Intrinsic finite-energy Cooper pairing in $j=3/2$ superconductors

TL;DR

This paper reveals that in $j=3/2$ superconductors with strong spin-orbit coupling, Cooper pairing can occur away from the Fermi surface between inter-band electrons with different magnetic quantum numbers, leading to unique finite-energy signatures.

Contribution

It introduces the concept of intrinsic finite-energy Cooper pairing in $j=3/2$ superconductors, a phenomenon not present in conventional spin-$1/2$ systems.

Findings

Finite-energy pairing manifests as gap-like structures at non-zero energies.

Density of states shows symmetric coherence peaks at finite energies.

Angular-momentum-resolved density of states reflects the $m_j$ composition of pairs.

Abstract

We show that Cooper pairing can occur intrinsically away from the Fermi surface in $j=3/2$ superconductors with strong spin-orbit coupling and equally curved bands in the normal state. In contrast to conventional pairing between spin-$1/2$ electrons, we derive that pairing can happen between inter-band electrons having different magnetic quantum numbers, for instance, $m_j=1/2$ and $m_j=3/2$. Such superconducting correlations manifest themselves by a pair of indirect gap-like structures at finite excitation energies. An observable signature of this exotic pairing is the emergence of a pair of symmetric superconducting coherence peaks in the density of states at finite energies. Moreover, the angular-momentum-resolved density of states in the presence of a perturbative Zeeman field reflects the $m_j$ composition of the Cooper pairs. We argue that such finite-energy pairing is a generic feature of $j=3/2$ superconductors, both in presence and absence of inversion symmetry.