THz optical response of Ba(Fe$_{1-x}$Ni$_x$)$_2$As$_2$ films analyzed within the three-band Eliashberg s$_\pm $-wave model

TL;DR

This study investigates the superconducting properties of Ba(Fe$_{1-x}$Ni$_x$)$_2$As$_2$ films using terahertz spectroscopy and analyzes the data within a three-band Eliashberg s$_$-wave model, supporting multiband s$_$-wave pairing.

Contribution

It provides a detailed analysis of the THz optical response of Ba(Fe$_{1-x}$Ni$_x$)$_2$As$_2$ films within a three-band Eliashberg framework, supporting the s$_$-wave pairing symmetry.

Findings

Superconducting gaps are consistent with s$_$-wave symmetry.

Data supports multiband superconductivity mediated by antiferromagnetic spin fluctuations.

Temperature dependence of superfluid density matches theoretical predictions.

Abstract

The uncertainty of the nature of the normal state and superconducting condensate of unconventional superconductors continues to stimulate considerable speculation about the mechanism of superconductivity in these materials. Of particular interest are the type of symmetry of the order parameter and the basic electronic characteristics of the superconducting and normal states. We report the derivation of temperature dependences of the superconducting condensate plasma frequency, superfluid density, and London penetration depth by measuring terahertz spectra of conductivity and dielectric permittivity of the Ba(Fe$_{1-x}$Ni$_x$)$_2$As$_2$ thin films with different Ni concentrations. A comprehensive analysis of the experimental data was performed in the framework of the simple three-band Eliashberg model under the assumption that the superconducting coupling mechanism is mediated by antiferromagnetic spin fluctuations. The results of independent experiments support the choice of model parameters. Based on calculations of the temperature dependences of superconducting gaps, we may conclude that the obtained results are compatible with the scenario, in which Ba(Fe$_{1-x}$Ni$_x$)$_2$As$_2$ is a multiband superconductor with s$_\pm $-wave pairing symmetry.