Determination of the electron phonon coupling and the superconducting gap in β-(BEDT-TTF)₂X crystals (X=I₃, IAuI)

We performed point-contact and tunneling measurements on the organic superconductors β-(BEDT-TTF)₂I₃ and β-(BEDT-TTF)₂IAuI, in the normal and superconducting states. The point-contact measurement in the normal state provides the Eliashberg function α²(ω)F (ω). This function has maxima at 4 and 15 mV, as well as a sharp peak at 1 mV which seems to indicate a very soft phonon strongly coupled to the electrons. The measurements in the superconducting state provide the superconducting gap, which for the tunneling measurements has a value 2Δ/k_B T_c≅4, just slightly above the BCS value.