Time-reversal symmetry breaking in the noncentrosymmetric superconductor Re6Hf : Further evidence for unconventional behavior in the α -Mn family of materials

Abstract

The discovery of new families of unconventional superconductors is important both experimentally and theoretically, especially if it challenges current models and thinking. By using muon spin relaxation in zero-field, time-reversal symmetry breaking has been observed in Re6Hf. Moreover, the temperature dependence of the superfluid density exhibits s-wave superconductivity with an enhanced electron-phonon coupling. This, coupled with the results from isostructural Re6Zr, shows that the Re6X family are indeed a new and important group of unconventional superconductors.

Time-reversal symmetry breaking in the noncentrosymmetric superconductor Re6Ti

“…In this regard, we systematically began the investigation on Re-based compounds with an α-Mn structure. The unconventional superconductivity was observed in a few members of the Re 6 X family, e.g., Re 6 Zr and Re 6 Hf, which provides the evidence of time-reversal symmetry breaking (TRSB) in the superconducting ground state [25,26]. The relaxation rates associated with the TRSB signal in both the compounds were similar.…”

“…The above function was fitted to the ZF asymmetry spectra collected at various temperatures above and below T c , which yields the temperature dependence of fit parameters σ ZF and as shown in Figs , which is close to the superconducting transition temperature. Such a systematic increase in σ ZF below T c was also identified in other members of the Re 6 X (X = Hf, Zr) [25,26] family by μSR measurements. This particular behavior was attributed to the formation of spontaneous magnetic fields below T c , which in turn confirmed time-reversal symmetry breaking in this compound.…”

Time-Reversal Symmetry Breaking in Re-Based Superconductors

“…18 and pure Re, the spontaneous magnetic fields revealed by zero-field µSR below T c indicate time-reversal symmetry breaking and thus unconventional superconductivity. The concomitant occurrence of TRSB in centrosymmetric Re and noncentrosymmetric ReT (T = transition metal), yet its preservation in the isostructural noncentrosymmetric superconductors Mg 10 Ir 19 B 16 and Nb 0.5 Os 0.5 , strongly suggests that the local electronic structure of Re is crucial for understanding the TRSB superconducting state in Re and ReT . We discuss the superconducting order parameter symmetries that are compatible with the observations.…”

“…In general, TRSB below T c and a lack of spatial-inversion symmetry of the crystal structure are independent events. Yet, in a few cases, such as in LaNiC 2 [13], La 7 Ir 3 [14], and, in particular, in the Re-based compounds Re 6 Zr [15], Re 6 Hf [16], Re 6 Ti [17], and Re 24 Ti 5 [18], TRSB below T c is concomitant with an existing lack of crystal inversion symmetry. Such an unusually frequent occurrence of TRSB among the superconducting ReT binary alloys (T = transition metal) is rather puzzling.…”

Quantitative theory of triplet pairing in the unconventional superconductor LaNiGa2

“…We diagonalize the mean-field Hamiltonian in Eqn. (14) using standard Bololiubov transformation [53] to obtain…”