THERMODYNAMICS OF LOCAL-PAIR SUPERCONDUCTORS WITH ANISOTROPIC LATTICE STRUCTURES

The effective-pseudospin model of the local-electron-pair superconductor, being equivalent to a hard-core charged Bose gas on a lattice, is analyzed for the case of anisotropic lattice structures and arbitrary electron concentration with the use of self-consistent random-phase approximation. The evolution of the ground-state phase diagrams, the low-temperature thermodynamic properties, and the superconducting critical temperature (T(c)), as a function of the lattice anisotropy and electron concentration (n), are determined. We also discuss the superfluid properties within the framework of the Landau quasiparticle model. In the quasi-two-dimensional case and for low concentrations, the changeover from T(c) approximately n2/3 to T(c) approximately n is found to occur for J perpendicular-to /J parallel-to approximately 10(-2), with J parallel-to and J perpendicular-to being the intraplanar and interplanar pair hopping amplitude parameters, respectively. It turns out that the increasing lattice anisotropy (i.e., reduction of interplanar or interchain couplings) extends the range of stability of the homogeneous superconducting phase and suppresses the charge-density-wave state. The results for the ground-state superconducting order parameter are in good agreement with recent Monte Carlo and exact-diagonalization studies for the two-dimensional square lattice. The relationship between the pseudospin and bosonic descriptions of local-electron-pair superconductor in the dilute limit is also further analyzed.