Nodeless superconductivity in the stoichiometric superconductor LiFeAs

The in- and out-of-plane London penetration depths were measured in single crystals of the intrinsic LiFeAs superconductor using a tunnel diode resonator (TDR) down to $0.03 T_c$. This compound appears to be in the clean limit with a residual resistivity of $\rho_0 \approx 5~\mu\Omega\cdot$cm and $RRR = 65$; it can be placed at a slightly overdoped value when compared to the charge-doped pnictides. The low-temperature region of the penetration depth, which is sensitive to the superconducting gap symmetry, is exponentially flat implying a nodeless gap. The superfluid density is well described by the self-consistent two-gap $\gamma$-model, where the larger gap is $\Delta_1/T_c\sim2$ and the smaller gap is $\Delta_2/T_c\sim1$. Together with the previous data, our results support the $s_\pm$ symmetry that evolves from nodeless to a nodal gap structure upon departure from optimal doping in Fe-based superconductors. We also conclude that pairbreaking scattering plays an important role in the deviations of the low-temperature behavior from exponential in $\lambda(T)$ of Fe-based compounds.