Half-metallic semiDirac point generated by quantum confinement in TiO$_2$/VO$_2$ nanostructures

Multilayer VO$_2$/TiO$_2$ nanostructures ($d^1$ - $d^0$ interfaces with no polar discontinuity) are studied with first principles density functional methods including structural relaxation. Quantum confinement of the {\it half metallic} VO$_2$ slab within insulating TiO$_2$ produces an unexpected and unprecendented two-dimensional new state, with a {\it semiDirac} point Fermi surface: spinless charge carriers are effective-mass like along one principal axis, but are massless along the other. Effects of interface imperfection are addressed.