Inhomogeneous cluster DMFT study on p-n junction Mott gap closing in Hubbard lattice

Mott gap closing induced by an external potential gradient is studied by considering the Hubbard model on a square lattice. Our model consists of hole- and electron-doped domains forming the p-n junction where the on-site potential varies smoothly across the interface. The effect of the potential drop direction, either along square edge or diagonal, is studied by describing the correlation effects of inhomogeneous 2x2 plaquette impurity clusters within a dynamical mean-field theory framework. It is found that the metallic phase is favored over the Mott insulators when the potential drop becomes comparable to the energy eigenvalue difference of an open 2x2 plaquette between the ground state and the first excited state. Given the same associated electric-field strength, the excitation energy is almost independent of the drop direction, which amounts ~0.35t at U=5.8t with the hopping parameter t. Upon insulator-to-metal transition, the correlated metal exhibits a coherent peak at zero energy whose intensity is greater for the drop direction along the square diagonal than along the edge. The main origin is attributed to the preserved degeneracy of two molecular orbitals for the former case, while it undergoes splitting in the energy levels for the latter. Our result is discussed regarding the experimentally measured Mott gap closing thresholds.