Nodeless two-gap superconductivity in stoichiometric iron pnictide LiFeAs

The variations of in- and inter- plane London penetration depths, $\Delta\lambda(T)$, were measured using a tunnel diode resonator in single crystals of the intrinsic pnictide superconductor LiFeAs. This compound appears to be in the clean limit with a residual resistivity of 4 ($T\to0$) to 8 ($T_c$) $\mu \Omega\cdot$cm and $RRR$ of 65 to 35, respectively. The superfluid density, $\rho_s(T)=\lambda^2(0)/\lambda^2(T)$, is well described by the self-consistent two-gap $\gamma-$model. Together with the previous data, our results support the universal evolution of the superconducting gap from nodeless to nodal upon departure from optimal doping. We also conclude that pairbreaking scattering plays an important role in the deviation of the low-temperature behavior of $\lambda(T)$ from exponential in Fe-based compounds.