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 ($\Delta$) in an organic superconductor. For a $\beta -{\left[bis\right(\mathrm{ethylenedithio}\left)\mathrm{tetrathiafulvalene}\right]}_2\mathrm{Au}{\mathrm{I}}_2$ single crystal, $\Delta$ 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.