$\mu$SR study of time-reversal symmetry constraints and bulk superfluid response in Li$_{0.95}$FeAs

Abstract

We report zero-field (ZF) and transverse-field (TF) muon-spin rotation/relaxation ($\mu$SR) measurements on superconducting Li$_{0.95}$FeAs ($T_{\rm c}\simeq16.0$ K) grown by a high-pressure self-flux method. The ZF-$\mu$SR data show no detectable change of the electronic relaxation rate on cooling through $T_{\rm c}$, providing no evidence for time-reversal-symmetry breaking in the superconducting state. TF-$\mu$SR measurements reveal a well-developed vortex response with strong flux pinning and a negligible nonsuperconducting contribution, confirming that superconductivity is a bulk property of the sample. From the second moment of the internal field distribution we determine a low-temperature in-plane magnetic penetration depth $\lambda_{ab}= 245(15)$ nm. The temperature dependence of the normalized superfluid density is well described by an effective two-gap model with $\Delta_1 = 2.0(2)$ meV and $\Delta_2 = 0.7(2)$ meV. A quantitative comparison with ARPES-based band weights shows that the $\mu$SR response is dominated by the Fermi-surface sheets carrying the intermediate and small superconducting gaps, whereas the band hosting the largest gap contributes only about 3\% to the total superfluid density and is therefore not resolved in the present analysis. Taken together, these results establish Li$_{0.95}$FeAs as a bulk multigap superconductor without detectable time-reversal symmetry breaking and show how $\mu$SR reconciles the gap scales reported by bulk and surface-sensitive probes in this multiband system.