Theory for high-Tc superconductors considering inhomogeneous charge distribution

Abstract

We propose a general theory for the dependence of the critical and pseudogap temperatures Tc and T* on the doping concentration for high-Tc oxides, taking into account the charge inhomogeneities in the CuO2 planes. Several recent experiments have revealed that the charge density ρ in a given compound (mostly underdoped) is intrinsic inhomogeneous with large spatial variations which leads to a local charge density ρ(r). These differences in the local charge concentration yield insulator and metallic regions, either in an intrinsic granular or in a stripe morphology. In the metallic region, the inhomogeneous charge density produces also spatial or local distributions which form Cooper pairs at a local superconducting critical temperatures Tc(r) and zero temperature gap Δ0(r). For a given compound, the measured onset of the vanishing gap temperature is identified as the pseudogap temperature, that is, T*, which is the maximum of all Tc(r). Below T*, due to the distribution of Tc(r)’s, there are some superconducting regions surrounded by an insulator or a metallic medium. The transition to a coherent superconducting state corresponds to the percolation threshold among the superconducting regions with different Tc(r)’s. The charge inhomogeneities have been studied by recent scanning tunneling microscopy experiments which provided a model for our phenomenological distribution. To make definite calculations and compare with the experimental results, we derive phase diagrams for the Bi2Sr2CaCu2O8+x(Bi2212), La2−xSrxCuO4(LSCO), and YBa2Cu3O7−y (YBCO) families, with a mean field theory for superconductivity using an extended Hubbard Hamiltonian. We show also that this approach provides insights into several experimental features of high-Tc oxides.

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