09. Local Measurement of the Superfluid Density in the Pnictide Superconductor Ba(Fe1-xCox)2As2 across the Superconducting Dome

We measure the penetration depth λ a b ( T ) in Ba ( Fe 1 − x Co x ) 2 As 2 using local techniques that do not average over the sample. The superfluid density ρ s ( T ) ≡ 1 / λ a b ( T ) 2 has three main features. First, ρ s ( T = 0 ) falls sharply on the underdoped side of the dome. Second, λ a b ( 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, ρ s ( T ) varies steeply near T c for optimal and underdoping. These observations are consistent with an interplay between magnetic and superconducting phases.