Temperature-driven change in band structure reflecting spin-charge separation of Mott and Kondo insulators

The electronic band structure can change with temperature in Mott and Kondo insulators, even without a phase transition. Here, to clarify the underlying mechanism, the spectral function at nonzero temperature is studied. By considering the selection rules, the spin excited states of Mott and Kondo insulators, whose excitation energies are lower than the charge gap, are shown to emerge in the electronic spectral function at nonzero temperature, exhibiting momentum-shifted magnetic dispersion relations from the band edges, as in the case of the doping-driven Mott transition at zero temperature. Based on this characteristic, we interpret the numerical results for the temperature-driven change in the band structure in the one- and two-dimensional Hubbard models, Hubbard ladder, and one-dimensional periodic Anderson model at half-filling obtained using the cluster perturbation theory with the low-temperature Lanczos method. This characteristic also explains why the band structures of Mott and Kondo insulators can change with temperature even in the energy regime far higher than the temperature and why spectral weights emerge in the energy regime within the band gap, where the excitation energies are lower than the lowest electronic excitation energy from the ground state. Furthermore, if the bandwidth of the spin excitation is comparable to the electronic band gap, the emergent electronic modes can cross the Fermi level and gain spectral weight as the temperature increases, which leads to an insulator-metal crossover. These features are primarily caused by the spin excited states that are transparent in electronic measurements at zero temperature, in contrast to the conventional view where thermal effects on electron-added states, electron-removed states, and static spin correlations are considered to mainly affect the band structure. This innovative perspective provides a different understanding from the conventional view on the electronic states at nonzero temperatures.