Fermi-surface and charge-spin excitations in strongly correlated electron systems

Fermi-surface and low-energy excitations in a variety of strongly correlated electron systems are studied by using the numerically exact diagonalization method in two-dimensional square lattice. In the one-band Hubbard and extended Hubbard models, the electronic structure in both insulating and metallic states are examined. The structure in the periodic Anderson model is also studied and compared with that in the above models. In the tJ model as well as the negative U Hubbard model, the excitation spectra in the superconducting states are calculated. In particular, in the tJ model the superconductivity with d(x² – y²)-wave pairing is found to be mapped onto the BCS state. It is shown that the systematic study in a variety of models provides an opportunity to construct a unified picture of strongly correlated systems.