Evidence of the superconducting energy gap in the optical spectra of αt − (BEDT − TTF)2I3

The far-infrared in-plane reflectivity of the organic superconductor αt-(BEDT-TTF)2I3 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 2Δ/kBTc = 4.4 in agreement with moderate coupling. Even well below Tc = 8 K a considerable absorption remains for ℏω < 2Δ. We discuss different posibilities to explain this behaviour.