Superconductivity on the verge of metal-insulator transition in Cu$_{1-x}$Zn$_x$Ir$_2$S$_4$ probed by $μ$SR

Abstract

The thiospinel CuIr$_2$S$_4$ undergoes a metal-insulator transition below $\approx$230 K, which is suppressed by substitution of Cu with Zn (Cu$_{1-x}$Zn$_x$Ir$_2$S$_4$) to induce superconductivity for $0.2\lesssim x\lesssim0.8$. We show that the temperature/field dependence of superfluid density in samples with $x = 0.3$ and 0.4 ($T_{\rm c} \approx 3$ K and 2.5 K) investigated by muon spin rotation and relaxation ($μ$SR) is consistent with a fully gapped $s$-wave pairing. Meanwhile, the relatively high resistivity (10$^{-3}$-10$^{-2}$ $Ω\:$cm) in their normal state suggests that the superconductivity is in the "dirty limit" where the mean free path is much shorter than the coherence length ($\ell \ll ξ_0$). This indicates that the potential anisotropy associated with unconventional pairing mechanisms expected under the strong electron correlations is smeared out by the electron scattering. Based on these observations, we discuss potential link between the Zn substitution-induced superconductivity and that recently discovered in CuIr$_2$S$_4$ under high pressure ($>18$ GPa) where the existence of strong electron scattering is also suggested.