Evidence of the superconducting energy gap in the optical spectra of   $\alpha_t$-(BEDT-TTF)$_{2}$I$_{3}$
  The far-infrared in-plane reflectivity of the organic superconductor $\alpha_t$-(BEDT-TTF)$_{2}$I$_{3}$ is measured down to frequencies of 10 cm$^{-1}$; the optical conductivity is obtained by a Kramers-Kronig procedure. In the superconducting state an energy gap opens at 25 cm$^{-1}$ which corresponds to $2\Delta/k_BT_c = 4.4$ in agreement with moderate coupling. Even well below $T_c=8$ K a considerable absorption remains for $\hbar\omega<2\Delta$. We discuss different posibilities to explain this behaviour. 
N. Drichko, P. Haas, B. Gorshunov, D. Schweitzer, and M. Dressel