Electron-phonon coupling in the superconducting single crystal   Rb$_{0.8}$Fe$_{1.68}$Se$_2$
  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. 
B. Xu, Y. M. Dai, J. Han, K. Wang, R. Yang, Y. X. Yang, W. Zhang, H.   Xiao, X. G. Qiu
cond-mat.supr-con