Local measurement of the superfluid density in the pnictide superconductor Ba(Fe_1-xCox₂)As₂ across the superconducting dome

We measure the penetration depth λab(T) in Ba(Fe1-xCox)2As2 using local techniques that do not average over the sample. The superfluid density ρs(T)≡1/λab(T)2 has three main features. First, ρs(T=0) falls sharply on the underdoped side of the dome. Second, λab(T) is flat at low T at optimal doping, indicating fully gapped superconductivity, but varies more strongly in underdoped and overdoped samples, consistent with either a power law or a small second gap. Third, ρs(T) varies steeply near Tc for optimal and underdoping. These observations are consistent with an interplay between magnetic and superconducting phases.