Subtle Effects of a nonmagnetic impurity on the sub-gap spectrum of a d-wave superconductor in the Fulde-Ferrell-Larkin-Ovchinnikov state

TL;DR

This study explores how a nonmagnetic impurity influences the sub-gap electronic spectrum in a d-wave superconductor within the FFLO state, revealing impurity-induced local modifications of the order parameter and sub-gap states.

Contribution

It provides a detailed numerical analysis of impurity effects on the order parameter and sub-gap spectrum in the FFLO state of d-wave superconductors, highlighting the pinning of saddle points and extrema.

Findings

Impurity pins the order parameter saddle point.

Local density of states shows subtle impurity effects.

Order parameter sign changes in momentum and real space.

Abstract

The effect of a nonmagnetic impurity on the subgap electronic spectrum in a d-wave superconductor in the Fulde-Ferrell-Larkin-Ovchinnikov state is investigated. In a pure system, the spatial variation of the order parameter (OP) of the FFLO state forms a two-dimensional lattice, and there exist two types of sub-gap quasi-particle states. Our numerical iterations indicate that an OP saddle point, and slightly less so energetically, an OP extremum favor to be pinned at the impurity site. The local density of states and the fixed-bias map reveal the subtle effects of an impurity on the subgap energy spectrum, because the OP changes sign in both the (relative) momentum space and the real space.