Hans-Joachim Elmers

Non-centrosymmetric magnetic materials are a promising platform for stabilizing chiral spin textures, such as skyrmions and cycloidal magnetic states. This is particularly true in epitaxial thin film geometries, where strain and interface effects offer additional control. Herein, we report on the first epitaxial thin films of EuPdSi$_3$ grown by molecular beam epitaxy on MgO(001). X-ray diffraction confirms an epitaxial relationship of tetragonal EuPdSi$_3$ in the BaNiSn$_3$ structure with out-of-plane c-axis orientation and parallel in-plane a-axes. Hard x-ray photoelectron spectroscopy reveals a stable Eu valence of 2.0, yielding a large magnetic moment of approximately 7\,$\upmu_{\text{B}}$ per Eu atom in accordance with Hund’s rule. Owing to the non-centrosymmetric crystal structure, non-collinear magnetic states such as N'{e}el-type skyrmions and cycloidal phases are allowed by symmetry. Electronic transport measurements reveal two magnetic phase transitions at 19\,K and 15\,K in zero applied field. Under magnetic fields applied along the crystallographic [100] and [001] directions, distinct temperature dependent magnetic phases emerge, demonstrating the sensitivity of the magnetic ground state to field orientation in epitaxial EuPdSi$_3$ thin films.

Strain‐induced modifications of the electronic structure in ‐ were studied using hard X‐ray angle‐resolved photoemission spectroscopy (HARPES). Partial substitution of Mo with W stabilizes the metastable low‐temperature orthorhombic phase of . Samples were first characterized by angle‐resolved photoemission spectroscopy experiments with soft X‐rays and higher resolution, showing good agreement with ab initio calculations of electronic states related to topology. The modification of the bulk electronic structure due to the applied strain was assessed using operando HARPES. Applying tensile strain along the ‐axis with amplitudes up to 0.34% result in changes in the bulk electronic structure as predicted by previous ab‐initio calculations. In particular, the results demonstrate a strain‐driven depletion of electronic states with heavy masses and high scattering rates, leading to an increase in conductivity. In contrast, tensile strain along the ‐axis results in a significant increase in resistivity, which we confirmed by transport studies.

Altermagnetism, a novel class of magnetic order, has recently attracted the scientific interest. Combining the favourable properties of ferromagnets, such as spin polarised conduction electrons, with the advantages of antiferromagnets, such as zero net magnetisation and high-frequency switching, it is a promising area of research. While the ground state properties of altermagnets have been studied, their dynamic properties, such as magnon dispersions, remain elusive. Understanding these properties includes the investigation of magnon excitations, which is essential for the development of spintronic and magnonic devices based on altermagnets. In this study, we measure the energy splitting of altermagnetic magnons with opposite chirality in the hexagonal altermagnet CrSb using a unique combination of angle-resolved photoemission spectroscopy and resonant inelastic X-ray scattering. We selectively excite electronic and magnonic states with opposite spins and quantify their energy splitting. We find that the non-degeneracy of electronic and magnonic states exists away from high-symmetry directions in good agreement with recent ab initio calculations. Our experimental proof of chiral magnons demonstrates a new degree of freedom in magnetism that is both of fundamental importance and opens the door to the exploration of the non-reciprocal transport properties of these bosons in devices.

A new type of objective lens has recently been proposed for use in x-ray photoemission electron microscopes (XPEEMs) and momentum microscopes. Adding a ring electrode concentric with the extractor allows the field in the gap between the sample and the extractor to be shaped. Forming a lens field in this gap reduces the field strength at the sample by up to an order of magnitude. This mitigates the risk of field emission, particularly for cleaved samples with sharp edges. A retarding field can redirect all slow electrons, thus eliminating the primary contribution to the space-charge interaction. Here, we present the first experimental investigation of the new lens, examining its performance at photon energies ranging from the extreme ultraviolet (XUV) produced by a high-harmonic generation-based source to soft and hard x rays at two synchrotron facilities. The gap lens in a region without electrodes enables large working distances up to 23 mm. Reduced aberrations allow for larger fields of view in both k-space and real-space imaging, with resolutions comparable to those of conventional cathode lenses. However, field strengths are an order of magnitude smaller. The zero-field mode enables the study of 3D structured objects and is, therefore, beneficial for small cleaved samples as well as for operando devices involving top electrodes. The repeller mode reduces space-charge effects but results in a smaller k-field diameter. This reduction ranges from 10% at hard x-ray energies to 50% in the XUV range. The usable energy interval is also reduced by a factor of two. In time-of-flight XPEEM mode, the raw data show a resolution of 250 nm, which can be improved to better than 100 nm through data processing.

The intricate fine structure of Kikuchi diffraction plays a vital role in probing phase transformations and strain distributions in functional materials, particularly in electron microscopy. Beyond these applications, it also proves essential in photoemission spectroscopy (PES) at high photon energies, aiding in the disentanglement of complex angle-resolved PES data and enabling emitter-site-specific studies. However, the detection and analysis of these rich faint structures in photoelectron diffraction, especially in the hard x-ray regime, remain highly challenging, with only a limited number of simulations successfully reproducing these patterns. The strong energy dependence of Kikuchi patterns further complicates their interpretation, necessitating advanced theoretical approaches. To enhance structural analysis, we present a comprehensive theoretical study of fine diffraction patterns and their evolution with energy by simulating core-level emissions from Ge(100) and Si(100). Using multiple-scattering theory and the fully relativistic one-step photoemission model, we simulate faint pattern networks for various core levels across different kinetic energies (106–4174 eV), avoiding cluster size convergence issues inherent in cluster-based methods. Broadening in patterns is discussed via the inelastic scattering treatment. For the first time, circular dichroism has been observed and successfully reproduced in the angular distribution of Si(100) 1 s , revealing detailed features and asymmetries up to 31%. Notably, we successfully replicate experimental bulk and more “surface-sensitivity” diffraction features, further validating the robustness of our simulations. The results show remarkable agreement with the experimental data obtained using circularly polarized radiations, demonstrating the potential of this methodology for advancing high-energy PES investigations.

This study uses angle-resolved photoemission spectroscopy to examine the low-temperature electronic structure of Cs(V$_{0.95}$Nb$_{0.05}$)$_3$Sb$_5$, demonstrating that partially substituting V atoms with isoelectronic Nb atoms results in \blue{an increase of the band width} and enhanced gap opening at the Dirac-like crossings due to the resulting chemical pressure. This increases the magnetic circular dichroism signal in the angular distribution (MCDAD) compared to CsV$_3$Sb$_5$, enabling detailed analysis of magnetic circular dichroism in several bands near the Fermi level. These results \blue{substantiate} the predicted coupling of orbital magnetic moments to three van Hove singularities near the Fermi level at M points. Previous studies have observed that Nb doping \blue{lowers the charge density transition temperature} and increases the critical temperature for superconductivity. This article demonstrates that Nb doping concomitantly increases the magnetic circular dichroism signal attributed to orbital moments.

Spin-polarized two-dimensional electron gases (2DEGs) are of particular interest for functional oxide electronics applications. The redox-created 2DEG residing on the strontium titanate, SrTiO 3 (STO), side of a europium monoxide ( EuO ) / SrTiO 3 (001) interface is expected to be significantly spin polarized due to the proximity to the strong ( 7 μ B / f . u . ) Heisenberg ferromagnet EuO. We apply magnetic circular dichroism in the angular distribution (MCDAD) of photoemitted electrons to investigate whether and how the induced spin polarization of the 2DEG depends on the dimensionality of the overlaying EuO layer. The experimental data are complemented by density functional theory calculations with a Hubbard U term ( DFT + U ). We show that the EuO/STO interfacial 2DEG is spin polarized even for ultrathin EuO overlayers, starting at an EuO threshold thickness of only two monolayers. Additional EuO monolayers even increase the induced magnetic Ti moment and thus the spin polarization of the 2DEG. Our results and the potential to enhance the magnetic order of EuO by other proximity effects indicate that the EuO/STO(001) interface is an ideal template for creating (multi-)functional spin-polarized 2DEGs for application in oxide electronics. Published by the American Physical Society 2025