Effect of disorder on the far-infrared conductivity and on the microwave conductivity of two-band superconductors

TL;DR

This study develops strong coupling models for the far-infrared and microwave conductivities of two-band superconductors with impurities, comparing results with MgB₂ experiments to understand impurity effects on conductivity features.

Contribution

It introduces a comprehensive theoretical framework for two-band superconductor conductivity considering impurity scattering, validated against experimental data for MgB₂.

Findings

Experimental far-infrared conductivity aligns with multi-band superconductivity and strong interband impurity scattering.

Microwave conductivity results depend on the ratio of scattering rates, showing different coherence peaks.

High interband impurity scattering suppresses low-temperature coherence peaks in microwave conductivity.

Abstract

We consider the far-infrared and the microwave conductivities of a two-band superconductor with non-magnetic impurities. The strong coupling expressions for the frequency and temperature dependent conductivity of a two-band superconductor are developed assuming isotropic bands and interactions. Our numerical results obtained using realistic interaction parameters for MgB$_{2}$ are compared with experiments on this compound. We find that the available experimental results for the far-infrared conductivity of MgB$_{2}$ are consistent with multi-band superconductivity in the presence of a sufficiently strong interband impurity scattering. On the other hand, our numerical results for the microwave conductivity in the superconducting state indicate that the experimental results obtained on samples with the highest transition temperature $T_{c}$ are consistent with a low interband impurity scattering rate but depend sensitively on the ratio of the total scattering rates in the two bands. For the $\pi$-band scattering rate $\gamma_{\pi}$ not greater than the $\sigma$-band scattering rate $\gamma_{\sigma}$ there is a single, broad, low-temperature (at about 0.5$T_{c}$) coherence peak in the microwave conductivity. For $\gamma_{\pi}/\gamma_{\sigma}$=4--7 a high-temperature (at about 0.9$T_{c}$) coherence peak is dominant, but there is also a low-temperature peak/shoulder resulting from the contribution of the $\pi$-band carriers to the microwave conductivity. For $\gamma_{\pi}/\gamma_{\sigma}\gg$1 only the high-temperature coherence peak should be observable.