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

We report superconducting (SC) properties of stoichiometric LiFeAs (${T}_{c}=17\text{ }\text{ }\mathrm{K}$) 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 ${\ensuremath{\lambda}}_{ab}(0)=210\ifmmode\pm\else\textpm\fi{}20\text{ }\text{ }\mathrm{nm}$, 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 ${\ensuremath{\lambda}}_{ab}$ is best described by a single isotropic SC gap ${\ensuremath{\Delta}}_{0}=3.0\ifmmode\pm\else\textpm\fi{}0.2\text{ }\text{ }\mathrm{meV}$, which agrees with the ARPES value of ${\ensuremath{\Delta}}_{0}^{\mathrm{ARPES}}=3.1\ifmmode\pm\else\textpm\fi{}0.3\text{ }\text{ }\mathrm{meV}$ and corresponds to the ratio $2\ensuremath{\Delta}/{k}_{B}{T}_{c}=4.1\ifmmode\pm\else\textpm\fi{}0.3$, approaching the weak-coupling limit predicted by the BCS theory. This classifies LiFeAs as a weakly coupled single-gap superconductor.