Phase separation instability in the Hubbard model

Abstract

The simple cubic Hubbard model is examined at zero temperature, for the case of strong interactions and average densities near but not at the half-filled band. It is shown that the ferromagnetic state which is usually assumed to be the ground state is unstable against phase separation. A new phase diagram is proposed, featuring a two-phase region in which the equilibrium state is separated into macroscopic antiferromagnetic and ferromagnetic phases (of differing density).