Electronic structure studies ofV6O13by soft x-ray emission spectroscopy: Band-like and excitonic vanadium states

Abstract

Resonant soft x-ray emission (SXE) spectra of the mixed valence vanadium oxide ${\mathrm{V}}_{6}{\mathrm{O}}_{13}$ have been recorded for a series of excitation energies across the V $L$-absorption band. Resonant excitation allows one to distinguish between charge neutral low-energy excitations and continuum-excited, more band-like V $3d$ valence band states in the spectra. We find that the V $L$-emission spectra of ${\mathrm{V}}_{6}{\mathrm{O}}_{13}$ consist of two distinct components that can be assigned to nearly pure V $3d$ states, and to V $3d$ states that are strongly hybridized with O $2p$ states, respectively. Band structure calculations of the density functional theory support the assignment of these features. At threshold excitation the V $L$-emission spectra of ${\mathrm{V}}_{6}{\mathrm{O}}_{13}$ show strong signatures from excitonic states, the energy dependence of which shows Raman-like behavior. We compare these spectral features in the resonant SXE spectra with cluster model calculations and assign them to $\mathrm{dd}$ excitations and charge-transfer excitations, respectively. Finally, we discuss changes in the V $L$-absorption and emission spectra that take place when changing the sample temperature from 295 K to 120 K. We relate the changes to redistributions in the V $3d$ partial density of states, which occur at the transition temperature for the metal-semiconductor-transition ${T}_{\mathrm{MST}}=150\mathrm{K}$ and find support in our temperature dependent band structure calculations.