Evidence for multiple superconducting gaps in optimally doped BaFe$_{1.87}$Co$_{0.13}$As$_{2}$ from infrared spectroscopy

TL;DR

This study uses infrared spectroscopy to reveal at least three distinct superconducting energy gaps in optimally doped BaFe$_{1.87}$Co$_{0.13}$As$_{2}$, advancing understanding of its complex superconducting state.

Contribution

It provides experimental evidence for multiple superconducting gaps in BaFe$_{1.87}$Co$_{0.13}$As$_{2}$ using infrared spectroscopy, and models these gaps with a BCS framework.

Findings

Evidence for at least three different energy gaps.

A BCS-model with isotropic gaps fits the data well.

Determined the in-plane magnetic penetration depth as 270 nm.

Abstract

We performed combined infrared reflection and ellipsometry measurements of the in-plane optical reponse of single crystals of the pnictide high temperature superconductor BaFe$_{1.87}$Co$_{0.13}$As$_{2}$ with $T_{c}$ = 24.5 K. We observed characteristic superconductivity-induced changes which provide evidence for at least three different energy gaps. We show that a BCS-model of isotropic gaps with 2$\Delta/k_{B}T_{c}$ of 3.1, 4.7, and 9.2 reproduces the experimental data rather well. We also determine the low-temperature value of the in-plane magnetic penetration depth of 270 nm.