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- ItemVisualization of localized perturbations on a (001) surface of the ferromagnetic semimetal EuB6(College Park, MD : American Physical Society, 2020) Rößler, S.; Jiao, L.; Seiro, S.; Rosa, P.F.S.; Fisk, Z.; Rößler, U.K.; Wirth, S.We performed scanning tunneling microscopy (STM) and spectroscopy on a (001) surface of the ferromagnetic semimetal EuB6. Large-amplitude oscillations emanating from the elastic scattering of electrons by the surface impurities are observed in topography and in differential conductance maps. Fourier transform of the conductance maps embracing these regions indicate a holelike dispersion centered around the Γ point of the two-dimensional Brillouin zone. Using density functional theory slab calculations, we identify a spin-split surface state, which stems from the dangling pz orbitals of the apical boron atom. Hybridization with bulk electronic states leads to a resonance enhancement in certain regions around the Γ point, contributing to the remarkably strong real-space response around static point defects, which are observed in STM measurements.
- ItemSynthetic gravitational horizons in low-dimensional quantum matter(College Park, MD : American Physical Society, 2021) Morice, C.; Moghaddam, A.; Chernyavsky, D.; van Wezel, J.; van den Brink, J.We propose a class of lattice models realizable in a wide range of setups whose low-energy dynamics exactlyreduces to Dirac fields subjected to (1+1)-dimensional [(1+1)D] gravitational backgrounds, including (anti-)deSitter space-time. Wave packets propagating on the lattice exhibit an eternal slowdown for power-law position-dependent hopping integralst(x)∝xγwhenγ 1, signaling the formation of black hole event horizons. Forγ<1 instead the wave packets behave radically different and bounce off the horizon. We show that the eternalslowdown relates to a zero-energy spectral singularity of the lattice model and that the semiclassical wave packetstrajectories coincide with the geodesics on (1+1)D dilaton gravity, paving the way for new and experimentallyfeasible routes to mimic black hole horizons and realize (1+1)D space-times as they appear in certain gravitytheories.
- ItemThickness dependence of the anomalous Nernst effect and the Mott relation of Weyl semimetal Co2MnGa thin films(College Park, MD : American Physical Society, 2020) Park, G.-H.; Reichlova, H.; Schlitz, R.; Lammel, M.; Markou, A.; Swekis, P.; Ritzinger, P.; Kriegner, D.; Noky, J.; Gayles, J.; Sun, Y.; Felser, C.; Nielsch, K.; Goennenwein, S.T.B.; Thomas, A.We report a robust anomalous Nernst effect in Co2MnGa thin films in the thickness regime between 20 and 50 nm. The anomalous Nernst coefficient varied in the range of -2.0 to -3.0 μV/K at 300 K. We demonstrate that the anomalous Hall and Nernst coefficients exhibit similar behavior and fulfill the Mott relation. We simultaneously measure all four transport coefficients of the longitudinal resistivity, transversal resistivity, Seebeck coefficient, and anomalous Nernst coefficient. We connect the values of the measured and calculated Nernst conductivity by using the remaining three magnetothermal transport coefficients, where the Mott relation is still valid. The intrinsic Berry curvature dominates the transport due to the relation between the longitudinal and transversal transport. Therefore, we conclude that the Mott relationship is applicable to describe the magnetothermoelectric transport in Weyl semimetal Co2MnGa as a function of film thickness.
- ItemGeneralization of coupled S-parameter calculation to compute beam impedances in particle accelerators(College Park, MD : American Physical Society, 2020) Flisgen, Thomas; Gjonaj, Erion; Glock, Hans-Walter; Tsakanian, AndranikIn this article, a decomposition approach for the computation of beam coupling impedances is proposed. This approach can account for the mutual electromagnetic coupling in long accelerator structures consisting of several consecutive segments. The method is based on the description of the individual segments using a multimodal network matrix formulation in which the charged particle beam is considered as an additional port. Then, the generalized multimodal network matrices of all segments are combined to a multimodal network matrix of the complete structure. The beam coupling impedance as well as the scattering parameters of the full structure are recovered as particular matrix elements in this multimodal network matrix. The new method generalizes Coupled S-Parameter Calculation (CSC) introduced in earlier work such that charged particle beams are considered. Consequently, the introduced scheme is referred to as CSC. Application examples for realistic accelerator components such as the simulation of a full TESLA 1.3 GHz-cavity of the European XFEL are provided. These simulations demonstrate the high accuracy and numerical performance of the proposed method.
- ItemInfluence of vacuum chamber port terminations on beam coupling impedances(College Park, MD : American Physical Society, 2023) Flisgen, Thomas; Gorgi Zadeh, Shahnam; Gjonaj, ErionVacuum chambers of particle accelerators are typically equipped with radio-frequency couplers. The couplers are employed to excite modes for particle acceleration, to extract the energy of higher-order modes, or for diagnostic purposes. From a network theory perspective, these couplers represent terminal ports by which means the structure can exchange energy with its exterior. Usually, these ports are terminated with fixed impedances corresponding to the characteristic impedances of the coaxial lines attached to them. In this paper, we investigate the influence of the termination conditions of vacuum chambers on beam coupling impedances. For this purpose, we introduce a novel approach that allows us to determine beam coupling impedances for arbitrary port terminations. A full-wave Maxwell solver is employed to determine a generalized scattering matrix of the vacuum chamber and its couplers terminated with prespecified reference impedances. Often, these impedances are chosen to be the characteristic line impedances of the waveguides so that coupler ports are free of reflection. Using the generalized scattering matrix, the beam coupling impedances can be readily determined by means of a computationally inexpensive postprocessing step that takes into account arbitrary impedance loads at the coupler ports. Thus, the influence of various port terminations on the beam coupling impedances can be conveniently examined. This is relevant to improve older structures that were designed when no sophisticated design tools were available or to improve the operation of existing structures for a purpose they were initially not designed for. Using the proposed approach, we investigate the 33-cell 200 MHz traveling-wave accelerating structures of the SPS at CERN. It is shown that port termination conditions do have an important influence on the beam coupling impedance and, therefore, must be taken into account in beam stability considerations.
- ItemValence effect on the thermopower of Eu systems(College Park, MD : American Physical Society, 2020) Stockert, U.; Seiro, S.; Seiro, S.; Caroca-Canales, N.; Hassinger, E.; Hassinger, E.; Geibel, C.We investigated the thermoelectric transport properties of EuNi2P2 and EuIr2Si2 to evaluate the relevance of Kondo interaction and valence fluctuations in these materials. While the thermal conductivities behave conventionally, the thermopower curves exhibit large values with pronounced maxima as typically observed in Ce- and Yb-based heavy-fermion materials. However, neither the positions of these maxima nor the absolute thermopower values at low temperature are in line with the heavy-fermion scenario and the moderately enhanced effective charge carrier masses. Instead, we may relate the thermopower in our materials to the temperature-dependent Eu valence by taking into account changes in the chemical potential. Our analysis confirms that valence fluctuations play an important role in EuNi2P2 and EuIr2Si2.
- ItemField tunable three-dimensional magnetic nanotextures in cobalt-nickel nanowires(College Park, MD : American Physical Society, 2021) Andersen, I.; Wolf, D.; Rodriguez, L.; Lubk, A.; Oliveros, D.; Bran, C.; Niermann, T.; Rößler, U.; Vazquez, M.; Gatel, C.; Snoeck, E.Cylindrical magnetic nanowires with large transversal magnetocrystalline anisotropy have been shown to sustain nontrivial magnetic configurations resulting from the interplay of spatial confinement, exchange, and anisotropies. Exploiting these peculiar three-dimensional (3D) spin configurations and their solitonic inhomogeneities is expected to improve magnetization switching in future spintronics, such as power-saving magnetic memory and logic applications. Here we employ holographic vector-field electron tomography to reconstruct the remanent magnetic states in CoNi nanowires with 10 nm resolution in 3D, with a particular focus on domain walls between remanent states and ubiquitous real-structure effects stemming from irregular morphology and anisotropy variations. By tuning the applied magnetic field direction, both longitudinal and transverse multivortex states of different chiralities and peculiar 3D features such as shifted vortex cores are stabilized. The chiral domain wall between the longitudinal vortices of opposite chiralities exhibits a complex 3D shape characterized by a push out of the central vortex line and a gain in exchange and anisotropy energy. A similar complex 3D texture, including bent vortex lines, forms at the domain boundary between transverse-vortex states and longitudinal configurations. Micromagnetic simulations allow an understanding of the origin of the observed complex magnetic states.
- ItemSuccessful user operation of a superconducting radio-frequency photoelectron gun with Mg cathodes(College Park, MD : American Physical Society, 2021) Teichert, J.; Arnold, A.; Ciovati, G.; Deinert, J.-C.; Evtushenko, P.; Justus, M.; Klopf, J.M.; Kneisel, P.; Kovalev, S.; Kuntzsch, M.; Lehnert, U.; Lu, P.; Ma, S.; Murcek, P.; Michel, P.; Ryzhov, A.; Schaber, J.; Schneider, C.; Schurig, R.; Steinbrück, R.; Vennekate, H.; Will, I.; Xiang, R.At the electron linac for beams with high brilliance and low emittance (ELBE) center for high-power radiation sources, the second version of a superconducting radio-frequency (SRF) photoinjector has been put into operation and has been routinely applied for user operation at the ELBE electron accelerator. SRF guns are suitable for generating a continuous wave electron beam with high average currents and high beam brightness. The SRF gun at ELBE has the goal to generate short electron pulses with bunch charges of 200–300 pC at typical repetition rates of 100 kHz for the production of superradiant, coherent terahertz radiation. The SRF gun includes a 3.5-cell, 1.3-GHz niobium cavity and a superconducting solenoid. A support system with liquid nitrogen (LN2) cooling allows the operation of normal-conducting, high quantum efficiency photocathodes. We present the design and performance of the SRF gun as well as beam measurement results of the operation with Mg photocathodes at an acceleration gradient of 8 MV/m (4 MeV kinetic energy). In the last section, we discuss the SRF gun application for production of coherent terahertz radiation at the ELBE facility.