Epitaxial-strain-induced insulating phase in 1:1 SrCrO$_3$/SrTiO$_3$ superlattices: A first-principles study

Using first principles calculations, we studied the structure and electronic properties of the 1:1 superlattice combining the magnetic metallic oxide SrCrO$_3$ and the band insulator SrTiO$_3$. We determined the epitaxial-strain dependent ground-state structures of the superlattice using the ``stacking'' method. An insulating polar ground state is found when the tensile strain is greater than 2.2\%. The insulating character of this phase is related to the $d_{xy}^1d_{yz}^1d_{xz}^0$ orbital ordering of the Cr $t_{2g}$ electrons. Specifically, the 1:1 periodic superlattice structure eliminates the Cr-O-Cr bonds in the direction normal to the interface, which reduces the hopping of 3d electrons in this direction. The band widths of $d_{yz}$ and $d_{xz}$ for Cr are thus significantly reduced, enhancing the effect of the in-plane polar distortions in the SrCrO$_3$ layer responsible for opening the the band gap. The elucidation of this mechanism opens an new approach to band engineering, encouraging further investigations on related metallic/dielectric superlattice systems.