Electron correlations in high-T-c superconductors

The electronic structure and excitation spectra in high-T-c superconducting cuprates are examined by using the exact diagonalization method on finite-size clusters. The low-energy electronic states are mapped onto the two-dimensional tJ model. Several excitation spectra in the tJ model are derived as functions of electron concentration and parameter (J/t) value where J is the antiferromagnetic exchange interaction between Cu spins and t is the hopping parameter of charge carriers introduced into the cuprates. The spectra are found to be in accord with those calculated in BCS theory with d(x(2) - y(2))-wave pairing.