Two-dimensional ferromagnetic extension of a topological insulator

Inducing a magnetic gap at the Dirac point of the topological surface state (TSS) in a three-dimensional (3D) topological insulator (TI) is a route to dissipationless charge and spin currents. Ideally, magnetic order is present only at the surface, as through proximity of a ferromagnetic (FM) layer. However, experimental evidence of such a proximity-induced Dirac mass gap is missing, likely due to an insufficient overlap of TSS and the FM subsystem. Here, we take a different approach, namely ferromagnetic extension (FME), using a thin film of the 3D TI Bi 2 Te 3 , interfaced with a monolayer of the lattice-matched van der Waals ferromagnet MnBi 2 Te 4 . Robust 2D ferromagnetism with out-of-plane anisotropy and a critical temperature of Tc ≈ 15 K is demonstrated by x- ray magnetic dichroism and electrical transport measurements. Using angle-resolved photoelectron spectroscopy, we observe the opening of a sizable magnetic gap in the 2D FM phase, while the surface remains gapless in the paramagnetic phase above Tc. Ferromagnetic extension paves the way to explore the interplay of strictly 2D magnetism and topological surface states, providing perspectives for realizing robust quantum anomalous Hall and chiral Majorana states.

Phys. Rev. Research 5, L022019 (2023)

Simple extension of the plane-wave final state in photoemission: Bringing understanding to the photon-energy dependence of two-dimensional materials

Angle-resolved photoemission spectroscopy (ARPES) is a method that measures orbital and band structure contrast through the momentum distribution of photoelectrons. Its simplest interpretation is obtained in the plane-wave approximation, according to which photoelectrons propagate freely to the detector. The photoelectron momentum distribution is then essentially given by the Fourier transform of the real-space orbital. While the plane-wave approximation is remarkably successful in describing the momentum distributions of aromatic compounds, it generally fails to capture kinetic-energy-dependent final-state interference and dichroism effects. Focusing our present study on quasi-freestanding monolayer graphene as the archetypical two-dimensional (2D) material, we observe an exemplary Ekin-dependent modulation of, and a redistribution of spectral weight within, its characteristic horseshoe signature around the ¯K and ¯K′ points: both effects indeed cannot be rationalized by the plane-wave final state. Our data are, however, in remarkable agreement with ab initio time-dependent density functional simulations of a freestanding graphene layer and can be explained by a simple extension of the plane-wave final state, permitting the two dipole-allowed partial waves emitted from the C 2pz orbitals to scatter in the potential of their immediate surroundings...

Phys. Rev. Research 5, 033075 (2023)

Specific Capacitance of RuO2(110) Depends Sensitively on Surface Order

We report the specific capacitance, Cs, of variously ordered RuO2(110) surfaces measured in a 1 M H2SO4 electrolyte with unprecedented precision. Employing ∼10 nm thick, atomically flat epitaxial RuO2 films along with a geometric surface area-controlled electrochemical cell, we determine an upper limit of Cs = (35.35 ± 0.53) μFcm–2 of the idealized, i.e., well-ordered and stoichiometric RuO2(110) surface by electrochemical impedance spectroscopy (EIS) and scan-rate-dependent cyclic voltammetry. We demonstrate, however, that a slight decrease in surface quality immediately translates into pronounced cyclo-voltammetric differences and a significant increase of Cs. This needs to be considered when determining electrochemically active surface areas (ECSAs) of geometrically ill-defined RuO2 catalysts─a benchmark to measure electrochemical performance─from their measured double-layer capacitance.

J. Phys. Chem. C 127, 3682 (2023)

Nanophysics at surfaces

The research activities of our group are concerned with the physics of low-dimensional systems, where the electron states resulting from dimensional confinement lead to unusual conduction properties and to phase transitions as a function of temperature.

Oxide interfaces

Our group focusses on the electronic structure of correlated systems in transition metal oxides (TMOs). Special interest lies in the interplay of different degrees of freedom (charge, spin, orbital, lattice) in the light of metal-insulator and other phase transitions.

Neutron and resonant X-ray spectroscopy

In our group we investigate complex, functional materials such as transition metal oxides, which are used in the emerging field of correlated nanoelectronics. Unlike with conventional semiconductors, exotic superconducting, orbital and magnetic states can be realized at the interfaces in layered structures comprising such materials.