2018, Kellner, J., Bihlmayer, G., Liebmann, M., Otto, S., Pauly, C., Boschker, J.E., Bragaglia, V., Cecchi, S., Wang, R.N., Deringer, V.L., Küppers, P., Bhaskar, P., Golias, E., Sánchez-Barriga, J., Dronskowski, R., Fauster, T., Rader, O., Calarco, R., Morgenstern, M.

Phase change alloys are used for non-volatile random-access memories exploiting the conductivity contrast between amorphous and metastable, crystalline phase. However, this contrast has never been directly related to the electronic band structure. Here we employ photoelectron spectroscopy to map the relevant bands for metastable, epitaxial GeSbTe films. The constant energy surfaces of the valence band close to the Fermi level are hexagonal tubes with little dispersion perpendicular to the (111) surface. The electron density responsible for transport belongs to the tails of this bulk valence band, which is broadened by disorder, i.e., the Fermi level is 100 meV above the valence band maximum. This result is consistent with transport data of such films in terms of charge carrier density and scattering time. In addition, we find a state in the bulk band gap with linear dispersion, which might be of topological origin.

2023, Krivenkov, M., Marchenko, D., Golias, E., Sajedi, M., Frolov, A.S., Sánchez-Barriga, J., Fedorov, A., Yashina, L.V., Rader, O., Varykhalov, A.

Two-dimensional (2D) Dirac materials are electronically and structurally very sensitive to proximity effects. We demonstrate, however, the opposite effect: that the deposition of a monolayer 2D material could exercise a substantial influence on the substrate electronic structure. Here we investigate TiC(111) and show that a graphene overlayer produces a proximity effect, changing the Fermi surface topology of the TiC from six electron pockets to one hole pocket on the depth of several atomic layers inside the substrate. In addition, the graphene electronic structure undergoes an extreme modification as well. While the Dirac cone remains gapless, it experiences an energy shift of 1.0 eV beyond what was recently achieved for the Lifshitz transition of overdoped graphene. Due to this shift, the antibonding π∗ band at the M¯ point becomes occupied and observable by photoemission.

2021, Golias, E., Kumberg, I., Gelen, I., Thakur, S., Gördes, J., Hosseinifar, R., Guillet, Q., Dewhurst, J.K., Sharma, S., Schüßler-Langeheine, C., Pontius, N., Kuch, W.

We present evidence for an ultrafast optically induced ferromagnetic alignment of antiferromagnetic Mn in Co/Mn multilayers. We observe the transient ferromagnetic signal at the arrival of the pump pulse at the Mn L3 resonance using x-ray magnetic circular dichroism in reflectivity. The timescale of the effect is comparable to the duration of the excitation and occurs before the magnetization in Co is quenched. Theoretical calculations point to the imbalanced population of Mn unoccupied states caused by the Co interface for the emergence of this transient ferromagnetic state.