Metal-insulator transitions in TiO2/VO2 superlattices

We have fabricated a series of superlattices composed of V1-xWxO2 (x=0 or 0.08 with 1 + 2x d electron per V atom) and TiO2 (no d electron) to investigate the interface and carrier-confinement effects of the metal-insulator phenomena of VO2. This study was also motivated by the prediction of a half-metallic state with a semi-Dirac point at the TiO2/VO2 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, VO2 exhibiting a paramagnetic metal to spin-singlet insulator transition at around room temperature, V0.92W0.08O2 (W:VO2) being metallic down to the lowest temperature, and TiO2 being a wide-gap band insulator. We found no metallic ground state in these superlattices in contradiction with the theoretical prediction. The TiO2/VO2 superlattices always show a resistive transition, corresponding to the metal-insulator transition in the VO2 single layer, at around 300 K for the VO2-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 VO2 along c axis, suggesting the robust spin-singlet bond formation as well as the dimeric lattice distortion persistent even at the TiO2/VO2 interface. In the case of TiO2/W:VO2 superlattices, the insulating ground state revives while the metal-insulator transition temperature decreases from 230 to 95 K as the W:VO2 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 W:VO2 thickness.