Delocalization of dark and bright excitons in flat-band materials and the optical properties of V$_2$O$_5$

TL;DR

This study reveals how excitons in V$_2$O$_5$ exhibit both high binding energy and large electron-hole separation, with anisotropic delocalization influenced by crystal structure, combining theoretical and experimental approaches.

Contribution

It demonstrates the coexistence of large binding energy and delocalized excitons in V$_2$O$_5$, and explains anisotropic exciton behavior through combined first-principles, experimental, and modeling methods.

Findings

Localized charge-transfer excitons have large binding energy and extended electron-hole separation.

Exciton anisotropy is determined by local structural anisotropy.

Bright and dark excitons coexist with differing binding energies and anisotropies.

Abstract

The simplest picture of excitons in materials with atomic-like localization of electrons is that of Frenkel excitons, where electrons and holes stay close together, which is associated with a large binding energy. Here, using the example of the layered oxide V$_2$O$_5$ , we show how localized charge-transfer excitations combine to form excitons that also have a huge binding energy but, at the same time, a large electron-hole distance, and we explain this seemingly contradictory finding. The anisotropy of the exciton delocalization is determined by the local anisotropy of the structure, whereas the exciton extends orthogonally to the chains formed by the crystal structure. Moreover, we show that the bright exciton goes together with a dark exciton of even larger binding energy and more pronounced anisotropy. These findings are obtained by combining first principles many-body perturbation theory calculations, ellipsometry experiments, and tight binding modelling, leading to very good agreement and a consistent picture. Our explanation is general and can be extended to other materials.