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Abstract

A polycrystalline Rb₂CsC₆₀ sample with T_c=32.5 K (onset value) is investigated in the superconducting state by scanning tunneling spectroscopy (STS) using a low temperature scanning tunneling microscope (STM). At 4.5 K the superconducting energy gap Δ is evaluated from the measured quasiparticle density of states (DOS). Most gap values range from 3.3 to 5.7 meV, with an average of4.2±0.6meV (2Δ/k_BT_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 (≈6.2 to 9.5 meV). These large values cannot be understood within standard BCS theory (1.76×k_BT_c=4.9 meV). We therefore suggest an alternative explanation by assuming a superconductor-insulator-superconductor (SIS) tunneling junction. In this case a gap width of 4Δ (peak-to-peak distance in the DOS) is expected, which is in good agreement with our measurements.