Atomic wires on substrates: Physics between one and two dimensions

Wires having a width of one or two atoms are the smallest possible physical objects that may exhibit one-dimensional properties. In order to be experimentally accessible at finite temperatures, such wires must stabilized by interactions in two and even three dimensions. These interactions modify and partly destroy their one-dimensional properties, but introduce new phenomena of coupling and correlation that entangle both charge and spin. We explore this fascinating field by first giving an overview of the present status of theoretical knowledge on 1D physics, including coupling between chains and to the substrate, before we set out for experimental results on ordered arrays of atomic wires on both flat and vicinal Si(111) surfaces comprising Si(111)-In, Si(hhk)-Au, Si(557)-Pb, Si(557)-Ag, on Ge(001)-Au and of rare earth silicide wires... 

Surf. Sci. Rep. 79,  (2024)

Bias-free access to orbital angular momentum in two-dimensional quantum materials

The demonstration of a topological band inversion constitutes the most elementary proof of a quantum spin Hall insulator (QSHI). On a fundamental level, such an inverted band gap is intrinsically related to the bulk Berry curvature, a gauge-invariant fingerprint of the wave function’s quantum geometric properties in Hilbert space. Intimately tied to orbital angular momentum (OAM), the Berry curvature can be, in principle, extracted from circular dichroism in angle-resolved photoemission spectroscopy (CD-ARPES), were it not for interfering final state photoelectron emission channels that obscure the initial state OAM signature. Here, we outline a full-experimental strategy to avoid such interference artifacts and isolate the clean OAM from the CD-ARPES response. Bench-marking this strategy for the recently discovered atomic monolayer system indenene, we demonstrate its distinct QSHI character and establish CD-ARPES as a scalable bulk probe to experimentally classify the topology of two-dimensional quantum materials with time reversal symmetry

Phys. Rev. Lett. 132, 196401 (2024) Editors' suggestion

Absence of magnetic order in RuO₂: insights from μSR spectroscopy and neutron diffraction

Altermagnets are a novel class of magnetic materials, where magnetic order is staggered both in coordinate and momentum space. The metallic rutile oxide RuO₂ , long believed to be a textbook Pauli paramagnet, recently emerged as a putative workhorse altermagnet when resonant X-ray and neutron scattering studies reported nonzero magnetic moments and long-range collinear order. While some experiments seem consistent with altermagnetism, magnetic order in RuO₂ remains controversial. We show that RuO₂ is nonmagnetic, both in bulk and thin film. Muon spectroscopy complemented by density-functional theory finds at most 1.14 × 10⁻⁴ μB/Ru in bulk and at most 7.5 × 10⁻⁴ μB/Ru in 11 nm epitaxial films, at our spectrometers’ detection limit, and dramatically smaller than previously reported neutron results that were used to rationalize altermagnetic behavior. Our own neutron diffraction measurements on RuO₂ single crystals identify multiple scattering as the source for the false signal in earlier studies.

npj Spintronics 2, 50 (2024).

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.