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We show that the I-V characteristics measured at 4 K of single tunnel junctions between the tip of a scanning tunneling microscope and a metallic sample are sensitive to the materials used. The superconducting energy gap of lead is observed for tips without surface oxides, but eliminated for tips with thick oxides. Probing bulk etched tungsten, observation of the Coulomb staircase suggests capacitive effects due to a small metallic-oxide impurity. This provides direct evidence that such impurities can dominate junction response, sometimes imitating a single-junction Coulomb blockade.
Wilkins et al. (1990) studied this question.