Impurity-induced bound states as a signature of pairing symmetry in multiband superconducting CeCu$_{2}$Si$_{2}$

TL;DR

This study investigates impurity-induced bound states in multiband superconductor CeCu$_{2}$Si$_{2}$ to identify its pairing symmetry, using first-principles calculations and impurity scattering models, providing a way to distinguish between different pairing states.

Contribution

It demonstrates that impurity-induced bound states vary with pairing symmetry, offering a method to determine the superconducting gap structure in CeCu$_{2}$Si$_{2}$.

Findings

Different pairing symmetries produce distinct impurity bound states.

Intra-gap states for $d_{x^2-y^2}$-wave and loop-nodal $s$-wave are distinguishable.

The results can be tested by STM/STS experiments.

Abstract

Multiband superconductivity with dominant two-gap features are recently proposed to challenge the earlier accepted nodal $d$-wave pairing in the first unconventional superconductor CeCu$_{2}$Si$_{2}$. Here we obtain multiband Fermi-surface topology of CeCu$_{2}$Si$_{2}$ via first-principles calculations, and study the problem within an effective two hybridization band model including detailed band-structure. Within T-matrix approximation, our calculations reveal that different pairing candidates could yield qualitatively distinct features characterised by impurity-induced bound states. Except for the nodeless $s^{\pm}$-wave, both loop-nodal $s$-wave and $d$-wave pairings can give rise to intra-gap impurity bound states. In particular, the intra-gap states for the $d_{x^2-y^2}$-wave and loop-nodal $s$-wave are distinguishable and locate either near or far away from the Fermi energy, respectively. These features can be readily verified by high-resolution scanning tunneling microscopy/spectroscopy and provide an unambiguous justification for the ongoing debate about the superconducting gap symmetry of CeCu$_{2}$Si$_{2}$ at ambient pressure.