Excitons and dielectric response in bulk and monolayer V2O5

V2O5 is a layered structure material showing large exciton binding en- ergies and a considerable discrepancy between the quasiparticle (QP) and optical gap. We present quasiparticle self-consistent QSGW calculations of the band structure in which W includes ladder diagrams [Cunningham et al. PRB108, 165104 (2023)]. The Bethe Salpeter equation (BSE) approach is used for the optical response functions. We study the gap as a function of interlayer distance L to determine the monolayer limit and find signifi- cantly higher QP gap which is largely compensated by an increase in exciton binding energy in the optical gap. We find a complex spectrum with various dark excitons and study their real space and k-space exciton extent. The excitons examined correspond primarily to the top valence and lowest con- duction bands only but sprad widely in k-space and are localized within a few neighbors in real space.. They exhibit similar extent in the bulk as in the monolayer. We study the spatial extent of the electron for fixed hole position and vice versa. For the polarization perpendicular to the plane the imaginary part of he dielectric function is strongly reduced by BSE compared to the independent particle approcimation in the range 4-10 eV but shows sharp peaks above this range for both bulk and monolayer. We hypothesize that this is an affect of strong local field effects which requires further study.