Superconductivity in the CuO Double Chain of Pr₂Ba₄Cu₇O_15−δ on the Basis of Tomonaga–Luttinger Liquid Theory

Abstract

Recently, Matsukawa et al. discovered a new superconductor, Pr₂Ba₄Cu₇O_15−δ, in which metallic CuO double chains are responsible for the superconductivity. To investigate its superconductivity, we employ the d–p double chain model where the tight-binding parameters are determined so as to fit a band structure in the local density approximation (LDA). On the basis of the Tomonaga–Luttinger liquid theory, we obtain a phase diagram that includes the superconducting phase in the weak coupling limit. We also calculate the Luttinger liquid parameter K_ρ as a function of the electron density n using the Hartree–Fock approximation. With increasing n from quarter filling, K_ρ increases and then exceeds 1/2 when the superconducting correlation becomes most dominant. K_ρ has a maximum at an optimal density between quarter- and half-filling. These results are consistent with the experimental observation.