Measurement of the Energy Gap in an Organic Superconductor: Evidence for Extremely Strong Coupling

Abstract

Point-contact tunneling is used to make what we believe to be the first measurement of a superconducting energy gap ($\ensuremath{\Delta}$) in an organic superconductor. For a $\ensuremath{\beta}\ensuremath{-}{[bis(\mathrm{ethylenedithio})\mathrm{tetrathiafulvalene}]}_{2}\mathrm{Au}{\mathrm{I}}_{2}$ single crystal, $\ensuremath{\Delta}$ shows consistent field and temperature dependences, but is more than 4 times larger than weak-coupling BCS value, implying extremely strong coupling. We speculate that very low-frequency modes may be responsible.