Photoemission study of the metal-insulator transition inVO2/TiO2(001): Evidence for strong electron-electron and electron-phonon interaction

Abstract

We have made a detailed temperature-dependent photoemission study of ${\mathrm{VO}}_2/{\mathrm{TiO}}_2\mathrm{}\left(001\right)\mathrm{}$ thin films, which show a metal-insulator transition at ∼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.