High Temperature Superconductivity from Strong Correlation

It is important to understand the mechanism of high-temperature superconductivity. It is obvious that the interaction with large energy scale is responsible for high critical temperature $T_c$. The Coulomb interaction is one of candidates that bring about high-temperature superconductivity because its characteristic energy is of the order of eV. There have been many works for the Hubbard model including three-band d-p model with the on-site Coulomb repulsion to investigate a possibility of high-temperature superconductivity. It is, of course, not trivial whether the on-site Coulomb interaction leads to a pairing interaction between two electrons. We argue that high-temperature superconductivity is possible in the strongly correlated region by using the variational Monte Carlo method for the two-dimensional t-U-J-V model. The exchange interaction J and the nearest-neighbour attractive interaction V cooperate with U and will act to enhance the critical temperature.Comment: in Proceedings of International Symposium on Superconductivity 201

Mott transition in cuprate high-temperature superconductors

In this study, we investigate the metal-insulator transition of charge transfer type in high-temperature cuprates. We first show that we must introduce a new band parameter in the three-band d-p model to reproduce the Fermi surface of high temperature cuprates such as BSCCO, YBCO and Hg1201. We present a new wave function of a Mott insulator based on the improved Gutzwiller function, and show that there is a transition from a metal to a charge-transfer insulator for such parameters by using the variational Monte Carlo method. This transition occurs when the level difference $\Delta_{dp}\equiv \epsilon_p-\epsilon_d$ between d and p orbitals reaches a critical value $(\Delta_{dp})_c$. The energy gain $\Delta E$, measured from the limit of large $\Delta_{dp}$, is proportional to $1/\Delta_{dp}$ for $\Delta_{dp}>(\Delta_{dp})_c$: $\Delta E\propto -t_{dp}^2/\Delta_{dp}$. We obtain $(\Delta_{dp})_c\simeq 2t_{dp}$ using the realistic band parameters