Broadband microwave absorption of Fe-based superconductors

Preliminary microwave measurements of the penetration depth in single crystals of the hole-doped iron-based superconductor Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$, using cavity perturbation and ac susceptometry, have been performed by our group. These results hint at a gap with nodes plus the presence of scattering. As a complement to these techniques, we have also undertaken broadband microwave absorption (i.e., surface resistance) measurements on the same pnictide samples. Our initial results of the microwave conductivity are consistent with a system possessing a finite density of states at low temperature due to impurity scattering which then evolves with temperature above about 5 K.