Photo-excited states in correlated band insulators

We study the photo-excitation dynamics of correlated band insulators, using non-equilibrium dynamical mean-field theory for the ionic Hubbard model. We find two distinct behaviors, depending on the ratio of the on-site interaction $U$ and the bare band gap $\Delta$. For small interactions, the relaxation is characterized by intra-band carrier scattering in relatively rigid bands, leading to a non-thermal intermediate state with separate thermal distributions of electrons and holes. This behavior can be viewed as typical for a band insulator with weak interactions. For larger interaction, on the other hand, we observe a strong modification of the electronic spectrum and a filling-in of the gap after photo-excitation, along with a rapid thermalization of the system. The two behaviors therefore provide a dynamical distinction of a correlated band insulator and a band-insulator, which can differ even when the spectra of the two systems are similar in equilibrium. The crossover happens when the interaction $U$ is comparable to $\Delta$.