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

TL;DR

This study investigates the unconventional superconductivity and time-reversal symmetry breaking in Re24Ti5, revealing s-wave pairing with an enhanced gap and spontaneous magnetic fields below Tc, suggesting a common origin in this family of compounds.

Contribution

It provides the first evidence of TRS breaking in Re24Ti5 using muon-spin relaxation and tunneling techniques, highlighting its unconventional superconducting nature.

Findings

Evidence of s-wave superconductivity with an enhanced gap.

Observation of spontaneous magnetic fields below Tc indicating TRS breaking.

TRS breaking also occurs in related Re-based compounds, suggesting a common mechanism.

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 ($\mu$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 $\mu$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.