Optical Properties of Superconducting K$_{0.8}$Fe$_{1.7}$(Se$_{0.73}$S$_{0.27}$)$_2$ Single Crystals

TL;DR

This study investigates the optical properties of superconducting K$_{0.8}$Fe$_{1.7}$(Se$_{0.73}$S$_{0.27}$)$_2$ single crystals, revealing the superconducting gap, plasma frequency, and penetration depth through infrared spectroscopy and ellipsometry.

Contribution

First detailed optical analysis of this specific superconducting compound, identifying key superconducting parameters and gap features.

Findings

Superconducting gap of 11.8 meV observed.

Superconducting plasma frequency of 213 cm$^{-1}$ measured.

Magnetic penetration depth of 7.5 μm determined.

Abstract

The optical properties of the superconducting K$_{0.8}$Fe$_{1.7}$(Se$_{0.73}$S$_{0.27}$)$_2$ single crystals with a critical temperature $T_c\approx 26$ K have been measured in the {\it ab} plane in a wide frequency range using both infrared Fourier-transform spectroscopy and spectroscopic ellipsometry at temperatures of 4--300 K. The normal-state reflectance of K$_{0.8}$Fe$_{1.7}$(Se$_{0.73}$S$_{0.27}$)$_2$ is analyzed using a Drude-Lorentz model with one Drude component. The temperature dependences of the plasma frequency, optical conductivity, scattering rate, and dc resistivity of the Drude contribution in the normal state are presented. In the superconducting state, we observe a signature of the superconducting gap opening at $2\Delta $(5~K) = 11.8~meV. An abrupt decrease in the low-frequency dielectric permittivity $\varepsilon _1(\omega )$ at $T < T_c$ also evidences the formation of the superconducting condensate. The superconducting plasma frequency $\omega _{pl,s} = (213\pm 5)$~cm$^{-1}$ and the magnetic penetration depth $\lambda =(7.5\pm 0.2)$~$\mu $m at $T=5$~K are determined.