Distinctive doping- and temperature-dependent metallic conductivity in moderately doped Y-based cuprate superconductors

The enigmatic pseudogap state and anomalous charge-transport properties of doped high-Tc cuprate superconductors are two hallmark features that distinguish these exotic materials from conventional metals. Here, we investigate the systematic evolution of the emergent pseudogap and the distinctive metallic conductivity in Y-based cuprate superconductors as a function of doping x and temperature T. We argue that the charge carriers in moderately doped cuprates, from underdoped to overdoped regimes, are large polarons and preformed polaronic Cooper pairs, emerging as composite bosons at a temperature T* higher than the superconducting transition temperature Tc and condensing into a Bose superfluid at Tc, thereby driving high-Tc superconductivity that is fairly well described by the Dzhumanov’s theory of Bose-liquid superfluidity. We consider the distinctive in-plane metallic conductivity in these materials as the conductivity of three different types of charge carriers (large polarons, bosonic Cooper pairs and excited polaronic components of these pairs) scattering off lattice vibrations in thin CuO2 layers of finite thickness. We use the appropriate Boltzmann transport equations and the BCS-like pseudogap equation (applicable below T*) to calculate the conductivity of polaronic carriers above T* and that of the two other above-mentioned types of charge carriers below T* in high-Tc cuprates within the relaxation-time approximation. By comparing the calculated doping- and temperature-dependent in-plane resistivity ρab(x,T) with experimental data for various Y-based cuprate superconductors, we find that the T-linear dependence of ρab(x,T) above T* originates from polaron-phonon scattering. In contrast, the pronounced nonlinear temperature dependence of the in-plane resistivity and its various downward and upward deviations from T-linear behavior below T* at different doping levels x arise from the competition between the dominant conductivity of bosonic Cooper pairs and the pseudogap-suppressed conductivity of the excited polaronic components of these pairs. Our results will shed new light on the microscopic origins of the enigmatic pseudogap state and strange-metal states in unconventional high-Tc cuprate superconductors.