Tobias Wichmann

Van der Waals (vdW) heterostructures allow the engineering of magnetic and electronic properties via interfacial orbital hybridization and charge transfer. The resulting electric fields can give rise to Rashba spin-orbit coupling and Dzyaloshinskii-Moriya interactions, which may stabilize novel two-dimensional magnetic phases. While such emergent magnetic ‘interphases’ may offer a promising platform for spin-based electronics, direct spectroscopic evidence for them is still lacking. Here, we report electric-field tunable Zeeman effects with Landé g -factors up to ≈ 230 at the interface of graphene and the vdW ferromagnet Fe ₃ GeTe ₂ . They arise from an interplay of local-moment canting and the itinerant orbital moments of Fe ₃ GeTe ₂ , producing a giant asymmetric level splitting. Exploiting the inelastic phonon gap of graphene, scanning tunneling spectroscopy allows us to access the buried interface properties, and quantify Faraday-like screening. Our findings provide new insights and opportunities for electrically controlled interface magnetism, e.g. for spintronics and orbitronics devices.

Bi 4 Br 4 is a prototypical quasi-one-dimensional (1D) material in which covalently bonded bismuth bromide chains are arranged in parallel, side-by-side and layer-by-layer, with van der Waals (vdW) gaps in between. So far, two different structures have been reported for this compound, α − Bi 4 Br 4 and β − Bi 4 Br 4 , in both of which neighboring chains are shifted by b / 2 , i.e., half a unit-cell vector in the plane, but which differ in their vertical stacking. While the different layer stacking is known to result in distinct electronic properties—higher-order and weak topological insulators, respectively—the effect of different in-plane arrangements of the atomic chains remains an open question. Here, using scanning tunneling microscopy and spectroscopy (STM/STS), we report an unusual Bi 4 Br 4 (001) structure, with a shift of b / 3 between neighboring chains in the plane and AB layer stacking. We determine shear strain to be the origin of this structure, which can readily result in shifts of neighboring atomic chains because of the weak interchain bonding. For the observed b / 3 structure, the (residual) atomic chain shift corresponds to an in-plane shear strain of γ ≈ 7.5 % . STS reveals a bulk insulating gap and metallic edge states at monolayer surface steps, indicating that, just like for α − Bi 4 Br 4 , monolayers of the b / 3 structure are also quantum spin Hall insulators, in agreement with density functional theory (DFT) calculations. Published by the American Physical Society 2025

Recent years have witnessed a steady progress towards blending two-dimensional quantum materials into technology, with future applications often rooted in the electronic structure. Since crossings and inversions of electronic bands with different orbital characters determine intrinsic quantum transport properties, knowledge of the orbital character is essential. Here, we benchmark angle-resolved photoelectron emission spectroscopy (ARPES) as a tool to experimentally derive orbital characters. For this purpose we study the valence electronic structure of two technologically relevant quantum materials, graphene and WSe 2 , and focus on circular dichroism that is believed to provide sensitivity to the orbital angular momentum. We analyze the contributions related to angular atomic photoionization profiles, interatomic interference, and multiple scattering. Regimes in which initial-state properties could be disentangled from the ARPES maps are critically discussed and the potential of using circular dichroic ARPES as a tool to investigate the spin polarization of initial bands is explored. For the purpose of generalization, results from two additional materials, GdMn 6 Sn 6 and PtTe 2 , are presented in addition. This research demonstrates rich complexity of the underlying physics of circular dichroic ARPES, providing insights that will shape the interpretation of both past and future circular-dichroic ARPES studies.