Fully-gapped superconductivity in single crystals of noncentrosymmetric Re$_6$Zr with broken time-reversal symmetry

TL;DR

This study reveals that Re$_6$Zr, a noncentrosymmetric superconductor with broken time-reversal symmetry, exhibits fully-gapped superconductivity consistent with a single s-wave gap and moderate coupling strength.

Contribution

It provides the first detailed investigation of the superconducting gap structure in Re$_6$Zr, demonstrating fully-gapped behavior using penetration depth and specific heat measurements.

Findings

Exponential temperature dependence of penetration depth indicates a fully-open gap.

Superfluid density and specific heat fit well with a single s-wave gap model.

The superconducting gap magnitude exceeds the weak coupling BCS value.

Abstract

The noncentrosymmetric superconductor Re$_6$Zr has attracted much interest due to the observation of broken time-reversal symmetry in the superconducting state. Here we report an investigation of the superconducting gap structure of Re$_6$Zr single crystals by measuring the magnetic penetration depth shift $\Delta\lambda(T)$ and electronic specific heat $C_e(T)$. $\Delta\lambda(T)$ exhibits an exponential temperature dependence behavior for $T~\ll~T_c$, which indicates a fully-open superconducting gap. Our analysis shows that a single gap $s$-wave model is sufficient to describe both the superfluid density $\rho_s(T)$ and $C_e(T)$ results, with a fitted gap magnitude larger than the weak coupling BCS value, providing evidence for fully-gapped superconductivity in Re$_6$Zr with moderate coupling.