Superfluid Density in the 111 Fe Pnictide Superconductors

We performed muon spin relaxation studies in two kinds of '111' iron pnictides, Li$_{1.1}$FeAs and Li$_{0.9}$FeP. The zero field spectra of the latter show a fast relaxation in a small volume fraction (approximately $13\%$) due probably to magnetism. In the case of the arsenide, the zero field spectra show a completely paramagnetic state. Below Tc, an applied transverse field allowed us to measure the superfluid density (via relaxation rate $\sigma$) for both compounds. We found that both of them have very high superfluid density and low Tc: $\sigma(T=2K)\simeq1.8 \mu s^{-1}$ with Tc$\simeq$18K for Li$_ {1.1}$FeAs, and $\sigma(T=2K)\simeq2.0 \mu s^{-1}$ with Tc$\simeq$4K for Li$_{0.9}$FeP.