Tunneling spectroscopy on an organic superconductor (BEDT-TTF)2Cu(NCS)2

We have constructed a scanning tunneling microscope that is capable of operation at low temperatures. Spectroscopic measurements were performed on single crystals of an organic superconductor, κ-(BEDT-TTF)2Cu(NCS)2, at temperatures between 1.8 and 12 K. Current-voltage characteristics measured well below the superconducting critical temperature Tc=11 K clearly showed superconducting energy gap, though the tip was supposed to be in contact with the sample surface. Its magnitude and temperature dependence were comparable to the value according to the Bardeen–Cooper–Schrieffer theory. Line shape of differential conductance dI/dV, however, tended to deviate from the shape according to the theory. Current-voltage characteristics typical of charging effect were also observed.