Hyperbolic fringe signal for twin impurity quasiparticle interference

TL;DR

This paper investigates hyperbolic fringe patterns in quasiparticle interference caused by twin impurities on superconductor surfaces, revealing their dependence on pairing symmetry and impurity type, and proposing a new spectroscopic tool.

Contribution

It introduces hyperbolic fringe patterns as a signature in QPI for identifying superconducting order parameters and impurity effects, expanding the analysis methods for superconductor surface states.

Findings

Hyperbolic fringes appear in QPI due to twin impurities.

HF patterns indicate chiral or sign-changing SC order.

Twin impurity QPI offers a new spectroscopic diagnostic.

Abstract

We study the quasiparticle interference (QPI) pattern emanating from a pair of adjacent impurities on the surface of a gapped superconductor (SC). We find that hyperbolic fringes (HF) in the QPI signal can appear due to the loop contribution of the two-impurity scattering, where the location of the two impurities are the hyperbolic focus points. For a single pocket Fermiology, an HF pattern signals chiral SC order for non-magnetic impurities and requires magnetic impurities for non-chiral SC. For a multi-pocket scenario, a sign-changing order parameter such as $s_{\pm}$-wave likewise yields an HF signature. We discuss twin impurity QPI as a new tool to complement the analysis of superconducting order from local spectroscopy.