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 $2\mathrm{\Delta }\mathrm{/}{k}_B{T}_c=4.4$ in agreement with moderate coupling. Even well below $T_c=8$ K a considerable absorption remains for $\mathrm{\hslash }\omega <2\mathrm{\Delta }$. We discuss different posibilities to explain this behaviour.