Theory of high temperature superconductivity based on Fermi liquid theory

Starting with the Fermi liquid state, we explain the superconductivity in s trongly correlated electron systems. For the overdoped cuprates we adopt th e perturbation theory with respect to Coulomb repulsion U and obtain superc onducting states of sufficiently strong correlation. For the optimally dope d cuprates we use the fluctuation-exchange approximation (FLEX) describing the spin fluctuation and calculate the transition temperature to the superc onductivity. For the underdoped ones we take into account the self-energy c orrection due to the strong superconducting fluctuations and successfully e xplain the pseudogap phenomena. In the underdoped case the imaginary part of the self-energy has a peak at the Fermi energy and its real part has a positive slope in the frequency de pendence. These behaviors are opposite to those of the Fermi liquid and exp lain the reduced T-linear term of the specific heat. By including the self- energy correction. we obtain the reduced superconducting transition tempera ture. In this case also the renormalized quasi-particles play an essential role in realizing superconductivity. Even in the pseudogap region. there ex ist the quasi-particles at the Fermi energy, which give rise to the superco nductivity through the gap equation. (C) 2000 Elsevier Science Ltd. All rig hts reserved.