ENERGY-GAP DETERMINATION ON POLYCRYSTALLINE RB2CSC60 BY SCANNING TUNNELING SPECTROSCOPY

A polycrystalline Rb2CsC60 sample with T-c = 32.5 K (onset value) is i nvestigated in the superconducting state by scanning tunneling spectro scopy (STS) using a low temperature scanning tunneling microscope (STM ). At 4.5 K the superconducting energy gap Delta is evaluated fron the measured quasiparticle density of states (DOS). Most gap values range from 3.3 to 5.7 meV, with an average of 4.2 +/- 0.6 meV (2 Delta/k(B) T(c) = 3.2 +/- 0.5). The scattering is attributed to local variations in stoichiometry on the sample surface. However, a few spectra exhibit much larger energy gaps (approximate to 6.2 to 9.5 meV). These large values cannot be understood within standard BCS theory (1.76 x k(B)T(c ) = 4.9 meV). We therefore suggest an alternative explanation by assum ing a superconductor-insulator-superconductor (SIS) tunneling junction . In this case a gap width of 4 Delta (peak-to-peak distance in the DO S) is expected, which is in good agreement with our measurements. (C) 1997 Elsevier Science Ltd.