Non-exponential London penetration depth in Ba${}_{1-}$K${}_x$Fe${}_2$As${}_2$ single crystals

Abstract

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 $\hat{I}{»}_{ab}$ does not show exponential saturation at low temperature, as would be expected from a fully gapped superconductor. Instead, $\hat{I}{»}_{ab}(T)$ shows a power-law behavior, $\hat{I}»\propto T^n$ ($n\approx 2$), down to $T\approx 0.02T_c$, similar to the electron doped Ba(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$. The penetration depth anisotropy ${\hat{I}}_{\hat{I}}^3»=\hat{I}{»}_c(T)/\hat{I}{»}_{ab}(T)$ increases upon cooling, opposite to the trend observed in the anisotropy of the upper critical field, ${\hat{I}}_{\hat{I}}^3¾=H_{c2}^{\perp c}(0)/H_{c2}^{\parallel c}(0)$. These are universal characteristics of both the electron and hole doped 122 systems, suggesting unconventional superconductivity. The behavior of the in-plane superfluid density $\ddot{I}{}_{ab}(T)$ is discussed in light of existing theoretical models proposed for the iron pnictides superconductors.