@article{MiedemaBeyeKoenneckeetal.2014, author = {Miedema, Piter Sybren and Beye, Martin and Koennecke, R. and Schiwietz, Gregor and F{\"o}hlisch, Alexander}, title = {Thermal evolution of the band edges of 6H-SiC: X-ray methods compared to the optical band gap}, series = {Journal of electron spectroscopy and related phenomena : the international journal on theoretical and experimental aspects of electron spectroscopy}, volume = {197}, journal = {Journal of electron spectroscopy and related phenomena : the international journal on theoretical and experimental aspects of electron spectroscopy}, publisher = {Elsevier}, address = {Amsterdam}, issn = {0368-2048}, doi = {10.1016/j.elspec.2014.08.003}, pages = {37 -- 42}, year = {2014}, abstract = {The band gap of semiconductors like silicon and silicon carbide (SIC) is the key for their device properties. In this research, the band gap of 6H-SiC and its temperature dependence were analyzed with silicon 2p X-ray absorption spectroscopy (XAS), X-ray emission spectroscopy (XES) and resonant inelastic X-ray scattering (RIXS) allowing for a separate analysis of the conduction-band minimum (CBM) and valence-band maximum (VBM) components of the band gap. The temperature-dependent asymmetric band gap shrinking of 6H-SiC was determined with a valence-band slope of +2.45 x 10(-4) eV/K and a conduction-band slope of -1.334 x 10(-4) eV/K. The apparent asymmetry, e.g., that two thirds of the band-gap shrinking with increasing temperature is due to the VBM evolution in 6H-SiC, is similar to the asymmetry obtained for pure silicon before. The overall band gap temperature-dependence determined with XAS and nonresonant XES is compared to temperature-dependent optical studies. The core-excitonic binding energy appearing in the Si 2p XAS is extracted as the main difference. In addition, the energy loss of the onset of the first band in RIXS yields to values similar to the optical band gap over the tested temperature range. (C) 2014 Elsevier B.V. All rights reserved.}, language = {en} } @article{NavirianSchickGaaletal.2014, author = {Navirian, Hengameh A. and Schick, Daniel and Gaal, Peter and Leitenberger, Wolfram and Shayduk, Roman and Bargheer, Matias}, title = {Thermoelastic study of nanolayered structures using time-resolved X-ray diffraction at high repetition rate}, series = {Applied physics letters}, volume = {104}, journal = {Applied physics letters}, number = {2}, publisher = {American Institute of Physics}, address = {Melville}, issn = {0003-6951}, doi = {10.1063/1.4861873}, pages = {4}, year = {2014}, abstract = {We investigate the thermoelastic response of a nanolayered sample composed of a metallic SrRuO3 electrode sandwiched between a ferroelectric Pb(Zr0.2Ti0.8)O-3 film with negative thermal expansion and a SrTiO3 substrate. SrRuO3 is rapidly heated by fs-laser pulses with 208 kHz repetition rate. Diffraction of X-ray pulses derived from a synchrotron measures the transient out-of-plane lattice constant c of all three materials simultaneously from 120 ps to 5 mu s with a relative accuracy up to Delta c/c = 10(-6). The in-plane propagation of sound is essential for understanding the delayed out-of-plane compression of Pb(Zr0.2Ti0.8)O-3.}, language = {en} } @article{KurfuerstFeldmeierKrticka2014, author = {Kurfuerst, P. and Feldmeier, Achim and Krticka, Jiri}, title = {Time-dependent modeling of extended thin decretion disks of critically rotating stars}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {569}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {0004-6361}, doi = {10.1051/0004-6361/201424272}, pages = {7}, year = {2014}, abstract = {Context. During their evolution massive stars can reach the phase of critical rotation when a further increase in rotational speed is no longer possible. Direct centrifugal ejection from a critically or near-critically rotating surface forms a gaseous equatorial decretion disk. Anomalous viscosity provides the efficient mechanism for transporting the angular momentum outwards. The outer part of the disk can extend up to a very large distance from the parent star. Aims. We study the evolution of density, radial and azimuthal velocity, and angular momentum loss rate of equatorial decretion disks out to very distant regions. We investigate how the physical characteristics of the disk depend on the distribution of temperature and viscosity. Methods. We calculated stationary models using the Newton-Raphson method. For time-dependent hydrodynamic modeling we developed the numerical code based on an explicit finite difference scheme on an Eulerian grid including full Navier-Stokes shear viscosity. Results. The sonic point distance and the maximum angular momentum loss rate strongly depend on the temperature profile and are almost independent of viscosity. The rotational velocity at large radii rapidly drops accordingly to temperature and viscosity distribution. The total amount of disk mass and the disk angular momentum increase with decreasing temperature and viscosity. Conclusions. The time-dependent one-dimensional models basically confirm the results obtained in the stationary models as well as the assumptions of the analytical approximations. Including full Navier-Stokes viscosity we systematically avoid the rotational velocity sign change at large radii. The unphysical drop of the rotational velocity and angular momentum loss at large radii (present in some models) can be avoided in the models with decreasing temperature and viscosity.}, language = {en} } @misc{CardinalettiKestersBerthoetal.2014, author = {Cardinaletti, Ilaria and Kesters, Jurgen and Bertho, Sabine and Conings, Bert and Piersimoni, Fortunato and Lutsen, Laurence and Nesladek, Milos and Van Mele, Bruno and Van Assche, Guy and Vandewal, Koen and Salleo, Alberto and Vanderzande, Dirk and Maes, Wouter and Manca, Jean V.}, title = {Toward bulk heterojunction polymer solar cells with thermally stable active layer morphology}, series = {Journal of photonics for energy}, volume = {4}, journal = {Journal of photonics for energy}, publisher = {SPIE}, address = {Bellingham}, issn = {1947-7988}, doi = {10.1117/1.JPE.4.040997}, pages = {12}, year = {2014}, abstract = {When state-of-the-art bulk heterojunction organic solar cells with ideal morphology are exposed to prolonged storage or operation at elevated temperatures, a thermally induced disruption of the active layer blend can occur, in the form of a separation of donor and acceptor domains, leading to diminished photovoltaic performance. Toward the long-term use of organic solar cells in real-life conditions, an important challenge is, therefore, the development of devices with a thermally stable active layer morphology. Several routes are being explored, ranging from the use of high glass transition temperature, cross-linkable and/or side-chain functionalized donor and acceptor materials, to light-induced dimerization of the fullerene acceptor. A better fundamental understanding of the nature and underlying mechanisms of the phase separation and stabilization effects has been obtained through a variety of analytical, thermal analysis, and electro-optical techniques. Accelerated aging systems have been used to study the degradation kinetics of bulk heterojunction solar cells in situ at various temperatures to obtain aging models predicting solar cell lifetime. The following contribution gives an overview of the current insights regarding the intrinsic thermally induced aging effects and the proposed solutions, illustrated by examples of our own research groups. (C) The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License.}, language = {en} } @article{HarrisMellinger2014, author = {Harris, Scott and Mellinger, Axel}, title = {Towards a better understanding of dielectric barrier discharges in ferroelectrets: Paschen breakdown fields in micrometer sized voids}, series = {Journal of applied physics}, volume = {115}, journal = {Journal of applied physics}, number = {16}, publisher = {American Institute of Physics}, address = {Melville}, issn = {0021-8979}, doi = {10.1063/1.4871678}, pages = {5}, year = {2014}, abstract = {Charged cellular polypropylene foams (i.e., ferro-or piezoelectrets) demonstrate high piezoelectric activity upon being electrically charged. When an external electric field is applied, dielectric barrier discharges (DBDs) occur, resulting in a separation of charges which are subsequently deposited on dielectric surfaces of internal micrometer sized voids. This deposited space charge is responsible for the piezoelectric activity of the material. Previous studies have indicated charging fields larger than predicted by Townsend's model of Paschen breakdown applied to a multilayered electromechanical model; a discrepancy which prompted the present study. The actual breakdown fields for micrometer sized voids were determined by constructing single cell voids using polypropylene spacers with heights ranging from 8 to 75 mu m, "sandwiched" between two polypropylene dielectric barriers and glass slides with semi-transparent electrodes. Subsequently, a bipolar triangular charging waveform with a peak voltage of 6 kV was applied to the samples. The breakdown fields were determined by monitoring the emission of light due to the onset of DBDs using an electron multiplying CCD camera. The breakdown fields at absolute pressures from 101 to 251 kPa were found to be in good agreement with the standard Paschen curves. Additionally, the magnitude of the light emission was found to scale linearly with the amount of gas, i.e., the height of the voids. Emissions were homogeneous over the observed regions of the voids for voids with heights of 25 lm or less and increasingly inhomogeneous for void heights greater than 40 lm at high electric fields.}, language = {en} } @article{LangeReiterPaetzeletal.2014, author = {Lange, Ilja and Reiter, Sina and Paetzel, Michael and Zykov, Anton and Nefedov, Alexei and Hildebrandt, Jana and Hecht, Stefan and Kowarik, Stefan and Woell, Christof and Heimel, Georg and Neher, Dieter}, title = {Tuning the work function of polar zinc oxide surfaces using modified phosphonic acid self-assembled monolayers}, series = {Advanced functional materials}, volume = {24}, journal = {Advanced functional materials}, number = {44}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {1616-301X}, doi = {10.1002/adfm.201401493}, pages = {7014 -- 7024}, year = {2014}, abstract = {Zinc oxide (ZnO) is regarded as a promising alternative material for transparent conductive electrodes in optoelectronic devices. However, ZnO suffers from poor chemical stability. ZnO also has a moderate work function (WF), which results in substantial charge injection barriers into common (organic) semiconductors that constitute the active layer in a device. Controlling and tuning the ZnO WF is therefore necessary but challenging. Here, a variety of phosphonic acid based self-assembled monolayers (SAMs) deposited on ZnO surfaces are investigated. It is demonstrated that they allow the tuning the WF over a wide range of more than 1.5 eV, thus enabling the use of ZnO as both the hole-injecting and electron-injecting contact. The modified ZnO surfaces are characterized using a number of complementary techniques, demonstrating that the preparation protocol yields dense, well-defined molecular monolayers.}, language = {en} } @article{SchickHerzogBojahretal.2014, author = {Schick, Daniel and Herzog, Marc and Bojahr, Andre and Leitenberger, Wolfram and Hertwig, Andreas and Shayduk, Roman and Bargheer, Matias}, title = {Ultrafast lattice response of photoexcited thin films studied by X-ray diffraction}, series = {Structural dynamics}, volume = {1}, journal = {Structural dynamics}, number = {6}, publisher = {American Institute of Physics}, address = {Melville}, issn = {2329-7778}, doi = {10.1063/1.4901228}, pages = {13}, year = {2014}, abstract = {Using ultrafast X-ray diffraction, we study the coherent picosecond lattice dynamics of photoexcited thin films in the two limiting cases, where the photoinduced stress profile decays on a length scale larger and smaller than the film thickness. We solve a unifying analytical model of the strain propagation for acoustic impedance-matched opaque films on a semi-infinite transparent substrate, showing that the lattice dynamics essentially depend on two parameters: One for the spatial profile and one for the amplitude of the strain. We illustrate the results by comparison with high-quality ultrafast X-ray diffraction data of SrRuO3 films on SrTiO3 substrates. (C) 2014 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.}, language = {en} } @phdthesis{Trabant2014, author = {Trabant, Christoph}, title = {Ultrafast photoinduced phase transitions in complex materials probed by time-resolved resonant soft x-ray diffraction}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-71377}, school = {Universit{\"a}t Potsdam}, year = {2014}, abstract = {In processing and data storage mainly ferromagnetic (FM) materials are being used. Approaching physical limits, new concepts have to be found for faster, smaller switches, for higher data densities and more energy efficiency. Some of the discussed new concepts involve the material classes of correlated oxides and materials with antiferromagnetic coupling. Their applicability depends critically on their switching behavior, i.e., how fast and how energy efficient material properties can be manipulated. This thesis presents investigations of ultrafast non-equilibrium phase transitions on such new materials. In transition metal oxides (TMOs) the coupling of different degrees of freedom and resulting low energy excitation spectrum often result in spectacular changes of macroscopic properties (colossal magneto resistance, superconductivity, metal-to-insulator transitions) often accompanied by nanoscale order of spins, charges, orbital occupation and by lattice distortions, which make these material attractive. Magnetite served as a prototype for functional TMOs showing a metal-to-insulator-transition (MIT) at T = 123 K. By probing the charge and orbital order as well as the structure after an optical excitation we found that the electronic order and the structural distortion, characteristics of the insulating phase in thermal equilibrium, are destroyed within the experimental resolution of 300 fs. The MIT itself occurs on a 1.5 ps timescale. It shows that MITs in functional materials are several thousand times faster than switching processes in semiconductors. Recently ferrimagnetic and antiferromagnetic (AFM) materials have become interesting. It was shown in ferrimagnetic GdFeCo, that the transfer of angular momentum between two opposed FM subsystems with different time constants leads to a switching of the magnetization after laser pulse excitation. In addition it was theoretically predicted that demagnetization dynamics in AFM should occur faster than in FM materials as no net angular momentum has to be transferred out of the spin system. We investigated two different AFM materials in order to learn more about their ultrafast dynamics. In Ho, a metallic AFM below T ≈ 130 K, we found that the AFM Ho can not only be faster but also ten times more energy efficiently destroyed as order in FM comparable metals. In EuTe, an AFM semiconductor below T ≈ 10 K, we compared the loss of magnetization and laser-induced structural distortion in one and the same experiment. Our experiment shows that they are effectively disentangled. An exception is an ultrafast release of lattice dynamics, which we assign to the release of magnetostriction. The results presented here were obtained with time-resolved resonant soft x-ray diffraction at the Femtoslicing source of the Helmholtz-Zentrum Berlin and at the free-electron laser in Stanford (LCLS). In addition the development and setup of a new UHV-diffractometer for these experiments will be reported.}, language = {en} } @article{GaalSchickHerzogetal.2014, author = {Gaal, Peter and Schick, Daniel and Herzog, Marc and Bojahr, Andre and Shayduk, Roman and Goldshteyn, Jevgeni and Leitenberger, Wolfram and Vrejoiu, Ionela and Khakhulin, Dmitry and Wulff, Michael and Bargheer, Matias}, title = {Ultrafast switching of hard X-rays}, series = {Journal of synchrotron radiation}, volume = {21}, journal = {Journal of synchrotron radiation}, publisher = {Wiley-Blackwell}, address = {Hoboken}, issn = {0909-0495}, doi = {10.1107/S1600577513031949}, pages = {380 -- 385}, year = {2014}, abstract = {A new concept for shortening hard X-ray pulses emitted from a third-generation synchrotron source down to few picoseconds is presented. The device, called the PicoSwitch, exploits the dynamics of coherent acoustic phonons in a photo-excited thin film. A characterization of the structure demonstrates switching times of <= 5 ps and a peak reflectivity of similar to 10(-3). The device is tested in a real synchrotron-based pump-probe experiment and reveals features of coherent phonon propagation in a second thin film sample, thus demonstrating the potential to significantly improve the temporal resolution at existing synchrotron facilities.}, language = {en} } @phdthesis{Goswami2014, author = {Goswami, Bedartha}, title = {Uncertainties in climate data analysis}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-78312}, school = {Universit{\"a}t Potsdam}, year = {2014}, abstract = {Scientific inquiry requires that we formulate not only what we know, but also what we do not know and by how much. In climate data analysis, this involves an accurate specification of measured quantities and a consequent analysis that consciously propagates the measurement errors at each step. The dissertation presents a thorough analytical method to quantify errors of measurement inherent in paleoclimate data. An additional focus are the uncertainties in assessing the coupling between different factors that influence the global mean temperature (GMT). Paleoclimate studies critically rely on `proxy variables' that record climatic signals in natural archives. However, such proxy records inherently involve uncertainties in determining the age of the signal. We present a generic Bayesian approach to analytically determine the proxy record along with its associated uncertainty, resulting in a time-ordered sequence of correlated probability distributions rather than a precise time series. We further develop a recurrence based method to detect dynamical events from the proxy probability distributions. The methods are validated with synthetic examples and demonstrated with real-world proxy records. The proxy estimation step reveals the interrelations between proxy variability and uncertainty. The recurrence analysis of the East Asian Summer Monsoon during the last 9000 years confirms the well-known `dry' events at 8200 and 4400 BP, plus an additional significantly dry event at 6900 BP. We also analyze the network of dependencies surrounding GMT. We find an intricate, directed network with multiple links between the different factors at multiple time delays. We further uncover a significant feedback from the GMT to the El Ni{\~n}o Southern Oscillation at quasi-biennial timescales. The analysis highlights the need of a more nuanced formulation of influences between different climatic factors, as well as the limitations in trying to estimate such dependencies.}, language = {en} }