Electronic phase diagram of Cr-doped VO2 epitaxial films studied by in situ photoemission spectroscopy

TL;DR

This study maps the electronic phase diagram of Cr-doped VO2 epitaxial films, revealing the interplay of Peierls and Mott mechanisms in the metal-insulator transition through in situ photoemission spectroscopy.

Contribution

It provides new insights into how Cr doping and epitaxial strain influence the electronic phases and transition mechanisms in VO2, highlighting the dominance of Mott physics at higher doping levels.

Findings

TMIT is suppressed by epitaxial strain.

Pseudogap behavior observed at x=0.08.

Mott insulating phase stabilizes at x>0.08.

Abstract

Through in situ photoemission spectroscopy (PES), we investigated the changes in the electronic structure of Cr-doped VO2 films coherently grown on TiO2 (001) substrates. The electronic phase diagram of CrxV1-xO2 is drawn by a combination of electric and spectroscopic measurements. The phase diagram is similar to that of bulk CrxV1-xO2, while the temperature of metal-insulator transition (TMIT) is significantly suppressed by the epitaxial strain effect. In the range of x = 0-0.04, where TMIT remains unchanged as a function of x, the PES spectra show dramatic change across TMIT, demonstrating the characteristic spectral changes associated with the Peierls phenomenon. In contrast, for x > 0.04, the TMIT linearly increases, and the metal-insulator transition (MIT) may disappear at x = 0.08-0.12. The PES spectra at x = 0.08 exhibit pseudogap behavior near the Fermi level, whereas the characteristic temperature-induced change remains almost intact, suggesting the existence of local V-V dimerization. The suppression of V-V dimerization with increasing x was confirmed by polarization-dependent x-ray absorption spectroscopy. These spectroscopic investigations reveal that the energy gap and V 3d states are essentially unchanged with 0 $\le$ x $\le$ 0.08 despite the suppression of V-V dimerization. The invariance of the energy gap with respect to x suggests that the MIT in CrxV1-xO2 arises primarily from the strong electron correlations, namely the Peierls-assisted Mott transition. Meanwhile, the pseudogap at x = 0.08 eventually evolves to a full gap (Mott gap) at x = 0.12, which is consistent with the disappearance of the temperature-dependent MIT in the electronic phase diagram. These results demonstrate that a Mott insulating phase without V-V dimerization is stabilized at x > 0.08 as a result of the superiority of Mott instability over the Peierls one.