Nonunitary Superconductivity in Complex Quantum Materials

TL;DR

This paper explores nonunitary superconductivity in complex quantum materials, emphasizing its spectral signatures, especially in multi-orbital systems, and suggests it may be more prevalent than previously thought, often linked to time-reversal symmetry breaking.

Contribution

It provides a detailed analysis of nonunitary superconductivity in multi-orbital systems, highlighting spectral features and proposing its commonality in complex materials.

Findings

Nonunitary order parameters cause finite-energy band gap openings.

Spectral signatures can be linked to the superconducting fitness matrix.

Nonunitary superconductivity may be widespread in complex quantum materials.

Abstract

We present a comprehensive discussion on nonunitary superconductivity in complex quantum materials. Starting with a brief review of the notion of nonunitary superconductivity, we discuss its spectral signatures in simple models with only the spin as an internal degree of freedom. In complex materials with multiple internal degrees of freedom, there are many more possibilities for the development of nonunitary order parameters. We provide examples focusing on d-electron systems with two orbitals, applicable to a variety of materials. We discuss the consequences for the superconducting spectra, highlighting that gap openings of band crossings at finite energies can be attributed to a nonunitary order parameter if this is associated with a finite superconducting fitness matrix $\hat{F}_C(\mathbf{k})$. We speculate that nonunitary superconductivity in complex quantum materials is in fact very common and can be associated with multiple cases of time-reversal symmetry breaking superconductors.