Photoemission study of the metal-insulator transition in VO_2/TiO_2(001) : Evidence for strong electron-electron and electron-phonon interaction

TL;DR

This study uses temperature-dependent photoemission to investigate the metal-insulator transition in VO_2/TiO_2(001) films, revealing strong electron-electron and electron-phonon interactions and their effects on spectral features.

Contribution

It provides detailed experimental evidence of spectral weight transfer, incoherent spectral parts, and temperature-dependent band width, highlighting the roles of electron-electron and electron-phonon interactions.

Findings

Spectral weight transfer observed across the transition.

Strong incoherent spectral component in metallic phase.

Temperature-dependent V 3d band width indicating electron-phonon coupling.

Abstract

We have made a detailed temperature-dependent photoemission study of VO_2/TiO_2(001) thin films, which show a metal-insulator transition at \sim 300 K. Clean surfaces were obtained by annealing the films in an oxygen atmosphere. Spectral weight transfer between the coherent and incoherent parts accompanying the metal-insulator transition was clearly observed. We also observed a hysteretic behavior of the spectra for heating-cooling cycles. We have derived the ``bulk'' spectrum of the metallic phase and found that it has a strong incoherent part. The width of the coherent part is comparable to that given by band-structure calculation in spite of its reduced spectral weight, indicating that the momentum dependence of the self-energy is significant. This is attributed to by ferromagnetic fluctuation arising from Hund's rule coupling between different d orbitals as originally proposed by Zylbersztejn and Mott. In the insulating phase, the width of the V 3d band shows strong temperature dependence. We attribute this to electron-phonon interaction and have reproduced it using the independent boson model with a very large coupling constant.