MODEL OF STRONGLY CORRELATED 2D FERMI LIQUIDS BASED ON FERMION-CONDENSATION QUANTUM PHASE TRANSITION

摘要

A model of strongly correlated electron or hole liquids with the fermion condensate is presented and applied to the consideration of quasiparticle excitations in high temperature superconductors, in their superconducting and normal states. Within our model the appearance of the fermion condensate presents a quantum phase transition, that separates the regions of normal and strongly correlated electron liquids. Beyond the fermion condensation quantum phase transition point the quasiparticle system is divided into two subsystems, one containing normal quasiparticles and the other- fermion condensate localized at the Fermi surface. In the superconducting state the quasiparticle dispersion in systems with FC can be presented by two straight lines, characterized by effective masses M_(FC)~* and M_L~*, respectively, and intersecting near the binding energy which is of the order of the superconducting gap. This same quasiparticle picture persists in the normal state, thus manifesting itself over a wide range of temperatures as new energy scales. Arguments are presented that fermion systems with FC have features of a "quantum protectorate". A theory of high temperature superconductivity based on the combination of the fermion-condensation quantum phase transition and the conventional theory of superconductivity is presented. This theory describes maximum values of the superconducting gap which can be as big as Δ_1 ～ 0.1εF with εF being the Fermi level. We show that the critical temperature 2T_c approx= Δ_1. If there exists the pseudogap above T_c then 2T~* approx= Δ_1, and T~* is the temperature at which the pseudogap vanishes. A discontinuity in the specific heat at T_c is calculated. The transition from conventional superconductors to high-T_c ones as a function of the doping level is investigated. The single-particle excitations and their lineshape are also considered. Analyzing experimental data on the high temperature superconductivity in different materials induced by field-effect doping, we show that all these facts can be understood within a theory of the superconductivity based on the fermion condensation quantum phase transition, which can be conceived of as a universal cause of the superconductivity. The main features of a room-temperature superconductor are considered.