Pseudogap state and unusual metallic conductivity in high-Tc cuprate superconductors

Abstract

The BCS-like pairing of polaronic carriers in underdoped and optimally doped high-Tc cuprates above the superconducting transition temperature Tc is considered. Such a BCS-like pairing correlation leads to the formation of bosonic Cooper pairs and the appearance of a pseudogap on the Fermi surface at a characteristic temperature T* > Tc. It is shown that the calculated doping dependence of the BCS-like pseudogap agrees quite well with the experimentally observed doping dependence of the pseudogap in La2–xSrxCuO4 (where x is the doping level). The mechanisms of the unusual metallic transports of different types of charge carriers above and below T* along the CuO2 layers (i.e., ab planes) in high-Tc cuprates are identified. The conductivity of the three types of charge carriers (large polarons, excited Fermi components of bosonic Cooper pairs, and bosonic Cooper pairs themselves) at their scattering by lattice vibrations is considered. It is established that the linear temperature dependence of the in-plane resistivity ρab (T) observed, as a rule, in underdoped and optimally doped cuprates above T* is associated with the scattering of polaronic carriers by acoustic and optical phonons. Theoretical results compared with the experimental data allow for confirming that the different (upward and downward) deviations from the linearity in ρab (T) below T* and the most interesting resistive transitions [i.e., a clear jump and a sharp drop in ρab (T)] at T = T* are caused by the pseudogap effect on the conductivity of the excited Fermi components of bosonic Cooper pairs and by the excessive conductivity of bosonic Cooper pairs in high-Tc cuprates above Tc.