Emergent loop-nodal s$_\pm$-wave superconductivity in CeCu$_2$Si$_2$: similarities to the iron-based superconductors

TL;DR

This paper proposes that CeCu$_2$Si$_2$ hosts an $s_$-wave superconducting state with loop-shaped nodes, driven by magnetic and octupolar fluctuations, challenging previous expectations of $d$-wave pairing in this heavy-fermion superconductor.

Contribution

The study introduces a first-principles theoretical model predicting an $s_$-wave pairing symmetry with unique nodal structure in CeCu$_2$Si$_2$, highlighting the role of multi-orbital and high-rank fluctuations.

Findings

Identifies $s_$-wave pairing with loop-shaped nodes.

Highlights magnetic and octupolar fluctuations as pairing mechanisms.

Reveals similarities between heavy-fermion and iron-based superconductors.

Abstract

Heavy-fermion superconductors are prime candidates for novel electron-pairing states due to the spin-orbital coupled degrees of freedom and electron correlations. Superconductivity in CeCu$_2$Si$_2$ discovered in 1979, which is a prototype of unconventional (non-BCS) superconductors in strongly correlated electron systems, still remains unsolved. Here we provide the first report of superconductivity based on the advanced first-principles theoretical approach. We find that the promising candidate is an $s_\pm$-wave state with loop-shaped nodes on the Fermi surface, different from the widely expected line-nodal $d$-wave state. The dominant pairing glue is magnetic but high-rank octupolar fluctuations. This system shares the importance of multi-orbital degrees of freedom with the iron-based superconductors. Our findings reveal not only the long-standing puzzle in this material, but also urge us to reconsider the pairing states and mechanisms in all heavy-fermion superconductors.