The Determination of $\lambda _{ab}$(0) in Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ from Tunnel Diode Resonator Measurements

The tunnel diode resonator (TDR) technique allows for precision measurements of the change of the London penetration depth with temperature, $\Delta \lambda $(T), in superconductors, but before now this approach has been insensitive to the zero temperature value, $\lambda $(0), which is necessary for absolute calibration. A method for the determination of $\lambda $(0) in superconductors has been developed that utilizes the capabilities of the TDR system along with a technique in which samples are coated with a thin film of aluminum [1]. Using this procedure, $\lambda _{ab}$(0) has been measured for the Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ series for superconducting samples ranging from underdoped to overdoped concentrations. The resulting temperature dependence of the superfluid density, $\rho _{s }$= [$\lambda $(0)/ $\lambda $(T)]$^{2}$, constructed from penetration depth measurements also obtained using a TDR system, will be discussed in terms of current theoretical models. [1] R. Prozorov \textit{et al.}, Appl. Phys. Lett. \textbf{77}, 25 (2000).