Pseudogap phenomena and superconducting fluctuations in Hubbard model for high-T-c cuprates

The pseudogap phenomena in High-T-c cuprates are investigated on the basis of the Hubbard model which includes only the on-site repulsive interaction U. We consider the pairing scenario for the pseudogap. The pseudogap arises from the resonance scattering due to the strong super-conducting fluctuati ons. First, tile electronic state and the anti-ferromagnetic spin fluctuati ons are calculated by using the fluctuation exchange (FLEX) approximation. The T-matrix which is the propagator of the superconducting fluctuations is calculated by extending the Eliashberg equation. The characteristics of th e superconducting fluctuations due to the pairing interaction arising from the spin fluctuations are represented in the T-matrix. The self-energy due to the superconducting fluctuations is calculated by the T-matrix approxima tion. The pseudogap is shown in the single particle properties and the magn etic properties. Moreover, a comprehensive explanation of the doping depend ence of the pseudogap is obtained. Furthermore, we apply the theory to the electron-doped cuprates and obtain the consistent results with the recent e xperiments. Finally, the self-consistent calculation for the spin fluctuati ons, superconducting fluctuations and the single particle properties is car ried out within the FLEX and the self-consistent T-matrix approximations. T he relation between the superconducting fluctuations and the spin fluctuati ons is clarified. The calculated super-conducting critical temperature T-c is remarkably reduced from the results of the mean field (FLEX) calculation . In particular, it is shown that the critical temperature decreases with d ecreasing doping in the under-doped region with large U.