Design and realization of topological Dirac fermions on a triangular lattice

Large-gap quantum spin Hall insulators are promising materials for room-temperature applications based on Dirac fermions. Following Kane and Mele’s original suggestion, one approach is to synthesize monolayers of heavy atoms with honeycomb coordination accommodated on templates with hexagonal symmetry. Yet, in the majority of cases, this recipe leads to triangular lattices, typically hosting metals or trivial insulators. Here, we conceive and realize “indenene”, a triangular monolayer of indium on SiC exhibiting non-trivial valley physics driven by local spin-orbit coupling, which prevails over inversion-symmetry breaking terms. By means of tunneling microscopy of the 2D bulk we identify the quantum spin Hall phase of this triangular lattice and unveil how a hidden honeycomb connectivity emerges from interference patterns in Bloch p_x ± ip_y-derived wave functions. [...]

Nature Commun. 12, 5396 (2021)

Bulk spin polarization of magnetite from spin-resolved hard x-ray photoelectron spectroscopy

There is broad consensus that magnetite (Fe₃O₄) is a promising material for spintronics applications due to its high degree of spin polarization at the Fermi level. However, previous attempts to measure the spin polarization by spin-resolved photoemission spectroscopy have been hampered by the use of low photon energies resulting in high surface sensitivity. In this study we determine the intrinsic---i.e., bulk---spin polarization of magnetite by spin-resolved photoelectron spectroscopy on (111)-oriented thin films, epitaxially grown on ZnO(0001), with hard X-rays, making it a truly bulk-sensitive probe. This becomes possible by using a novel, specially adapted momentum microscope, featuring time-of-flight energy recording and an imaging spin-filter. [...]

Phys. Rev. B 104, 045129 (2021)

Controlling the electronic interface properties of AlO_x/SrTiO₃ heterostructures

Depositing disordered Al on top of SrTiO₃ is a cheap and easy way to create a two-dimensional electron system in the SrTiO₃ surface layers. To facilitate future device applications, we passivate the heterostructure by a disordered LaAlO₃ capping layer to study the electronic properties by complementary x-ray photoemission spectroscopy and transport measurements on the very same samples. We also tune the electronic interface properties by adjusting the oxygen pressure during film growth. [...]

Phys. Rev. Mater. 5, 065003 (2021)

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.