Metal-insulator transitions inTiO2/VO2superlattices

Abstract

We have fabricated a series of superlattices composed of ${\text{V}}_{1\ensuremath{-}x}{\text{W}}_{x}{\text{O}}_{2}$ ($x=0$ or 0.08 with $1+2x$ $d$ electron per V atom) and ${\text{TiO}}_{2}$ (no $d$ electron) to investigate the interface and carrier-confinement effects of the metal-insulator phenomena of ${\text{VO}}_{2}$. This study was also motivated by the prediction of a half-metallic state with a semi-Dirac point at the ${\text{TiO}}_{2}/{\text{VO}}_{2}$ interface [V. Pardo and W. E. Pickett, Phys. Rev. Lett. 102, 166803 (2009)]. The growth conditions of the superlattices were optimized so that we could reproduce the known electronic states for the constituent compounds in case of the single-layer films, namely, ${\text{VO}}_{2}$ exhibiting a paramagnetic metal to spin-singlet insulator transition at around room temperature, ${\text{V}}_{0.92}{\text{W}}_{0.08}{\text{O}}_{2}$ $(\text{W}:{\text{VO}}_{2})$ being metallic down to the lowest temperature, and ${\text{TiO}}_{2}$ being a wide-gap band insulator. We found no metallic ground state in these superlattices in contradiction with the theoretical prediction. The ${\text{TiO}}_{2}/{\text{VO}}_{2}$ superlattices always show a resistive transition, corresponding to the metal-insulator transition in the ${\text{VO}}_{2}$ single layer, at around 300 K for the ${\text{VO}}_{2}$-layer thickness varying from 15 monolayers (ML) down to 3 ML. The resistive transition is accompanied with the structural change, consistent with the V-V dimerization in ${\text{VO}}_{2}$ along $c$ axis, suggesting the robust spin-singlet bond formation as well as the dimeric lattice distortion persistent even at the ${\text{TiO}}_{2}/{\text{VO}}_{2}$ interface. In the case of ${\text{TiO}}_{2}/\text{W}:{\text{VO}}_{2}$ superlattices, the insulating ground state revives while the metal-insulator transition temperature decreases from 230 to 95 K as the $\text{W}:{\text{VO}}_{2}$ thickness is increased from 10 to 40 ML. These results indicate the persistent competition between the spin-singlet bond formation and the kinetic energy of correlated electrons regulated by the $\text{W}:{\text{VO}}_{2}$ thickness.