Interplay of Pomeranchuk instability and superconductivity in the two-dimensional repulsive Hubbard model

摘要

Interplay of Pomeranchuk instability (spontaneous symmetry breaking of the Fermi surface) and d-wave superconductivity is studied for the repulsive Hubbard model on the square lattice with the dynamical mean-field theory combined with the fluctuation exchange approximation (FLEX+DMFT). We show that the fourfold symmetric Fermi surface becomes unstable against a spontaneous distortion into twofold near the van Hove filling, where the symmetry of superconductivity coexisting with the Pomeranchuk-distorted Fermi surface is modified from the d-wave pairing to the (d + s) wave. By systematically shifting the position of van Hove filling with varied second- and third-neighbor hoppings, we find that the transition temperature T~(PI)_c for the Pomeranchuk instability is more sensitively affected by the position of van Hove filling than the superconducting T~(SC)_c . This implies that the filling region for strong Pomeranchuk instability and that for the T~(SC)_c dome can be separated, and that Pomeranchuk instability can appear even if the peak of T~(PI)_c is lower than the peak of T~(SC)_c . An interesting finding is that the Fermi surface distortion can enhance the superconducting T~(SC)_c in the overdoped regime, which is explained with a perturbational picture for small distortions.