Multi-gap superconductivity in a BaFe1.84Co0.16As2 film from optical measurements at terahertz frequencies

TL;DR

This study uses terahertz optical measurements to reveal multi-gap superconductivity in a BaFe1.84Co0.16As2 film, demonstrating the necessity of a two-gap model and identifying two distinct superconducting gaps.

Contribution

It provides the first detailed optical evidence for a two-gap superconducting state in BaFe1.84Co0.16As2, emphasizing the importance of a two-band, two-gap model for accurate description.

Findings

Identification of a superconducting gap near 15 cm$^{-1}$.

Evidence supporting a two-gap, nodeless isotropic superconducting state.

Quantitative determination of the two gaps with 2$ riangle/kT_c$ ratios close to 2 and 7.

Abstract

We measured the THz reflectance properties of a high quality epitaxial thin film of the Fe-based superconductor BaFe$_{1.84}$Co$_{0.16}$As$_2$ with T$_c$=22.5 K. The film was grown by pulsed laser deposition on a DyScO$_3$ substrate with an epitaxial SrTiO$_3$ intermediate layer. The measured $R_S/R_N$ spectrum, i.e. the reflectivity ratio between the superconducting and normal state reflectance, provides clear evidence of a superconducting gap $\Delta_A$ close to 15 cm$^{-1}$. A detailed data analysis shows that a two-band, two-gap model is absolutely necessary to obtain a good description of the measured $R_S/R_N$ spectrum. The low-energy $\Delta_A$ gap results to be well determined ($\Delta_A$=15.5$\pm$0.5 cm$^{-1}$), while the value of the high-energy gap $\Delta_B$ is more uncertain ($\Delta_B$=55$\pm$7 cm$^{-1}$). Our results provide evidence of a nodeless isotropic double-gap scenario, with the presence of two optical gaps corresponding to 2$\Delta/kT_c$ values close to 2 and 7.