Unusual electronic states in TiO$_2$/VO$_2$ (001) multilayers.

Abrupt interfaces between oxides display a wealth of unexpected behavior, and the interface between a band insulator and a Mott insulator is expected to display extra richness. Several multilayered TiO$_2$/VO$_2$ structures have been studied by ab initio density functional theory techniques, including the thin VO$_2$ regime corresponding to the quantum confined Mott insulator. VO$_2$ undergoes a metal-insulator transition near room temperature, but when deposited in thin films of thickness smaller than 5 nm, the metal-insulator transition disappears. Our calculations (using the correlated LDA+U method with modest values of U and J) show that the electronic character (metallic versus insulating) changes with the number of VO$_2$ layers embedded within insulating TiO$_2$ layers: metallic for five VO$_2$ layers, semimetallic and half-metallic for three layers, and insulating for a single VO$_2$ layer. These trends, and the peculiar nature of the three VO$_2$ layer case, will be discussed in some detail.