Revealing sign-reversal $s^{+-}$-wave pairing by quasiparticle interference in the heavy-fermion superconductor CeCu$_2$Si$_2$

TL;DR

This study uses quasiparticle interference calculations with realistic Fermi surface data to distinguish between sign-reversal and sign-preserving pairing in CeCu$_2$Si$_2$, aiding the identification of its true superconducting gap symmetry.

Contribution

It provides a theoretical framework for phase-sensitive QPI measurements to determine the pairing symmetry in CeCu$_2$Si$_2$, highlighting qualitative differences between $s^{+-}$ and $s^{++}$ states.

Findings

Distinct QPI signatures for $s^{+-}$ and $s^{++}$ pairing states.

Identification of intragap impurity resonance as a key difference.

Energy dependence of gap features varies between pairing types.

Abstract

Recent observations of two nodeless gaps in superconducting CeCu$_2$Si$_2$ have raised intensive debates as to its exact gap structure of either sign-reversal ($s^{+-}$) or sign-preserving ($s^{++}$) pairing. Here we investigate the quasiparticle interference (QPI) using realistic Fermi surface topology for both weak and strong interband impurity scatterings. Our calculations of the QPI and integrated antisymmetrized local density of states reveal qualitative distinctions between $s^{+-}$ and $s^{++}$ pairing states, which include the intragap impurity resonance and a significant energy-dependence difference between two gap energies. Our predictions provide a guide for phase-sensitive QPI measurements to uncover decisively the true pairing symmetry in the heavy-fermion superconductor CeCu$_2$Si$_2$.