Disordered d-wave superconductors with chiral symmetry

TL;DR

This paper investigates a 2D lattice model of disordered d-wave superconductors with chiral symmetry, revealing universal behavior and density of states characteristics at the band center depending on time-reversal symmetry.

Contribution

It identifies the universality classes of the field theory at the band center and analyzes the density of states behavior in different symmetry scenarios.

Findings

Extended states at the band center due to vanishing beta function

Density of states vanishes cubically with energy when time-reversal symmetry is broken

Density of states vanishes linearly with energy when time-reversal symmetry is unbroken

Abstract

A two-dimensional lattice model for d-wave superconductor with chiral symmetry is studied. The field theory at the band center is shown to be in the universality class of U(2n)/O(2n) and U(2n) nonlinear sigma model for the system with broken and unbroken time-reversal symmetry, respectively. Vanishing of the beta function implies extended states at the band center. Density of state vanishes as a cubic function of the energy at the band center for the former case, while linear for the latter.