Weak Superconducting Pairing and a Single Isotropic Energy Gap in Stoichiometric LiFeAs

We report superconducting (SC) properties of stoichiometric LiFeAs (T_c=17  K) studied by small-angle neutron scattering (SANS) and angle-resolved photoemission (ARPES). Although the vortex lattice exhibits no long-range order, well-defined SANS rocking curves indicate better ordering than in chemically doped 122 compounds. The London penetration depth λ_ab(0)=210±20  nm, determined from the magnetic field dependence of the form factor, is compared to that calculated from the ARPES band structure with no adjustable parameters. The temperature dependence of λ_ab is best described by a single isotropic SC gap Δ₀=3.0±0.2  meV, which agrees with the ARPES value of Δ₀^ARPES=3.1±0.3  meV and corresponds to the ratio 2Δ/k_BT_c=4.1±0.3, approaching the weak-coupling limit predicted by the BCS theory. This classifies LiFeAs as a weakly coupled single-gap superconductor.