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 LaCoO3. 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.