Evidence of the superconducting energy gap in the optical spectra of $\alpha_t$-(BEDT-TTF)$_{2}$I$_{3}$

TL;DR

This study measures the optical spectra of the organic superconductor $ ext{α}_t$-(BEDT-TTF)$_{2}$I$_{3}$, revealing an energy gap consistent with moderate coupling and persistent absorption below the gap at low temperatures.

Contribution

It provides experimental evidence of the superconducting energy gap in $ ext{α}_t$-(BEDT-TTF)$_{2}$I$_{3}$ through infrared reflectivity measurements and analysis.

Findings

Energy gap opens at 25 cm$^{-1}$ in the superconducting state.

The gap value corresponds to $2 riangle/k_B T_c = 4.4$, indicating moderate coupling.

Residual absorption persists below the gap even at temperatures well below $T_c$.

Abstract

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.