Evidence for multiple superconducting gaps in optimally doped BaFe${}_{1.87}$Co${}_{0.13}$As${}_2$ from infrared spectroscopy

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.

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