Multiband superconductivity in ${\rm BiS_2}$-based layered compounds

A mean-field treatment is presented of a square lattice two-orbital-model for ${\rm BiS_2}$ taking into account intra- and inter-orbital superconductivity. A rich phase diagram involving both types of superconductivity is presented as a function of the ratio between the couplings of electrons in the same and different orbitals (${\rm \eta= V_{XX}/V_{XY}}$) and electron doping $x$. With the help of a quantity we call orbital-mixing ratio, denoted as $R(\phi)$, the phase diagram is analyzed using a simple and intuitive picture based on how $R(\phi)$ varies as electron doping increases. The predictive power of $R(\phi)$ suggests that it could be a useful tool in qualitatively (or even semi-quantitatively) analyzing multiband superconductivity in BCS-like superconductors.