TY - JOUR A1 - Petreska, Irina A1 - Pejov, Ljupco A1 - Sandev, Trifce A1 - Kocarev, Ljupčo A1 - Metzler, Ralf T1 - Tuning of the dielectric relaxation and complex susceptibility in a system of polar molecules: a generalised model based on rotational diffusion with resetting JF - Fractal and fractional N2 - The application of the fractional calculus in the mathematical modelling of relaxation processes in complex heterogeneous media has attracted a considerable amount of interest lately. The reason for this is the successful implementation of fractional stochastic and kinetic equations in the studies of non-Debye relaxation. In this work, we consider the rotational diffusion equation with a generalised memory kernel in the context of dielectric relaxation processes in a medium composed of polar molecules. We give an overview of existing models on non-exponential relaxation and introduce an exponential resetting dynamic in the corresponding process. The autocorrelation function and complex susceptibility are analysed in detail. We show that stochastic resetting leads to a saturation of the autocorrelation function to a constant value, in contrast to the case without resetting, for which it decays to zero. The behaviour of the autocorrelation function, as well as the complex susceptibility in the presence of resetting, confirms that the dielectric relaxation dynamics can be tuned by an appropriate choice of the resetting rate. The presented results are general and flexible, and they will be of interest for the theoretical description of non-trivial relaxation dynamics in heterogeneous systems composed of polar molecules. KW - rotational diffusion KW - memory kernel KW - Fokker-Planck equation KW - non-exponential relaxation KW - autocorrelation function KW - complex KW - susceptibility Y1 - 2022 U6 - https://doi.org/10.3390/fractalfract6020088 SN - 2504-3110 VL - 6 IS - 2 PB - MDPI AG, Fractal Fract Editorial Office CY - Basel ER - TY - JOUR A1 - Petrov, Polina A1 - Singer, Leo P. A1 - Coughlin, Michael W. A1 - Kumar, Vishwesh A1 - Almualla, Mouza A1 - Anand, Shreya A1 - Bulla, Mattia A1 - Dietrich, Tim A1 - Foucart, Francois A1 - Guessoum, Nidhal T1 - Data-driven expectations for electromagnetic counterpart searches based on LIGO/Virgo public alerts JF - The astrophysical journal : an international review of spectroscopy and astronomical physics; part 1 N2 - Searches for electromagnetic counterparts of gravitational-wave signals have redoubled since the first detection in 2017 of a binary neutron star merger with a gamma-ray burst, optical/infrared kilonova, and panchromatic afterglow. Yet, one LIGO/Virgo observing run later, there has not yet been a second, secure identification of an electromagnetic counterpart. This is not surprising given that the localization uncertainties of events in LIGO and Virgo's third observing run, O3, were much larger than predicted. We explain this by showing that improvements in data analysis that now allow LIGO/Virgo to detect weaker and hence more poorly localized events have increased the overall number of detections, of which well-localized, gold-plated events make up a smaller proportion overall. We present simulations of the next two LIGO/Virgo/KAGRA observing runs, O4 and O5, that are grounded in the statistics of O3 public alerts. To illustrate the significant impact that the updated predictions can have, we study the follow-up strategy for the Zwicky Transient Facility. Realistic and timely forecasting of gravitational-wave localization accuracy is paramount given the large commitments of telescope time and the need to prioritize which events are followed up. We include a data release of our simulated localizations as a public proposal planning resource for astronomers. Y1 - 2022 U6 - https://doi.org/10.3847/1538-4357/ac366d SN - 1538-4357 VL - 924 IS - 2 PB - Institute of Physics Publ. CY - London ER - TY - THES A1 - Piankova, Diana T1 - Electron pair distribution function (ePDF) analysis and advanced transmission electron microscopy (TEM) techniques BT - a perfect tandem to study the evolution of short-range order in materials Y1 - 2022 ER - TY - JOUR A1 - Pietzsch, Annette A1 - Niskanen, Johannes A1 - Vaz da Cruz, Vinicius A1 - Büchner, Robby A1 - Eckert, Sebastian A1 - Fondell, Mattis A1 - Jay, Raphael Martin A1 - Lu, Xingye A1 - McNally, Daniel A1 - Schmitt, Thorsten A1 - Föhlisch, Alexander T1 - Cuts through the manifold of molecular H2O potential energy surfaces in liquid water at ambient conditions JF - Proceedings of the National Academy of Sciences of the United States of America N2 - The fluctuating hydrogen bridge bonded network of liquid water at ambient conditions entails a varied ensemble of the underlying constituting H2O molecular moieties. This is mirrored in a manifold of the H2O molecular potentials. Subnatural line width resonant inelastic X-ray scattering allowed us to quantify the manifold of molecular potential energy surfaces along the H2O symmetric normal mode and the local asymmetric O-H bond coordinate up to 1 and 1.5 angstrom, respectively. The comparison of the single H2O molecular potentials and spectroscopic signatures with the ambient conditions liquid phase H2O molecular potentials is done on various levels. In the gas phase, first principles, Morse potentials, and stepwise harmonic potential reconstruction have been employed and benchmarked. In the liquid phase the determination of the potential energy manifold along the local asymmetric O-H bond coordinate from resonant inelastic X-ray scattering via the bound state oxygen ls to 4a(1) resonance is treated within these frameworks. The potential energy surface manifold along the symmetric stretch from resonant inelastic X-ray scattering via the oxygen 1 s to 2b(2) resonance is based on stepwise harmonic reconstruction. We find in liquid water at ambient conditions H2O molecular potentials ranging from the weak interaction limit to strongly distorted potentials which are put into perspective to established parameters, i.e., intermolecular O-H, H-H, and O-O correlation lengths from neutron scattering. KW - water KW - potential ene rgy surface KW - RIXS Y1 - 2022 U6 - https://doi.org/10.1073/pnas.2118101119 SN - 1091-6490 VL - 119 IS - 28 PB - National Acad. of Sciences CY - Washington, DC ER - TY - JOUR A1 - Poelking, Carl A1 - Benduhn, Johannes A1 - Spoltore, Donato A1 - Schwarze, Martin A1 - Roland, Steffen A1 - Piersimoni, Fortunato A1 - Neher, Dieter A1 - Leo, Karl A1 - Vandewal, Koen A1 - Andrienko, Denis T1 - Open-circuit voltage of organic solar cells BT - interfacial roughness makes the difference JF - Communications physics N2 - Organic photovoltaics (PV) is an energy-harvesting technology that offers many advantages, such as flexibility, low weight and cost, as well as environmentally benign materials and manufacturing techniques. Despite growth of power conversion efficiencies to around 19 % in the last years, organic PVs still lag behind inorganic PV technologies, mainly due to high losses in open-circuit voltage. Understanding and improving open circuit voltage in organic solar cells is challenging, as it is controlled by the properties of a donor-acceptor interface where the optical excitations are separated into charge carriers. Here, we provide an electrostatic model of a rough donor-acceptor interface and test it experimentally on small molecule PV materials systems. The model provides concise relationships between the open-circuit voltage, photovoltaic gap, charge-transfer state energy, and interfacial morphology. In particular, we show that the electrostatic bias generated across the interface reduces the photovoltaic gap. This negative influence on open-circuit voltage can, however, be circumvented by adjusting the morphology of the donor-acceptor interface. Organic solar cells, despite their high power conversion efficiencies, suffer from open circuit voltage losses making them less appealing in terms of applications. Here, the authors, supported with experimental data on small molecule photovoltaic cells, relate open circuit voltage to photovoltaic gap, charge-transfer state energy, and donor-acceptor interfacial morphology. Y1 - 2022 U6 - https://doi.org/10.1038/s42005-022-01084-x SN - 2399-3650 VL - 5 IS - 1 PB - Nature portfolio CY - Berlin ER - TY - JOUR A1 - Pohl, Martin A1 - Macias, Oscar A1 - Coleman, Phaedra A1 - Gordon, Chris T1 - Assessing the impact of hydrogen absorption on the characteristics of the Galactic center excess JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - We present a new reconstruction of the distribution of atomic hydrogen in the inner Galaxy that is based on explicit radiation transport modeling of line and continuum emission and a gas-flow model in the barred Galaxy that provides distance resolution for lines of sight toward the Galactic center. The main benefits of the new gas model are (a) the ability to reproduce the negative line signals seen with the HI4PI survey and (b) the accounting for gas that primarily manifests itself through absorption. We apply the new model of Galactic atomic hydrogen to an analysis of the diffuse gamma-ray emission from the inner Galaxy, for which an excess at a few GeV was reported that may be related to dark matter. We find with high significance an improved fit to the diffuse gamma-ray emission observed with the Fermi-LAT, if our new H i model is used to estimate the cosmic-ray induced diffuse gamma-ray emission. The fit still requires a nuclear bulge at high significance. Once this is included there is no evidence of a dark-matter signal, be it cuspy or cored. But an additional so-called boxy bulge is still favored by the data. This finding is robust under the variation of various parameters, for example, the excitation temperature of atomic hydrogen, and a number of tests for systematic issues. Y1 - 2022 U6 - https://doi.org/10.3847/1538-4357/ac6032 SN - 0004-637X SN - 1538-4357 VL - 929 IS - 2 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Poppenhäger, Katja T1 - Helium absorption in exoplanet atmospheres is connected to stellar coronal abundances JF - Monthly notices of the Royal Astronomical Society N2 - Transit observations in the helium triplet around 10 830 Angstrom are a successful tool to study exoplanetary atmospheres and their mass loss. Forming those lines requires ionization and recombination of helium in the exoplanetary atmosphere. This ionization is caused by stellar photons at extreme ultraviolet (EUV) wavelengths; however, no currently active telescopes can observe this part of the stellar spectrum. The relevant part of the stellar EUV spectrum consists of individual emission lines, many of them being formed by iron at coronal temperatures. The stellar iron abundance in the corona is often observed to be depleted for high-activity low-mass stars due to the first ionization potential (FIP) effect. I show that stars with high versus low coronal iron abundances follow different scaling laws that tie together their X-ray emission and the narrow-band EUV flux that causes helium ionization. I also show that the stellar iron to oxygen abundance ratio in the corona can be measured reasonably well from X-ray CCD spectra, yielding similar results to high-resolution X-ray observations. Taking coronal iron abundance into account, the currently observed large scatter in the relationship of EUV irradiation with exoplanetary helium transit depths can be reduced, improving the target selection criteria for exoplanet transmission spectroscopy. In particular, previously puzzling non-detections of helium for Neptunic exoplanets are now in line with expectations from the revised scaling laws. KW - planets and satellites: atmospheres KW - stars: abundances KW - stars: coronae KW - stars: late-type KW - ultraviolet: stars KW - X-rays: stars Y1 - 2022 U6 - https://doi.org/10.1093/mnras/stac507 SN - 0035-8711 SN - 1365-2966 VL - 512 IS - 2 SP - 1751 EP - 1764 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Prol, Fabricio S. A1 - Smirnov, Artem G. A1 - Hoque, M. Mainul A1 - Shprits, Yuri T1 - Combined model of topside ionosphere and plasmasphere derived from radio-occultation and Van Allen Probes data JF - Scientific reports N2 - In the last years, electron density profile functions characterized by a linear dependence on the scale height showed good results when approximating the topside ionosphere. The performance above 800 km, however, is not yet well investigated. This study investigates the capability of the semi-Epstein functions to represent electron density profiles from the peak height up to 20,000 km. Electron density observations recorded by the Van Allen Probes were used to resolve the scale height dependence in the plasmasphere. It was found that the linear dependence of the scale height in the topside ionosphere cannot be directly used to extrapolate profiles above 800 km. We find that the dependence of scale heights on altitude is quadratic in the plasmasphere. A statistical model of the scale heights is therefore proposed. After combining the topside ionosphere and plasmasphere by a unified model, we have obtained good estimations not only in the profile shapes, but also in the Total Electron Content magnitude and distributions when compared to actual measurements from 2013, 2014, 2016 and 2017. Our investigation shows that Van Allen Probes can be merged to radio-occultation data to properly represent the upper ionosphere and plasmasphere by means of a semi-Epstein function. Y1 - 2022 U6 - https://doi.org/10.1038/s41598-022-13302-1 SN - 2045-2322 VL - 12 IS - 1 PB - Macmillan Publishers Limited, part of Springer Nature CY - London ER - TY - THES A1 - Raman Venkatesan, Thulasinath T1 - Tailoring applications-relevant properties in poly(vinylidene fluoride)-based homo-, co- and ter-polymers through modification of their three-phase structure T1 - Maßgeschneiderte anwendungsrelevante Eigenschaften in Homo-, Ko- und Ter-Polymeren auf der Basis von Poly(vinylidenfluorid) durch Modifikation ihrer Dreiphasenstruktur N2 - Poly(vinylidene fluoride) (PVDF)-based homo-, co- and ter-polymers are well-known for their ferroelectric and relaxor-ferroelectric properties. Their semi-crystalline morphology consists of crystalline and amorphous phases, plus interface regions in between, and governs the relevant electro-active properties. In this work, the influence of chemical, thermal and mechanical treatments on the structure and morphology of PVDF-based polymers and on the related ferroelectric/relaxor-ferroelectric properties is investigated. Polymer films were prepared in different ways and subjected to various treatments such as annealing, quenching and stretching. The resulting changes in the transitions and relaxations of the polymer samples were studied by means of dielectric, thermal, mechanical and optical techniques. In particular, the origin(s) behind the mysterious mid-temperature transition (T_{mid}) that is observed in all PVDF-based polymers was assessed. A new hypothesis is proposed to describe the T_{mid} transition as a result of multiple processes taking place within the temperature range of the transition. The contribution of the individual processes to the observed overall transition depends on both the chemical structure of the monomer units and the processing conditions which also affect the melting transition. Quenching results in a decrease of the overall crystallinity and in smaller crystallites. On samples quenched after annealing, notable differences in the fractions of different crystalline phases have been observed when compared to samples that had been slowly cooled. Stretching of poly(vinylidene fluoride-tetrafluoroethylene) (P(VDF-TFE)) films causes an increase in the fraction of the ferroelectric β-phase with simultaneous increments in the melting point (T_m) and the crystallinity (\chi_c) of the copolymer. While an increase in the stretching temperature does not have a profound effect on the amount of the ferroelectric phase, its stability appears to improve. Measurements of the non-linear dielectric permittivity \varepsilon_2^\prime in a poly(vinylidenefluoride-trifluoroethylene-chlorofluoroethylene) (P(VDF-TrFE- CFE)) relaxor-ferroelectric (R-F) terpolymer reveal peaks at 30 and 80 °C that cannot be identified in conventional dielectric spectroscopy. The former peak is associated with T_{mid}\ and may help to understand the non-zero \varepsilon_2^\prime values that are found for the paraelectric terpolymer phase. The latter peak can also be observed during cooling of P(VDF-TrFE) copolymer samples at 100 °C and is due to conduction processes and space-charge polarization as a result of the accumulation of real charges at the electrode-sample interface. Annealing lowers the Curie-transition temperature of the terpolymer as a consequence of its smaller ferroelectric-phase fraction, which by default exists even in terpolymers with relatively high CFE content. Changes in the transition temperatures are in turn related to the behavior of the hysteresis curves observed on differently heat-treated samples. Upon heating, the hysteresis curves evolve from those known for a ferroelectric to those of a typical relaxor-ferroelectric material. Comparing dielectric-hysteresis loops obtained at various temperatures, we find that annealed terpolymer films show higher electric-displacement values and lower coercive fields than the non-annealed samples − irrespective of the measurement temperature − and also exhibit ideal relaxor-ferroelectric behavior at ambient temperatures, which makes them excellent candidates for related applications at or near room temperature. However, non-annealed films − by virtue of their higher ferroelectric activity − show a larger and more stable remanent polarization at room temperature, while annealed samples need to be poled below 0 °C to induce a well-defined polarization. Overall, by modifying the three phases in PVDF-based polymers, it has been demonstrated how the preparation steps and processing conditions can be tailored to achieve the desired properties that are optimal for specific applications. N2 - Homo-, Ko- und Terpolymere auf der Basis von Poly(vinylidenfluorid) (PVDF) sind für ihre ferroelektrischen und relaxor-ferroelektrischen Eigenschaften bekannt. Die teilkristalline Morphologie dieser Fluorpolymere, bestehend aus kristallinen und amorphen sowie den dazwischen liegenden Grenzflächen, bestimmt die relevanten elektroaktiven Eigenschaften. In dieser Arbeit wird der Einfluss chemischer, thermischer und mechanischer Behandlungen auf die Struktur und Morphologie von Polymeren der PVDF-Familie untersucht, die wiederum direkt mit den ferroelektrischen/relaxor-ferroelektrischen Eigenschaften zusammenhängen. Daher wurden Polymerfilme mit verschiedenen Methoden hergestellt und vielfältigen Prozessschritten wie z.B. Tempern, Abschrecken und Recken unterzogen. Die sich daraus ergebenden Veränderungen bei Phasenübergängen und Relaxationen in den Proben wurden mit dielektrischen, thermischen, mechanischen und optischen Verfahren untersucht. Insbesondere wurden die Ursachen für den mysteriösen Übergang bei mittleren Temperaturen (T_{mid}) untersucht, der an allen PVDF-basierten Polymeren beobachtet werden kann. Es wurde eine neue Hypothese aufgestellt, die den T_{mid}-Übergang als Ergebnis mehrerer Prozesse beschreibt, die innerhalb eines Temperaturbereichs mehr oder weniger gleichzeitig ablaufen. Der Beitrag dieser einzelnen Übergänge zum Gesamtübergang hängt sowohl von der chemischen Struktur der Monomereinheiten als auch von den Verarbeitungsbedingungen ab. Die verschiedenen Verarbeitungsbedingungen wirken sich auch auf den Schmelzübergang der Polymere aus. Der Prozess des Abschreckens führt zu einer Abnahme der Gesamtkristallinität mit einer geringeren Kristallitgröße. Bei Proben, die nach dem Tempern abgeschreckt wurden, werden im Vergleich zu Proben, die langsam aus dem getemperten Zustand abgekühlt wurden, bemerkenswerte Unterschiede im Anteil der verschiedenen kristallinen Phasen festgestellt. Das Recken von Poly(vinylidenfluorid-Tetrafluorethylen) (P(VDF-TFE))-Filmen führt zu einem Anstieg des Anteils der ferroelektrischen β-Phase bei gleichzeitiger Erhöhung von Schmelzpunkt (T_m) und Kristallinität (\chi_c) des Kopolymers. Während eine Erhöhung der Strecktemperatur keinen tiefgreifenden Einfluss auf die Menge der ferroelektrischen Phase hat, scheint sich die Stabilität der ferroelektrischen Phase zu verbessern. Messungen der nichtlinearen dielektrischen Dielektrizitätskonstante \varepsilon_2^\prime zeigen in einem relaxor-ferroelektrischen (R-F) Terpolymer aus Poly(vinylidenfluorid-trifluorethylen-chlorfluorethylen) (P(VDF-TrFE- CFE)) Maxima bei 30 und 80 °C, die in der herkömmlichen dielektrischen Spektroskopie nicht identifiziert werden können. Das erste Maximum hängt mit T_{mid}\ zusammen und kann dabei helfen, die von Null verschiedenen \varepsilon_2^\prime-Werte zu verstehen, die an der paraelektrischen Phase des Terpolymers beobachtet werden. Das zweite Maximum kann auch während des Abkühlens von P(VDF-TrFE)-Kopolymerproben bei 100 °C beobachtet werden und ist auf elektrische Leitungsprozesse und Raumladungspolarisationen infolge der Ansammlung von realen Ladungen an den Grenzflächen der Elektroden zum Polymermaterial zurückzuführen. Das Tempern verringert die Curie-Übergangstemperatur des Terpolymers als Folge der Verringerung des ferroelektrischen Phasenanteils, der standardmäßig sogar in Terpolymeren mit relativ hohem CFE-Gehalt vorhanden ist. Die Änderungen der Übergangstemperaturen stehen wiederum im Zusammenhang mit dem Verhalten der Hysteresekurven bei unterschiedlich wärmebehandelten Proben. Während der Erwärmung kommt es zu einer deutlichen Veränderung der Hysteresekurven von einem typisch ferroelektrischen Verhalten hin zu relaxor-ferroelektrischem Verhalten. Vergleicht man die bei verschiedenen Temperaturen beobachteten dielektrischen Hystereseschleifen, so stellt man fest, dass getemperte Terpolymerfilme - unabhängig von der Messtemperatur - höhere dielektrische Verschiebungen und niedrigere Koerzitivfelder aufweisen als nicht getemperte Proben und dass sie auch bei Raumtemperatur ein ideales relaxor-ferroelektrisches Verhalten zeigen, was sie zu ausgezeichneten Kandidaten für Anwendungen in der Nähe der Raumtemperatur macht. Allerdings zeigen nicht getemperte Filme aufgrund ihrer höheren Ferroelektrizität einen höheren und stabilen Wert der remanenten Polarisation bei Raumtemperatur, während getemperte Proben unter 0 °C gepolt werden müssen, um eine eindeutige Polarisation aufzuweisen. Insgesamt konnte gezeigt werden, dass durch die Modifizierung der drei Phasen in PVDF-basierten Polymeren die Präparationsschritte und Verarbeitungsbedingungen so angepasst werden können, dass die gewünschten Eigenschaften für bestimmte Anwendungen optimal sind. KW - PVDF-based polymers KW - structure-property relationships KW - ferroelectric polymers KW - relaxor-ferroelectric polymers KW - mid-temperature transition KW - Polymere auf PVDF-Basis KW - ferroelektrische Polymere KW - Mitteltemperaturübergang KW - Relaxor-ferroelektrische Polymere KW - Struktur-Eigenschafts-Beziehungen Y1 - 2022 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-549667 ER - TY - JOUR A1 - Raman Venkatesan, Thulasinath A1 - Smykalla, David A1 - Ploss, Bernd A1 - Wübbenhorst, Michael A1 - Gerhard, Reimund T1 - Tuning the relaxor-ferroelectric properties of Poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) Terpolymer films by means of thermally induced micro- and nanostructures JF - Macromolecules : a publication of the American Chemical Society N2 - The effects of thermal processing on the micro- and nanostructural features and thus also on the relaxor-ferroelectric properties of a P(VDF-TrFE-CFE) terpolymer were investigated in detail by means of dielectric experiments, such as dielectric relaxation spectroscopy (DRS), dielectric hysteresis loops, and thermally stimulated depolarization currents (TSDCs). The results were correlated with those obtained from differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), and Fourier-transform infrared spectroscopy (FTIR). The results from DRS and DSC show that annealing reduces the Curie transition temperature of the terpolymer, whereas the results from WAXD scans and FTIR spectra help to understand the shift in the Curie transition temperatures as a result of reducing the ferroelectric phase fraction, which by default exists even in terpolymers with relatively high CFE contents. In addition, the TSDC traces reveal that annealing has a similar effect on the midtemperature transition by altering the fraction of constrained amorphous phase at the interphase between the crystalline and the amorphous regions. Changes in the transition temperatures are in turn related to the behavior of the hysteresis curves on differently heat-treated samples. During heating, evolution of the hysteresis curves from ferroelectric to relaxor-ferroelectric, first exhibiting single hysteresis loops and then double hysteresis loops near the Curie transition of the sample, is observed. When comparing the dielectric-hysteresis loops obtained at various temperatures, we find that annealed terpolymer films show higher electric-displacement values and lower coercive fields than the nonannealed sample, irrespective of the measurement temperature, and also exhibit ideal relaxor- ferroelectric behavior at ambient temperatures, which makes them excellent candidates for applications at or near room temperature. By tailoring the annealing conditions, it has been shown that the application temperature could be increased by fine tuning the induced micro- and nanostructures. KW - Annealing (metallurgy) KW - Hysteresis KW - Insulators KW - Phase transitions KW - Polarization Y1 - 2022 U6 - https://doi.org/10.1021/acs.macromol.2c00302 SN - 0024-9297 SN - 1520-5835 VL - 55 IS - 13 SP - 5621 EP - 5635 PB - American Chemical Society CY - Washington ER -