A Modified Mean Field Theory for Spin Systems with Orbital Degeneracy

In order to understand the ground state of spin systems with orbital degeneracy, we present a modified mean-field theory that includes four order parameters. Our mean-field results suggest that for a small Hund interaction, the flavor liquid state is still stable against the solid state, but long-range orders may be attained in the system with sufficient deviation from the SU(4) limit. Finally, the implications for the experimental observations on the system LaMnO₃ are discussed.