Doping evolution of the absolute value of the London penetration depth and superfluid density in single crystals of Ba ( Fe_1-x Co _x)₂ As₂

The zero-temperature value of the in-plane London penetration depth, λ_ab (0), has been measured in single crystals of Ba (Fe _1-x Co_x)₂ As₂ as a function of the Co concentration, x, across both the underdoped and overdoped superconducting regions of the phase diagram. For x≤0.047, λab (0) has been found to have values between 120±50 and 300±50nm. A pronounced increase in λ_ab (0), to a value as high as 950±50nm, has been observed for x≤0.047, corresponding to the region of the phase diagram where the itinerant antiferromagnetic and superconducting phases coexist and compete. Direct determination of the doping-dependent λ_ab (0) has allowed us to track the evolution of the temperature-dependent superfluid density, from which we infer the development of a pronounced superconducting gap anisotropy at the edges of the superconducting dome.