Superconductivity in Spin-Orbit coupled SU(8) Dirac Fermions on Honeycomb lattice

TL;DR

This paper classifies and analyzes various unconventional superconducting phases in a spin-orbit coupled SU(8) Dirac semi-metal on a honeycomb lattice, revealing complex pairing structures and potential experimental realizations.

Contribution

It provides a comprehensive classification of 12 superconducting states, including gapped and nodal types, in an SU(8) Dirac semi-metal with strong spin-orbit coupling, highlighting novel pairing phenomena.

Findings

72 superconducting bilinears identified

7 gapped and 5 nodal superconducting phases classified

Unusual non-unitary and finite momentum pairing observed

Abstract

We study superconducting (SC) phases that are naturally proximate to a spin-orbit coupled SU(8) Dirac semi-metal on a honeycomb lattice. This system, which offers enhanced low-energy symmetries, presents an interesting platform for realizing unconventional superconductivity in j=3/2 electrons. In particular, we find 72 superconducting charge-$2e$ fermion bilinears which, under classification of microscopic symmetries, lead to 12 different SCs -- four singlets, two doublets, and six triplets -- 7 of them are gapped and 5 are symmetry-protected nodal SCs. The strong spin-orbit coupling leads to locking of the spin of the Cooper pairs with real-space direction -- as is evident from the structure of the Cooper pair wave-functions -- leading to unusual non-unitary superconductors (even singlets), and with finite momentum pairing (for the triplets). This results, in many cases, in the magnitude of multiple pairing gaps being intricately dependent on the direction of the SC order-parameter. The present classification of SCs along with normal phases (Phys. Rev. B 108, 245106 (2023)) provides the complete list of naturally occurring phases in the vicinity of such a SU(8) Dirac semi-metal. This study allows for understanding the global phase diagram of such systems, stimulating further experimental work on candidate materials such as metallic halides (MX$_3$ with M=Zr, Hf, and X=Cl, Br). Further, it provides the starting point for the exploration of unconventional phase transitions in such systems.