Scanning Tunneling Spectroscopy of Superconducting LiFeAs Single Crystals: Evidence for Two Nodeless Energy Gaps and Coupling to a Bosonic Mode

The superconducting compound LiFeAs is studied by scanning tunneling microscopy and spectroscopy. A gap map of the unreconstructed surface indicates a high degree of homogeneity in this system. Spectra at 2 K show two nodeless superconducting gaps with Delta(1) = 5.3 +/- 0.1 meV and Delta(2) = 2.5 +/- 0.2 meV. The gaps close as the temperature is increased to the bulk T-c, indicating that the surface accurately represents the bulk. A dip-hump structure is observed below T-c with an energy scale consistent with a magnetic resonance recently reported by inelastic neutron scattering.