Title: Electronic Structure and Optical Properties of 2D Oxides
When: Tuesday, May 16, (2023), 12:00
Place: Department of Condensed Matter Physics, Faculty of Sciences, Module 3, Seminar Room (5th Floor).
Speaker: Walter Lambrecht, Case Western Reserve University, Ohio, USA.
Many oxides have layered structures and are thus amenable to exfoliation to create monolayer of few layer systems only a few atoms thick. These can have unique properties different from their bulk counterparts.
In this talk I will focus first on oxocobaltates, such as LiCoO2. Although this material is well known as a battery material, there are still some surprising findings. We find a Li related surface state on fully Li covered surfaces of LiCoO2 and relate its origins to topology of the Su-Schrieffer-Heeger model. It leads to a spin-polarized 2DEG on the surface. We find this is also present when we simply expand the distance between layers, which leads to a symmetry breaking phase transition. A similar effect occurs in NaxCoO2 when the distance between layers is expanded by hydration and this has been found to lead to superconductivity. Turning to larger alkali atoms, we find that KCoO2 has a unique crystal structure with large magnetic moments, which order antiferromagnetically. Surprisingly, the band gaps of these materials and optical properties are not yet well understood. We show that the quasiparticle gap is significantly larger than the optical gap and that excitonic effects are very large in LiCoO2. Finally, I will talk briefly about V2O5, a van der Waals bonded oxide where again we find very large exciton binding energies.