Recent Publications

Quantitative reconstruction of atomic orbital densities of neon from partial cross sections

The approach of photoemission orbital tomography, i.e., the orbital density reconstruction from photoemission of planar molecular layers by using a formalism equivalent to a Fourier transformation, is transferred to free atoms. Absolute radial orbital densities of neon 1s, 2s, and 2p orbitals are reconstructed with a central-field one-electron model, using well-known atomic photoionization data. The model parameters are optimized by a Markov chain Monte Carlo method with Bayesian inference from which uncertainties for the reconstructed orbital densities are derived. The presented model opens the path for photoemission orbital tomography as a powerful tool, as well as for a quantitative analysis.

Phys. Rev. A 109, 012814 (2024)

Orbital-selective metal skin induced by alkali-metal-dosing Mott-insulating Ca2RuO4

Doped Mott insulators are the starting point for interesting physics such as high temperature superconductivity and quantum spin liquids. For multi-band Mott insulators, orbital selective ground states have been envisioned. However, orbital selective metals and Mott insulators have been difficult to realize experimentally. Here we demonstrate by photoemission spectroscopy how Ca2RuO4, upon alkali-metal surface doping, develops a single-band metal skin. Our dynamical mean field theory calculations reveal that homogeneous electron doping of Ca2RuO4 results in a multi-band metal. All together, our results provide evidence for an orbital-selective Mott insulator breakdown, which is unachievable via simple electron doping. Supported by a cluster model and cluster perturbation theory calculations, we demonstrate a type of skin metal-insulator transition induced by surface dopants that orbital-selectively hybridize with the bulk Mott state and in turn produce coherent in-gap states.

Commun Phys 6, 323 (2023)

 

Electron Glass Phase with Resilient Zhang-Rice Singlets in LiCu3O3

LiCu3O3 is an antiferromagnetic mixed valence cuprate where trilayers of edge-sharing Cu(II)O (3d9) are sandwiched in between planes of Cu(I) (3d10) ions, with Li stochastically substituting Cu(II). Angle- resolved photoemission spectroscopy (ARPES) and density functional theory reveal two insulating electronic subsystems that are segregated in spite of sharing common oxygen atoms: a Cu dz2 =O p z derived valence band (VB) dispersing on the Cu(I) plane, and a Cu 3d x2−y2 / O 2px,y derived Zhang-Rice singlet (ZRS) band dispersing on the Cu(II)O planes. First-principle analysis shows the Li substitution to stabilize the insulating ground state, but only if antiferromagnetic correlations are present. Li further induces substitutional disorder and a 2D electron glass behavior in charge transport, reflected in a large 530 meV Coulomb gap and a linear suppression of VB spectral weight at E F that is observed by ARPES. Surprisingly, the disorder leaves the Cu(II)-derived ZRS largely unaffected. This indicates a local segregation of Li and Cu atoms onto the two separate corner-sharing CuðIIÞO2 sub-lattices of the edge- sharing Cu(II)O planes, and highlights the ubiquitous resilience of the entangled two hole ZRS entity against impurity scattering.

Phys. Rev. Lett. 132, 126502 (20244)

 

Research Groups

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.

Cooperations