Nodeless two-gap superconducting state in single crystals of the 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 -> 0) to 8 (T-c) mu Omega cm and a residual resistivity ratio 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 even in clean systems. We also conclude that pair-breaking scattering plays an important role in the deviation of the low-temperature behavior of lambda(T) from the exponential in Fe-based compounds.