Time-reversal symmetry breaking in Re-based superconductors: Recent developments

TL;DR

This review discusses recent findings on time-reversal symmetry breaking in Re-based superconductors, highlighting the role of muon-spin relaxation and the influence of rhenium content on TRS breaking phenomena.

Contribution

It provides a comprehensive overview of recent developments in understanding TRS breaking in Re-based superconductors, emphasizing the importance of muon-spin relaxation studies and rhenium's role.

Findings

Re content influences TRS breaking in superconductors

Muon-spin relaxation is crucial for detecting TRS breaking

TRS breaking varies between centrosymmetric and noncentrosymmetric structures

Abstract

In the recent search for unconventional- and topological superconductivity, noncentrosymmetric superconductors (NCSCs) rank among the most promising candidate materials. Surprisingly, some of them -- especially those containing rhenium -- seem to exhibit also time-reversal symmetry (TRS) breaking in their superconducting state, while TRS is preserved in many other isostructural NCSCs. To date, a satisfactory explanation for such discrepant behavior, albeit crucial for understanding the unconventional superconductivity of these materials, is still missing. Here we review the most recent developments regarding the Re-based class, where the muon-spin relaxation ($\mu$SR) technique plays a key role due to its high sensitivity to the weak internal fields associated with the TRS breaking phenomenon. We discuss different cases of Re-containing superconductors, comprising both centrosymmetric- and noncentrosymmetric crystal structures and ranging from pure rhenium, to Re$T$ ($T$ = 3$d$-5$d$ early transition metals), to the dilute-Re case of ReBe$_{22}$. $\mu$SR results suggest that the rhenium presence and its amount are two key factors for the appearance and the extent of TRS breaking in Re-based superconductors. Besides summarizing the existing findings, we also put forward future research ideas regarding the exciting field of materials showing TRS breaking.