Muon spin rotation measurement of the magnetic field penetration depth in Ba(Fe_0.926Co_0.074)₂As₂: Evidence for multiple superconducting gaps

We have performed transverse field muon spin rotation measurements of single crystals of Ba(Fe_0.926Co_0.074)₂As₂ with the applied magnetic field along the ĉ direction. Fourier transforms of the measured spectra reveal an anisotropic line-shape characteristic of an Abrikosov vortex lattice. We have fit the μSR spectra to a microscopic model in terms of the penetration depth λ and the Ginzburg-Landau parameter κ. We find that as a function of temperature, the penetration depth varies more rapidly than in standard weak-coupled BCS theory. For this reason we first fit the temperature dependence to a power law where the power varies from 1.6 to 2.2 as the field changes from 0.02 to 0.1 T. Due to the surprisingly strong field dependence of the power and the superfluid density we proceeded to fit the temperature dependence to a two-gap model, where the size of the two gaps is field independent. From this model, we obtained gaps of 2Δ₁=3.77k_BT_C and 2Δ₂=1.57k_BT_C, corresponding to roughly 6 and 3 meV, respectively.