London penetration depth in single crystals of Ba(Fe_1−xCo_x)₂As₂ spanning underdoped to overdoped compositions

The London penetration depth λ(T) has been measured in single crystals of Ba(Fe_1−xCo_x)₂As₂ using the tunnel diode resonator technique. The measured doping levels of x=0.038, 0.047, 0.058, 0.074, and 0.10 range from underdoped to overdoped concentrations. The measurements have shown that the density of carriers participating in superconductivity decreases sharply in the underdoped regime but the penetration depth as a function of temperature exhibits a robust power law, Δλ(T)∼Tⁿ, for all measured dopings with n being about 2 in underdoped samples and 2.5 in overdoped samples. We discuss the implications of these results and possible interpretations of such a robust behavior.