Superconducting states study in electron-overdoped BaFe1.8Co0.2As2 using terahertz and far-infrared spectroscopy

Abstract

Terahertz and far-infrared spectroscopy in the temperature range, 4-300 K were used to study the normal and superconducting states of superconductor BaFe<inf>1.8</inf>Co<inf>0.2</inf>As<inf>2</inf> with T<inf>c</inf> = 22.5 K. At T < T<inf>c</inf>, the vanishing of optical conductivity caused by the unity approach and flat behavior in reflectivity were observed below 45 cm-1. This feature indicates the formation of a superconducting energy gap due to the formation of Cooper pairs. The introduction of the two Drudes model well reproduced the normal state optical conductivity, indicating the multiband nature of this superconductor. Two superconducting energy gaps were estimated as = 2.90 meV and = 6.75 meV by the BCS model. Using the sum rule and dielectric function, the superconducting plasma frequency () can be estimated as 5170 ± 270 cm-1, yielding that the magnetic penetration depth (λ) is 3090 ± 160 . This material was observed to fall on the universal scaling line, N<inf>c</inf> ∼ 4.4. © 2015 IOP Publishing Ltd.