TY - JOUR A1 - Kühn, Danilo A1 - Müller, Moritz A1 - Sorgenfrei, Nomi A1 - Giangrisostomi, Erika A1 - Jay, Raphael Martin A1 - Ovsyannikov, Ruslan A1 - Martensson, Nils A1 - Sanchez-Portal, Daniel A1 - Föhlisch, Alexander T1 - Directional sub-femtosecond charge transfer dynamics and the dimensionality of 1T-TaS2 JF - Scientific reports N2 - For the layered transition metal dichalcogenide 1T-TaS2, we establish through a unique experimental approach and density functional theory, how ultrafast charge transfer in 1T-TaS2 takes on isotropic three-dimensional character or anisotropic two-dimensional character, depending on the commensurability of the charge density wave phases of 1T-TaS2. The X-ray spectroscopic core-hole-clock method prepares selectively in-and out-of-plane polarized sulfur 3p orbital occupation with respect to the 1T-TaS2 planes and monitors sub-femtosecond wave packet delocalization. Despite being a prototypical two-dimensional material, isotropic three-dimensional charge transfer is found in the commensurate charge density wave phase (CCDW), indicating strong coupling between layers. In contrast, anisotropic two-dimensional charge transfer occurs for the nearly commensurate phase (NCDW). In direct comparison, theory shows that interlayer interaction in the CCDW phase - not layer stacking variations - causes isotropic three-dimensional charge transfer. This is presumably a general mechanism for phase transitions and tailored properties of dichalcogenides with charge density waves. Y1 - 2019 U6 - https://doi.org/10.1038/s41598-018-36637-0 SN - 2045-2322 VL - 9 IS - 488 PB - Nature Publ. Group CY - London ER - TY - JOUR A1 - Woodfield, Emma E. A1 - Horne, Richard B. A1 - Glauert, S. A. A1 - Menietti, J. D. A1 - Shprits, Yuri Y. A1 - Kurth, William S. T1 - Formation of electron radiation belts at Saturn by Z-mode wave acceleration JF - Nature Communications N2 - At Saturn electrons are trapped in the planet’s magnetic field and accelerated to relativistic energies to form the radiation belts, but how this dramatic increase in electron energy occurs is still unknown. Until now the mechanism of radial diffusion has been assumed but we show here that in-situ acceleration through wave particle interactions, which initial studies dismissed as ineffectual at Saturn, is in fact a vital part of the energetic particle dynamics there. We present evidence from numerical simulations based on Cassini spacecraft data that a particular plasma wave, known as Z-mode, accelerates electrons to MeV energies inside 4 RS (1 RS = 60,330 km) through a Doppler shifted cyclotron resonant interaction. Our results show that the Z-mode waves observed are not oblique as previously assumed and are much better accelerators than O-mode waves, resulting in an electron energy spectrum that closely approaches observed values without any transport effects included. Y1 - 2018 U6 - https://doi.org/10.1038/s41467-018-07549-4 SN - 2041-1723 VL - 9 PB - Nature Publ. Group CY - London ER - TY - JOUR A1 - Cao, Xing A1 - Ni, Binbin A1 - Summers, Danny A1 - Shprits, Yuri Y. A1 - Gu, Xudong A1 - Fu, Song A1 - Lou, Yuequn A1 - Zhang, Yang A1 - Ma, Xin A1 - Zhang, Wenxun A1 - Huang, He A1 - Yi, Juan T1 - Sensitivity of EMIC wave-driven scattering loss of ring current protons to wave normal angle distribution JF - Geophysical research letters N2 - Electromagnetic ion cyclotron waves have long been recognized to play a crucial role in the dynamic loss of ring current protons. While the field-aligned propagation approximation of electromagnetic ion cyclotron waves was widely used to quantify the scattering loss of ring current protons, in this study, we find that the wave normal distribution strongly affects the pitch angle scattering efficiency of protons. Increase of peak normal angle or angular width can considerably reduce the scattering rates of <= 10 keV protons. For >10 keV protons, the field-aligned propagation approximation results in a pronounced underestimate of the scattering of intermediate equatorial pitch angle protons and overestimates the scattering of high equatorial pitch angle protons by orders of magnitude. Our results suggest that the wave normal distribution of electromagnetic ion cyclotron waves plays an important role in the pitch angle evolution and scattering loss of ring current protons and should be incorporated in future global modeling of ring current dynamics. Y1 - 2019 U6 - https://doi.org/10.1029/2018GL081550 SN - 0094-8276 SN - 1944-8007 VL - 46 IS - 2 SP - 590 EP - 598 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Giangrisostomi, Erika A1 - Ovsyannikov, Ruslan A1 - Sorgenfrei, Nomi A1 - Zhang, Teng A1 - Lindblad, Andreas A1 - Sassa, Yasmine A1 - Cappel, Ute B. A1 - Leitner, Torsten A1 - Mitzner, Rolf A1 - Svensson, Svante A1 - Martensson, Nils A1 - Föhlisch, Alexander T1 - Low Dose Photoelectron Spectroscopy at BESSY II BT - electronic structure of matter in its native state JF - Journal of electron spectroscopy and related phenomena : the international journal on theoretical and experimental aspects of electron spectroscopy N2 - The implementation of a high-transmission, angular-resolved time-of-Right electron spectrometer with a 1.25 MHz pulse selector at the PM4 soft X-ray dipole beamline of the synchrotron BESSY II creates unique capabilities to inquire electronic structure via photoelectron spectroscopy with a minimum of radiation dose. Solid-state samples can be prepared and characterized with standard UHV techniques and rapidly transferred from various preparation chambers to a 4-axis temperature-controlled measurement stage. A synchronized MHz laser system enables excited-state characterization and dynamical studies starting from the picosecond timescale. This article introduces the principal characteristics of the PM4 beamline and LowDosePES end-station. Recent results from graphene, an organic hole transport material for solar cells and the transition metal dichalcogenide MoS2 are presented to demonstrate the instrument performances. Y1 - 2018 U6 - https://doi.org/10.1016/j.elspec.2017.05.011 SN - 0368-2048 SN - 1873-2526 VL - 224 SP - 68 EP - 78 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Pontius, Niko A1 - Beye, Martin A1 - Trabant, Christoph A1 - Mitzner, Rolf A1 - Sorgenfrei, Nomi A1 - Kachel, Torsten A1 - Woestmann, Michael A1 - Roling, Sebastian A1 - Zacharias, Helmut A1 - Ivanov, Rosen A1 - Treusch, Rolf A1 - Buchholz, Marcel A1 - Metcalf, Pete A1 - Schuessler-Langeheine, Christian A1 - Föhlisch, Alexander T1 - Probing the non-equilibrium transient state in magnetite by a jitter-free two-color X-ray pump and X-ray probe experiment JF - Structural dynamics N2 - We present a general experimental concept for jitter-free pump and probe experiments at free electron lasers. By generating pump and probe pulse from one and the same X-ray pulse using an optical split-and-delay unit, we obtain a temporal resolution that is limited only by the X-ray pulse lengths. In a two-color X-ray pump and X-ray probe experiment with sub 70 fs temporal resolution, we selectively probe the response of orbital and charge degree of freedom in the prototypical functional oxide magnetite after photoexcitation. We find electronic order to be quenched on a time scale of (30 +/- 30) fs and hence most likely faster than what is to be expected for any lattice dynamics. Our experimental result hints to the formation of a short lived transient state with decoupled electronic and lattice degree of freedom in magnetite. The excitation and relaxation mechanism for X-ray pumping is discussed within a simple model leading to the conclusion that within the first 10 fs the original photoexcitation decays into low-energy electronic excitations comparable to what is achieved by optical pump pulse excitation. Our findings show on which time scales dynamical decoupling of degrees of freedom in functional oxides can be expected and how to probe this selectively with soft X-ray pulses. Results can be expected to provide crucial information for theories for ultrafast behavior of materials and help to develop concepts for novel switching devices. (C) 2018 Author(s). Y1 - 2018 U6 - https://doi.org/10.1063/1.5042847 SN - 2329-7778 VL - 5 IS - 5 PB - American Institute of Physics CY - Melville ER - TY - JOUR A1 - Wang, Dedong A1 - Shprits, Yuri Y. A1 - Zhelayskaya, Irina S. A1 - Agapitov, Oleksiy A1 - Drozdov, Alexander A1 - Aseev, Nikita T1 - Analytical chorus wave model derived from van Allen Probe Observations JF - Journal of geophysical research : Space physics N2 - Chorus waves play an important role in the dynamic evolution of energetic electrons in the Earth's radiation belts and ring current. Using more than 5 years of Van Allen Probe data, we developed a new analytical model for upper‐band chorus (UBC; 0.5fce < f < fce) and lower‐band chorus (LBC; 0.05fce < f < 0.5fce) waves, where fce is the equatorial electron gyrofrequency. By applying polynomial fits to chorus wave root mean square amplitudes, we developed regression models for LBC and UBC as a function of geomagnetic activity (Kp), L, magnetic latitude (λ), and magnetic local time (MLT). Dependence on Kp is separated from the dependence on λ, L, and MLT as Kp‐scaling law to simplify the calculation of diffusion coefficients and inclusion into particle tracing codes. Frequency models for UBC and LBC are also developed, which depends on MLT and magnetic latitude. This empirical model is valid in all MLTs, magnetic latitude up to 20°, Kp ≤ 6, L‐shell range from 3.5 to 6 for LBC and from 4 to 6 for UBC. The dependence of root mean square amplitudes on L are different for different bands, which implies different energy sources for different wave bands. This analytical chorus wave model is convenient for inclusion in quasi‐linear diffusion calculations of electron scattering rates and particle simulations in the inner magnetosphere, especially for the newly developed four‐dimensional codes, which require significantly improved wave parameterizations. KW - chorus waves KW - radiation belt electrons KW - ring current electrons KW - analytical model KW - wave-particle interactions KW - diffusion coefficients Y1 - 2019 U6 - https://doi.org/10.1029/2018JA026183 SN - 2169-9380 SN - 2169-9402 VL - 124 IS - 2 SP - 1063 EP - 1084 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Castillo, Angelica M. A1 - Shprits, Yuri Y. A1 - Ganushkina, Natalia A1 - Drozdov, Alexander A1 - Aseev, Nikita A1 - Wang, Dedong A1 - Dubyagin, Stepan T1 - Simulations of the inner magnetospheric energetic electrons using the IMPTAM-VERB coupled model JF - Journal of Atmospheric and Solar-Terrestrial Physics N2 - In this study, we present initial results of the coupling between the Inner Magnetospheric Particle Transport and Acceleration Model (IMPTAM) and the Versatile Electron Radiation Belt (VERB-3D) code. IMPTAM traces electrons of 10-100 keV energies from the plasma sheet (L = 9 Re) to inner L-shell regions. The flux evolution modeled by IMPTAM is used at the low energy and outer L* computational boundaries of the VERB code (assuming a dipole approximation) to perform radiation belt simulations of energetic electrons. The model was tested on the March 17th, 2013 storm, for a six-day period. Four different simulations were performed and their results compared to satellites observations from Van Allen probes and GOES. The coupled IMPTAM-VERB model reproduces evolution and storm-time features of electron fluxes throughout the studied storm in agreement with the satellite data (within similar to 0.5 orders of magnitude). Including dynamics of the low energy population at L* = 6.6 increases fluxes closer to the heart of the belt and has a strong impact in the VERB simulations at all energies. However, inclusion of magnetopause losses leads to drastic flux decreases even below L* = 3. The dynamics of low energy electrons (max. 10s of keV) do not affect electron fluxes at energies >= 900 keV. Since the IMPTAM-VERB coupled model is only driven by solar wind parameters and the Dst and Kp indexes, it is suitable as a forecasting tool. In this study, we demonstrate that the estimation of electron dynamics with satellite-data-independent models is possible and very accurate. KW - Electron populations KW - Radiation belts KW - IMPTAM KW - VERB Y1 - 2019 U6 - https://doi.org/10.1016/j.jastp.2019.05.014 SN - 1364-6826 SN - 1879-1824 VL - 191 PB - Elsevier CY - Oxford ER - TY - JOUR A1 - Woodfield, Emma E. A1 - Glauert, Saraha A. A1 - Menietti, J. Douglas A1 - Averkamp, Terrance F. A1 - Horne, Richard B. A1 - Shprits, Yuri Y. T1 - Rapid Electron Acceleration in Low‐Density Regions of Saturn's Radiation Belt by Whistler Mode Chorus Waves JF - Geophysical research letters N2 - Electron acceleration at Saturn due to whistler mode chorus waves has previously been assumed to be ineffective; new data closer to the planet show it can be very rapid (factor of 104 flux increase at 1 MeV in 10 days compared to factor of 2). A full survey of chorus waves at Saturn is combined with an improved plasma density model to show that where the plasma frequency falls below the gyrofrequency additional strong resonances are observed favoring electron acceleration. This results in strong chorus acceleration between approximately 2.5 R-S and 5.5 R-S outside which adiabatic transport may dominate. Strong pitch angle dependence results in butterfly pitch angle distributions that flatten over a few days at 100s keV, tens of days at MeV energies which may explain observations of butterfly distributions of MeV electrons near L = 3. Including cross terms in the simulations increases the tendency toward butterfly distributions. Plain Language Summary Radiation belts are hazardous regions found around several of the planets in our Solar System. They consist of very hot, electrically charged particles trapped in the magnetic field of the planet. At Saturn the most important way to heat these particles has for many years been thought to involve the particles drifting closer toward the planet. This paper adds to the emerging idea at Saturn that a different way to heat the particles is also possible where the heating is done by waves, in a similar way to what we find at the Earth. We use recent information from the Cassini spacecraft on the number and location of particles and also of the waves strength and location combined with computer simulations to show that a particular wave called chorus is excellent at heating the particles where the surrounding number of cold particles is low. Y1 - 2019 U6 - https://doi.org/10.1029/2019GL083071 SN - 0094-8276 SN - 1944-8007 VL - 46 IS - 13 SP - 7191 EP - 7198 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Denton, Richard E. A1 - Ofman, L. A1 - Shprits, Yuri Y. A1 - Bortnik, J. A1 - Millan, R. M. A1 - Rodger, C. J. A1 - da Silva, C. L. A1 - Rogers, B. N. A1 - Hudson, M. K. A1 - Liu, K. A1 - Min, K. A1 - Glocer, A. A1 - Komar, C. T1 - Pitch Angle Scattering of Sub-MeV Relativistic Electrons by Electromagnetic Ion Cyclotron Waves JF - Journal of geophysical research : Space physics N2 - Electromagnetic ion cyclotron (EMIC) waves have long been considered to be a significant loss mechanism for relativistic electrons. This has most often been attributed to resonant interactions with the highest amplitude waves. But recent observations have suggested that the dominant energy of electrons precipitated to the atmosphere may often be relatively low, less than 1 MeV, whereas the minimum resonant energy of the highest amplitude waves is often greater than 2 MeV. Here we use relativistic electron test particle simulations in the wavefields of a hybrid code simulation of EMIC waves in dipole geometry in order to show that significant pitch angle scattering can occur due to interaction with low-amplitude short-wavelength EMIC waves. In the case we examined, these waves are in the H band (at frequencies above the He+ gyrofrequency), even though the highest amplitude waves were in the He band frequency range (below the He+ gyrofrequency). We also present wave power distributions for 29 EMIC simulations in straight magnetic field line geometry that show that the high wave number portion of the spectrum is in every case mostly due to the H band waves. Though He band waves are often associated with relativistic electron precipitation, it is possible that the He band waves do not directly scatter the sub-megaelectron volts (sub-MeV) electrons, but that the presence of He band waves is associated with high plasma density which lowers the minimum resonant energy so that these electrons can more easily resonate with the H band waves. KW - electromagnetic ion cyclotron waves KW - EMIC KW - relativistic electron precipitation KW - pitch angle scattering KW - wave particle interaction KW - radiation belts Y1 - 2019 U6 - https://doi.org/10.1029/2018JA026384 SN - 2169-9402 VL - 124 IS - 7 SP - 5610 EP - 5626 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Kühn, Danilo A1 - Giangrisostomi, Erika A1 - Jay, Raphael Martin A1 - Sorgenfrei, Nomi A1 - Föhlisch, Alexander T1 - The influence of x-ray pulse length on space-charge effects in optical pump/x-ray probe photoemission JF - New journal of physics : the open-access journal for physics N2 - Pump-probe photoelectron spectroscopy (PES) is a versatile tool to investigate the dynamics of transient states of excited matter. Vacuum space-charge effects can mask these dynamics and complicate the interpretation of electron spectra. Here we report on space-charge effects in Au 4f photoemission from a polycrystalline gold surface, excited with moderately intense 90 ps (FWHM) soft x-ray probe pulses, under the influence of the Coulomb forces exerted by a pump electron cloud, which was produced by intense 40 fs laser pulses. The experimentally observed kinetic energy shift and spectral broadening of the Au 4f lines, measured with highly-efficient time-of-flight spectroscopy, are in good agreement with simulations utilizing a mean-field model of the electrostatic pump electron potential. This confirms that the line broadening is predominantly caused by variations in the take-off time of the probe electrons without appreciable influence of local scattering events. Our findings might be of general interest for pump-probe PES with picosecond-pulse-length sources. KW - space-charge effects KW - mean-field model KW - x-ray photoemission KW - electron spectroscopy KW - pump-probe KW - ARTOF Y1 - 2019 U6 - https://doi.org/10.1088/1367-2630/ab2f5c SN - 1367-2630 VL - 21 PB - IOP Publ. Ltd. CY - Bristol ER -