Mott transition, magnetic and orbital orders in the ground state of the two-band Hubbard model using variational slave-spin mean field formalism

Abstract

We study the ground state phase diagram of the degenerate two-band Hubbard model at integer fillings as a function of onsite Hubbard interaction U and Hund’s exchange coupling J. We use a variational slave-spin mean field method which allows symmetry broken states to be studied within the computationally less intensive slave-spin mean field formalism. The results show that at half-filling, the ground state at smaller U is a Slater antiferromagnet with substantial local charge fluctuations. As U is increased, the antiferromagnetic (AF) state develops a Heisenberg behavior, finally undergoing a first-order transition to a Mott insulating AF state at a critical interaction U c which is of the order of the bandwidth. Introducing the Hund’s coupling J correlates the system more and reduces U c drastically. At quarter-filling with one electron per site, the ground state at smaller U is paramagnetic metallic. At finite J, as interaction is increased beyond a lower critical value U c1, it goes to a fully spin polarized ferromagnetic state coexisting with an antiferro-orbital order. Further increase in U beyond a higher critical value U c2 results in the Mott insulating state where local charge fluctuation vanishes.