SPATIALLY-RESOLVED ELECTRON-TUNNELING SPECTROSCOPY ON SINGLE-CRYSTALLINE RB3C60

A Rb3C60 single crystal (T-c = 30.5 K) is investigated in the supercon ducting state at 2.8 K by scanning tunneling microscopy and scanning t unneling spectroscopy (STS). STS data reveals a spatial variation of t he superconducting energy gap Delta on a scale of 50 nm (Delta = 2.6-5 .2 meV; 2 Delta/k(B)T(c) = 2.0-4.0), This behavior is attributed to va rying stoichiometry on the sample surface. An investigation of a Rb3C6 0 facet shows that I(V) characteristics even vary on molecular scale. I(V) curves acquired between fullerene molecules exhibit a nonvanishin g slope at zero bias whereas I(V) characteristics measured above molec ules exhibit vanishing slope at zero bias. (C) 1996 American Vacuum So ciety.