Time-Reversal Symmetry Breaking in Re-Based Superconductors: Recent Developments

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 (μSR) 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, ranging from pure rhenium, to ReT (T = 3d-5d early transition metals), to the dilute-Re case of ReBe22. μSR 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.

Evidence of fully gapped superconductivity in NbReSi: A combined μSR and NMR study

“…Nevertheless, as we showed above, the two-gap feature in NbReSi is clearly reflected in its field-dependent superconducting relaxation rate σ sc (H ), specific-heat coefficient γ H (H ), and also in the temperature-dependent upper critical field H c2 (T ). The superconducting gap of NbReSi derived from TF-μSR is similar to that of other Re-based superconductors, e.g., ReT (T = transition metal) [29,30,32,44,45] and rhenium-boron compounds [33], the latter exhibiting a multigap SC, too.…”

Spontaneous magnetization in unitary superconductors with time reversal symmetry breaking

“…Another fascinating phenomenon is the spontaneous magnetization or spin-polarization (SP) in a time-reversal symmetry (TRS)-breaking superconductor (SC) [9][10][11], which continuously promotes extensive experimental and theoretical research [12,13]. The TRS-breaking candidate SCs include Sr 2 RuO 4 [14][15][16], ReT (T = transition metal) [17][18][19][20][21][22], UPt 3 [23][24][25], UGe 2 [26,27], URhGe [28], UCoGe [29,30], PrOs 4 Sb 12 [31], URu 2 Si 2 [32,33], SrP-tAs [34], Ru 7 B 3 [35,36], LaNiC 2 [37], LaNiGa 2 [38,39], Bi/Ni bilayers [40], CaPtAs [41], Zr 3 Ir [42], and others summarized in a recent paper [13]. More recently, ironbased SCs also exhibit TRS-breaking signatures [43,44].…”

Magnetically textured superconductivity in elemental rhenium

“…Such systems are expected to have important applications in spintronics [2] and topological quantum computing [3] however this is hindered by the lack of a general theory of unconventional superconductivity [4,5] which is normally associated with strong electron correlations or fluctuations of competing ordered phases. Recently, however, TRS breaking has been reported in seemingly ordinary superconductors where such exotic physics are not at play [6], including the chemical element Rhenium [7]. Here we show that TRS breaking in Re is due to a form of mixed singlettriplet pairing that has an atomic-scale magnetic texture.…”

“…This value of Λ EOT is 31.6% smaller than the experimentally-relevant value for Re. However, there is a large number of Re compounds and alloys that are superconducting, with some showing no signs of broken TRS and others displaying internal fields with a wide range of values [6]. It is therefore likely that a systematic investigation of such compounds may In summary a TRS breaking mechanism was identified in s-wave superconductors with strong spin-orbit coupling and non-symmorphic crystal structure.…”