The London penetration depth in single crystals of Ba(Fe_{1-x}Co_x)₂As₂ at various doping levels

The London penetration depth $\lambda(T)$ has been measured in single crystals of Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ 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, $\Delta\lambda(T)\sim T^{n}$, for all measured dopings, with $n$ between 2 and 2.5. We discuss the implications of these results and possible interpretations of such robust behavior.