Paring instability in the mixed state of d-wave superconductor

TL;DR

This paper proposes that Coulomb interactions can induce an excitonic gap along the nodal directions in d-wave cuprate superconductors under magnetic fields, explaining the observed thermal metal-insulator transition.

Contribution

It introduces a mechanism where magnetic field suppresses superfluid density, strengthening Coulomb interactions to generate a nodal gap in d-wave superconductors.

Findings

Nodal gap increases with magnetic field H.

Thermal conductivity is suppressed at zero temperature.

Explains the field-induced metal-insulator transition.

Abstract

We propose that an excitonic gap can be generated along nodal directions by Coulomb interaction in the mixed state of d-wave cuprate superconductors. In a superconductor, the Coulomb interaction usually can not generate any fermion gap since its strength is weakened by superfluidity. It becomes stronger as superfluid density is suppressed by external magnetic field, and is able to generate a gap for initially gapless nodal quasiparticles beyond some critical field $H_{c}$. By solving the gap equation, it is found that the nodal gap increases with growing field $H$, which leads to a suppression of thermal conductivity at zero temperature. This mechanism naturally produces the field-induced thermal metal-insulator transition observed in transport experiments.