arXiv subjects

R. Y. Engel

Publications and source records attributed to R. Y. Engel.

4 recordsLinked to original sources

arXiv2025

Influence of Hydrogen-Incorporation on the Bulk Electronic Structure and Chemical Bonding in Palladium

Palladium hydride is a model system for studying metal-hydrogen interactions. Yet, its bulk electronic structure has proven difficult to directly probe, with most studies to date limited to surface-sensitive photoelectron spectroscopy approaches. This work reports the first in-situ ambient-pressure hard X-ray photoelectron spectroscopy (AP-HAXPES) study of hydrogen incorporation in Pd thin films, providing direct access to bulk chemical and electronic information at elevated hydrogen pressures. Structural characterisation by in-situ X-ray diffraction and neutron reflectometry under comparable conditions establishes a direct correlation between hydrogen loading, lattice expansion, and electronic modifications. Comparison with density functional theory (DFT) reveals how hydrogen stoichiometry and site occupancy govern the density of occupied states near the Fermi level. These results resolve long-standing questions regarding PdH and establish AP-HAXPES as a powerful tool for probing the bulk electronic structure of metal hydrides under realistic conditions.

cond-mat.mtrl-sci↗

arXiv2025

Simultaneous mapping of the ultrafast time and fluence dependence of the laser-induced insulator-to-metal transition in magnetite

Pump-probe methods are a ubiquitous tool in the field of ultrafast dynamic measurements. In recent years, x-ray free-electron laser experiments have gained importance due to their ability to probe with high chemical selectivity and at atomic length scales. Measurements are typically repeated many thousands of times to collect sufficient statistics and vary parameters like delay or fluence, necessitating that initial conditions are restored each time. An alternative is presented by experiments which measure the relevant parameters in a single shot. Here, we present a time-to-space mapping imaging scheme that enables us to record a range of delays and laser fluences in any single shot of the x-ray probe. We demonstrate the use of this scheme by mapping the ultrafast dynamics of the optically induced insulator-to-metal Verwey transition in a magnetite thin film, probed by soft x-ray resonant diffraction. By extrapolating our results toward the conditions found at x-ray free-electron lasers with higher photon energy, we demonstrate that the presented data could be recorded in a single shot.

cond-mat.mtrl-sci↗

arXiv2024

Optical control of 4f orbital state in rare-earth metals

A change of orbital state alters the coupling between ions and their surroundings drastically. Orbital excitations are hence key to understand and control interaction of ions. Rare-earth (RE) elements with strong magneto-crystalline anisotropy (MCA) are important ingredients for magnetic devices. Thus, control of their localized 4f magnetic moments and anisotropy is one major challenge in ultrafast spin physics. With time-resolved X-ray absorption and resonant inelastic scattering experiments, we show for Tb metal that 4f-electronic excitations out of the ground state multiplet occur after optical pumping. These excitations are driven by inelastic 5d-4f-electron scattering, alter the 4f-orbital state and consequently the MCA with important implications for magnetization dynamics in 4f-metals, and more general for the excitation of localized electronic states in correlated materials.

cond-mat.mtrl-sci↗

arXiv2021

Ultrafast modification of the electronic structure of a correlated insulator

A non-trivial balance between Coulomb repulsion and kinematic effects determines the electronic structure of correlated electron materials. The use electromagnetic fields strong enough to rival these native microscopic interactions allows us to study the electronic response as well as the timescales and energies involved in using quantum effects for possible applications. We use element-specific transient x-ray absorption spectroscopy and high-harmonic generation to measure the response to ultrashort off-resonant optical fields in the prototypical correlated electron insulator NiO. Surprisingly, fields of up to 0.22 V/Å leads to no detectable changes on the correlated Ni 3d-orbitals contrary to previous predictions. A transient directional charge transfer is uncovered, a behavior that is captured by first-principles theory. Our results highlight the importance of retardation effects in electronic screening, and pinpoints a key challenge in functionalizing correlated materials for ultrafast device operation.

cond-mat.mtrl-sci↗