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- ItemIncreased static dielectric constant in ZnMnO and ZnCoO thin films with bound magnetic polarons(London : Nature Publishing Group, 2020) Vegesna, S.V.; Bhat, V.J.; Bürger, D.; Dellith, J.; Skorupa, I.; Schmidt, O.G.; Schmidt, H.A novel small signal equivalent circuit model is proposed in the inversion regime of metal/(ZnO, ZnMnO, and ZnCoO) semiconductor/Si3N4 insulator/p-Si semiconductor (MSIS) structures to describe the distinctive nonlinear frequency dependent capacitance (C-F) and conductance (G-F) behaviour in the frequency range from 50 Hz to 1 MHz. We modelled the fully depleted ZnO thin films to extract the static dielectric constant (εr) of ZnO, ZnMnO, and ZnCoO. The extracted enhancement of static dielectric constant in magnetic n-type conducting ZnCoO (εr ≥ 13.0) and ZnMnO (εr ≥ 25.8) in comparison to unmagnetic ZnO (εr = 8.3–9.3) is related to the electrical polarizability of donor-type bound magnetic polarons (BMP) in the several hundred GHz range (120 GHz for CdMnTe). The formation of donor-BMP is enabled in n-type conducting, magnetic ZnO by the s-d exchange interaction between the electron spin of positively charged oxygen vacancies Vo+ in the BMP center and the electron spins of substitutional Mn2+ and Co2+ ions in ZnMnO and ZnCoO, respectively. The BMP radius scales with the Bohr radius which is proportional to the static dielectric constant. Here we show how BMP overlap can be realized in magnetic n-ZnO by increasing its static dielectric constant and guide researchers in the field of transparent spintronics towards ferromagnetism in magnetic, n-ZnO.
- ItemReciprocal space slicing: A time-efficient approach to femtosecond x-ray diffraction(Melville, NY : AIP Publishing LLC, 2021) Zeuschner, S.P.; Mattern, M.; Pudell, J.-E.; von Reppert, A.; Rössle, M.; Leitenberger, W.; Schwarzkopf, J.; Boschker, J.E.; Herzog, M.; Bargheer, M.An experimental technique that allows faster assessment of out-of-plane strain dynamics of thin film heterostructures via x-ray diffraction is presented. In contrast to conventional high-speed reciprocal space-mapping setups, our approach reduces the measurement time drastically due to a fixed measurement geometry with a position-sensitive detector. This means that neither the incident (ω) nor the exit ( 2θ ) diffraction angle is scanned during the strain assessment via x-ray diffraction. Shifts of diffraction peaks on the fixed x-ray area detector originate from an out-of-plane strain within the sample. Quantitative strain assessment requires the determination of a factor relating the observed shift to the change in the reciprocal lattice vector. The factor depends only on the widths of the peak along certain directions in reciprocal space, the diffraction angle of the studied reflection, and the resolution of the instrumental setup. We provide a full theoretical explanation and exemplify the concept with picosecond strain dynamics of a thin layer of NbO2.
- ItemThe boson peak and the first sharp diffraction peak in (As2S3)x(GeS2)1–x glasses(Kyiv : Inst. of Semiconductor Physics, 2021) Stronski, A.V.; Kavetskyy, T.S.; Revutska, L.O.; Kaban, I.; Jóvári, P.; Shportko, K.V.; Sergienko, V.P.; Popovych, M.V.The parameters of the boson peak (BP) and the first sharp diffraction peak (FSDP) in (As2S3)x(GeS2)1x glasses measured using high-resolution Raman spectroscopy and high-energy synchrotron X-ray diffraction measurements are examined as a function of x. It has been found that there is no correlation between the positions of BP and FSDP. The BP position shows a nonlinear composition behavior with a maximum at about x = 0.4, whereas the FSDP position changes virtually linearly with x. The intensities of both BP and FSDP show nonlinear composition dependences with the slope changes at x = 0.4, although there is no direct proportionality. Analysis of the partial structure factors for the glasses with x = 0.2, 0.4 and 0.6 obtained in another study has shown that the cation-cation atomic pairs of Ge–Ge, Ge–As and As–As make the largest contribution to FSDP, where the Ge–Ge and Ge–As pairs are dominant.
- ItemToward ultrafast magnetic depth profiling using time-resolved x-ray resonant magnetic reflectivity(Melville, NY : AIP Publishing LLC, 2021) Chardonnet, Valentin; Hennes, Marcel; Jarrier, Romain; Delaunay, Renaud; Jaouen, Nicolas; Kuhlmann, Marion; Ekanayake, Nagitha; Léveillé, Cyril; von Korff Schmising, Clemens; Schick, Daniel; Yao, Kelvin; Liu, Xuan; Chiuzbăian, Gheorghe S.; Lüning, Jan; Vodungbo, Boris; Jal, EmmanuelleDuring the last two decades, a variety of models have been developed to explain the ultrafast quenching of magnetization following femtosecond optical excitation. These models can be classified into two broad categories, relying either on a local or a non-local transfer of angular momentum. The acquisition of the magnetic depth profiles with femtosecond resolution, using time-resolved x-ray resonant magnetic reflectivity, can distinguish local and non-local effects. Here, we demonstrate the feasibility of this technique in a pump–probe geometry using a custom-built reflectometer at the FLASH2 free-electron laser (FEL). Although FLASH2 is limited to the production of photons with a fundamental wavelength of 4 nm (≃310 eV), we were able to probe close to the Fe L3 edge (706.8 eV) of a magnetic thin film employing the third harmonic of the FEL. Our approach allows us to extract structural and magnetic asymmetry signals revealing two dynamics on different time scales which underpin a non-homogeneous loss of magnetization and a significant dilation of 2 Å of the layer thickness followed by oscillations. Future analysis of the data will pave the way to a full quantitative description of the transient magnetic depth profile combining femtosecond with nanometer resolution, which will provide further insight into the microscopic mechanisms underlying ultrafast demagnetization.
- ItemOn the role of non-diagonal system-environment interactions in bridge-mediated electron transfer(Melville, NY : American Institute of Physics, 2020) Acharyya, Nirmalendu; Ovcharenko, Roman; Fingerhut, Benjamin P.Bridge-mediated electron transfer (ET) between a donor and an acceptor is prototypical for the description of numerous most important ET scenarios. While multi-step ET and the interplay of sequential and direct superexchange transfer pathways in the donor-bridge-acceptor (D-B-A) model are increasingly understood, the influence of off-diagonal system-bath interactions on the transfer dynamics is less explored. Off-diagonal interactions account for the dependence of the ET coupling elements on nuclear coordinates (non-Condon effects) and are typically neglected. Here, we numerically investigate with quasi-adiabatic propagator path integral simulations the impact of off-diagonal system-environment interactions on the transfer dynamics for a wide range of scenarios in the D-B-A model. We demonstrate that off-diagonal system-environment interactions can have profound impact on the bridge-mediated ET dynamics. In the considered scenarios, the dynamics itself does not allow for a rigorous assignment of the underlying transfer mechanism. Furthermore, we demonstrate how off-diagonal system-environment interaction mediates anomalous localization by preventing long-time depopulation of the bridge B and how coherent transfer dynamics between donor D and acceptor A can be facilitated. The arising non-exponential short-time dynamics and coherent oscillations are interpreted within an equivalent Hamiltonian representation of a primary reaction coordinate model that reveals how the complex vibronic interplay of vibrational and electronic degrees of freedom underlying the non-Condon effects can impose donor-to-acceptor coherence transfer on short timescales. © 2020 Author(s).
- ItemLong-range magnetic order in the ~S=1/2 triangular lattice antiferromagnet KCeS2(Amsterdam : SciPost Foundation, 2020) Bastien, Gaël; Rubrecht, Bastian; Haeussler, Ellen; Schlender, Philipp; Zangeneh, Ziba; Avdoshenko, Stanislav; Sarkar, Rajib; Alfonsov, Alexey; Luther, Sven; Onykiienko, Yevhen A.; Walker, Helen C.; Kühne, Hannes; Grinenko, Vadim; Guguchia, Zurab; Kataev, Vladislav; Klauss, Hans-Henning; Hozoi, Liviu; van den Brink, Jeroen; Inosov, Dmytro S.; Büchner, Bernd; Wolter, Anja U.B.; Doert, ThomasRecently, several putative quantum spin liquid (QSL) states were discovered in ~S=1/2 rare-earth based triangular-lattice antiferromagnets (TLAF) with the delafossite structure. A way to clarify the origin of the QSL state in these systems is to identify ways to tune them from the putative QSL state towards long-range magnetic order. Here, we introduce the Ce-based TLAF KCeS2 and show via low-temperature specific heat and μSR investigations that it yields magnetic order below TN=0.38 K despite the same delafossite structure. We identify a well separated ~S=1/2 ground state for KCeS2 from inelastic neutron scattering and embedded-cluster quantum chemical calculations. Magnetization and electron spin resonance measurements on single crystals indicate a strong easy-plane g~factor anisotropy, in agreement with the ab initio calculations. Finally, our specific-heat studies reveal an in-plane anisotropy of the magnetic field-temperature phase diagram which may indicate anisotropic magnetic interactions in KCeS2.
- ItemDirect measurement of Coulomb-laser coupling([London] : Macmillan Publishers Limited, part of Springer Nature, 2021) Azoury, Doron; Krüger, Michael; Bruner, Barry D.; Smirnova, Olga; Dudovich, NiritThe Coulomb interaction between a photoelectron and its parent ion plays an important role in a large range of light-matter interactions. In this paper we obtain a direct insight into the Coulomb interaction and resolve, for the first time, the phase accumulated by the laser-driven electron as it interacts with the Coulomb potential. Applying extreme-ultraviolet interferometry enables us to resolve this phase with attosecond precision over a large energy range. Our findings identify a strong laser-Coulomb coupling, going beyond the standard recollision picture within the strong-field framework. Transformation of the results to the time domain reveals Coulomb-induced delays of the electrons along their trajectories, which vary by tens of attoseconds with the laser field intensity.
- ItemXPS chemical state analysis of sputter depth profiling measurements for annealed TiAl-SiO2 and TiAl-W layer stacks(Chichester [u.a.] : Wiley, 2020) Oswald, Steffen; Lattner, Eric; Seifert, MariettaFor the application of surface acoustic wave sensors at high temperatures, both a high-temperature stable piezoelectric substrate and a suitable metallization for the electrodes are needed. Our current attempt is to use TiAl thin films as metallization because this material is also known to be high temperature stable. In this study, Ti/Al multilayers and Ti-Al alloy layers were prepared in combination with an SiO2 cover layer or a W barrier layer at the interface to the substrate (thermally oxidized Si or Ca3TaGa3Si2O14) as an oxidation protection. To form the high-temperature stable γ-TiAl phase and to test the thermal stability of the layer systems, thermal treatments were done in vacuum at several temperatures. We used X-ray photoelectron spectroscopy (XPS) sputter depth-profiling to investigate the film composition and oxidation behavior. In this paper, we demonstrate how the semiautomatic peak fitting can help to extract beside the elemental information also the chemical information from the measured depth profiles. © 2020 The Authors. Surface and Interface Analysis published by John Wiley & Sons Ltd
- ItemSwitching between Proton Vacancy and Excess Proton Transfer Pathways in the Reaction between 7-Hydroxyquinoline and Formate(Washington, DC : Soc., 2021) Codescu, Marius-Andrei; Weiß, Moritz; Brehm, Martin; Kornilov, Oleg; Sebastiani, Daniel; Nibbering, Erik T. J.Bifunctional or amphoteric photoacids simultaneously present donor (acidic) and acceptor (basic) properties making them useful tools to analyze proton transfer reactions. In protic solvents, the proton exchange between the acid and the base is controlled by the acidity or basicity strength and typically occurs on two different pathways known as protolysis and hydrolysis. We report here how the addition of a formate base will alter the relative importance of the possible reaction pathways of the bifunctional photoacid 7-hydroxyquinoline (7HQ), which has been recently understood to predominantly involve a hydroxide/methoxide transport mechanism between the basic proton-accepting quinoline nitrogen site toward the proton-donating OH group with a time constant of 360 ps in deuterated methanol (CD3OD). We follow the reaction dynamics by probing the IR-active marker modes of the different charged forms of photoexcited 7HQ, and of formic acid (HCOOD) in CD3OD solution. A comparison of the transient IR spectra as a function of formate concentration, and classical molecular dynamics simulations enables us to identify distinct contributions of “tight” (meaning “contact”) and “loose” (i.e., “solvent-separated”) 7HQ–formate reaction pairs in our data. Our results suggest that depending on the orientation of the OH group with respect to the quinoline aromatic ring system, the presence of the formate molecule in a proton relay pathway facilitates a net proton transfer from the proton-donating OH group of 7HQ-N* via the methanol/formate bridge toward the quinoline N site.
- ItemV4 tetrahedral units in AV4X8 lacunar spinels: Near degeneracy, charge fluctuations, and configurational mixing within a valence space of up to 21 d orbitals(2020) Hozoi, L.; Eldeeb, M.S.; Rößler, U.K.All properties of a given molecule or solid are determined by the way valence electrons are distributed over single-particle energy levels. For multiple, closely spaced single-particle levels, different occupation patterns may provide many-electron quantum states that are close in energy, interact, and admix. We address such near-degeneracy electron correlation effects for V4 vanadium tetrahedral units as encountered in the lacunar spinel GaV4S8, explicitly taking into account up to 21 vanadium valence orbitals, and find effective orbital occupation numbers much different as compared to the picture previously laid out on the basis of mean-field calculations. In light of these results, a modified theoretical frame seems necessary to explain the peculiar magnetic properties of lacunar spinels and of related compounds.