Electron-phonon coupling in the superconducting single crystal Rb$_{0.8}$Fe$_{1.68}$Se$_2$

TL;DR

This study investigates the optical properties of the superconducting Rb$_{0.8}$Fe$_{1.68}$Se$_2$ crystal, revealing low carrier density and increasing electron-phonon coupling as temperature decreases, which may influence its superconductivity.

Contribution

First detailed optical analysis of Rb$_{0.8}$Fe$_{1.68}$Se$_2$ showing phonon behavior and electron-phonon interactions in this superconductor.

Findings

Low carrier density indicated by small Drude response.

Phonon frequencies increase as temperature decreases.

Enhanced phonon asymmetry suggests stronger electron-phonon coupling at low temperatures.

Abstract

The optical properties of the superconducting single crystal Rb$_{0.8}$Fe$_{1.68}$Se$_2$ with $T_{c}$ $\simeq$ 31 K have been measured over a wide frequency range in the $ab$ plane. We found that the optical conductivity is dominated by a series of infrared-active phonon modes at low-frequency region as well as several other high-frequency bound excitations. The low-frequency optical conductivity has rather low value and shows quite small Drude-like response, indicating low carriers density in this material. Furthermore, the phonon modes increase continuously in frequency with decreasing temperature; specifically, the phonon mode around 200 cm$^{-1}$ shows an enhanced asymmetry effect at low temperatures, suggesting an increasing electron-phonon coupling in this system.