Charge Fluctuation in the d-p Model —Effects of the Repulsive Interaction between d- and p-Holes—

We investigate effects of the repulsive interaction U_pd between holes on the nearest-neighbor Cu- and O-sites in the d-p model, a model suitable to describe the CuO₂ plane in high-temperature superconductors. It is found that the condition for a metal-insulator (M-I) transition to occur at the half-filled case is modified. Furthermore, the charge susceptibility is found to be enhanced by U_pd, except for the region around the critical point of the M-I transition; it indeed diverges at a certain value of U_pd. Possible relevance of the enhanced charge susceptibility to the mechanism of the high T_c superconductivity is discussed.