Gap symmetry of the heavy fermion superconductor CeCu$_2$Si$_2$ at ambient pressure

TL;DR

This paper proposes a phenomenological model considering Fermi surface topology and quantum critical scatterings to explain the nodeless $s^$-wave superconducting gap symmetry in CeCu$_2$Si$_2$, aligning with experimental observations.

Contribution

It introduces a theoretical approach that accounts for multiple Fermi surfaces and interband interactions to explain the superconducting gap symmetry in CeCu$_2$Si$_2$.

Findings

Nodeless $s^$-wave solution obtained with strong interband pairing.

Model aligns well with experimental data on CeCu$_2$Si$_2$.

Highlights the role of multiple Fermi surfaces in heavy fermion superconductivity.

Abstract

Recent observations of two nodeless gaps in superconducting CeCu$_2$Si$_2$ have raised intensive debates on its exact gap symmetry, while a satisfactory theoretical basis is still lacking. Here we propose a phenomenological approach to calculate the superconducting gap functions, taking into consideration both the realistic Fermi surface topology and the intra- and interband quantum critical scatterings. Our calculations yield a nodeless $s^\pm$-wave solution in the presence of strong interband pairing interaction, in good agreement with experiments. This provides a possible basis for understanding the superconducting gap symmetry of CeCu$_2$Si$_2$ at ambient pressure and indicates the potential importance of multiple Fermi surfaces and interband pairing interaction in understanding heavy fermion superconductivity.