Walter Lambrecht (CWRU)

Date: Mon. September 19th, 2022, 12:45 pm-1:45 pm
Location: Rockefeller 221 (Foldy) & Zoom

Some surprising findings in 2D oxides

Walter R. L. Lambrecht

Department of Physics, Case Western Reserve University

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Abstract.–Layered and few-layer 2D oxides have remarkable properties. In this talk I will tell you about two oxides, V₂O₅ and LiCoO₂. In both materials the quasiparticle self-consistent GW method is found to strongly overestimate the optical band gap. This is surprising because  the GW method is usually quite accurate in predicting band gaps.   The question is: what is missing in GW? Is it electron-phonon effects or electron-hole effects on the screening of the screened Coulomb interaction W? Or some other 2D effect?  We’ll delve into this mystery story by pursuing each of the hypotheses.  Both turn out to play a role but are insufficient to explain the gap discrepancy. The main reason for the discrepancy is that excitons have a huge binding energy in these materials and strongly modify the optical absorption. In LiCoO₂ they are strongly localized Frenkel excitons but in V₂O₅ they are surprisingly delocalized for having such high binding energy. This talk summarizes our work on these topics over the last decade. Along the way, I’ll also talk about an interesting blue shift of certain phonon modes in V₂O₅  and a recent experimental confirmation  of this prediction.