Evolution of the absolute value of the London penetration depth in single crystals of Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ upon cobalt doping

The absolute value of the London penetration depth, $\lambda _{0}$, has been measured in single crystals of Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ over a wide range of cobalt doping levels using a tunnel diode resonator (TDR) [1]. Normally, the TDR technique can only measure the relative change, $\Delta \lambda $(T), but the absolute value is needed to construct $\lambda $(T) and the superfluid density, $\rho _{s}$=[$\lambda _{0}$/$\lambda $(T)]$^{2}$, which can be compared to theory. A more elaborate approach in which the samples are coated with a thin film of Al allows for a direct estimate of $\lambda _{0}$ by using the same TDR apparatus [2]. The resulting doping-dependent $\lambda _{0}$(x) as well as the calculated $\rho _{s}$(T,x) curves will be discussed within current theoretical models. In particular, attention will be devoted to the effects of possible strong pair breaking predictions which state $\Delta \lambda \propto $T$^{n}$/T$_{c}^{3}$, with n$\approx $2 [3]. [1] R. T. Gordon \textit{et al}., Phys. Rev. B \textbf{79}, 100506(R) (2009). [2] R. Prozorov \textit{et al}., Appl. Phys. Lett. \textbf{77}, 25 (2000). [3] V. G. Kogan, arXiv:0910.4728