Superconductivity from strong correlation: direct transition between a non-degenerate Mott insulator and a superconductor

We introduce a generalized Hubbard model with orbital degrees of freedom in which the atomic limit is non-degenerate. The Mott insulating state, which is reached by increasing the electron-electron repulsion, is therefore a zero-entropy state. The zero-entropy property is shown to introduce significant changes to the usual scenario for the Mott transition within the dynamical mean field theory (DMFT). Namely, the metallic phase and the insulating phase with a spin-gap cannot be continuously connected. Our DMFT study shows that superconductivity appears as a sort of bridge between the two phases. The superconducting gap opens for U-sc < U-Mott, opening a spin-gap in the system that continuously evolves in the insulating gap. The renormalization of the bare repulsive interaction leading to pairing between the electrons is also discussed.