Nodeless superconductivity and time-reversal symmetry breaking in the noncentrosymmetric superconductor Re24Ti5

Abstract

The noncentrosymmetric superconductor Re${}_{24}$Ti${}_5$, a time-reversal symmetry (TRS) breaking candidate with $T_c=6$\,K, was studied by means of muon-spin rotation/relaxation ($\hat{I}¼$SR) and tunnel-diode oscillator (TDO) techniques. At a macroscopic level, its bulk superconductivity was investigated via electrical resistivity, magnetic susceptibility, and heat capacity measurements. The low-temperature penetration depth, superfluid density and electronic heat capacity all evidence an $s$-wave coupling with an enhanced superconducting gap. The spontaneous magnetic fields revealed by zero-field $\hat{I}¼$SR below $T_c$ indicate a time-reversal symmetry breaking and thus the unconventional nature of superconductivity in Re${}_{24}$Ti${}_5$. The concomitant occurrence of TRS breaking also in the isostructural Re${}_6$(Zr,Hf) compounds, hints at its common origin in this superconducting family and that an enhanced spin-orbital coupling does not affect pairing symmetry.

5 pages, 4 figures, to be appear on Physical Review B Rapid Communication