We have studied the in- and out-of-plane magnetic penetration depths in the hole- doped iron based superconductor Ba$_{ 1-x}$K$_{ x}$Fe$_{ 2}$As$_{ 2}$ ($T_{ c}\approx$ 30K). The study was performed on single crystals grown from different fluxes and we find that the results are nearly the same. The in-plane London penetration depth $\lambda_{ ab}$ does not show exponential saturation at low temperature, as would be expected from a fully gapped superconductor. Instead, $\lambda_{ ab}(T)$ shows a power-law behavior, $\lambda\propto T^{ n}$ ($n\approx 2$), down to $T\approx 0.02 T_{ c}$, similar to the electron doped Ba(Fe$_{ 1-x}$Co$_{ x}$)$_{ 2}$As$_{ 2}$. The penetration depth anisotropy $\gamma_{ \lambda}=\lambda_{ c}(T)/\lambda_{ ab}(T)$ increases upon cooling, opposite to the trend observed in the anisotropy of the upper critical field, $\gamma_{ \xi}=H_{ c2}^{ \perp c}(0)/H_{ c2}^{ \parallel c}(0)$. These are universal characteristics of both the electron and hole doped 122 systems, suggesting unconventional superconductivity. The behavior of the in-plane superfluid density $\rho_{ ab}(T)$ is discussed in light of existing theoretical models proposed for the iron pnictides superconductors.
Categories: cond-mat.supr-con cond-mat.str-el