Evidence of the superconducting energy gap in the optical spectra of $\hat{I}{\pm }_t$-(BEDT-TTF)${}_2$I${}_3$

Abstract

The far-infrared in-plane reflectivity of the organic superconductor $\hat{I}{\pm }_t$-(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 $2\hat{I}”/k_B{T}_c=4.4$ in agreement with moderate coupling. Even well below $T_c=8$ K a considerable absorption remains for $\hslash \ddot{I}‰<2\hat{I}”$. We discuss different posibilities to explain this behaviour.

5 pages, 2 figures