methods
JF  - Physica : D, Nonlinear phenomena
N2  - Recently, the term explainable AI came into discussion as an approach to produce models from artificial intelligence which allow interpretation. For a long time, symbolic regression has been used to produce explainable and mathematically tractable models. In this contribution, we extend previous work on symbolic regression methods to infer the optimal control of a dynamical system given one or several optimization criteria, or cost functions. In earlier publications, network control was achieved by automated machine learning control using genetic programming. Here, we focus on the subsequent path continuation analysis of the mathematical expressions which result from the machine learning model. In particular, we use AUTO to analyze the solution properties of the controlled oscillator system which served as our model. As a result, we show that there is a considerable advantage of explainable symbolic regression models over less accessible neural networks. In particular, the roadmap of future works may be to integrate such analyses into the optimization loop itself to filter out robust solutions by construction.
KW  - Explainable AI
KW  - Machine learning control
KW  - Dynamical systems
KW  - Synchronization control
KW  - Genetic programming
Y1  - 2020
U6  - https://doi.org/10.1016/j.physd.2020.132582
SN  - 0167-2789
SN  - 1872-8022
VL  - 412
PB  - Elsevier
CY  - Amsterdam
ER  - 
TY  - JOUR
A1  - Sangian, Danial
T1  - Improving the performance of dielectric-elastomer actuators at elevated operating temperatures by means of thermal softening
JF  - Smart materials and structures
N2  - Dielectric elastomer devices operate on the principle of Maxwell stress and their operating performance significantly rely on the elastomer and compliant electrode's electrical and mechanical properties. This paper reports that performing actuation tests at elevated temperatures resulted in an enhanced performance due to the reduction of Young's modulus and the increase of dielectric permittivity. As a result, considerably higher isometric forces and isotonic strains were achieved above the ambient operating temperature. For actuators made of silicone, polyurethane and acrylic elastomers, 166%, 70% and 266% higher isometric forces and 450%, 250% and 54% higher isotonic strains were observed, respectively, when tested at the temperature of 100 degrees C in comparison to ambient temperature values using the same operating voltages. Values of up to 0.4 J kg(-1) and 3.1 W kg(-1) were achieved for the work and power outputs per mass, respectively, on a silicone elastomer driven with a voltage of 1.5 kV at a temperature of 100 degrees C.
KW  - elevated-temperature effects on the operation of dielectric-elastomer
KW  - actuators
KW  - thermally enhanced actuators
KW  - thermal stimulation of
KW  - electro-electrets
Y1  - 2020
U6  - https://doi.org/10.1088/1361-665X/ab6079
SN  - 0964-1726
SN  - 1361-665X
VL  - 29
IS  - 2
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Perovic, Milena
A1  - Qin, Qing
A1  - Oschatz, Martin
T1  - From molecular precursors to nanoparticles
BT  - tailoring the adsorption properties of porous carbon materials by controlled chemical functionalization
JF  - Advanced functional materials
N2  - Nanoporous carbon materials (NCMs) provide the "function" of high specific surface area and thus have large interface area for interactions with surrounding species, which is of particular importance in applications related to adsorption processes. The strength and mechanism of adsorption depend on the pore architecture of the NCMs. In addition, chemical functionalization can be used to induce changes of electron density and/or electron density distribution in the pore walls, thus further modifying the interactions between carbons and guest species. Typical approaches for functionalization of nanoporous materials with regular atomic construction like porous silica, metal-organic frameworks, or zeolites, cannot be applied to NCMs due to their less defined local atomic construction and abundant defects. Therefore, synthetic strategies that offer a higher degree of control over the process of functionalization are needed. Synthetic approaches for covalent functionalization of NCMs, that is, for the incorporation of heteroatoms into the carbon backbone, are critically reviewed with a special focus on strategies following the concept "from molecules to materials." Approaches for coordinative functionalization with metallic species, and the functionalization by nanocomposite formation between pristine carbon materials and heteroatom-containing carbons, are introduced as well. Particular focus is given to the influences of these functionalizations in adsorption-related applications.
KW  - composites
KW  - heteroatoms
KW  - metal species
KW  - porous carbon materials
KW  - surface
KW  - functionalization
Y1  - 2020
U6  - https://doi.org/10.1002/adfm.201908371
SN  - 1616-301X
SN  - 1616-3028
VL  - 30
IS  - 41
PB  - Wiley-VCH
CY  - Weinheim
ER  - 
TY  - JOUR
A1  - Fumani, F. Khastehdel
A1  - Nemati, Somayyeh
A1  - Mahdavifar, Saeed
T1  - Quantum critical lines in the ground state phase diagram of spin-1/2 frustrated transverse-field ising chains
JF  - Annalen der Physik
N2  - This paper focuses on the ground state phase diagram of a 1D spin-1/2 quantum Ising model with competing first and second nearest neighbour interactions known as the axial next nearest neighbour Ising model in the presence of a transverse magnetic field. Here, using quantum correlations, both numerically and analytically, some evidence is provided to clarify the identification of the ground state phase diagram. Local quantum correlations play a crucial role in detecting the critical lines either revealed or hidden by symmetry-breaking. A non-symmetry-breaking disorder transition line can be identified by the first derivative of both entanglement of formation and quantum discord between nearest neighbour spins. In addition, the quantum correlations between the second neighbour spins can also be used to reveal Kosterlitz-Thouless phase transition when their interaction strength grows and becomes closer to the first nearest neighbour one. The results obtained using the Jordan-Wigner transformation confirm the accuracy of the numerical case.
KW  - axial next nearest neighbour Ising chains
KW  - quantum correlations
KW  - quantum
KW  - phase transitions
Y1  - 2020
U6  - https://doi.org/10.1002/andp.202000384
SN  - 0003-3804
SN  - 1521-3889
VL  - 533
IS  - 2
PB  - Wiley-VCH
CY  - Weinheim
ER  - 
TY  - JOUR
A1  - Siddiqi, Muhammad Ali
A1  - Dörr, Christian
A1  - Strydis, Christos
T1  - IMDfence
BT  - architecting a secure protocol for implantable medical devices
JF  - IEEE access
N2  - Over the past decade, focus on the security and privacy aspects of implantable medical devices (IMDs) has intensified, driven by the multitude of cybersecurity vulnerabilities found in various existing devices. However, due to their strict computational, energy and physical constraints, conventional security protocols are not directly applicable to IMDs. Custom-tailored schemes have been proposed instead which, however, fail to cover the full spectrum of security features that modern IMDs and their ecosystems so critically require. In this paper we propose IMDfence, a security protocol for IMD ecosystems that provides a comprehensive yet practical security portfolio, which includes availability, non-repudiation, access control, entity authentication, remote monitoring and system scalability. The protocol also allows emergency access that results in the graceful degradation of offered services without compromising security and patient safety. The performance of the security protocol as well as its feasibility and impact on modern IMDs are extensively analyzed and evaluated. We find that IMDfence achieves the above security requirements at a mere less than 7% increase in total IMD energy consumption, and less than 14 ms and 9 kB increase in system delay and memory footprint, respectively.
KW  - protocols
KW  - implants
KW  - authentication
KW  - ecosystems
KW  - remote monitoring
KW  - scalability
KW  - authentication protocol
KW  - battery-depletion attack
KW  - battery
KW  - DoS
KW  - denial-of-service attack
KW  - IMD
KW  - implantable medical device
KW  - non-repudiation
KW  - smart card
KW  - zero-power defense
Y1  - 2020
U6  - https://doi.org/10.1109/ACCESS.2020.3015686
SN  - 2169-3536
VL  - 8
SP  - 147948
EP  - 147964
PB  - Institute of Electrical and Electronics Engineers
CY  - Piscataway
ER  - 
TY  - JOUR
A1  - Laing, Carlo R.
A1  - Omel'chenko, Oleh
T1  - Moving bumps in theta neuron networks
JF  - Chaos : an interdisciplinary journal of nonlinear science
N2  - We consider large networks of theta neurons on a ring, synaptically coupled with an asymmetric kernel. Such networks support stable "bumps" of activity, which move along the ring if the coupling kernel is asymmetric. We investigate the effects of the kernel asymmetry on the existence, stability, and speed of these moving bumps using continuum equations formally describing infinite networks. Depending on the level of heterogeneity within the network, we find complex sequences of bifurcations as the amount of asymmetry is varied, in strong contrast to the behavior of a classical neural field model.
Y1  - 2020
U6  - https://doi.org/10.1063/1.5143261
SN  - 1054-1500
SN  - 1089-7682
VL  - 30
IS  - 4
PB  - American Institute of Physics
CY  - Melville
ER  - 
TY  - JOUR
A1  - Granado, Felipe Le Vot
A1  - Abad, Enrique
A1  - Metzler, Ralf
A1  - Yuste, Santos B.
T1  - Continuous time random walk in a velocity field
BT  - role of domain growth, Galilei-invariant advection-diffusion, and kinetics of particle mixing
JF  - New Journal of Physics
N2  - We consider the emerging dynamics of a separable continuous time random walk (CTRW) in the case when the random walker is biased by a velocity field in a uniformly growing domain. Concrete examples for such domains include growing biological cells or lipid vesicles, biofilms and tissues, but also macroscopic systems such as expanding aquifers during rainy periods, or the expanding Universe. The CTRW in this study can be subdiffusive, normal diffusive or superdiffusive, including the particular case of a Lévy flight. We first consider the case when the velocity field is absent. In the subdiffusive case, we reveal an interesting time dependence of the kurtosis of the particle probability density function. In particular, for a suitable parameter choice, we find that the propagator, which is fat tailed at short times, may cross over to a Gaussian-like propagator. We subsequently incorporate the effect of the velocity field and derive a bi-fractional diffusion-advection equation encoding the time evolution of the particle distribution. We apply this equation to study the mixing kinetics of two diffusing pulses, whose peaks move towards each other under the action of velocity fields acting in opposite directions. This deterministic motion of the peaks, together with the diffusive spreading of each pulse, tends to increase particle mixing, thereby counteracting the peak separation induced by the domain growth. As a result of this competition, different regimes of mixing arise. In the case of Lévy flights, apart from the non-mixing regime, one has two different mixing regimes in the long-time limit, depending on the exact parameter choice: in one of these regimes, mixing is mainly driven by diffusive spreading, while in the other mixing is controlled by the velocity fields acting on each pulse. Possible implications for encounter–controlled reactions in real systems are discussed.
KW  - diffusion
KW  - expanding medium
KW  - continuous time random walk
Y1  - 2020
U6  - https://doi.org/10.1088/1367-2630/ab9ae2
SN  - 1367-2630
VL  - 22
PB  - Dt. Physikalische Ges.
CY  - Bad Honnef
ER  - 
TY  - JOUR
A1  - Großmann, Robert
A1  - Aranson, Igor S.
A1  - Peruani, Fernando
T1  - A particle-field approach bridges phase separation and collective motion in active matter
JF  - Nature Communications
N2  - Whereas self-propelled hard discs undergo motility-induced phase separation, self-propelled rods exhibit a variety of nonequilibrium phenomena, including clustering, collective motion, and spatio-temporal chaos. In this work, we present a theoretical framework representing active particles by continuum fields. This concept combines the simplicity of alignment-based models, enabling analytical studies, and realistic models that incorporate the shape of self-propelled objects explicitly. By varying particle shape from circular to ellipsoidal, we show how nonequilibrium stresses acting among self-propelled rods destabilize motility-induced phase separation and facilitate orientational ordering, thereby connecting the realms of scalar and vectorial active matter. Though the interaction potential is strictly apolar, both, polar and nematic order may emerge and even coexist. Accordingly, the symmetry of ordered states is a dynamical property in active matter. The presented framework may represent various systems including bacterial colonies, cytoskeletal extracts, or shaken granular media. Interacting self-propelled particles exhibit phase separation or collective motion depending on particle shape. A unified theory connecting these paradigms represents a major challenge in active matter, which the authors address here by modeling active particles as continuum fields.
Y1  - 2020
U6  - https://doi.org/10.1038/s41467-020-18978-5
SN  - 2041-1723
VL  - 11
IS  - 1
PB  - Nature Publishing Group
CY  - London
ER  - 
TY  - JOUR
A1  - Fernandez, Amanda Diez
A1  - Charchar, Patrick
A1  - Cherstvy, Andrey G.
A1  - Metzler, Ralf
A1  - Finnis, Michael W.
T1  - The diffusion of doxorubicin drug molecules in silica nanoslits is non-Gaussian, intermittent and anticorrelated
JF  - Physical chemistry, chemical physics
N2  - In this study we investigate, using all-atom molecular-dynamics computer simulations, the in-plane diffusion of a doxorubicin drug molecule in a thin film of water confined between two silica surfaces. We find that the molecule diffuses along the channel in the manner of a Gaussian diffusion process, but with parameters that vary according to its varying transversal position. Our analysis identifies that four Gaussians, each describing particle motion in a given transversal region, are needed to adequately describe the data. Each of these processes by itself evolves with time at a rate slower than that associated with classical Brownian motion due to a predominance of anticorrelated displacements. Long adsorption events lead to ageing, a property observed when the diffusion is intermittently hindered for periods of time with an average duration which is theoretically infinite. This study presents a simple system in which many interesting features of anomalous diffusion can be explored. It exposes the complexity of diffusion in nanoconfinement and highlights the need to develop new understanding.
Y1  - 2020
U6  - https://doi.org/10.1039/d0cp03849k
SN  - 1463-9076
SN  - 1463-9084
VL  - 22
IS  - 48
SP  - 27955
EP  - 27965
PB  - Royal Society of Chemistry
CY  - Cambridge
ER  - 
TY  - JOUR
A1  - Omelʹchenko, Oleh E.
T1  - Nonstationary coherence-incoherence patterns in nonlocally coupled heterogeneous phase oscillators
JF  - Chaos : an interdisciplinary journal of nonlinear science
N2  - We consider a large ring of nonlocally coupled phase oscillators and show that apart from stationary chimera states, this system also supports nonstationary coherence-incoherence patterns (CIPs). For identical oscillators, these CIPs behave as breathing chimera states and are found in a relatively small parameter region only. It turns out that the stability region of these states enlarges dramatically if a certain amount of spatially uniform heterogeneity (e.g., Lorentzian distribution of natural frequencies) is introduced in the system. In this case, nonstationary CIPs can be studied as stable quasiperiodic solutions of a corresponding mean-field equation, formally describing the infinite system limit. Carrying out direct numerical simulations of the mean-field equation, we find different types of nonstationary CIPs with pulsing and/or alternating chimera-like behavior. Moreover, we reveal a complex bifurcation scenario underlying the transformation of these CIPs into each other. These theoretical predictions are confirmed by numerical simulations of the original coupled oscillator system.
KW  - chimera states
KW  - synchronization
KW  - networks
KW  - Kuramoto
KW  - populations
KW  - dynamics
KW  - bumps
KW  - model
Y1  - 2020
U6  - https://doi.org/10.1063/1.5145259
SN  - 1054-1500
SN  - 1089-7682
VL  - 30
IS  - 4
PB  - American Institute of Physics
CY  - Melville
ER  - 
TY  - JOUR
A1  - Wang, Wei
A1  - Cherstvy, Andrey G.
A1  - Chechkin, Aleksei V.
A1  - Thapa, Samudrajit
A1  - Seno, Flavio
A1  - Liu, Xianbin
A1  - Metzler, Ralf
T1  - Fractional Brownian motion with random diffusivity
BT  - emerging residual nonergodicity below the correlation time
JF  - Journal of physics : A, Mathematical and theoretical
N2  - Numerous examples for a priori unexpected non-Gaussian behaviour for normal and anomalous diffusion have recently been reported in single-particle tracking experiments. Here, we address the case of non-Gaussian anomalous diffusion in terms of a random-diffusivity mechanism in the presence of power-law correlated fractional Gaussian noise. We study the ergodic properties of this model via examining the ensemble- and time-averaged mean-squared displacements as well as the ergodicity breaking parameter EB quantifying the trajectory-to-trajectory fluctuations of the latter. For long measurement times, interesting crossover behaviour is found as function of the correlation time tau characterising the diffusivity dynamics. We unveil that at short lag times the EB parameter reaches a universal plateau. The corresponding residual value of EB is shown to depend only on tau and the trajectory length. The EB parameter at long lag times, however, follows the same power-law scaling as for fractional Brownian motion. We also determine a corresponding plateau at short lag times for the discrete representation of fractional Brownian motion, absent in the continuous-time formulation. These analytical predictions are in excellent agreement with results of computer simulations of the underlying stochastic processes. Our findings can help distinguishing and categorising certain nonergodic and non-Gaussian features of particle displacements, as observed in recent single-particle tracking experiments.
KW  - stochastic processes
KW  - anomalous diffusion
KW  - fractional Brownian motion
KW  - diffusing diffusivity
KW  - weak ergodicity breaking
Y1  - 2020
U6  - https://doi.org/10.1088/1751-8121/aba467
SN  - 1751-8113
SN  - 1751-8121
VL  - 53
IS  - 47
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Kurilovich, Aleksandr A.
A1  - Mantsevich, Vladimir
A1  - Stevenson, Keith J.
A1  - Chechkin, Aleksei V.
A1  - Palyulin, V. V.
T1  - Complex diffusion-based kinetics of photoluminescence in semiconductor nanoplatelets
JF  - Physical chemistry, chemical physics : a journal of European Chemical Societies
N2  - We present a diffusion-based simulation and theoretical models for explanation of the photoluminescence (PL) emission intensity in semiconductor nanoplatelets. It is shown that the shape of the PL intensity curves can be reproduced by the interplay of recombination, diffusion and trapping of excitons. The emission intensity at short times is purely exponential and is defined by recombination. At long times, it is governed by the release of excitons from surface traps and is characterized by a power-law tail. We show that the crossover from one limit to another is controlled by diffusion properties. This intermediate region exhibits a rich behaviour depending on the value of diffusivity. The proposed approach reproduces all the features of experimental curves measured for different nanoplatelet systems.
Y1  - 2020
U6  - https://doi.org/10.1039/d0cp03744c
SN  - 1463-9076
SN  - 1463-9084
VL  - 22
IS  - 42
SP  - 24686
EP  - 24696
PB  - Royal Society of Chemistry
CY  - Cambridge
ER  - 
TY  - JOUR
A1  - Steppa, Constantin
A1  - Egberts, Kathrin
T1  - Modelling the Galactic very-high-energy gamma-ray source population
JF  - Astronomy and astrophysics : an international weekly journal
N2  - Context. The High Energy Stereoscopic System Galactic plane survey (HGPS) is to date the most comprehensive census of Galactic gamma -ray sources at very high energies (VHE; 100 GeV <= E <= 100 TeV). As a consequence of the limited sensitivity of this survey, the 78 detected gamma -ray sources comprise only a small and biased subsample of the overall population. The larger part consists of currently unresolved sources, which contribute to large-scale diffuse emission to a still uncertain amount.Aims. We study the VHE gamma -ray source population in the Milky Way. For this purpose population-synthesis models are derived based on the distributions of source positions, extents, and luminosities.Methods. Several azimuth-symmetric and spiral-arm models are compared for spatial source distribution. The luminosity and radius function of the population are derived from the source properties of the HGPS data set and are corrected for the sensitivity bias of the HGPS. Based on these models, VHE source populations are simulated and the subsets of sources detectable according to the HGPS are compared with HGPS sources.Results. The power-law indices of luminosity and radius functions are determined to range between -1.6 and -1.9 for luminosity and -1.1 and -1.6 for radius. A two-arm spiral structure with central bar is discarded as spatial distribution of VHE sources, while azimuth-symmetric distributions and a distribution following a four-arm spiral structure without bar describe the HGPS data reasonably well. The total number of Galactic VHE sources is predicted to be in the range from 800 to 7000 with a total luminosity and flux of (1.6-6.3) x 10(36) ph s(-1) and (3-15) x 10(-10) ph cm(-2) s(-1), respectively.Conclusions. Depending on the model, the HGPS sample accounts for (68-87)% of the emission of the population in the scanned region. This suggests that unresolved sources represent a critical component of the diffuse emission measurable in the HGPS. With the foreseen jump in sensitivity of the Cherenkov Telescope Array, the number of detectable sources is predicted to increase by a factor between 5 and 9.
KW  - astroparticle physics
KW  - gamma rays: general
KW  - gamma rays: diffuse
KW  - background
KW  - methods: observational
KW  - methods: numerical
Y1  - 2020
U6  - https://doi.org/10.1051/0004-6361/202038172
SN  - 0004-6361
SN  - 1432-0746
VL  - 643
PB  - EDP Sciences
CY  - Les Ulis
ER  - 
TY  - JOUR
A1  - Kontro, Inkeri
A1  - Buschhüter, David
T1  - Validity of Colorado Learning Attitudes about Science Survey for a high-achieving, Finnish population
JF  - Physical review. Physics education research
N2  - The Colorado Learning Attitudes about Science Survey (CLASS) is an instrument which is widely used in physics education to characterize students' attitudes toward physics and learning physics and compare them with those of experts. While CLASS has been extensively validated for use in the context of higher education institutions in the United States, there has been less information about its use with European students. We have studied the structural, content, and substantive aspects of validity of CLASS by first doing a confirmatory factor analysis of N = 642 sets of student answers from the University of Helsinki, Finland. The students represented a culturally and demographically different subset of university physics students than in previous studies. The confirmatory factor analysis used a 3-factor, 15-item factor structure as a starting point and the resulting factor structure was similar to the original. Just minor modifications were needed for fit parameters to be in the acceptable range. We explored the differences by student interviews and consultation of experts. With the exception of one item, they supported the new 14-item, 3-factor structure. The results show that the interpretations made from CLASS results are mostly transferable, and CLASS remains a useful instrument for a wide variety of populations.
Y1  - 2020
U6  - https://doi.org/10.1103/PhysRevPhysEducRes.16.020104
SN  - 2469-9896
VL  - 16
IS  - 2
PB  - American Physical Society
CY  - College Park, MD
ER  - 
TY  - JOUR
A1  - Beckus, Siegfried
A1  - Pinchover, Yehuda
T1  - Shnol-type theorem for the Agmon ground state
JF  - Journal of spectral theory
N2  - LetH be a Schrodinger operator defined on a noncompact Riemannianmanifold Omega, and let W is an element of L-infinity (Omega; R). Suppose that the operator H + W is critical in Omega, and let phi be the corresponding Agmon ground state. We prove that if u is a generalized eigenfunction ofH satisfying vertical bar u vertical bar <= C-phi in Omega for some constant C > 0, then the corresponding eigenvalue is in the spectrum of H. The conclusion also holds true if for some K is an element of Omega the operator H admits a positive solution in (Omega) over bar = Omega \ K, and vertical bar u vertical bar <= C psi in (Omega) over bar for some constant C > 0, where psi is a positive solution of minimal growth in a neighborhood of infinity in Omega. Under natural assumptions, this result holds also in the context of infinite graphs, and Dirichlet forms.
KW  - Shnol theorem
KW  - Caccioppoli inequality
KW  - Schrodinger operators
KW  - generalized eigenfunction
KW  - ground state
KW  - positive solutions
KW  - weighted
KW  - graphs
Y1  - 2020
U6  - https://doi.org/10.4171/JST/296
SN  - 1664-039X
SN  - 1664-0403
VL  - 10
IS  - 2
SP  - 355
EP  - 377
PB  - EMS Publishing House
CY  - Zürich
ER  - 
TY  - JOUR
A1  - Ghaffarvand Mokari, Payam
A1  - Sardhaei, Nasim Mahdinezhad
T1  - Predictive power of cepstral coefficients and spectral moments in the classification of Azerbaijani fricatives
JF  - The journal of the Acoustical Society of America
N2  - This study compares the classification of Azerbaijani fricatives based on two sets of features: (a) spectral moments, spectral peak, amplitude, duration, and (b) cepstral coefficients employing Hidden Markov Models to divide each fricative into three regions such that the variances of the measures within each region are minimized. The cepstral coefficients were found to be more reliable predictors in the classification of all nine Azerbaijani fricatives and the cepstral measures yielded highly successful classification rates (91.21% across both genders) in the identification of the full set of fricatives of Azerbaijani.
Y1  - 2020
U6  - https://doi.org/10.1121/10.0000830
SN  - 0001-4966
SN  - 1520-8524
VL  - 147
IS  - 3
SP  - EL228
EP  - EL234
PB  - American Institute of Physics
CY  - Melville
ER  - 
TY  - JOUR
A1  - Krylov, Dmitrii
A1  - Dylov, Dmitry V.
A1  - Rosenblum, Michael
T1  - Reinforcement learning for suppression of collective activity in oscillatory ensembles
JF  - Chaos : an interdisciplinary journal of nonlinear science
N2  - We present the use of modern machine learning approaches to suppress self-sustained collective oscillations typically signaled by ensembles of degenerative neurons in the brain. The proposed hybrid model relies on two major components: an environment of oscillators and a policy-based reinforcement learning block. We report a model-agnostic synchrony control based on proximal policy optimization and two artificial neural networks in an Actor-Critic configuration. A class of physically meaningful reward functions enabling the suppression of collective oscillatory mode is proposed. The synchrony suppression is demonstrated for two models of neuronal populations-for the ensembles of globally coupled limit-cycle Bonhoeffer-van der Pol oscillators and for the bursting Hindmarsh-Rose neurons using rectangular and charge-balanced stimuli.
Y1  - 2020
U6  - https://doi.org/10.1063/1.5128909
SN  - 1054-1500
SN  - 1089-7682
VL  - 30
IS  - 3
PB  - American Institute of Physics
CY  - Melville
ER  - 
TY  - JOUR
A1  - Awad, Emad
A1  - Metzler, Ralf
T1  - Crossover dynamics from superdiffusion to subdiffusion
BT  - models and solutions
JF  - Fractional calculus and applied analysis : an international journal for theory and applications
N2  - The Cattaneo or telegrapher's equation describes the crossover from initial ballistic to normal diffusion. Here we study and survey time-fractional generalisations of this equation that are shown to produce the crossover of the mean squared displacement from superdiffusion to subdiffusion. Conditional solutions are derived in terms of Fox H-functions and the dth-order moments as well as the diffusive flux of the different models are derived. Moreover, the concept of the distribution-like is proposed as an alternative to the probability density function.
KW  - Cattaneo equation
KW  - telegrapher's equation
KW  - crossover dynamics
KW  - fractional dynamic equations
KW  - anomalous diffusion
KW  - superdiffusion and
KW  - subdiffusion
KW  - Fox H-functions
Y1  - 2020
U6  - https://doi.org/10.1515/fca-2020-0003
SN  - 1311-0454
SN  - 1314-2224
VL  - 23
IS  - 1
SP  - 55
EP  - 102
PB  - De Gruyter
CY  - Berlin ; Boston
ER  - 
TY  - JOUR
A1  - Gómez-Nava, Luis
A1  - Grossmann, Robert
A1  - Hintsche, Marius
A1  - Beta, Carsten
A1  - Peruani, Fernando
T1  - A novel approach to chemotaxis
BT  - active particles guided by internal clocks
JF  - epl : a letters journal exploring the frontiers of physics
N2  - Motivated by the observation of non-exponential run-time distributions of bacterial swimmers, we propose a minimal phenomenological model for taxis of active particles whose motion is controlled by an internal clock. The ticking of the clock depends on an external concentration field, e.g., a chemical substance. We demonstrate that these particles can detect concentration gradients and respond to them by moving up- or down-gradient depending on the clock design, albeit measurements of these fields are purely local in space and instantaneous in time. Altogether, our results open a new route in the study of directional navigation: we show that the use of a clock to control motility actions represents a generic and versatile toolbox to engineer behavioral responses to external cues, such as light, chemical, or temperature gradients.
Y1  - 2020
U6  - https://doi.org/10.1209/0295-5075/130/68002
SN  - 0295-5075
SN  - 1286-4854
VL  - 130
IS  - 6
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Rüdiger, Günther
A1  - Schultz, Manfred
T1  - Large-scale dynamo action of magnetized Taylor-Couette flows
JF  - Monthly notices of the Royal Astronomical Society
N2  - A conducting Taylor-Couette flow with quasi-Keplerian rotation law containing a toroidal magnetic field serves as a mean-field dynamo model of the Tayler-Spruit type. The flows are unstable against non-axisymmetric perturbations which form electromotive forces defining a effect and eddy diffusivity. If both degenerated modes with m = +/- 1 are excited with the same power then the global a effect vanishes and a dynamo cannot work. It is shown, however, that the Tayler instability produces finite alpha effects if only an isolated mode is considered but this intrinsic helicity of the single-mode is too low for an alpha(2) dynamo. Moreover, an alpha Omega dynamo model with quasi-Keplerian rotation requires a minimum magnetic Reynolds number of rotation of Rm similar or equal to 2000 to work. Whether it really works depends on assumptions about the turbulence energy. For a steeper-than-quadratic dependence of the turbulence intensity on the magnetic field, however, dynamos are only excited if the resulting magnetic eddy diffusivity approximates its microscopic value, eta(T) similar or equal to eta. By basically lower or larger eddy diffusivities the dynamo instability is suppressed.
KW  - dynamo
KW  - instabilities
KW  - MHD
KW  - magnetic fields
Y1  - 2020
U6  - https://doi.org/10.1093/mnras/staa293
SN  - 0035-8711
SN  - 1365-2966
VL  - 493
IS  - 1
SP  - 1249
EP  - 1260
PB  - Oxford Univ. Press
CY  - Oxford
ER  - 
TY  - JOUR
A1  - Caetano, Daniel L. Z.
A1  - Carvalho, Sidney Jurado de
A1  - Metzler, Ralf
A1  - Cherstvy, Andrey G.
T1  - Critical adsorption of multiple polyelectrolytes onto a nanosphere
BT  - splitting the adsorption-desorption transition boundary
JF  - Interface : journal of the Royal Society
N2  - Employing extensive Monte Carlo computer simulations, we investigate in detail the properties of multichain adsorption of charged flexible polyelectrolytes (PEs) onto oppositely charged spherical nanoparticles (SNPs). We quantify the conditions of critical adsorption-the phase-separation curve between the adsorbed and desorbed states of the PEs-as a function of the SNP surface-charge density and the concentration of added salt. We study the degree of fluctuations of the PE-SNP electrostatic binding energy, which we use to quantify the emergence of the phase subtransitions, including a series of partially adsorbed PE configurations. We demonstrate how the phase-separation adsorption-desorption boundary shifts and splits into multiple subtransitions at low-salt conditions, thereby generalizing and extending the results for critical adsorption of a single PE onto the SNP. The current findings are relevant for finite concentrations of PEs around the attracting SNP, such as the conditions for PE adsorption onto globular proteins carrying opposite electric charges.
KW  - nanoparticles
KW  - polyelectrolytes
KW  - electrostatics
KW  - critical adsorption
KW  - phase-transition boundary
Y1  - 2020
U6  - https://doi.org/10.1098/rsif.2020.0199
SN  - 1742-5689
SN  - 1742-5662
VL  - 17
IS  - 167
PB  - Royal Society
CY  - London
ER  - 
TY  - JOUR
A1  - Stojkoski, Viktor
A1  - Sandev, Trifce
A1  - Basnarkov, Lasko
A1  - Kocarev, Ljupco
A1  - Metzler, Ralf
T1  - Generalised geometric Brownian motion
BT  - theory and applications to option pricing
JF  - Entropy
N2  - Classical option pricing schemes assume that the value of a financial asset follows a geometric Brownian motion (GBM). However, a growing body of studies suggest that a simple GBM trajectory is not an adequate representation for asset dynamics, due to irregularities found when comparing its properties with empirical distributions. As a solution, we investigate a generalisation of GBM where the introduction of a memory kernel critically determines the behaviour of the stochastic process. We find the general expressions for the moments, log-moments, and the expectation of the periodic log returns, and then obtain the corresponding probability density functions using the subordination approach. Particularly, we consider subdiffusive GBM (sGBM), tempered sGBM, a mix of GBM and sGBM, and a mix of sGBMs. We utilise the resulting generalised GBM (gGBM) in order to examine the empirical performance of a selected group of kernels in the pricing of European call options. Our results indicate that the performance of a kernel ultimately depends on the maturity of the option and its moneyness.
KW  - geometric Brownian motion
KW  - Fokker– Planck equation
KW  - Black– Scholes model
KW  - option pricing
Y1  - 2020
U6  - https://doi.org/10.3390/e22121432
SN  - 1099-4300
VL  - 22
IS  - 12
PB  - MDPI
CY  - Basel
ER  - 
TY  - JOUR
A1  - Kuentzer, Felipe A.
A1  - Juracy, Leonardo R.
A1  - Moreira, Matheus T.
A1  - Amory, Alexandre M.
T1  - Testing the blade resilient asynchronous template
JF  - Analog integrated circuits and signal processing : an international journal
N2  - As VLSI design moves into ultra-deep-submicron technologies, timing margins added to the clock period are mandatory, to ensure correct circuit behavior under worst-case conditions. Timing resilient architectures emerged as a promising solution to alleviate these worst-case timing margins. These architectures allow improving system performance and reducing energy consumption. Asynchronous systems, on the other hand, have the potential to improve energy efficiency and performance. Blade is an asynchronous timing resilient template that leverages the advantages of both asynchronous and timing resilient techniques. However, Blade still presents challenges regarding its testability, which hinders its commercial or large-scale application. This paper demonstrates that scan chains can be prohibitive for Blade due to their high silicon costs., which can reach more than 100%. Then, it proposes an alternative test approach that allows concurrent testing, stuck-at, and delay testing. The test approach is based on the reuse the Blade features to provide testability, with silicon area overheads between 4 and 7%.
KW  - asynchronous design
KW  - blade
KW  - delay faults
KW  - design for Testability
KW  - stuck-at faults
KW  - timing resilient design
Y1  - 2020
U6  - https://doi.org/10.1007/s10470-020-01651-8
SN  - 0925-1030
SN  - 1573-1979
VL  - 106
IS  - 1
SP  - 219
EP  - 234
PB  - Springer
CY  - Dordrecht
ER  - 
TY  - JOUR
A1  - Tröndle, Tim
A1  - Lilliestam, Johan
A1  - Marelli, Stefano
A1  - Pfenninger, Stefan
T1  - Trade-offs between geographic scale, cost, and infrastructure requirements for fully renewable electricity in Europe
JF  - Joule
N2  - The European potential for renewable electricity is sufficient to enable fully renewable supply on different scales, from self-sufficient, subnational regions to an interconnected continent. We not only show that a continental-scale system is the cheapest, but also that systems on the national scale and below are possible at cost penalties of 20% or less. Transmission is key to low cost, but it is not necessary to vastly expand the transmission system. When electricity is transmitted only to balance fluctuations, the transmission grid size is comparable to today's, albeit with expanded cross-border capacities. The largest differences across scales concern land use and thus social acceptance: in the continental system, generation capacity is concentrated on the European periphery, where the best resources are. Regional systems, in contrast, have more dispersed generation. The key trade-off is therefore not between geographic scale and cost, but between scale and the spatial distribution of required generation and transmission infrastructure.
KW  - energy decarbonization
KW  - self-sufficiency
KW  - cooperation
KW  - trade
KW  - transmission
KW  - regional equity
KW  - land use
KW  - acceptance
KW  - flexibility
Y1  - 2020
U6  - https://doi.org/10.1016/j.joule.2020.07.018
SN  - 2542-4351
VL  - 4
IS  - 9
SP  - 1929
EP  - 1948
PB  - Cell Press
CY  - Cambridge , Mass.
ER  - 
TY  - JOUR
A1  - Vollbrecht, Joachim
A1  - Brus, Viktor V.
T1  - On charge carrier density in organic solar cells obtained via capacitance spectroscopy
JF  - Advanced electronic materials
N2  - The determination of the voltage-dependent density of free charge carriers via capacitance spectroscopy is considered an important step in the analysis of emerging photovoltaic technologies, such as organic and perovskite solar cells. In particular, an intimate knowledge of the density of free charge carriers is required for the determination of crucial parameters such as the effective mobility, charge carrier lifetime, nongeminate recombination coefficients, average extraction times, and competition factors. Hence, it is paramount to verify the validity of the commonly employed approaches to obtain the density of free charge carriers. The advantages, drawbacks, and limitations of the most common approaches are investigated in detail and strategies to mitigate misleading values are explored. To this end, two types of nonfullerene organic solar cells based on a PTB7-Th:ITIC-2F blend and a PM6:Y6 blend, respectively, are used as a case study to assess how subsequent analyses of the nongeminate recombination dynamics depend on the chosen approach to calculate the density of free charge carriers via capacitance spectroscopy.
KW  - bulk-heterojunction solar cells
KW  - capacitance spectroscopy
KW  - charge
KW  - carrier density
KW  - impedance spectroscopy
KW  - organic photovoltaics
Y1  - 2020
U6  - https://doi.org/10.1002/aelm.202000517
SN  - 2199-160X
VL  - 6
IS  - 10
PB  - Wiley
CY  - Hoboken
ER  - 
TY  - JOUR
A1  - Vollbrecht, Joachim
A1  - Brus, Viktor V.
T1  - On the recombination order of surface recombination under open circuit conditions
JF  - Organic electronics : physics, materials and applications
N2  - Understanding the recombination dynamics of organic and perovskite solar cells has been a crucial prerequisite in the steadily increasing performance of these promising new types of photovoltaics. Surface recombination in particular has turned out to be one of the last remaining roadblocks, which specifically reduces the open circuit voltage. In this study, the relationship between the rate of surface recombination and the density of charge carriers is analyzed, revealing a cubic dependence between these two parameters. This hypothesis is then tested and verified with the recombination dynamics of an organic solar cell known to exhibit significant surface recombination and a high energy proton irradiated CH3NH3PbI3 pemvskite solar cell during white light illumination. Incidentally, these results can also explain recombination orders exceeding the commonly known threshold for bimolecular recombination that have been observed in some studies without the need for a charge carrier dependent bimolecular recombination coefficient.
KW  - surface recombination
KW  - recombination order
KW  - organic photovoltaics
KW  - Perovskite solar cells
KW  - charge carrier density
KW  - Shockley-Read-Hall
KW  - statistics
Y1  - 2020
U6  - https://doi.org/10.1016/j.orgel.2020.105905
SN  - 1566-1199
SN  - 1878-5530
VL  - 86
PB  - Elsevier Science
CY  - Amsterdam [u.a.]
ER  - 
TY  - JOUR
A1  - Holmes, Zoe
A1  - Anders, Janet
A1  - Mintert, Florian
T1  - Enhanced energy transfer to an optomechanical piston from indistinguishable photons
JF  - Physical review letters
N2  - Thought experiments involving gases and pistons, such as Maxwell's demon and Gibbs' mixing, are central to our understanding of thermodynamics. Here, we present a quantum thermodynamic thought experiment in which the energy transfer from two photonic gases to a piston membrane grows quadratically with the number of photons for indistinguishable gases, while it grows linearly for distinguishable gases. This signature of bosonic bunching may be observed in optomechanical experiments, highlighting the potential of these systems for the realization of thermodynamic thought experiments in the quantum realm.
Y1  - 2020
U6  - https://doi.org/10.1103/PhysRevLett.124.210601
SN  - 0031-9007
SN  - 1079-7114
VL  - 124
IS  - 21
PB  - American Physical Society
CY  - College Park, Md.
ER  - 
TY  - JOUR
A1  - Eberhard, Julius
A1  - Schaik, N. Loes M. B.
A1  - Schibalski, Anett
A1  - Gräff, Thomas
T1  - Simulating future salinity dynamics in a coastal marshland under different climate scenarios
JF  - Vadose zone journal
N2  - Salinization is a well-known problem in agricultural areas worldwide. In the last 20-30 yr, rising salinity in the upper, unconfined aquifer has been observed in the Freepsumer Meer, a grassland near the German North Sea coast. For investigating long-term development of salinity and water balance during 1961-2099, the one-dimensional Soil-Water-Atmosphere-Plant (SWAP) model was set up and calibrated for a soil column in the area. The model setup involves a deep aquifer as the source of salt through upward seepage. In the vertical salt transport equation, dispersion and advection are included. Six different regional outputs of statistical downscaling methods were used as climate scenarios. These comprise different rates of increasing surface temperature and different trends in seasonal rainfall. The simulation results exhibit opposing salinity trends for topsoil and deeper layers. Although projections of some scenarios entail decreasing salinities near the surface, most of them project a rise in subsoil salinity, with the strongest trends of up to +0.9 mg cm(-3) 100 yr(-1) at -65 cm. The results suggest that topsoil salinity trends in the study area are affected by the magnitude of winter rainfall trends, whereas high subsoil salinities correspond to low winter rainfall and high summer temperature. How these projected trends affect the vegetation and thereby future land use will depend on the future management of groundwater levels in the area.
Y1  - 2020
U6  - https://doi.org/10.1002/vzj2.20008
SN  - 1539-1663
VL  - 19
IS  - 1
PB  - Wiley
CY  - Hoboken
ER  - 
TY  - JOUR
A1  - Galka, Andreas
A1  - Moontaha, Sidratul
A1  - Siniatchkin, Michael
T1  - Constrained expectation maximisation algorithm for estimating ARMA models in state space representation
JF  - EURASIP journal on advances in signal processing
N2  - This paper discusses the fitting of linear state space models to given multivariate time series in the presence of constraints imposed on the four main parameter matrices of these models. Constraints arise partly from the assumption that the models have a block-diagonal structure, with each block corresponding to an ARMA process, that allows the reconstruction of independent source components from linear mixtures, and partly from the need to keep models identifiable. The first stage of parameter fitting is performed by the expectation maximisation (EM) algorithm. Due to the identifiability constraint, a subset of the diagonal elements of the dynamical noise covariance matrix needs to be constrained to fixed values (usually unity). For this kind of constraints, so far, no closed-form update rules were available. We present new update rules for this situation, both for updating the dynamical noise covariance matrix directly and for updating a matrix square-root of this matrix. The practical applicability of the proposed algorithm is demonstrated by a low-dimensional simulation example. The behaviour of the EM algorithm, as observed in this example, illustrates the well-known fact that in practical applications, the EM algorithm should be combined with a different algorithm for numerical optimisation, such as a quasi-Newton algorithm.
KW  - Kalman filtering
KW  - state space modelling
KW  - expectation maximisation algorithm
Y1  - 2020
U6  - https://doi.org/10.1186/s13634-020-00678-3
SN  - 1687-6180
VL  - 2020
IS  - 1
PB  - Springer
CY  - Heidelberg
ER  - 
TY  - JOUR
A1  - Xu, Pengbo
A1  - Deng, Weihua
A1  - Sandev, Trifce
T1  - Levy walk with parameter dependent velocity
BT  - hermite polynomial approach and numerical simulation
JF  - Journal of physics : A, Mathematical and theoretical
N2  - To analyze stochastic processes, one often uses integral transform (Fourier and Laplace) methods. However, for the time-space coupled cases, e.g. the Levy walk, sometimes the integral transform method may fail. Here we provide a Hermite polynomial expansion approach, being complementary to the integral transform method, to the Levy walk. Two approaches are compared for some already known results. We also consider the generalized Levy walk with parameter dependent velocity. Namely, we consider the Levy walk with velocity which depends on the walking length or on the duration of each step. Some interesting features of the generalized Levy walk are observed, including the special shapes of the probability density function, the first passage time distributions, and various diffusive behaviors of the mean squared displacement.
KW  - Hermite polynomial expansion
KW  - Levy walk
KW  - anomalous diffusion
KW  - parameter
KW  - dependent velocity
Y1  - 2020
U6  - https://doi.org/10.1088/1751-8121/ab7420
SN  - 1751-8113
SN  - 1751-8121
VL  - 53
IS  - 11
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Schneider, Sebastian
A1  - Hoffmann-Vogel, Regina
T1  - Electrostatic forces above graphene nanoribbons and edges interpreted as partly hydrogen-free
JF  - Nanoscale
N2  - Graphene nanoribbons' electronic transport properties strongly depend on the type of edge, armchair, zigzag or other, and on edge functionalization that can be used for band-gap engineering. For only partly hydrogenated edges interesting magnetic properties are predicted. Electric charge accumulates at edges and corners. Scanning force microscopy has so far shown the centre of graphene nanoribbons with atomic resolution using a quartz crystal tuning fork sensor of high stiffness. Weak long-range electrostatic forces related to the charge accumulation on the edges of graphene nanoribbons could not be imaged so far. Here, we show the electrostatic forces at the corners and edges of graphene nanoribbons are amenable to measurement. We use soft cantilevers and a bimodal imaging technique to combine enhanced sensitivity to weak long-range electrostatic forces with the high resolution of the second-frequency shift. Additionally, in our work the edges of the nanoribbons are mainly hydrogen-free, opening to the route to investigations of partly hydrogenated magnetic nanoribbons.
Y1  - 2020
U6  - https://doi.org/10.1039/d0nr03348k
SN  - 2040-3364
SN  - 2040-3372
VL  - 12
IS  - 34
SP  - 17895
EP  - 17901
PB  - Royal Society of Chemistry
CY  - Cambridge
ER  - 
TY  - JOUR
A1  - Schulz, Christian
A1  - Lieutenant, Klaus
A1  - Xiao, Jie
A1  - Hofmann, Tommy
A1  - Wong, Deniz
A1  - Habicht, Klaus
T1  - Characterization of the soft X-ray spectrometer PEAXIS at BESSY II
JF  - Journal of synchrotron radiation
N2  - The performance of the recently commissioned spectrometer PEAXIS for resonant inelastic soft X-ray scattering (RIXS) and X-ray photoelectron spectroscopy and its hosting beamline U41-PEAXIS at the BESSY II synchrotron are characterized. The beamline provides linearly polarized light from 180 eV to 1600 eV allowing for RIXS measurements in the range 200-1200 eV. The monochromator optics can be operated in different configurations to provide either high flux with up to 10(12) photons s(-1) within the focal spot at the sample or high energy resolution with a full width at half maximum of <40 meV at an incident photon energy of similar to 400 eV. The measured total energy resolution of the RIXS spectrometer is in very good agreement with theoretically predicted values obtained by ray-tracing simulations. PEAXIS features a 5 m-long RIXS spectrometer arm that can be continuously rotated about the sample position by 106 degrees within the horizontal photon scattering plane, thus enabling the study of momentum-transfer-dependent excitations. Selected scientific examples are presented to demonstrate the instrument capabilities, including measurements of excitations in single-crystalline NiO and in liquid acetone employing a fluid cell sample manipulator. Planned upgrades of the beamline and the RIXS spectrometer to further increase the energy resolution to similar to 100 meV at 1000 eV incident photon energy are discussed.
KW  - resonant inelastic X-ray scattering
KW  - X-ray photoelectron spectroscopy
KW  - soft X-ray spectroscopy
KW  - soft X-ray beamline
KW  - X-ray emission
KW  - X-ray
KW  - absorption
KW  - BESSY II
Y1  - 2020
U6  - https://doi.org/10.1107/S1600577519014887
SN  - 1600-5775
VL  - 27
SP  - 238
EP  - 249
PB  - International Union of Crystallography
CY  - Chester
ER  - 
TY  - JOUR
A1  - Yeste, Maria Pilar
A1  - Primus, Philipp-Alexander
A1  - Alcantara, Rodrigo
A1  - Cauqui, Miguel Angel
A1  - Calvino, Juan Jose
A1  - Pintado, José María
A1  - Blanco, Ginesa
T1  - Surface characterization of two Ce0.62Zr0.38O2 mixed oxides with different reducibility
JF  - Applied surface science : a journal devoted to applied physics and chemistry of surfaces and interfaces
N2  - This paper presents a study of the surface properties of two Ce/Zr mixed oxides with different reducibility, obtained by applying distinct thermal ageing treatments to an oxide with the composition Ce0.62Zr0.38O2. The surface composition was investigated by XPS. Chemical reactivity of the surface was studied by adsorption of the probe molecules CO2, D-2 and methanol. Nanostructural characterization was carried out by XRD, Raman and high-resolution Eu3+ spectroscopy (FLNS). The characterization showed only slight variations in surface composition and bulk Ce-Zr distribution, but hardy differences concerning the type and strength of acidic surface centres, as well as strong differences in the ability to dissociate hydrogen. Structural variations between both samples were identified by comparing the optical spectra of Eu3+ in surface doped samples.
KW  - Ce/Zr
KW  - Surface properties
KW  - Reactive adsorption
KW  - Hydrogen activation
Y1  - 2020
U6  - https://doi.org/10.1016/j.apsusc.2019.144255
SN  - 0169-4332
SN  - 1873-5584
VL  - 503
PB  - Elsevier
CY  - Amsterdam
ER  - 
TY  - JOUR
A1  - Sandev, Trifce
A1  - Iomin, Alexander
A1  - Kocarev, Ljupco
T1  - Hitting times in turbulent diffusion due to multiplicative noise
JF  - Physical review : E, Statistical, nonlinear and soft matter physics
N2  - We study a distribution of times of the first arrivals to absorbing targets in turbulent diffusion, which is due to a multiplicative noise. Two examples of dynamical systems with a multiplicative noise are studied. The first one is a random process according to inhomogeneous diffusion, which is also known as a geometric Brownian motion in the Black-Scholes model. The second model is due to a random processes on a two-dimensional comb, where inhomogeneous advection is possible only along the backbone, while Brownian diffusion takes place inside the branches. It is shown that in both cases turbulent diffusion takes place as the one-dimensional random process with the log-normal distribution in the presence of absorbing targets, which are characterized by the Levy-Smirnov distribution for the first hitting times.
Y1  - 2020
U6  - https://doi.org/10.1103/PhysRevE.102.042109
SN  - 2470-0045
SN  - 2470-0053
VL  - 102
IS  - 4
PB  - American Institute of Physics
CY  - Woodbury, NY
ER  - 
TY  - JOUR
A1  - Massolt, Joost  Willem
A1  - Borowski, Andreas
T1  - Perceived relevance of university physics problems by pre-service physics teachers
BT  - personal constructs
JF  - International journal of science education
N2  - Pre-service physics teachers often do not recognise the relevance for their future career in their university content knowledge courses. A lower perceived relevance can, however, have a negative effect on their motivation and on their academic success. Several intervention studies have been undertaken with the goal to increase this perceived relevance. A previous study shows that conceptual physics problems used in university physics courses are perceived by pre-service physics teachers as more relevant for their future career than regular, quantitative problems. It is however not clear, what the students' meaning of the construct 'relevance' is: what makes a problem more relevant to them than another problem? To answer this question, N = 7 pre-service teachers were interviewed using the repertory grid technique, based on the personal construct theory. Nine physics problems were discussed with regards to their perceived relevance and with regards to problem properties that distinguish these problems from each other. We are able to identify six problem properties that have a positive influence on the perceived relevance. Physics problems that are based on these properties should therefore potentially have a higher perceived relevance, which can have a positive effect on the motivation of the pre-service teachers who solve these problems.
KW  - Motivation
KW  - physics education
KW  - pre-service teachers
KW  - repertory grid
Y1  - 2020
U6  - https://doi.org/10.1080/09500693.2019.1705424
SN  - 0950-0693
SN  - 1464-5289
VL  - 42
IS  - 2
SP  - 167
EP  - 189
PB  - Routledge, Taylor & Francis Group
CY  - Abingdon
ER  - 
TY  - JOUR
A1  - Evsevleev, Sergei
A1  - Paciornik, Sidnei
A1  - Bruno, Giovanni
T1  - Advanced deep learning-based 3D microstructural characterization of multiphase metal matrix composites
JF  - Advanced engineering materials
N2  - The quantitative analysis of microstructural features is a key to understanding the micromechanical behavior of metal matrix composites (MMCs), which is a premise for their use in practice. Herein, a 3D microstructural characterization of a five-phase MMC is performed by synchrotron X-ray computed tomography (SXCT). A workflow for advanced deep learning-based segmentation of all individual phases in SXCT data is shown using a fully convolutional neural network with U-net architecture. High segmentation accuracy is achieved with a small amount of training data. This enables extracting unprecedently precise microstructural parameters (e.g., volume fractions and particle shapes) to be input, e.g., in micromechanical models.
KW  - computed tomography
KW  - convolutional neural networks
KW  - deep learning
KW  - metal
KW  - matrix composites
KW  - segmentations
Y1  - 2020
U6  - https://doi.org/10.1002/adem.201901197
SN  - 1438-1656
SN  - 1527-2648
VL  - 22
IS  - 4
PB  - Wiley-VCH
CY  - Weinheim
ER  - 
TY  - JOUR
A1  - Mishurova, Tatiana
A1  - Bruno, Giovanni
A1  - Evsevleev, Sergei
A1  - Sevostianov, Igor
T1  - Determination of macroscopic stress from diffraction experiments
BT  - a critical discussion
JF  - Journal of applied physics
N2  - The paper is motivated by some inconsistencies and contradictions present in the literature on the calculation of the so-called diffraction elastic constants. In an attempt at unifying the views that the two communities of Materials Science and Mechanics of Materials have on the subject, we revisit and define the terminology used in the field. We also clarify the limitations of the commonly used approaches and show that a unified methodology is also applicable to textured materials with a nearly arbitrary grain shape. We finally compare the predictions based on this methodology with experimental data obtained by in situ synchrotron radiation diffraction on additively manufactured Ti-6Al-4V alloy. We show that (a) the transverse isotropy of the material yields good agreement between the best-fit isotropy approximation (equivalent to the classic Kroner's model) and the experimental data and (b) the use of a general framework allows the calculation of all components of the tensor of diffraction elastic constants, which are not easily measurable by diffraction methods. This allows us to extend the current state-of-the-art with a predictive tool.
Y1  - 2020
U6  - https://doi.org/10.1063/5.0009101
SN  - 0021-8979
SN  - 1089-7550
VL  - 128
IS  - 2
PB  - American Institute of Physics
CY  - Melville
ER  - 
TY  - JOUR
A1  - Makwana, Kirit D.
A1  - Yan, Huirong
T1  - Properties of magnetohydrodynamic modes in compressively driven plasma turbulence
JF  - Physical Review X
N2  - We study properties of magnetohydrodynamic (MHD) eigenmodes by decomposing the data of MHD simulations into linear MHD modes-namely, the Alfven, slow magnetosonic, and fast magnetosonic modes. We drive turbulence with a mixture of solenoidal and compressive driving while varying the Alfven Mach number (M-A), plasma beta, and the sonic Mach number from subsonic to transsonic. We find that the proportion of fast and slow modes in the mode mixture increases with increasing compressive forcing. This proportion of the magnetosonic modes can also become the dominant fraction in the mode mixture. The anisotropy of the modes is analyzed by means of their structure functions. The Alfven-mode anisotropy is consistent with the Goldreich-Sridhar theory. We find a transition from weak to strong Alfvenic turbulence as we go from low to high M-A. The slow-mode properties are similar to the Alfven mode. On the other hand, the isotropic nature of fast modes is verified in the cases where the fast mode is a significant fraction of the mode mixture. The fast-mode behavior does not show any transition in going from low to high M-A. We find indications that there is some interaction between the different modes, and the properties of the dominant mode can affect the properties of the weaker modes. This work identifies the conditions under which magnetosonic modes can be a major fraction of turbulent astrophysical plasmas, including the regime of weak turbulence. Important astrophysical implications for cosmic-ray transport and magnetic reconnection are discussed.
KW  - Astrophysics
KW  - Plasma Physics
Y1  - 2020
U6  - https://doi.org/10.1103/PhysRevX.10.031021
SN  - 2160-3308
VL  - 10
IS  - 3
PB  - American Physical Society (APS)
CY  - College Park
ER  - 
TY  - JOUR
A1  - Chen, Cong
A1  - Müller, Bernd R.
A1  - Prinz, Carsten
A1  - Stroh, Julia
A1  - Feldmann, Ines
A1  - Bruno, Giovanni
T1  - The correlation between porosity characteristics and the crystallographic texture in extruded stabilized aluminium titanate for diesel particulate filter applications
JF  - Journal of the European Ceramic Society
N2  - Porous ceramic diesel particulate filters (DPFs) are extruded products that possess macroscopic anisotropic mechanical and thermal properties. This anisotropy is caused by both morphological features (mostly the orientation of porosity) and crystallographic texture. We systematically studied those two aspects in two aluminum titanate ceramic materials of different porosity using mercury porosimetry, gas adsorption, electron microscopy, X-ray diffraction, and X-ray refraction radiography. We found that a lower porosity content implies a larger isotropy of both the crystal texture and the porosity orientation. We also found that, analogous to cordierite, crystallites do align with their axis of negative thermal expansion along the extrusion direction. However, unlike what found for cordierite, the aluminium titanate crystallite form is such that a more pronounced (0 0 2) texture along the extrusion direction implies porosity aligned perpendicular to it.
KW  - preferred orientation
KW  - X-ray refraction
KW  - pore orientation
KW  - crystal
KW  - structure
KW  - extrusion
KW  - microstructure-property relations
Y1  - 2020
U6  - https://doi.org/10.1016/j.jeurceramsoc.2019.11.076
SN  - 0955-2219
SN  - 1873-619X
VL  - 40
IS  - 4
SP  - 1592
EP  - 1601
PB  - Elsevier
CY  - Oxford
ER  - 
TY  - JOUR
A1  - Bastian, Philipp U.
A1  - Yu, Leixiao
A1  - de Guereñu Kurganova, Anna Lopez
A1  - Haag, Rainer
A1  - Kumke, Michael Uwe
T1  - Bioinspired confinement of upconversion nanoparticles for improved performance in aqueous solution
JF  - The journal of physical chemistry : C, Nanomaterials and interfaces
N2  - The resonance energy transfer (RET) from NaYF4:Yb,Er upconverting nanoparticles (UNCPs) to a dye (5-carboxytetramethylrhodamine (TAMRA)) was investigated by photoluminescence experiments and microscale thermophoresis (MST). The dye was excited via RET from the UCNPs which was excited in the near-infrared (NIR). The change of the dye diffusion speed (free vs coupled) was investigated by MST. RET shows significant changes in the decay times of the dye as well as of the UCNPs. MST reveals significant changes in the diffusion speed. A unique amphiphilic coating polymer (customized mussel protein (CMP) polymer) for UCNP surface coating was used, which mimics blood protein adsorption and mussel food protein adhesion to transfer the UCNP into the aqueous phase and to allow surface functionalization. The CMP provides very good water dispersibility to the UCNPs and minimizes ligand exchange and subsequent UCNP aging reactions because of the interlinkage of the CMP on the UCNP surface. Moreover, CMP provides N-3-functional groups for dick chemistry-based functionalization demonstrated with the dye 5-carboxytetramethylrhodamine (TAMRA). This establishes the principle coupling scheme for suitable biomarkers such as antibodies. The CMP provides very stable aqueous UCNP dispersions that are storable up to 3 years in a fridge at 5 degrees C without dissolution or coagulation. The outstanding properties of CMP in shielding the UCNP from unwanted solvent effects is reflected in the distinct increase of the photoluminescence decay times after UCNP functionalization. The UCNP-to-TAMRA energy transfer is also spectroscopically investigated at low temperatures (4-200 K), revealing that one of the two green Er(III) emission bands contributes the major part to the energy transfer. The TAMRA fluorescence decay time increases by a factor of 9500 from 2.28 ns up to 22 mu s due to radiationless energy transfer from the UCNP after NIR excitation of the latter. This underlines the unique properties of CMP as a versatile capping ligand for distinctly improving the UCNPs' performance in aqueous solutions, for coupling of biomolecules, and for applications for in vitro and in vivo experiments using UCNPs as optical probes in life science applications.
Y1  - 2020
U6  - https://doi.org/10.1021/acs.jpcc.0c09798
SN  - 1932-7447
SN  - 1932-7455
VL  - 124
IS  - 52
SP  - 28623
EP  - 28635
PB  - American Chemical Society
CY  - Washington, DC
ER  - 
TY  - JOUR
A1  - Fernandez, Ricardo
A1  - Bruno, Giovanni
A1  - Garces, Gerardo
A1  - Nieto-Luis, H.
A1  - Gonzalez-Doncel, Gaspar
T1  - Fractional brownian motion of dislocations during creep deformation of metals
JF  - Materials science & engineering. A, Structural materials
N2  - The present work offers an explanation on how the long-range interaction of dislocations influences their movement, and therefore the strain, during creep of metals. It is proposed that collective motion of dislocations can be described as a fractional Brownian motion. This explains the noisy appearance of the creep strain signal as a function of time. Such signal is split into a deterministic and a stochastic part. These terms can be related to two kinds of dislocation motions: individual and collective, respectively. The description is consistent with the fractal nature of strain-induced dislocation structures predicated in previous works. Moreover, it encompasses the evolution of the strain rate during all stages of creep, including the tertiary one. Creep data from Al99.8% and Al3.85%Mg tested at different temperatures and stresses are used to validate the proposed ideas: it is found that different creep stages present different diffusion characters, and therefore different dislocation motion character.
KW  - Creep
KW  - Aluminum alloys
KW  - Dislocation motion
KW  - Diffusion
KW  - Fractal
KW  - structures
Y1  - 2020
U6  - https://doi.org/10.1016/j.msea.2020.140013
SN  - 0921-5093
SN  - 1873-4936
VL  - 796
PB  - Elsevier
CY  - Lausanne
ER  - 
TY  - JOUR
A1  - Laquai, Rene
A1  - Müller, Bernd R.
A1  - Schneider, Judith Ann
A1  - Kupsch, Andreas
A1  - Bruno, Giovanni
T1  - Using SXRR to probe the nature of discontinuities in SLM additive manufactured inconel 718 specimens
JF  - Metallurgical and Materials Transactions A
N2  - The utilization of additive manufacturing (AM) to fabricate robust structural components relies on understanding the nature of internal anomalies or discontinuities, which can compromise the structural integrity. While some discontinuities in AM microstructures stem from similar mechanisms as observed in more traditional processes such as casting, others are unique to the AM process. Discontinuities in AM are challenging to detect, due to their submicron size and orientation dependency. Toward the goal of improving structural integrity, minimizing discontinuities in an AM build requires an understanding of the mechanisms of formation to mitigate their occurrence. This study utilizes various techniques to evaluate the shape, size, nature and distribution of discontinuities in AM Inconel 718, in a non-hot isostatic pressed (HIPed) as-built, non-HIPed and direct age, and HIPed with two step age samples. Non-destructive synchrotron radiation refraction and transmission radiography (SXRR) provides additional information beyond that obtained with destructive optical microscopy. SXRR was able to distinguish between voids, cracks and lack of melt in, due to its sensitivity to the orientation of the discontinuity.
Y1  - 2020
U6  - https://doi.org/10.1007/s11661-020-05847-5
SN  - 1073-5623
SN  - 1543-1940
VL  - 51
IS  - 8
SP  - 4146
EP  - 4157
PB  - Springer
CY  - New York
ER  - 
TY  - JOUR
A1  - Fernandez, Ricardo
A1  - Gonzalez-Doncel, Gaspar
A1  - Garces, Gerardo
A1  - Bruno, Giovanni
T1  - Towards a comprehensive understanding of creep
BT  - microstructural dependence of the pre-exponential term in Al
JF  - Materials science & engineering. A, Structural materials: properties, microstructure and processing
N2  - We show that the equation proposed by Takeuchi and Argon to explain the creep behavior of Al-Mg solid solution can be used to describe also the creep behavior of pure aluminum. In this frame, it is possible to avoid the use of the classic pre-exponential fitting parameter in the power law equation to predict the minimum creep strain rate. The effect of the fractal arrangement of dislocations, developed at the mesoscale, must be considered to fully explain the experimental data. These ideas allow improving the recently introduced SSTC model, fully describing the primary and secondary creep regimes of aluminum alloys without the need for fitting. Creep data from commercially pure A199.8% and Al-Mg alloys tested at different temperatures and stresses are used to validate the proposed ideas.
KW  - creep
KW  - Aluminum alloys
KW  - dislocations
KW  - fractal
KW  - stress exponent
Y1  - 2020
U6  - https://doi.org/10.1016/j.msea.2020.139036
SN  - 0921-5093
SN  - 1873-4936
VL  - 776
PB  - Elsevier
CY  - Lausanne
ER  - 
TY  - JOUR
A1  - Mishurova, Tatiana
A1  - Sydow, Benjamin
A1  - Thiede, Tobias
A1  - Sizova, Irina
A1  - Ulbricht, Alexander
A1  - Bambach, Markus
A1  - Bruno, Giovanni
T1  - Residual stress and microstructure of a Ti-6Al-4V Wire Arc Additive Manufacturing hybrid demonstrator
JF  - Metals
N2  - Wire Arc Additive Manufacturing (WAAM) features high deposition rates and, thus, allows production of large components that are relevant for aerospace applications. However, a lot of aerospace parts are currently produced by forging or machining alone to ensure fast production and to obtain good mechanical properties; the use of these conventional process routes causes high tooling and material costs. A hybrid approach (a combination of forging and WAAM) allows making production more efficient. In this fashion, further structural or functional features can be built in any direction without using additional tools for every part. By using a combination of forging basic geometries with one tool set and adding the functional features by means of WAAM, the tool costs and material waste can be reduced compared to either completely forged or machined parts. One of the factors influencing the structural integrity of additively manufactured parts are (high) residual stresses, generated during the build process. In this study, the triaxial residual stress profiles in a hybrid WAAM part are reported, as determined by neutron diffraction. The analysis is complemented by microstructural investigations, showing a gradient of microstructure (shape and size of grains) along the part height. The highest residual stresses were found in the transition zone (between WAAM and forged part). The total stress range showed to be lower than expected for WAAM components. This could be explained by the thermal history of the component.
KW  - residual stress
KW  - WAAM
KW  - Ti-6Al-4V
KW  - additive manufacturing
KW  - neutron
KW  - diffraction
KW  - hybrid manufacturing
Y1  - 2020
U6  - https://doi.org/10.3390/met10060701
SN  - 2075-4701
VL  - 10
IS  - 6
PB  - MDPI
CY  - Basel
ER  - 
TY  - JOUR
A1  - Magkos, Sotirios
A1  - Kupsch, Andreas
A1  - Bruno, Giovanni
T1  - Direct iterative reconstruction of computed tomography trajectories reconstruction from limited number of projections with DIRECTT
JF  - Review of scientific instruments : a monthly journal devoted to scientific instruments, apparatus, and techniques
N2  - X-ray computed tomography has many applications in materials science and non-destructive testing. While the standard filtered back-projection reconstruction of the radiographic datasets is fast and simple, it typically fails in returning accurate results from missing or inconsistent projections. Among the alternative techniques that have been proposed to handle such data is the Direct Iterative REconstruction of Computed Tomography Trajectories (DIRECTT) algorithm. We describe a new approach to the algorithm, which significantly decreases the computational time while achieving a better reconstruction quality than that of other established algorithms.
Y1  - 2020
U6  - https://doi.org/10.1063/5.0013111
SN  - 0034-6748
SN  - 1089-7623
SN  - 1527-2400
VL  - 91
IS  - 10
PB  - American Institute of Physics
CY  - Melville
ER  - 
TY  - JOUR
A1  - Rüdiger, Günther
A1  - Küker, Manfred
A1  - Käpylä, Petri J.
T1  - Electrodynamics of turbulent fluids with fluctuating electric conductivity
JF  - Journal of plasma physics
N2  - Consequences of fluctuating microscopic conductivity in mean-field electrodynamics of turbulent fluids are formulated and discussed. If the conductivity fluctuations are assumed to be uncorrelated with the velocity fluctuations then only the turbulence-originated magnetic diffusivity of the fluid is reduced and the decay time of a large-scale magnetic field or the cycle times of oscillating turbulent dynamo models are increased. If, however, the fluctuations of conductivity and flow in a certain well-defined direction are correlated, an additional diamagnetic pumping effect results, transporting the magnetic field in the opposite direction to the diffusivity flux vector <eta'u'>. In the presence of global rotation, even for homogeneous turbulence fields, an alpha effect appears. If the characteristic values of the outer core of the Earth or the solar convection zone are applied, the dynamo number of the new alpha effect does not reach supercritical values to operate as an alpha(2)-dynamo but oscillating alpha Omega-dynamos with differential rotation are not excluded.
KW  - astrophysical plasmas
KW  - plasma flows
Y1  - 2020
U6  - https://doi.org/10.1017/S0022377820000665
SN  - 0022-3778
SN  - 1469-7807
VL  - 86
IS  - 3
PB  - Cambridge Univ. Press
CY  - London
ER  - 
TY  - JOUR
A1  - Xu, Yong
A1  - Liu, Xuemei
A1  - Li, Yongge
A1  - Metzler, Ralf
T1  - Heterogeneous diffusion processes and nonergodicity with Gaussian colored noise in layered diffusivity landscapes
JF  - Physical review : E, Statistical, nonlinear and soft matter physics
N2  - Heterogeneous diffusion processes (HDPs) with space-dependent diffusion coefficients D(x) are found in a number of real-world systems, such as for diffusion of macromolecules or submicron tracers in biological cells. Here, we examine HDPs in quenched-disorder systems with Gaussian colored noise (GCN) characterized by a diffusion coefficient with a power-law dependence on the particle position and with a spatially random scaling exponent. Typically, D(x) is considered to be centerd at the origin and the entire x axis is characterized by a single scaling exponent a. In this work we consider a spatially random scenario: in periodic intervals ("layers") in space D(x) is centerd to the midpoint of each interval. In each interval the scaling exponent alpha is randomly chosen from a Gaussian distribution. The effects of the variation of the scaling exponents, the periodicity of the domains ("layer thickness") of the diffusion coefficient in this stratified system, and the correlation time of the GCN are analyzed numerically in detail. We discuss the regimes of superdiffusion, subdiffusion, and normal diffusion realisable in this system. We observe and quantify the domains where nonergodic and non-Gaussian behaviors emerge in this system. Our results provide new insights into the understanding of weak ergodicity breaking for HDPs driven by colored noise, with potential applications in quenched layered systems, typical model systems for diffusion in biological cells and tissues, as well as for diffusion in geophysical systems.
Y1  - 2020
U6  - https://doi.org/10.1103/PhysRevE.102.062106
SN  - 2470-0045
SN  - 2470-0053
VL  - 102
IS  - 6
PB  - American Physical Society
CY  - College Park
ER  - 
TY  - JOUR
A1  - Keles, Engin
A1  - Kitzmann, Daniel
A1  - Mallonn, Matthias
A1  - Alexoudi, Xanthippi
A1  - Fossati, Luca
A1  - Pino, Lorenzo
A1  - Seidel, Julia Victoria
A1  - Caroll, Thorsten A.
A1  - Steffen, M.
A1  - Ilyin, Ilya
A1  - Poppenhäger, Katja
A1  - Strassmeier, Klaus G.
A1  - von Essen, Carolina
A1  - Nascimbeni, Valerio
A1  - Turner, Jake D.
T1  - Probing the atmosphere of HD189733b with the Na i and K i lines
JF  - Monthly Notices of the Royal Astronomical Society
N2  - High spectral resolution transmission spectroscopy is a powerful tool to characterize exoplanet atmospheres. Especially for hot Jupiters, this technique is highly relevant, due to their high-altitude absorption, e.g. from resonant sodium (Na i) and potassium (K i) lines. We resolve the atmospheric K i absorption on HD189733b with the aim to compare the resolved K i line and previously obtained high-resolution Na i-D line observations with synthetic transmission spectra. The line profiles suggest atmospheric processes leading to a line broadening of the order of ∼10 km/s for the Na i-D lines and only a few km/s for the K i line. The investigation hints that either the atmosphere of HD189733b lacks a significant amount of K i or the alkali lines probe different atmospheric regions with different temperature, which could explain the differences we see in the resolved absorption lines.
Y1  - 2020
U6  - https://doi.org/10.1093/mnras/staa2435
VL  - 498
IS  - 1
SP  - 1033
PB  - Oxford Univ. Press
CY  - Oxford
ER  - 
TY  - JOUR
A1  - Capała, Karol
A1  - Padash, Amin
A1  - Chechkin, Aleksei V.
A1  - Shokri, Babak
A1  - Metzler, Ralf
A1  - Dybiec, Bartłomiej
T1  - Levy noise-driven escape from arctangent potential wells
JF  - Chaos : an interdisciplinary journal of nonlinear science
N2  - The escape from a potential well is an archetypal problem in the study of stochastic dynamical systems, representing real-world situations from chemical reactions to leaving an established home range in movement ecology. Concurrently, Levy noise is a well-established approach to model systems characterized by statistical outliers and diverging higher order moments, ranging from gene expression control to the movement patterns of animals and humans. Here, we study the problem of Levy noise-driven escape from an almost rectangular, arctangent potential well restricted by two absorbing boundaries, mostly under the action of the Cauchy noise. We unveil analogies of the observed transient dynamics to the general properties of stationary states of Levy processes in single-well potentials. The first-escape dynamics is shown to exhibit exponential tails. We examine the dependence of the escape on the shape parameters, steepness, and height of the arctangent potential. Finally, we explore in detail the behavior of the probability densities of the first-escape time and the last-hitting point.
Y1  - 2020
U6  - https://doi.org/10.1063/5.0021795
SN  - 1054-1500
SN  - 1089-7682
VL  - 30
IS  - 12
PB  - American Institute of Physics
CY  - Woodbury, NY
ER  - 
TY  - JOUR
A1  - Li, Yongge
A1  - Mei, Ruoxing
A1  - Xu, Yong
A1  - Kurths, Jürgen
A1  - Duan, Jinqiao
A1  - Metzler, Ralf
T1  - Particle dynamics and transport enhancement in a confined channel with position-dependent diffusivity
JF  - New Journal of Physics
N2  - This work focuses on the dynamics of particles in a confined geometry with position-dependent diffusivity, where the confinement is modelled by a periodic channel consisting of unit cells connected by narrow passage ways. We consider three functional forms for the diffusivity, corresponding to the scenarios of a constant (D ₀), as well as a low (D ₘ) and a high (D d) mobility diffusion in cell centre of the longitudinally symmetric cells. Due to the interaction among the diffusivity, channel shape and external force, the system exhibits complex and interesting phenomena. By calculating the probability density function, mean velocity and mean first exit time with the Itô calculus form, we find that in the absence of external forces the diffusivity D d will redistribute particles near the channel wall, while the diffusivity D ₘ will trap them near the cell centre. The superposition of external forces will break their static distributions. Besides, our results demonstrate that for the diffusivity D d, a high dependence on the x coordinate (parallel with the central channel line) will improve the mean velocity of the particles. In contrast, for the diffusivity D ₘ, a weak dependence on the x coordinate will dramatically accelerate the moving speed. In addition, it shows that a large external force can weaken the influences of different diffusivities; inversely, for a small external force, the types of diffusivity affect significantly the particle dynamics. In practice, one can apply these results to achieve a prominent enhancement of the particle transport in two- or three-dimensional channels by modulating the local tracer diffusivity via an engineered gel of varying porosity or by adding a cold tube to cool down the diffusivity along the central line, which may be a relevant effect in engineering applications. Effects of different stochastic calculi in the evaluation of the underlying multiplicative stochastic equation for different physical scenarios are discussed.
KW  - diffusion
KW  - channel
KW  - space-dependent diffusivity
Y1  - 2020
U6  - https://doi.org/10.1088/1367-2630/ab81b9
SN  - 1367-2630
VL  - 22
PB  - Dt. Physikalische Ges.
CY  - Bad Honnef
ER  - 
TY  - JOUR
A1  - Mkrtchian, Vanik E.
A1  - Henkel, Carsten
T1  - Green function solution of generalised boundary value problems
JF  - Physics Letters A
N2  - We construct an expression for the Green function of a differential operator satisfying nonlocal, homogeneous boundary conditions starting from the fundamental solution of the differential operator. This also provides the solution to the boundary value problem of an inhomogeneous partial differential equation with inhomogeneous, nonlocal boundary conditions. The construction applies for a broad class of linear partial differential equations and linear boundary conditions.
KW  - Boundary value problem
KW  - Green function
Y1  - 2020
U6  - https://doi.org/10.1016/j.physleta.2020.126573
SN  - 0375-9601
SN  - 1873-2429
SN  - 0031-9163
VL  - 384
IS  - 23
PB  - Elsevier
CY  - Amsterdam
ER  - 
TY  - JOUR
A1  - Henkel, Carsten
T1  - Thermally excited quasiparticles in metals, dispersion forces, and the thermal anomaly
JF  - Modern physics letters : A, Particles and fields, gravitation, cosmology, nuclear physics
N2  - We provide a brief review of the contribution of thermally excited carriers to dispersion forces. In a metal, these carriers generate charge and current fluctuations whose spectral frequencies are comparable to k(B)T/(h) over bar. They are very likely responsible for the "plasma vs. Drude" anomaly.
KW  - Dispersion force
KW  - quasi-particles
Y1  - 2020
U6  - https://doi.org/10.1142/S021773232040009X
SN  - 0217-7323
SN  - 1793-6632
VL  - 35
IS  - 3
PB  - World Scientific
CY  - Singapore
ER  - 
TY  - JOUR
A1  - Yochelis, Arik
A1  - Beta, Carsten
A1  - Gov, Nir S.
T1  - Excitable solitons
BT  - annihilation, crossover, and nucleation of pulses in mass-conserving activator-inhibitor media
JF  - Physical review : E, Statistical, nonlinear and soft matter physics
N2  - Excitable pulses are among the most widespread dynamical patterns that occur in many different systems, ranging from biological cells to chemical reactions and ecological populations. Traditionally, the mutual annihilation of two colliding pulses is regarded as their prototypical signature. Here we show that colliding excitable pulses may exhibit solitonlike crossover and pulse nucleation if the system obeys a mass conservation constraint. In contrast to previous observations in systems without mass conservation, these alternative collision scenarios are robustly observed over a wide range of parameters. We demonstrate our findings using a model of intracellular actin waves since, on time scales of wave propagations over the cell scale, cells obey conservation of actin monomers. The results provide a key concept to understand the ubiquitous occurrence of actin waves in cells, suggesting why they are so common, and why their dynamics is robust and long-lived.
Y1  - 2020
U6  - https://doi.org/10.1103/PhysRevE.101.022213
SN  - 2470-0045
SN  - 2470-0053
VL  - 101
IS  - 2
PB  - American Physical Society
CY  - Melville, NY
ER  - 
TY  - JOUR
A1  - Weimar, Jannis
A1  - Köhli, Markus
A1  - Budach, Christian
A1  - Schmidt, Ulrich
T1  - Large-scale boron-lined neutron detection systems as a 3He alternative for Cosmic Ray Neutron Sensing
JF  - Frontiers in water
N2  - Cosmic-Ray neutron sensors are widely used to determine soil moisture on the hectare scale. Precise measurements, especially in the case of mobile application, demand for neutron detectors with high counting rates and high signal-to-noise ratios. For a long time Cosmic Ray Neutron Sensing (CRNS) instruments have relied on He-3 as an efficient neutron converter. Its ongoing scarcity demands for technological solutions using alternative converters, which are Li-6 and B-10. Recent developments lead to a modular neutron detector consisting of several B-10-lined proportional counter tubes, which feature high counting rates via its large surface area. The modularity allows for individual shieldings of different segments within the detector featuring the capability of gaining spectral information about the detected neutrons. This opens the possibility for active signal correction, especially useful when applied to mobile measurements, where the influence of constantly changing near-field to the overall signal should be corrected. Furthermore, the signal-to-noise ratio could be increased by combining pulse height and pulse length spectra to discriminate between neutrons and other environmental radiation. This novel detector therefore combines high-selective counting electronics with large-scale instrumentation technology.
KW  - CRNS
KW  - neutron
KW  - detector
KW  - soil moisture
KW  - readout electronics
KW  - boron-10
KW  - helium-3 alternative
Y1  - 2020
U6  - https://doi.org/10.3389/frwa.2020.00016
SN  - 2624-9375
VL  - 2
PB  - Frontiers Media
CY  - Lausanne
ER  - 
TY  - JOUR
A1  - Petreska, Irina
A1  - de Castro, Antonio S. M.
A1  - Sandev, Trifce
A1  - Lenzi, Ervin K.
T1  - The time-dependent Schrödinger equation in non-integer dimensions for constrained quantum motion
JF  - Modern physics letters : A, Particles and fields, gravitation, cosmology, nuclear physics
N2  - We propose a theoretical model, based on a generalized Schroedinger equation, to study the behavior of a constrained quantum system in non-integer, lower than two-dimensional space. The non-integer dimensional space is formed as a product space X x Y, comprising x-coordinate with a Hausdorff measure of dimension alpha(1) = D -1 (1 < D < 2) and y-coordinate with the Lebesgue measure of dimension of length (alpha(2) = 1). Geometric constraints are set at y = 0. Two different approaches to find the Green's function are employed, both giving the same form in terms of the Fox H-function. For D = 2, the solution for two-dimensional quantum motion on a comb is recovered. (C) 2020 Elsevier B.V. All rights reserved.
KW  - Schrödinger equation
KW  - non-integer dimension
KW  - Green's function
KW  - Bessel functions
KW  - Fox H-function
Y1  - 2020
U6  - https://doi.org/10.1016/j.physleta.2020.126866
SN  - 0375-9601
SN  - 1873-2429
VL  - 384
IS  - 34
PB  - Elsevier
CY  - Amsterdam
ER  - 
TY  - JOUR
A1  - Mazzio, Katherine A.
A1  - Kojda, Sandrino Danny
A1  - Rubio-Govea, Rodrigo
A1  - Niederhausen, Jens
A1  - Ryll, Britta
A1  - Raja-Thulasimani, Monika
A1  - Habicht, Klaus
A1  - Raoux, Simone
T1  - P-type-to-n-type transition in hybrid AgxTe/PEDOT:PSS thermoelectric materials via stoichiometric control during solution-based synthesis
JF  - ACS applied energy materials
N2  - This work demonstrates the production of high-performing p- type and n-type hybrid AgxTe/poly(3,4-ethylenedioxythiopene):polystyrene sulfonic acid (PE-DOT:PSS) thermoelectric materials from the same Te/PEDOT:PSS parent structure during aqueous-based synthesis. All samples were solution-processed and analyzed in thin- film architectures. We were able to demonstrate a power factor of 44 mu W m(-1) K-2 for our highest-performing n-type material. In addition, we were also able to realize a 68% improvement in the power factor of our p-type compositions relative to the parent structure through manipulation of the inorganic nanostructure composition. We demonstrate control over the thermoelectric properties by varying the stoichiometry of AgxTe nanoparticles in AgxTe/PEDOT:PSS hybrid materials via a topotactic chemical transformation process at room temperature. This process offers a simple, low-temperature, and cost-effective route toward the production of both efficient n-type and p-type hybrid thermoelectric materials.
KW  - thermoelectrics
KW  - hybrid material
KW  - PEDOT:PSS
KW  - tellurium
KW  - silver
KW  - telluride
KW  - hybrid synthesis
Y1  - 2020
U6  - https://doi.org/10.1021/acsaem.0c01774
SN  - 2574-0962
VL  - 3
IS  - 11
SP  - 10734
EP  - 10743
PB  - ACS Publications
CY  - Washington, DC
ER  - 
TY  - JOUR
A1  - Crovetto, Andrea
A1  - Hempel, Hannes
A1  - Rusu, Marin
A1  - Choubrac, Leo
A1  - Kojda, Sandrino Danny
A1  - Habicht, Klaus
A1  - Unold, Thomas
T1  - Water adsorption enhances electrical conductivity in transparent p-type CuI
JF  - ACS applied materials & interfaces
N2  - CuI has been recently rediscovered as a p-type transparent conductor with a high figure of merit. Even though many metal iodides are hygroscopic, the effect of moisture on the electrical properties of CuI has not been clarified. In this work, we observe a 2-fold increase in the conductivity of CuI after exposure to ambient humidity for 5 h, followed by slight long-term degradation. Simultaneously, the work function of CuI decreases by almost 1 eV, which can explain the large spread in the previously reported work function values. The conductivity increase is partially reversible and is maximized at intermediate humidity levels. On the basis of the large intragrain mobility measured by THz spectroscopy, we suggest that hydration of grain boundaries may be beneficial for the overall hole mobility.
KW  - transparent conductors
KW  - CuI
KW  - copper iodide
KW  - conductivity
KW  - humidity
KW  - p-type
KW  - work function
Y1  - 2020
U6  - https://doi.org/10.1021/acsami.0c11040
SN  - 1944-8244
SN  - 1944-8252
VL  - 12
IS  - 43
SP  - 48741
EP  - 48747
PB  - American Chemical Society
CY  - Washington, DC
ER  - 
TY  - JOUR
A1  - Saggioro, Elena
A1  - de Wiljes, Jana
A1  - Kretschmer, Marlene
A1  - Runge, Jakob
T1  - Reconstructing regime-dependent causal relationships from observational time series
JF  - Chaos : an interdisciplinary journal of nonlinear science
N2  - Inferring causal relations from observational time series data is a key problem across science and engineering whenever experimental interventions are infeasible or unethical. Increasing data availability over the past few decades has spurred the development of a plethora of causal discovery methods, each addressing particular challenges of this difficult task. In this paper, we focus on an important challenge that is at the core of time series causal discovery: regime-dependent causal relations. Often dynamical systems feature transitions depending on some, often persistent, unobserved background regime, and different regimes may exhibit different causal relations. Here, we assume a persistent and discrete regime variable leading to a finite number of regimes within which we may assume stationary causal relations. To detect regime-dependent causal relations, we combine the conditional independence-based PCMCI method [based on a condition-selection step (PC) followed by the momentary conditional independence (MCI) test] with a regime learning optimization approach. PCMCI allows for causal discovery from high-dimensional and highly correlated time series. Our method, Regime-PCMCI, is evaluated on a number of numerical experiments demonstrating that it can distinguish regimes with different causal directions, time lags, and sign of causal links, as well as changes in the variables' autocorrelation. Furthermore, Regime-PCMCI is employed to observations of El Nino Southern Oscillation and Indian rainfall, demonstrating skill also in real-world datasets.
Y1  - 2020
U6  - https://doi.org/10.1063/5.0020538
SN  - 1054-1500
SN  - 1089-7682
SN  - 1527-2443
VL  - 30
IS  - 11
PB  - American Institute of Physics
CY  - Melville
ER  - 
TY  - JOUR
A1  - Rosenau, Philip
A1  - Pikovskij, Arkadij
T1  - Solitary phase waves in a chain of autonomous oscillators
JF  - Chaos : an interdisciplinary journal of nonlinear science
N2  - In the present paper, we study phase waves of self-sustained oscillators with a nearest-neighbor dispersive coupling on an infinite lattice. To analyze the underlying dynamics, we approximate the lattice with a quasi-continuum (QC). The resulting partial differential model is then further reduced to the Gardner equation, which predicts many properties of the underlying solitary structures. Using an iterative procedure on the original lattice equations, we determine the shapes of solitary waves, kinks, and the flat-like solitons that we refer to as flatons. Direct numerical experiments reveal that the interaction of solitons and flatons on the lattice is notably clean. All in all, we find that both the QC and the Gardner equation predict remarkably well the discrete patterns and their dynamics.
Y1  - 2020
U6  - https://doi.org/10.1063/1.5144939
SN  - 1054-1500
SN  - 1089-7682
VL  - 30
IS  - 5
PB  - American Institute of Physics, AIP
CY  - Melville, NY
ER  - 
TY  - JOUR
A1  - Wiljes, Jana de
A1  - Tong, Xin T.
T1  - Analysis of a localised nonlinear ensemble Kalman Bucy filter with complete and accurate observations
JF  - Nonlinearity
N2  - Concurrent observation technologies have made high-precision real-time data available in large quantities. Data assimilation (DA) is concerned with how to combine this data with physical models to produce accurate predictions. For spatial-temporal models, the ensemble Kalman filter with proper localisation techniques is considered to be a state-of-the-art DA methodology. This article proposes and investigates a localised ensemble Kalman Bucy filter for nonlinear models with short-range interactions. We derive dimension-independent and component-wise error bounds and show the long time path-wise error only has logarithmic dependence on the time range. The theoretical results are verified through some simple numerical tests.
KW  - data assimilation
KW  - stability and accuracy
KW  - dimension independent bound
KW  - localisation
KW  - high dimensional
KW  - filter
KW  - nonlinear
Y1  - 2020
U6  - https://doi.org/10.1088/1361-6544/ab8d14
SN  - 0951-7715
SN  - 1361-6544
VL  - 33
IS  - 9
SP  - 4752
EP  - 4782
PB  - IOP Publ.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Wojcik, Michal
A1  - Brinkmann, Pia
A1  - Zdunek, Rafał
A1  - Riebe, Daniel
A1  - Beitz, Toralf
A1  - Merk, Sven
A1  - Cieslik, Katarzyna
A1  - Mory, David
A1  - Antonczak, Arkadiusz
T1  - Classification of copper minerals by handheld laser-induced breakdown spectroscopy and nonnegative tensor factorisation
JF  - Sensors
N2  - Laser-induced breakdown spectroscopy (LIBS) analysers are becoming increasingly common for material classification purposes. However, to achieve good classification accuracy, mostly noncompact units are used based on their stability and reproducibility. In addition, computational algorithms that require significant hardware resources are commonly applied. For performing measurement campaigns in hard-to-access environments, such as mining sites, there is a need for compact, portable, or even handheld devices capable of reaching high measurement accuracy. The optics and hardware of small (i.e., handheld) devices are limited by space and power consumption and require a compromise of the achievable spectral quality. As long as the size of such a device is a major constraint, the software is the primary field for improvement. In this study, we propose a novel combination of handheld LIBS with non-negative tensor factorisation to investigate its classification capabilities of copper minerals. The proposed approach is based on the extraction of source spectra for each mineral (with the use of tensor methods) and their labelling based on the percentage contribution within the dataset. These latent spectra are then used in a regression model for validation purposes. The application of such an approach leads to an increase in the classification score by approximately 5% compared to that obtained using commonly used classifiers such as support vector machines, linear discriminant analysis, and the k-nearest neighbours algorithm.
KW  - LIBS
KW  - NTF
KW  - HALS
KW  - classification
KW  - copper minerals
Y1  - 2020
U6  - https://doi.org/10.3390/s20185152
SN  - 1424-8220
VL  - 20
IS  - 18
PB  - MDPI
CY  - Basel
ER  - 
TY  - JOUR
A1  - Moutal, Nicolas
A1  - Grebenkov, Denis S.
T1  - The localization regime in a nutshell
JF  - Journal of magnetic resonance : JMR
N2  - High diffusion-sensitizing magnetic field gradients have been more and more often applied nowadays to achieve a better characterization of the microstructure. As the resulting spin-echo signal significantly deviates from the conventional Gaussian form, various models have been employed to interpret these deviations and to relate them with the microstructural properties of a sample. In this paper, we argue that the non-Gaussian behavior of the signal is a generic universal feature of the Bloch-Torrey equation. We provide a simple yet rigorous description of the localization regime emerging at high extended gradients and identify its origin as a symmetry breaking at the reflecting boundary. We compare the consequent non-Gaussian signal decay to other diffusion NMR regimes such as slow-diffusion, motional-narrowing and diffusion-diffraction regimes. We emphasize limitations of conventional perturbative techniques and advocate for non-perturbative approaches which may pave a way to new imaging modalities in this field.
KW  - Localization regime
KW  - Bloch-Torrey equation
KW  - Diffusion NMR
KW  - Spin-echo
KW  - Non-perturbative analysis
Y1  - 2020
U6  - https://doi.org/10.1016/j.jmr.2020.106836
SN  - 1090-7807
SN  - 1096-0856
VL  - 320
PB  - Elsevier
CY  - San Diego, Calif. [u.a.]
ER  - 
TY  - JOUR
A1  - Serrano-Munoz, Itziar
A1  - Fritsch, Tobias
A1  - Mishurova, Tatiana
A1  - Trofimov, Anton
A1  - Apel, Daniel
A1  - Ulbricht, Alexander
A1  - Kromm, Arne
A1  - Hesse, Rene
A1  - Evans, Alexander
A1  - Bruno, Giovanni
T1  - On the interplay of microstructure and residual stress in LPBF IN718
JF  - Journal of materials science
N2  - The relationship between residual stresses and microstructure associated with a laser powder bed fusion (LPBF) IN718 alloy has been investigated on specimens produced with three different scanning strategies (unidirectional Y-scan, 90 degrees XY-scan, and 67 degrees Rot-scan). Synchrotron X-ray energy-dispersive diffraction (EDXRD) combined with optical profilometry was used to study residual stress (RS) distribution and distortion upon removal of the specimens from the baseplate. The microstructural characterization of both the bulk and the near-surface regions was conducted using scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). On the top surfaces of the specimens, the highest RS values are observed in the Y-scan specimen and the lowest in the Rot-scan specimen, while the tendency is inversed on the side lateral surfaces. A considerable amount of RS remains in the specimens after their removal from the baseplate, especially in the Y- and Z-direction (short specimen dimension and building direction (BD), respectively). The distortion measured on the top surface following baseplate thinning and subsequent removal is mainly attributed to the amount of RS released in the build direction. Importantly, it is observed that the additive manufacturing microstructures challenge the use of classic theoretical models for the calculation of diffraction elastic constants (DEC) required for diffraction-based RS analysis. It is found that when the Reuss model is used for the calculation of RS for different crystal planes, as opposed to the conventionally used Kroner model, the results exhibit lower scatter. This is discussed in context of experimental measurements of DEC available in the literature for conventional and additively manufactured Ni-base alloys.
Y1  - 2020
U6  - https://doi.org/10.1007/s10853-020-05553-y
SN  - 0022-2461
SN  - 1573-4803
VL  - 56
IS  - 9
SP  - 5845
EP  - 5867
PB  - Springer
CY  - New York
ER  - 
TY  - JOUR
A1  - Fischer, Eric W.
A1  - Werther, Michael
A1  - Bouakline, Foudhil
A1  - Saalfrank, Peter
T1  - A hierarchical effective mode approach to phonon-driven multilevel vibrational relaxation dynamics at surfaces
JF  - The journal of chemical physics : bridges a gap between journals of physics and journals of chemistry
N2  - We discuss an efficient Hierarchical Effective Mode (HEM) representation of a high-dimensional harmonic oscillator bath, which describes phonon-driven vibrational relaxation of an adsorbate-surface system, namely, deuterium adsorbed on Si(100). Starting from the original Hamiltonian of the adsorbate-surface system, the HEM representation is constructed via iterative orthogonal transformations, which are efficiently implemented with Householder matrices. The detailed description of the HEM representation and its construction are given in the second quantization representation. The hierarchical nature of this representation allows access to the exact quantum dynamics of the adsorbate-surface system over finite time intervals, controllable via the truncation order of the hierarchy. To study the convergence properties of the effective mode representation, we solve the time-dependent Schrodinger equation of the truncated system-bath HEM Hamiltonian, with the help of the multilayer extension of the Multiconfigurational Time-Dependent Hartree (ML-MCTDH) method. The results of the HEM representation are compared with those obtained with a quantum-mechanical tier-model. The convergence of the HEM representation with respect to the truncation order of the hierarchy is discussed for different initial conditions of the adsorbate-surface system. The combination of the HEM representation with the ML-MCTDH method provides information on the time evolution of the system (adsorbate) and multiple effective modes of the bath (surface). This permits insight into mechanisms of vibration-phonon coupling of the adsorbate-surface system, as well as inter-mode couplings of the effective bath.
Y1  - 2020
U6  - https://doi.org/10.1063/5.0017716
SN  - 0021-9606
SN  - 1089-7690
VL  - 153
IS  - 6
PB  - American Institute of Physics
CY  - Melville
ER  - 
TY  - JOUR
A1  - Bouakline, Foudhil
T1  - Does nuclear permutation symmetry allow dynamical localization in symmetric double-well achiral molecules?
JF  - The journal of chemical physics : bridges a gap between journals of physics and journals of chemistr
N2  - We discuss the effect of molecular symmetry on coherent tunneling in symmetric double-well potentials whose two molecular equilibrium configurations are interconverted by nuclear permutations. This is illustrated with vibrational tunneling in ammonia molecules, electronic tunneling in the dihydrogen cation, and laser-induced rotational tunneling of homonuclear diatomics. In this contribution, we reexamine the textbook picture of coherent tunneling in such potentials, which is depicted with a wavepacket shuttling back and forth between the two potential-wells. We show that the common application of this picture to the aforementioned molecules contravenes the principle of the indistinguishability of identical particles. This conflict originates from the sole consideration of the dynamics of the tunneling-mode, connecting the double-well energy minima, and complete omission of all the remaining molecular degrees of freedom. This gives rise to double-well wavepackets that are nonsymmetric under nuclear permutations. To obey quantum statistics, we show that the double-well eigenstates composing these wavepackets must be entangled with the wavefunctions that describe all the omitted molecular modes. These wavefunctions have compensating and opposite nuclear permutation symmetry. This in turn leads to complete quenching of interference effects behind localization in one potential-well or another. Indeed, we demonstrate that the reduced density of probability of the symmetrized molecular wavefunction, where all the molecular coordinates but the tunneling-mode are integrated out, is symmetrically distributed over the two potential-wells, at all times. This applies to any multilevel wavepacket of isotropic or fully aligned symmetric double-well achiral molecules. However, in the case of coherent electronic or vibrational tunneling, fully aligned molecules may exhibit dynamical localization in the space-fixed frame, where the tunneling-mode density shuttles between the opposite directions of the alignment axis. This dynamical spatial-localization results from linear combinations of molecular states that have opposite parity. In summary, this study shows that dynamical localization of the tunneling-mode density on either of the two indistinguishable molecular equilibrium configurations of symmetric double-well achiral molecules is forbidden by quantum statistics, whereas its dynamical localization in the space-fixed frame is allowed by parity. The subtle distinction between these two types of localization has far-reaching implications in the interpretation of many ultrafast molecular dynamics experiments.
Y1  - 2020
U6  - https://doi.org/10.1063/1.5141746
SN  - 0021-9606
SN  - 1089-7690
VL  - 152
IS  - 24
PB  - American Institute of Physics
CY  - Melville
ER  - 
TY  - JOUR
A1  - Sposini, Vittoria
A1  - Grebenkov, Denis S.
A1  - Metzler, Ralf
A1  - Oshanin, Gleb
A1  - Seno, Flavio
T1  - Universal spectral features of different classes of random-diffusivity processes
JF  - New Journal of Physics
N2  - Stochastic models based on random diffusivities, such as the diffusing-diffusivity approach, are popular concepts for the description of non-Gaussian diffusion in heterogeneous media. Studies of these models typically focus on the moments and the displacement probability density function. Here we develop the complementary power spectral description for a broad class of random-diffusivity processes. In our approach we cater for typical single particle tracking data in which a small number of trajectories with finite duration are garnered. Apart from the diffusing-diffusivity model we study a range of previously unconsidered random-diffusivity processes, for which we obtain exact forms of the probability density function. These new processes are different versions of jump processes as well as functionals of Brownian motion. The resulting behaviour subtly depends on the specific model details. Thus, the central part of the probability density function may be Gaussian or non-Gaussian, and the tails may assume Gaussian, exponential, log-normal, or even power-law forms. For all these models we derive analytically the moment-generating function for the single-trajectory power spectral density. We establish the generic 1/f²-scaling of the power spectral density as function of frequency in all cases. Moreover, we establish the probability density for the amplitudes of the random power spectral density of individual trajectories. The latter functions reflect the very specific properties of the different random-diffusivity models considered here. Our exact results are in excellent agreement with extensive numerical simulations.
KW  - diffusion
KW  - power spectrum
KW  - random diffusivity
KW  - single trajectories
Y1  - 2020
U6  - https://doi.org/10.1088/1367-2630/ab9200
SN  - 1367-2630
VL  - 22
IS  - 6
PB  - Dt. Physikalische Ges.
CY  - Bad Honnef
ER  - 
TY  - JOUR
A1  - Grebenkov, Denis S.
A1  - Metzler, Ralf
A1  - Oshanin, Gleb
T1  - From single-particle stochastic kinetics to macroscopic reaction rates
BT  - fastest first-passage time of N random walkers
JF  - New Journal of Physics
N2  - We consider the first-passage problem for N identical independent particles that are initially released uniformly in a finite domain Ω and then diffuse toward a reactive area Γ, which can be part of the outer boundary of Ω or a reaction centre in the interior of Ω. For both cases of perfect and partial reactions, we obtain the explicit formulas for the first two moments of the fastest first-passage time (fFPT), i.e., the time when the first out of the N particles reacts with Γ. Moreover, we investigate the full probability density of the fFPT. We discuss a significant role of the initial condition in the scaling of the average fFPT with the particle number N, namely, a much stronger dependence (1/N and 1/N² for partially and perfectly reactive targets, respectively), in contrast to the well known inverse-logarithmic behaviour found when all particles are released from the same fixed point. We combine analytic solutions with scaling arguments and stochastic simulations to rationalise our results, which open new perspectives for studying the relevance of multiple searchers in various situations of molecular reactions, in particular, in living cells.
KW  - diffusion
KW  - first-passage
KW  - fastest first-passage time of N walkers
Y1  - 2020
U6  - https://doi.org/10.1088/1367-2630/abb1de
SN  - 1367-2630
VL  - 22
PB  - Dt. Physikalische Ges.
CY  - Bad Honnef
ER  - 
TY  - JOUR
A1  - Grebenkov, Denis S.
A1  - Sposini, Vittoria
A1  - Metzler, Ralf
A1  - Oshanin, Gleb
A1  - Seno, Flavio
T1  - Exact distributions of the maximum and range of random diffusivity processes
JF  - New Journal of Physics
N2  - We study the extremal properties of a stochastic process xt defined by the Langevin equation ẋₜ =√2Dₜ ξₜ, in which ξt is a Gaussian white noise with zero mean and Dₜ is a stochastic‘diffusivity’, defined as a functional of independent Brownian motion Bₜ.We focus on threechoices for the random diffusivity Dₜ: cut-off Brownian motion, Dₜt ∼ Θ(Bₜ), where Θ(x) is the Heaviside step function; geometric Brownian motion, Dₜ ∼ exp(−Bₜ); and a superdiffusive process based on squared Brownian motion, Dₜ ∼ B²ₜ. For these cases we derive exact expressions for the probability density functions of the maximal positive displacement and of the range of the process xₜ on the time interval ₜ ∈ (0, T).We discuss the asymptotic behaviours of the associated probability density functions, compare these against the behaviour of the corresponding properties of standard Brownian motion with constant diffusivity (Dₜ = D0) and also analyse the typical behaviour of the probability density functions which is observed for a majority of realisations of the stochastic diffusivity process.
KW  - random diffusivity
KW  - extremal values
KW  - maximum and range
KW  - diffusion
KW  - Brownian motion
Y1  - 2021
U6  - https://doi.org/10.1088/1367-2630/abd313
SN  - 1367-2630
VL  - 23
PB  - Dt. Physikalische Ges.
CY  - Bad Honnef
ER  - 
TY  - JOUR
A1  - Hajduk, Marcin
A1  - Todt, Helge Tobias
A1  - Hamann, Wolf-Rainer
A1  - Borek, Karolina
A1  - van Hoof, Peter A. M.
A1  - Zijlstra, Albert A.
T1  - The cooling-down central star of the planetary nebula SwSt 1
BT  - a late thermal pulse in a massive post-AGB star?
JF  - Monthly notices of the Royal Astronomical Society
N2  - SwSt 1 (PN G001.5-06.7) is a bright and compact planetary nebula containing a late [WC]-type central star. Previous studies suggested that the nebular and stellar lines are slowly changing with time. We studied new and archival optical and ultraviolet spectra of the object. The [O III] 4959 and 5007 angstrom to H beta line flux ratios decreased between about 1976 and 1997/2015. The stellar spectrum also shows changes between these epochs. We modelled the stellar and nebular spectra observed at different epochs. The analyses indicate a drop of the stellar temperature from about 42 kK to 40.5 kK between 1976 and 1993. We do not detect significant changes between 1993 and 2015. The observations show that the star performed a loop in the H-R diagram. This is possible when a shell source is activated during its post-AGB evolution. We infer that a late thermal pulse (LTP) experienced by a massive post-AGB star can explain the evolution of the central star. Such a star does not expand significantly as the result of the LTP and does not became a born-again red giant. However, the released energy can remove the tiny H envelope of the star.
KW  - stars: AGB and post-AGB
KW  - stars: atmospheres
KW  - stars: evolution
KW  - planetary
KW  - nebulae: general
KW  - planetary nebulae: individual: SwSt1
Y1  - 2020
U6  - https://doi.org/10.1093/mnras/staa2274
SN  - 0035-8711
SN  - 1365-2966
VL  - 498
IS  - 1
SP  - 1205
EP  - 1220
PB  - Oxford Univ. Press
CY  - Oxford
ER  - 
TY  - JOUR
A1  - Gruner, David
A1  - Barnes, Sydney A.
T1  - Rotation periods for cool stars in the open cluster Ruprecht 147 (NGC 6774) Implications for gyrochronology
JF  - Astronomy and astrophysics : an international weekly journal
N2  - Context:
Gyrochronology allows the derivation of ages for cool main sequence stars based on their observed rotation periods and masses, or a suitable proxy thereof. It is increasingly well-explored for FGK stars, but requires further measurements for older ages and K - M-type stars.

Aims:
We study the 2.7 Gyr-old open cluster Ruprecht 147 to compare it with the previously-studied, but far more distant, NGC 6819 cluster, and especially to measure cooler stars than was previously possible there.

Methods:
We constructed an inclusive list of 102 cluster members from prior work, including Gaia DR2, and for which light curves were also obtained during Campaign 7 of the Kepler/K2 space mission. We placed them in the cluster color-magnitude diagram and checked the related information against appropriate isochrones. The light curves were then corrected for data systematics using Principal Component Analysis on all observed K2 C07 stars and subsequently subjected to periodicity analysis.

Results:
Periodic signals are found for 32 stars, 21 of which are considered to be both highly reliable and to represent single, or effectively single, Ru 147 stars. These stars cover the spectral types from late-F to mid-M stars, and they have periods ranging from 6 d - 33 d, allowing for a comparison of Ruprecht 147 to both other open clusters and to models of rotational spindown. The derived rotation periods connect reasonably to, overlap with, and extend to lower masses the known rotation period distribution of the 2.5 Gyr-old cluster NGC 6819.

Conclusions:
The data confirm that cool stars lie on a single surface in rotation period-mass-age space, and they simultaneously challenge its commonly assumed shape. The shape at the low mass region of the color-period diagram at the age of Ru 147 favors a recently-proposed model which requires a third mass-dependent timescale in addition to the two timescales required by a former model, suggesting that a third physical process is required to model rotating stars effectively.
KW  - stars: late-type
KW  - stars: low-mass
KW  - stars: rotation
KW  - stars: solar-type
Y1  - 2020
U6  - https://doi.org/10.1051/0004-6361/202038984
SN  - 0004-6361
SN  - 1432-0746
VL  - 644
PB  - EDP Sciences
CY  - Les Ulis
ER  - 
TY  - JOUR
A1  - Wang, Wei
A1  - Seno, Flavio
A1  - Sokolov, Igor M.
A1  - Chechkin, Aleksei V.
A1  - Metzler, Ralf
T1  - Unexpected crossovers in correlated random-diffusivity processes
JF  - New Journal of Physics
N2  - The passive and active motion of micron-sized tracer particles in crowded liquids and inside living biological cells is ubiquitously characterised by 'viscoelastic' anomalous diffusion, in which the increments of the motion feature long-ranged negative and positive correlations. While viscoelastic anomalous diffusion is typically modelled by a Gaussian process with correlated increments, so-called fractional Gaussian noise, an increasing number of systems are reported, in which viscoelastic anomalous diffusion is paired with non-Gaussian displacement distributions. Following recent advances in Brownian yet non-Gaussian diffusion we here introduce and discuss several possible versions of random-diffusivity models with long-ranged correlations. While all these models show a crossover from non-Gaussian to Gaussian distributions beyond some correlation time, their mean squared displacements exhibit strikingly different behaviours: depending on the model crossovers from anomalous to normal diffusion are observed, as well as a priori unexpected dependencies of the effective diffusion coefficient on the correlation exponent. Our observations of the non-universality of random-diffusivity viscoelastic anomalous diffusion are important for the analysis of experiments and a better understanding of the physical origins of 'viscoelastic yet non-Gaussian' diffusion.
KW  - diffusion
KW  - anomalous diffusion
KW  - non-Gaussianity
KW  - fractional Brownian motion
Y1  - 2020
U6  - https://doi.org/10.1088/1367-2630/aba390
SN  - 1367-2630
VL  - 22
PB  - Dt. Physikalische Ges.
CY  - Bad Honnef
ER  - 
TY  - JOUR
A1  - Kulgemeyer, Christoph
A1  - Borowski, Andreas
A1  - Buschhüter, David
A1  - Enkrott, Patrick
A1  - Kempin, Maren
A1  - Reinhold, Peter
A1  - Riese, Josef
A1  - Schecker, Horst
A1  - Schröder, Jan
A1  - Vogelsang, Christoph
T1  - Professional knowledge affects action-related skills
BT  - the development of preservice physics teachers' explaining skills during a field experience
JF  - Journal of research in science teaching : the official journal of the National Association for Research in Science Teaching
N2  - Professional knowledge is an important source of science teachers' actions in the classroom (e.g., personal professional content knowledge [pedagogical content knowledge, PCK] is the source of enacted PCK in the refined consensus model [RCM] for PCK). However, the evidence for this claim is ambiguous at best. This study applied a cross-lagged panel design to examine the relationship between professional knowledge and actions in one particular instructional situation: explaining physics. Pre- and post a field experience (one semester), 47 preservice physics teachers from four different universities were tested for their content knowledge (CK), PCK, pedagogical knowledge (PK), and action-related skills in explaining physics. The study showed that joint professional knowledge (the weighted sum of CK, PCK, and PK scores) at the beginning of the field experience impacted the development of explaining skills during the field experience (beta = .38**). We interpret this as a particular relationship between professional knowledge and science teachers' action-related skills (enacted PCK): professional knowledge is necessary for the development of explaining skills. That is evidence that personal PCK affects enacted PCK. In addition, field experiences are often supposed to bridge the theory-practice gap by transforming professional knowledge into instructional practice. Our results suggest that for field experiences to be effective, preservice teachers should start with profound professional knowledge.
KW  - enacted PCK
KW  - field experience
KW  - instructional explanation
KW  - instructional
KW  - quality
KW  - practicum
KW  - professional knowledge
KW  - school internship
Y1  - 2020
U6  - https://doi.org/10.1002/tea.21632
SN  - 0022-4308
SN  - 1098-2736
VL  - 57
IS  - 10
SP  - 1554
EP  - 1582
PB  - Wiley
CY  - Hoboken
ER  - 
TY  - JOUR
A1  - Emanuel, Marc D.
A1  - Cherstvy, Andrey G.
A1  - Metzler, Ralf
A1  - Gompper, Gerhard
T1  - Buckling transitions and soft-phase invasion of two-component icosahedral shells
JF  - Physical review / publ. by The American Physical Society. E, Statistical, nonlinear, and soft matter physics
N2  - What is the optimal distribution of two types of crystalline phases on the surface of icosahedral shells, such as of many viral capsids? We here investigate the distribution of a thin layer of soft material on a crystalline convex icosahedral shell. We demonstrate how the shapes of spherical viruses can be understood from the perspective of elasticity theory of thin two-component shells. We develop a theory of shape transformations of an icosahedral shell upon addition of a softer, but still crystalline, material onto its surface. We show how the soft component "invades" the regions with the highest elastic energy and stress imposed by the 12 topological defects on the surface. We explore the phase diagram as a function of the surface fraction of the soft material, the shell size, and the incommensurability of the elastic moduli of the rigid and soft phases. We find that, as expected, progressive filling of the rigid shell by the soft phase starts from the most deformed regions of the icosahedron. With a progressively increasing soft-phase coverage, the spherical segments of domes are filled first (12 vertices of the shell), then the cylindrical segments connecting the domes (30 edges) are invaded, and, ultimately, the 20 flat faces of the icosahedral shell tend to be occupied by the soft material. We present a detailed theoretical investigation of the first two stages of this invasion process and develop a model of morphological changes of the cone structure that permits noncircular cross sections. In conclusion, we discuss the biological relevance of some structures predicted from our calculations, in particular for the shape of viral capsids.
Y1  - 2020
U6  - https://doi.org/10.1103/PhysRevE.102.062104
SN  - 2470-0045
SN  - 2470-0053
SN  - 2470-0061
SN  - 1538-4519
VL  - 102
IS  - 6
PB  - Woodbury
CY  - New York
ER  - 
TY  - JOUR
A1  - Chemura, Sitshengisiwe
A1  - Haubitz, Toni
A1  - Primus, Philipp A.
A1  - Underberg, Martin
A1  - Hülser, Tim
A1  - Kumke, Michael Uwe
T1  - Europium-doped Ceria-Gadolinium mixed oxides
BT  - PARAFAC analysis and high-resolution emission spectroscopy under cryogenic conditions for structural analysis
JF  - The journal of physical chemistry : A, Molecules, spectroscopy, kinetics, environment & general theory
N2  - Gadolinium-doped ceria or gadolinium-stabilized ceria (GDC) is an important technical material due to its ability to conduct O2- ions, e.g., used in solid oxide fuel cells operated at intermediate temperature as an electrolyte, diffusion barrier, and electrode component. We have synthesized Ce1-xGdxO2-y:Eu3+ (0 <= x <= 0.4) nanoparticles (11-15 nm) using a scalable spray pyrolysis method, which allows the continuous large-scale technical production of such materials. Introducing Eu3+ ions in small amounts into ceria and GDC as spectroscopic probes can provide detailed information about the atomic structure and local environments and allows us to monitor small structural changes. This study presents a novel approach to structurally elucidate europium-doped Ce1-xGdxO2-y:Eu3+ nanoparticles by way of Eu3+ spectroscopy, processing the spectroscopic data with the multiway decomposition method parallel factor (PARAFAC) analysis. In order to perform the deconvolution of spectra, data sets of excitation wavelength, emission wavelength, and time are required. Room temperature, time-resolved emission spectra recorded at lambda(ex) = 464 nm show that Gd3+ doping results in significantly altered emission spectra compared to pure ceria. The PARAFAC analysis for the pure ceria samples reveals a high-symmetry species (which can also be probed directly via the CeO2 charge transfer band) and a low-symmetry species. The GDC samples yield two low-symmetry spectra in the same experiment. High-resolution emission spectra recorded under cryogenic conditions after probing the D-5(0)-F-7(0) transition at lambda(ex) = 575-583 nm revealed additional variation in the low-symmetry Eu3+ sites in pure ceria and GDC. The total luminescence spectra of CeO2-y:Eu3+ showed Eu3+ ions located in at least three slightly different coordination environments with the same fundamental symmetry, whereas the overall hypsochromic shift and increased broadening of the D-5(0)-F-7(0) excitation in the GDC samples, as well as the broadened spectra after deconvolution point to less homogeneous environments. The data of the Gd3+-containing samples indicates that the average charge density around the Eu3+ ions in the lattice is decreased with increasing Gd3+ and oxygen vacancy concentration. For reference, the Judd-Ofelt parameters of all spectra were calculated. PARAFAC proves to be a powerful tool to analyze lanthanide spectra in crystalline solid materials, which are characterized by numerous Stark transitions and where measurements usually yield a superposition of different contributions to any given spectrum.
Y1  - 2020
U6  - https://doi.org/10.1021/acs.jpca.0c03188
SN  - 1089-5639
SN  - 1520-5215
VL  - 124
IS  - 24
SP  - 4972
EP  - 4983
PB  - American Chemical Society
CY  - Washington
ER  - 
TY  - JOUR
A1  - Li, Hua
A1  - Xu, Yong
A1  - Li, Yongge
A1  - Metzler, Ralf
T1  - Transition path dynamics across rough inverted parabolic potential barrier
JF  - The European physical journal : Plus
N2  - Transition path dynamics have been widely studied in chemical, physical, and technological systems. Mostly, the transition path dynamics is obtained for smooth barrier potentials, for instance, generic inverse-parabolic shapes. We here present analytical results for the mean transition path time, the distribution of transition path times, the mean transition path velocity, and the mean transition path shape in a rough inverted parabolic potential function under the driving of Gaussian white noise. These are validated against extensive simulations using the forward flux sampling scheme in parallel computations. We observe how precisely the potential roughness, the barrier height, and the noise intensity contribute to the particle transition in the rough inverted barrier potential.
Y1  - 2020
U6  - https://doi.org/10.1140/epjp/s13360-020-00752-7
SN  - 2190-5444
VL  - 135
IS  - 9
PB  - Springer
CY  - Berlin ; Heidelberg
ER  - 
TY  - JOUR
A1  - Vinke, Kira
A1  - Gabrysch, Sabine
A1  - Paoletti, Emanuela
A1  - Rockström, Johan
A1  - Schellnhuber, Hans Joachim
T1  - Corona and the climate
BT  - a comparison of two emergencies
JF  - Global sustainability
N2  - Lessons from the corona crisis can help manage the even more daunting challenge of anthropogenic global warming.
KW  - adaptation and mitigation
KW  - ecology and biodiversity
KW  - human behaviour
KW  - natural resources (biological and non-biological)
KW  - policies
KW  - politics
KW  - and governance
Y1  - 2020
U6  - https://doi.org/10.1017/sus.2020.20
SN  - 2059-4798
VL  - 3
PB  - Cambridge Univ. Press
CY  - Cambridge
ER  - 
TY  - JOUR
A1  - Aldiyarov, Abdurakhman
A1  - Sokolov, Dmitriy
A1  - Akylbayeva, Aigerim
A1  - Nurmukan, Assel
A1  - Tokmoldin, Nurlan
T1  - On thermal stability of cryovacuum deposited CH4+H2O films
JF  - Low temperature physics
N2  - Whereas stable homogenous states of aqueous hydrocarbon solutions are typically observed at high temperatures and pressures far beyond the critical values corresponding to individual components, the stability of such system may be preserved upon transition into the region of metastable states at low temperatures and low pressures. This work is dedicated to the study of the thermal stability of a water-methane mixture formed by cryogenic vapor phase deposition. The obtained thin films were studied using vibrational spectroscopy in the temperature range of 16-180 K. During thermal annealing of the samples, characteristic vibrational C-H modes of methane were monitored alongside the chamber pressure to register both structural changes and desorption of the film material. The obtained results reveal that upon the co-deposition of methane and water, methane molecules appear both in non-bound and trapped states. The observed broadening of the characteristic C-H stretching mode at 3010 cm(-1) upon an increase in temperature of the sample from 16 to 90 K, followed by narrowing of the peak as the temperature is reduced back to 16 K, indicates localization of methane molecules within the water matrix at lower temperatures.
KW  - molecular crystals
KW  - water-methane films
KW  - vibrational spectroscopy
KW  - low
KW  - temperature
KW  - methane localization
Y1  - 2020
U6  - https://doi.org/10.1063/10.0002156
SN  - 1063-777X
SN  - 1090-6517
VL  - 46
IS  - 11
SP  - 1121
EP  - 1124
PB  - American Institute of Physics
CY  - Melville
ER  - 
TY  - JOUR
A1  - Gostkowska-Lekner, Natalia Katarzyna
A1  - Wallacher, Dirk
A1  - Grimm, Nico
A1  - Habicht, Klaus
A1  - Hofmann, Tommy
T1  - A novel electrochemical anodization cell for the synthesis of mesoporous silicon
JF  - Review of scientific instruments : a monthly journal devoted to scientific instruments, apparatus, and techniques
N2  - A novel design of an electrochemical anodization cell dedicated to the synthesis of mesoporous, single-crystalline silicon is presented. First and foremost, the design principle follows user safety since electrochemical etching of silicon requires highly hazardous electrolytes based on hydrofluoric (HF) acid. The novel cell design allows for safe electrolyte handling prior, during, and post-etching. A peristaltic pump with HF-resistant fluoroelastomer tubing transfers electrolytes between dedicated reservoirs and the anodization cell. Due to the flexibility of the cell operation, different processing conditions can be realized providing a large parameter range for the attainable sample thickness, its porosity, and the mean pore size. Rapid etching on the order of several minutes to synthesize micrometer-thick porous silicon epilayers on bulk silicon is possible as well as long-time etching with continuous, controlled electrolyte flow for several days to prepare up to 1000 mu m thick self-supporting porous silicon membranes. A highly adaptable, LabVIEW((TM))-based control software allows for user-defined etching profiles.
Y1  - 2020
U6  - https://doi.org/10.1063/5.0008536
SN  - 0034-6748
SN  - 1089-7623
VL  - 91
IS  - 10
PB  - American Institute of Physics
CY  - Melville, NY
ER  - 
TY  - JOUR
A1  - Jelken, Joachim
A1  - Henkel, Carsten
A1  - Santer, Svetlana
T1  - Polarization controlled fine structure of diffraction spots from an optically induced grating
JF  - Applied physics letters
N2  - We report on the remote control of the fine structure of a diffraction spot from optically induced dual gratings within a photosensitive polymer film. The material contains azobenzene in the polymer side chains and develops a surface relief under two-beam holographic irradiation. The diffraction of a polarized probe beam is sensitive to the orientation of the azobenzene groups forming a permanently stored birefringence grating within the film. We demonstrate that the fine structure of the probe diffraction spot switches from a Gaussian to a hollow or a hollow to a "Saturn"-like structure by a change in polarization. This makes it potentially useful in photonic devices because the beam shape can be easily inverted by an external stimulus.
Y1  - 2020
U6  - https://doi.org/10.1063/1.5140067
SN  - 0003-6951
SN  - 1077-3118
VL  - 116
IS  - 5
PB  - American Institute of Physics
CY  - Melville
ER  - 
TY  - JOUR
A1  - Rietze, Clemens
A1  - Titov, Evgenii
A1  - Granucci, Giovanni
A1  - Saalfrank, Peter
T1  - Surface hopping dynamics for azobenzene photoisomerization
BT  - effects of packing density on surfaces, fluorination, and excitation wavelength
JF  - The journal of physical chemistry : C, Nanomaterials and interfaces
N2  - Azobenzenes easily photoswitch in solution, while their photoisomerization at surfaces is often hindered. In recent work, it was demonstrated by nonadiabatic molecular dynamics with trajectory surface hopping [Titov et al., J. Phys. Chem. Lett. 2016, 7, 3591-3596] that the experimentally observed suppression of trans -> cis isomerization yields in azobenzenes in a densely packed SAM (self-assembled monolayer) [Gahl et al., J. Am. Chem. Soc. 2010, 132, 1831-1838] is dominated by steric hindrance. In the present work, we systematically study by ground-state Langevin and nonadiabatic surface hopping dynamics, the effects of decreasing packing density on (i) UV/vis absorption spectra, (ii) trans -> cis isomerization yields, and (iii) excited-state lifetimes of photoexcited azobenzene. Within the quantum mechanics/ molecular mechanics models adopted here, we find that above a packing density of similar to 3 molecules/nm(2), switching yields are strongly reduced, while at smaller packing densities, the "monomer limit" is quickly approached. The UV/vis absorption spectra, on the other hand, depend on packing density over a larger range (down to at least similar to 1 molecule/nm(2)). Trends for excited-state lifetimes are less obvious, but it is found that lifetimes of pi pi* excited states decay monotonically with decreasing coverage. Effects of fluorination of the switches are also discussed for single, free molecules. Fluorination leads to comparatively large trans -> cis yields, in combination with long pi pi* lifetimes. Furthermore, for selected systems, also the effects of n pi* excitation at longer excitation wavelengths have been studied, which is found to enhance trans -> cis yields for free molecules but can lead to an opposite behavior in densely packed SAMs.
KW  - Computational chemistry
KW  - Energy
KW  - Molecules
KW  - Monomers
KW  - Oligomers
Y1  - 2020
U6  - https://doi.org/10.1021/acs.jpcc.0c08052
SN  - 1932-7447
SN  - 1932-7455
VL  - 124
IS  - 48
SP  - 26287
EP  - 26295
PB  - American Chemical Society
CY  - Washington
ER  - 
TY  - JOUR
A1  - von Zander, Robert Edler
A1  - Saalfrank, Peter
T1  - On the borate-catalyzed electrochemical reduction of phosphine oxide
BT  - a computational study
JF  - The journal of physical chemistry : A, Molecules, spectroscopy, kinetics, environment & general theory
N2  - Recently, Nocera and co-workers (J. Am. Chem. Soc. 2018, 140, 13711) demonstrated that triaryl borate Lewis acids facilitate the direct electrochemical reduction of triphenylphosphine oxide (TPPO) to triphenylphosphine (TPP). In the present contribution, we report a quantum chemical study unravelling details of the reaction, which also supports the proposed ECrECi mechanism. Alternative electrochemical routes to TPPO reduction facilitated by other Lewis acids (CH3+), or by photocatalysis at semiconductor surfaces, are also briefly discussed.
Y1  - 2020
U6  - https://doi.org/10.1021/acs.jpca.0c07805
SN  - 1089-5639
SN  - 1520-5215
VL  - 124
IS  - 49
SP  - 10239
EP  - 10245
PB  - American Chemical Society
CY  - Washington
ER  - 
TY  - JOUR
A1  - Spelzhausen, Simon
A1  - Ionian, Mario-Rafael
A1  - Gerhard, Reimund
A1  - Plath, Ronald
T1  - Time-resolved measurement of space-charge evolution in dielectric films or slabs by means of repeatable laser-induced pressure pulses
JF  - Review of scientific instruments : a monthly journal devoted to scientific instruments, apparatus, and techniques
N2  - A new variant of the Laser-Induced Pressure-Pulse (LIPP) method for repeatable, time-resolved space-charge profile measurements is proposed and demonstrated. Automated deposition of a fresh laser-target film before each illumination leads to good repeatability of the LIPP and thus allows for the detection of time-resolved changes in the space-charge distribution over many hours. We describe and discuss the experimental setup and its features, compare the repeatability of the LIPP measurements on the same sample without and with re-preparation of the test cell, and present the time-resolved evolution of the space-charge profile in a two-layer arrangement of a silicone-grease and a silicone-elastomer film as an example. Finally, the temperature dependence of the space-charge evolution during polarization under high voltage and during depolarization in short circuit is shown. Possible uses and future developments of the new LIPP approach are also discussed.
Y1  - 2020
U6  - https://doi.org/10.1063/1.5142443
SN  - 0034-6748
SN  - 1089-7623
VL  - 91
IS  - 5
PB  - American Institute of Physics
CY  - Melville
ER  - 
TY  - JOUR
A1  - Qiu, Xunlin
A1  - Benjamin, Aravindan Joseph
A1  - Raman Venkatesan, Thulasinath
A1  - Schmidt, Georg C.
A1  - Soler, Ricardo Alonso Quintana
A1  - Panicker, Pramul Muraleedhara
A1  - Gerhard, Reimund
A1  - Hübler, Arved Carl
T1  - Dielectric and electroacoustic assessment of screen-printed piezoelectric polymer layers as flexible transducers
BT  - influence of the electrode material
JF  - IEEE transactions on dielectrics and electrical insulation
N2  - Here, piezoelectric transducers consisting of a P(VDF-TrFE) layer with either silver or PEDOT:PSS screen-printed electrodes are studied. The influence of electrodes on the dielectric and electroacoustic properties are studied in dielectric-spectroscopy and ferroelectric-hysteresis measurements. Only when both the bottom and the top electrodes are made of silver, the typical dielectric relaxation of the P(VDF-TrFE) layer is clearly observed. When one or two of the electrodes are of PEDOT:PSS, a Debye-like relaxation is present. Compared with silver electrodes, PEDOT:PSS electrodes allow for moderate self-healing. Consequently, samples with bottom and top PEDOT:PSS electrodes can be poled to saturation, while samples with silver electrodes can hardly be poled to saturation due to destructive electric breakdown. Acoustic transducer measurements show that silver electrodes facilitate higher and broader frequency operation, while PEDOT:PSS electrodes bring slightly lower total harmonic distortion. Overall, the acoustic performance shows no significant deviations between differently electroded samples so that silver electrodes do not offer any advantages for the transducers studied here due to their much higher tendency for destructive electric breakdown.
KW  - poly(vinylidenefluoride-trifluoroethylene) P(VDF-TrFE)
KW  - dielectric
KW  - spectroscopy
KW  - ferroelectricity and piezoelectricity in polymers
KW  - screen
KW  - printing
KW  - printed electroacoustic thin-film transducers
Y1  - 2020
U6  - https://doi.org/10.1109/TDEI.2020.008864
SN  - 1070-9878
SN  - 1558-4135
SN  - 0018-9367
VL  - 27
IS  - 5
SP  - 1683
EP  - 1690
PB  - Institute of Electrical and Electronics Engineers
CY  - New York, NY
ER  - 
TY  - JOUR
A1  - Wang, Jingwen
A1  - Rychkov, Dmitry
A1  - Nguyen, Quyet Doan
A1  - Gerhard, Reimund
T1  - The influence of orthophosphoric-acid surface modification on charge-storage enhancement in polypropylene electrets
JF  - Journal of applied physics
N2  - Bipolar electrets from polypropylene (PP) are essential, e.g., in electret air filters and in cellular-foam ferroelectrets. Therefore, the mechanism of surface-charge stability enhancement on PP electrets via orthophosphoric-acid surface treatment is investigated in detail. It is shown that the significant charge-stability enhancement can be mainly attributed to deeper surface traps originating from deposited chemicals and topographic features on the modified surfaces. Thermally stimulated discharge of chemically treated and non-treated PP films with different surface-charge densities is used to test the limits of the newly formed deep traps in terms of the capacity for hosting surface charges. When the initial surface-charge density is very high, more charges are forced into shallower original traps on the surface or in the bulk of the treated PP samples, reducing the effect of the deeper surface traps brought by the surface modification. The well-known crossover phenomenon (of the surface-charge decay curves) has been observed between modified PP electrets charged to +/- 2kV and to +/- 3kV. Acoustically probed charge distributions in the thickness direction of PP electrets at different stages of thermal discharging indicate that the deep surface trapping sites may have preference for negative charges, resulting in the observed asymmetric charge stability of the modified PP films.
Y1  - 2020
U6  - https://doi.org/10.1063/5.0013805
SN  - 0021-8979
SN  - 1089-7550
VL  - 128
IS  - 3
PB  - American Institute of Physics
CY  - Melville
ER  - 
TY  - JOUR
A1  - Wang, Jingwen
A1  - Rychkov, Dmitry
A1  - Nguyen, Quyet Doan
A1  - Gerhard, Reimund
T1  - Unexpected bipolar space-charge polarization across transcrystalline interfaces in polypropylene electret films
JF  - Journal of applied physics
N2  - A double-layer transcrystalline polypropylene (PP) film with a flat central interface layer between its two transcrystalline layers is obtained by recrystallization from the melt between two polytetrafluoroethylene (PTFE) surfaces on both sides of the PP film. Its electret properties are studied and compared with those of a single-layer transcrystalline PP film re-crystallized in contact with only one PTFE surface. Within experimental uncertainty, the two types of transcrystalline films exhibit the same thermal properties and crystallinities. After thermal poling, however, two hetero-charge layers of opposite polarity are found on the internal interfaces of the double-layer transcrystalline films and may together be considered as micrometer-sized dipoles. The unexpected phenomenon does not occur in single-layer transcrystalline samples without a central interface layer, suggesting that the interfaces between the transcrystalline layers and the micrometer-thick central interface layer may be the origin of deeper traps rather than the crystalline structures in the transcrystallites or the spherulites. The origin of the interfacial charges was also studied by means of an injection-blocking charging method, which revealed that intrinsic charge carriers introduced during recrystallization are most likely responsible for the interfacial charges. It is fascinating that a material as familiar as PP can exhibit such intriguing properties with a special bipolar space-charge polarization across the central interface layer after quasi-epitaxial surface moulding into a double-layer transcrystalline form. In addition to applications in electret (micro-)devices for electro-mechanical transduction, the highly ordered structures may also be employed as a new paradigm for studying charge storage and transport in polymer electrets and in dielectrics for DC electrical insulation.
Y1  - 2020
U6  - https://doi.org/10.1063/5.0022071
SN  - 0021-8979
SN  - 1089-7550
VL  - 128
IS  - 13
PB  - American Institute of Physics, AIP
CY  - Melville, NY
ER  - 
TY  - JOUR
A1  - Assagra, Yuri A.O.
A1  - Altafim, Ruy Alberto Pisani
A1  - do Carmo, Joao P.
A1  - Altafim, Ruy A.C.
A1  - Rychkov, Dmitry
A1  - Wirges, Werner
A1  - Gerhard, Reimund
T1  - A new route to piezo-polymer transducers: 3D printing of polypropylene ferroelectrets
JF  - IEEE transactions on dielectrics and electrical insulation
N2  - Here, a promising approach for producing piezo-polymer transducers in a one-step process is presented. Using 3D-printing technology and polypropylene (PP) filaments, we are able to print a two-layered film structure with regular cavities of precisely controlled size and shape. It is found that the 3D-printed samples exhibit piezoelectric coefficients up to 200 pC/N, similar to those of other PP ferroelectrets, and their temporal and thermal behavior is in good agreement with those known of PP ferroelectrets. The piezoelectric response strongly decreases for applied pressures above 20 kPa, as the pressure in the air-filled cavities strongly influences the overall elastic modulus of ferroelectrets.
KW  - 3D printing
KW  - polymer ferroelectrets
KW  - sensors and actuators
KW  - piezoelectrets
KW  - electret polymers
KW  - soft electro-active materials
KW  - functional materials
KW  - soft matter
Y1  - 2020
U6  - https://doi.org/10.1109/TDEI.2020.008461
SN  - 1070-9878
SN  - 1558-4135
VL  - 27
IS  - 5
SP  - 1668
EP  - 1674
PB  - Inst. of Electr. and Electronics Engineers
CY  - Piscataway
ER  - 
TY  - JOUR
A1  - Jay, Raphael M.
A1  - Vaz da Cruz, Vinicius
A1  - Eckert, Sebastian
A1  - Fondell, Mattis
A1  - Mitzner, Rolf
A1  - Föhlisch, Alexander
T1  - Probing solute-solvent interactions of transition metal complexes using L-edge absorption spectroscopy
JF  - The journal of physical chemistry : B, Condensed matter, materials, surfaces, interfaces & biophysical chemistry
N2  - In order to tailor solution-phase chemical reactions involving transition metal complexes, it is critical to understand how their valence electronic charge distributions are affected by the solution environment. Here, solute-solvent interactions of a solvatochromic mixed-ligand iron complex were investigated using X-ray absorption spectroscopy at the transition metal L-2,L-3-edge. Due to the selectivity of the corresponding core excitations to the iron 3d orbitals, the method grants direct access to the valence electronic structure around the iron center and its response to interactions with the solvent environment. A linear increase of the total L-2,L-3-edge absorption cross section as a function of the solvent Lewis acidity is revealed. The effect is caused by relative changes in different metal-ligand-bonding channels, which preserve local charge densities while increasing the density of unoccupied states around the iron center. These conclusions are corroborated by a combination of molecular dynamics and spectrum simulations based on time-dependent density functional theory. The simulations reproduce the spectral trends observed in the X-ray but also optical absorption experiments. Our results underscore the importance of solute-solvent interactions when aiming for an accurate description of the valence electronic structure of solvated transition metal complexes and demonstrate how L-2,L-3-edge absorption spectroscopy can aid in understanding the impact of the solution environment on intramolecular covalency and the electronic charge distribution.
KW  - basis-sets
KW  - charge-transfer
KW  - density
KW  - dynamics
KW  - electron localization
KW  - iron
KW  - solvation
KW  - spin-crossover
KW  - tranfer excited-state
KW  - x-ray-absorption
Y1  - 2020
U6  - https://doi.org/10.1021/acs.jpcb.0c00638
SN  - 1520-6106
SN  - 1520-5207
VL  - 124
IS  - 27
SP  - 5636
EP  - 5645
PB  - American Chemical Society
CY  - Washington
ER  - 
TY  - JOUR
A1  - Jay, Raphael M.
A1  - Eckert, Sebastian
A1  - Mitzner, Rolf
A1  - Fondell, Mattis
A1  - Föhlisch, Alexander
T1  - Quantitative evaluation of transient valence orbital occupations in a 3d transition metal complex as seen from the metal and ligand perspective
JF  - Chemical physics letters
N2  - It is demonstrated for the case of photo-excited ferrocyanide how time-resolved soft X-ray absorption spectroscopy in transmission geometry at the ligand K-edge and metal L-3-edge provides quantitatively equivalent valence electronic structure information, where signatures of photo-oxidation are assessed locally at the metal as well as the ligand. This allows for a direct and independent quantification of the number of photo-oxidized molecules at two soft X-ray absorption edges highlighting the sensitivity of X-ray absorption spectroscopy to the valence orbital occupation of 3d transition metal complexes throughout the soft X-ray range.
KW  - iron cyanides
KW  - photochemistry
KW  - soft X-ray absorption
Y1  - 2020
U6  - https://doi.org/10.1016/j.cplett.2020.137681
SN  - 0009-2614
SN  - 1873-4448
VL  - 754
PB  - Elsevier
CY  - Amsterdam
ER  - 
TY  - JOUR
A1  - Arya, Pooja
A1  - Jelken, Joachim
A1  - Feldmann, David
A1  - Lomadze, Nino
A1  - Santer, Svetlana
T1  - Light driven diffusioosmotic repulsion and attraction of colloidal particles
JF  - The journal of chemical physics : bridges a gap between journals of physics and journals of chemistr
N2  - In this paper, we introduce the phenomenon of light driven diffusioosmotic long-range attraction and repulsion of porous particles under irradiation with UV light. The change in the inter-particle interaction potential is governed by flow patterns generated around single colloids and results in reversible aggregation or separation of the mesoporous silica particles that are trapped at a solid surface. The range of the interaction potential extends to several times the diameter of the particle and can be adjusted by varying the light intensity. The "fuel" of the process is a photosensitive surfactant undergoing photo-isomerization from a more hydrophobic trans-state to a rather hydrophilic cis-state. The surfactant has different adsorption affinities to the particles depending on the isomerization state. The trans-isomer, for example, tends to accumulate in the negatively charged pores of the particles, while the cis-isomer prefers to remain in the solution. This implies that when under UV irradiation cis-isomers are being formed within the pores, they tend to diffuse out readily and generate an excess concentration near the colloid's outer surface, ultimately resulting in the initiation of diffusioosmotic flow. The direction of the flow depends strongly on the dynamic redistribution of the fraction of trans- and cis-isomers near the colloids due to different kinetics of photo-isomerization within the pores as compared to the bulk. The unique feature of the mechanism discussed in the paper is that the long-range mutual repulsion but also the attraction can be tuned by convenient external optical stimuli such as intensity so that a broad variety of experimental situations for manipulation of a particle ensemble can be realized.
Y1  - 2020
U6  - https://doi.org/10.1063/5.0007556
SN  - 0021-9606
SN  - 1089-7690
VL  - 152
IS  - 19
PB  - American Institute of Physics
CY  - Melville, NY
ER  - 
TY  - JOUR
A1  - Jelken, Joachim
A1  - Henkel, Carsten
A1  - Santer, Svetlana
T1  - Formation of half-period surface relief gratings in azobenzene containing polymer films
JF  - Applied physics : B, Lasers and optics
N2  - We study the peculiar response of photo-sensitive polymer films irradiated with a certain type of interference pattern where one interfering beam is S-polarized, while the second one is P-polarized. The polymer film, although in a glassy state, deforms following the local polarization distribution of the incident light, and a surface relief grating (SRG) appears whose period is half the optical one. All other types of interference patterns result in the matching of both periods. The topographical response is triggered by the alignment of photo-responsive azobenzene containing polymer side chains orthogonal to the local electrical field, resulting in a bulk birefringence grating (BBG). We investigate the process of dual grating formation (SRG and BBG) in a polymer film utilizing a dedicated set-up that combines probe beam diffraction and atomic force microscopy (AFM) measurements, and permits acquiring in situ and in real-time information about changes in local topography and birefringence. We find that the SRG maxima appear at the positions of linearly polarized light (tilted by 45 degrees relative to the grating vector), causing the formation of the half-period topography. This permits to inscribe symmetric and asymmetric topography gratings with sub-wavelength period, while changing only slightly the polarization of one of the interfering beams. We demonstrate an easy generation of sawtooth profiles (blazed gratings) with adjustable shape. With these results, we have taken a significant step in understanding the photo-induced deformation of azo-polymer films.
Y1  - 2020
U6  - https://doi.org/10.1007/s00340-020-07500-w
SN  - 0946-2171
SN  - 1432-0649
VL  - 126
IS  - 9
PB  - Springer
CY  - Heidelberg
ER  -