Anisotropic London Penetration Depth in Iron-based Pnictide Superconductors

The temperature dependent London penetration depth, $\lambda \left( T \right)$, is linked directly to the structure of the superconducting gap, thus providing valuable insight into the pairing mechanism. I will summarize measurements of the penetration depth in single crystals of iron-based pnictide superconductors comparing the ``1111'', ``11'' and ``122'' families. Compatibility of our results with other gap sensitive probes, such as ARPES and thermal conductivity, will be addressed. A detailed discussion of the doping dependent penetration depth will be given for the well-characterized ``122'' family, (Ba$_{1-y}$K$_{y})$(Fe$_{1-x}$T$_{x})_{2}$As$_{2}$ (T=Co, Ni, Pd, Rh). Overall, $\lambda \left( T \right)$ exhibits a power law variation at low temperatures, $\lambda \left( T \right)=\lambda \left( 0 \right)+bT^n$ (down to 80 mK in the case of FeNi-122). The exponent $n$ is typically less than 2.8, which is clearly different from $n\approx 4$ that parameterizes the exponential behavior expected for conventional fully gapped s-wave superconductors. The low-temperature parameters, $\lambda \left( 0 \right)$, $b$ and $n$ depend on the doping level and the orientation of a magnetic field with respect to the crystal axes. This evolution is best observed in the out-of-plane penetration depths, $\lambda _c \left( T \right)$, which at least in the FeNi-122 system, changes from a high power in the underdoped regime to $T-$linear in overdoped samples. Simultaneously, the in-plane penetration depth, $\lambda _{ab} \left( T \right)$, evolves towards a sub-quadratic behavior with $n\approx 1.7$. Furthermore, analysis of the superfluid density in the full temperature range is consistent with two-gap superconductivity. However, the temperature dependencies of the anisotropies, ${\lambda _c } \mathord{\left/ {\vphantom {{\lambda _c } {\lambda _{ab} }}} \right. \kern-\nulldelimiterspace} {\lambda _{ab} }$ and ${\xi _{ab} } \mathord{\left/ {\vphantom {{\xi _{ab} } {\xi _c }}} \right. \kern-\nulldelimiterspace} {\xi _c }$, are opposite compared to another two-gap superconductor, MgB$_{2}$. Consistency of these results with theories that explain the power law behavior to be due to scattering in a two-dimensional $s_\pm $ model will be discussed. Overall, our results suggest that the superconducting gap in iron-based pnictide superconductors develops nodal structure in the overdoped regime with nodes located at finite $k_z $ wave vectors on a three-dimensional Fermi surface. \\[4pt] \textbf{References:} C. Martin \textit{et al.}, Phys. Rev. Lett. \textbf{102}, 247002 (2009); R. T. Gordon \textit{et al.}, Phys. Rev. Lett. \textbf{102}, 127004 (2009); R. T. Gordon \textit{et al.}, Phys. Rev. B \textbf{79}, 100506(R) (2009)