Traces of the evolution from Mott insulator to a band insulator in the pair excitation spectra

We inspect the fundamental difference between the correlated band insulators (BI) and the Mott insulators (MI) from the perspective of the dynamical pair excitations. To this end, we investigated the physics of the two-plane Hubbard model by employing the well-tested dynamical mean field theory (DMFT) together with the quantum Monte Carlo (QMC) method. At half-filling our results clearly indicate that while the spectral weight of the pair excitation becomes minimal at MI which corresponds to a diminishing of the double occupancy, the opposite occurs at BI. We then discuss the effect of doping and find that the correlated band insulator and the Mott insulator robust at low doping concentration and the metallic state emerges at larger doping. The pair spectral function demonstrates that the metallic state of doped MI is strongly different from that of doped BI and it is readily reflected in the lineshape of the spectra. We discuss the implication of our results in the context of the two-particle spectroscopy