Disproportionation and metallization at low-spin to high-spin transition in multiorbital Mott systems.

We study the thermally driven spin state transition in a two-orbital Hubbard model with crystal-field splitting, which provides a minimal description of the physics of LaCoO(3). We employ the dynamical mean-field theory with a quantum Monte Carlo impurity solver. At intermediate temperatures we find a spin disproportionated phase characterized by a checkerboard order of sites with small and large spin moments. The high temperature transition from the disproportionated to a homogeneous phase is accompanied by a vanishing of the charge gap. With the increasing crystal-field splitting the temperature range of the disproportionated phase shrinks and eventually disappears completely.