Microwave conductivity in the ferropnictides with specific application to Ba$_{1-x}$K$_x$Fe$_2$As$_2$

TL;DR

This paper models microwave conductivity in Ba$_{1-x}$K$_x$Fe$_2$As$_2$ using a two-band superconductor with s± symmetry, revealing strong evidence for anisotropic gaps with potential nodes, and compares results with experimental data and other models.

Contribution

It introduces a detailed BCS-based model including inelastic scattering to analyze microwave conductivity and gap anisotropy in ferropnictides, comparing with experimental and other theoretical approaches.

Findings

Strong evidence for large anisotropies in the electron pocket s-wave gap

Best fit suggests the presence of nodes on the Fermi surface

Discrepancies with penetration depth measurements regarding gap nodes

Abstract

We calculate the microwave conductivity of a two band superconductor with $s^\pm$ 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 (TFM) is provided as well as with the case of the cuprates where the gap has d-wave symmetry and with experimental results in Ba$_{1-x}$K$_x$Fe$_2$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 node-less gap as do also thermal conductivity and nuclear magnetic resonance data.