Electronic phase separation and emergence of a nondimerized insulating phase in VO${}_2$ $(110{)}_R$ ultrathin films

Abstract

Using in situ photoemission spectroscopy and x-ray absorption spectroscopy, we investigated the thickness dependence of the electronic structure and V-V dimerization in VO${}_2$/TiO${}_2$ (110) ultrathin films, in which the one-dimensional V-V chains along the $c_R$ axis lie in the film plane. In VO${}_2$ $(110{)}_R$ films, the reduction in dimensionality along the surface-normal direction is not expected to impose a geometric constraint on V-V dimerization, unlike in VO${}_2$ $(001{)}_R$ films. Nevertheless, the characteristic spectral changes associated with the temperature-driven metal-insulator transition observed in thick films persist down to 1.5 nm, whereas at 1 nm an insulating electronic phase is observed without V-V dimerization. This behavior is highly similar to that reported for VO${}_2$ $(001{)}_R$, suggesting that the enhancement of Mott instability resulting from reduced dimensionality is a common and essential driving force for the emergence of the nondimerized insulating phase in VO${}_2$ ultrathin films. Meanwhile, unlike in VO${}_2$ $(001{)}_R$, the nondimerized insulating phase in VO${}_2$ $(110{)}_R$ coexists with the phase exhibiting the temperature-driven metal-insulator transition. Its fraction increases exponentially with decreasing thickness and becomes dominant at 1 nm. The corresponding effective critical thickness is estimated to be 2.2 nm. These results imply that the geometric orientation of the V-V chains dictates the spatial evolution of electronic phase separation via strain-mediated phase competition.

30 pages (incl. 8-page Supplement); 4 figures in the main text and 5 in the Supplement. Contact author: Daisuke Shiga