Andreev states near short-ranged pairing potential impurities

TL;DR

This paper investigates Andreev states caused by atomic-scale pairing potential variations in superconductors, revealing conditions for their existence and comparing theoretical LDOS patterns with experimental observations around impurities.

Contribution

It introduces a phase impurity model explaining LDOS patterns near impurities in high-Tc superconductors, distinguishing it from other models.

Findings

Resonances near the Fermi level occur with sign-changing order parameters.

LDOS patterns resemble those observed around Zn impurities in BSCCO.

States exist close to the gap edge for moderate gap reductions.

Abstract

We study Andreev states near atomic scale modulations in the pairing potential in both $s$- and d-wave superconductors with short coherence lengths. For a moderate reduction of the local gap, the states exist only close to the gap edge. If one allows for local sign changes of the order parameter, however, resonances can occur at energies close to the Fermi level. The local density of states (LDOS) around such pairing potential defects strongly resembles the patterns observed by tunneling measurements around Zn impurities in Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$ (BSCCO). We discuss how this phase impurity model of the Zn LDOS pattern can be distinguished from other proposals experimentally.