Superfluid Density Measurements of Ba(Co$_{x}$Fe$_{1-x})_{2}$As$_{2}$ Films near Optimal Doping

We report the first direct, low-frequency measurements of superfluid density, n$_{s}$(T) $\propto \quad \lambda ^{-2}$(T), in Ba(Co$_{x}$Fe$_{1-x})_{2}$As$_{2}$ thin films, near optimal doping. 100 nm thick films are fabricated by Pulsed Laser Deposition (PLD) in high vacuum and SrTiO$_{3}$ is used as template to match Ba layer in ba-122. Temperature dependence of superfluid density is measured by our two-coil mutual inductance apparatus down to 1.3K. The magnetic penetration depth, $\lambda $, at T $\approx $ 0 is 350 to 430 nm. The T-dependence of $\lambda ^{-2}$ is well characterized by a small s-wave gap, 2$\Delta $(0)/k$_{B}$T$_{c}$ = 2.2 $\pm $ 0.1. In detail, $\lambda $ has power-law behavior at low T: $\lambda $(T)/$\lambda $(0) -- 1 = 0.60*(T/T$_{c})^{2.5\pm 0.1}$. A tail of superfluid density near T$_{c}$ is the only possible indication of a bigger gap. The small gap, together with power-law behavior at low-T, suggests strong intraband scattering on the larger-gap Fermi surface plus significant interband scattering between large-gap and small-gap Fermi surfaces.