Electron-phonon coupling in the superconducting single crystal Rb0.8Fe1.68Se2

The optical properties of the superconducting single crystal Rb0.8Fe1.68Se2 with Tc ≃ 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.