A statistical model for the intrinsically broad superconducting-to-normal transition in quasi-two-dimensional crystalline organic metals

Abstract

Although quasi-two-dimensional organic superconductors such as κ-(BEDT-TTF)₂Cu(NCS)₂ (BEDT-TTF≡bis(ethylene-dithio)tetrathiafulvalene) seem to be very clean systems, with apparent quasiparticle mean free paths of several thousand ångströms, the superconducting transition is intrinsically broad (e.g. ~1 K wide for T_c≈10 K). We propose that this is due to the extreme anisotropy of these materials, which greatly exacerbates the statistical effects of spatial variations in the potential experienced by the quasiparticles. Using a statistical model, we are able to account for the experimental observations. A parameter , which characterizes the spatial potential variations, may be derived from Shubnikov-de Haas oscillation experiments. Using this value, we are able to predict a transition width which is in good agreement with that observed in megahertz penetration-depth measurements on the same sample.