Markus Donath

Image-potential surface states are simple model systems, where spin-dependent effects can be studied in view of spintronic applications. The Rashba effect in surface states at high- Z materials is related to both the strong spin-orbit interaction in heavy atoms and the orbital angular momentum arising from the inversion-symmetry breaking at the surface. Our study on image-potential states at Bi 2 Se 3 and Bi 2 Te 3 showcases that the orbital angular momentum causes an intrinsically reversed Rashba parameter α R ≈ − 100 meV Å , i.e., a reversed spin splitting compared with the prototypical Rashba-split Au(111) crystal-induced surface state. We present a consistent picture of spin-resolved experimental data from inverse photoemission and three-photon photoemission together with calculations from density-functional theory and many-body perturbation theory.

The suitability of transition metal dichalcogenides (TMDCs) for the use as scattering targets in electron spin-polarization detectors is put to a paradigmatic test: a case study of MoS 2 is performed. Therefore, the electron reflectivity, the Sherman function, and the figure of merit are measured for the following samples: a single layer of MoS 2 on Au(111) and cleaved MoS 2 single-crystal surfaces. MoS 2 offers promising features for the use in spin-polarization detection: working points with a reversed sign of the Sherman function and a long-term target stability. To overcome the low figure of merit of MoS 2 , the closely-related 2D material WS 2 with higher spin-orbit interaction is proposed, which may resemble the promising features with higher Sherman function. This study demonstrates the potential suitability of 2D materials such as TMDCs as scattering targets in spin-polarization detectors.

The detection of vacuum-ultraviolet photons in inverse-photoemission experiments is usually carried out with gas-filled counting tubes. Iodine/argon gas fillings have been used for Geiger-Müller-type photon detection. Later, the gas filling was replaced by acetone to increase stability and the operation was changed to proportional mode. While doing this, the counting tube geometry was not necessarily changed. We carried out a systematic study to understand the photon-detection process in acetone-filled counting tubes and to optimize the geometry accordingly. In the range of optimal gas pressures, we found that the photon mean free path in acetone is in the order of a few millimeters instead of a few centimeters in the iodine/argon case. As a consequence, the position of the cathode wire close to the entrance window becomes a critical parameter for optimizing the detection efficiency.

In angle-resolved photoelectron spectroscopy (ARPES) experiments, the use of polarized light for excitation provides access to the orbital symmetries of the electronic states under investigation. In angle-resolved inverse photoemission (IPE), the equivalent approach is the polarization analysis of the emitted light. So far, this light-polarization analysis has been neglected for intensity reasons due to the much lower cross section of the transitions in IPE compared with ARPES and the lack of suitable polarizers in the vacuum ultraviolet (VUV) spectral range. In this paper, we present the first VUV polarimeter for IPE. A mirror with a polarizing coating of high reflectivity in Brewster-angle geometry is used. An easy-to-use attachment to established photon detectors with high reflectivity and polarization power is presented and tested. We performed IPE measurements of the well-studied L-gap surface state of Cu(111) with separate detection of p- and s-polarized light. The data show strongly enhanced photon intensity for p polarization compared with the intensity for s polarization. Our results are in accordance with the polarization-dependent ARPES measurements of the occupied part of the surface state.

We present an experimental study that offers first insights into the unoccupied electronic states of a pristine ( 2 × 2 ) R 45 ∘ reconstructed Fe 3 O 4 (100) surface. We initially demonstrate the successful sample preparation by analyzing the surface periodicity and chemical composition by low-energy electron diffraction and Auger electron spectroscopy, respectively. Using angle-resolved inverse photoemission, we study the unoccupied electronic states. We observe three nondispersive emissions, one dominant feature around 2 eV above the Fermi level and two additional ones at higher energies. For the low-lying triple-split feature, we propose two possible interpretations based on former theoretical studies. We either ascribe it to 3 d -related bandgroups originating from tetrahedrally and octahedrally coordinated bulk iron atoms or attribute it to a mixture of 3 d states originating from octahedrally coordinated iron atoms within the bulk and the reconstructed surface layer. The high-lying features are ascribed to bulk bands based on the experimentally observed characteristics.

In a combined experimental and theoretical study, we investigate the interplay of spin–orbit interaction (SOI) and exchange interaction (XI) in the electronic structure of ultrathin Ni films on W(110). Using spin- and angle-resolved inverse photoemission, we observe that the size of the spin splitting of Ni-related exchange-split states differs for opposite magnetization directions. A quenched spin splitting for one of the magnetization directions reveals a contribution of SOI on an equal footing with XI. Using density-functional theory calculations, we explore the underlying mechanisms responsible for the experimentally observed coupling of SOI and XI. We find that a hybridization between adsorbate and substrate states, along with a high probability density of the respective states at the heavy W nuclei, cause the strong influence of SOI on the Ni-related exchange-split states.