Absolute value and temperature dependence of the magnetic penetration depth in Ba(Co0.074Fe0.926)2As2

Abstract

The absolute value and temperature dependence of the in-plane magnetic penetration depth $\ensuremath{\lambda}$ have been measured on a single crystal of Ba(Co${}_{0.074}$Fe${}_{0.926}{)}_{2}$As${}_{2}$ using low-energy muon-spin rotation and microwave cavity perturbation. The magnetic field profiles in the Meissner state are consistent with a local London model beyond a depth of 15 nm. We determine the gap symmetry through measurements of the temperature dependence of the superfluid density which follows a two-gap $s$-wave model over the entire temperature range below ${T}_{\mathrm{c}}$. While the intermediate to high temperature data is well fit by an energy gap model in the BCS-like (weak-coupling) limit, a second smaller gap becomes apparent at low temperatures.