Microwave conductivity in the ferropnictides with specific application toBa1−xKxFe2As2

Abstract

We calculate the microwave conductivity of a two band superconductor with ${s}^{\ifmmode\pm\else\textpm\fi{}}$ gap symmetry. Inelastic scattering is included approximately in a BCS model augmented by a temperature-dependent quasiparticle scattering rate assumed, however, to be frequency independent. The possibility that the $s$-wave gap on one or the other of the electron or hole pockets is anisotropic is explored including cases with and without gap nodes on the Fermi surface. A comparison of our BCS results with those obtained in the two-fluid model is provided as well as with the case of the cuprates where the gap has $d$-wave symmetry and with experimental results in ${\text{Ba}}_{1\ensuremath{-}\ensuremath{\delta}}{\text{K}}_{\ensuremath{\delta}}{\text{Fe}}_{2}{\text{As}}_{2}$. The presently available microwave conductivity data in this material provides strong evidence for large anisotropies in the electron pocket $s$-wave gap. While a best fit favors a gap with nodes on the Fermi surface this disagrees with some but not all penetration-depth measurements which would favor a nodeless gap as do also thermal-conductivity and nuclear-magnetic-resonance data.