Two-dimensional superconductivity in atomic layers has recently attracted considerable attention owing to its intriguing different physical properties compared to that of the three-dimensional cases. By using an in situ four-point probe method in ultrahigh vacuum, we measured the transport properties of Pb atomic layers with different thicknesses and structures grown on a Ge(111) substrate at low temperature. The transition to zero resistance was observed around 4 and 6K for deposition amounts of Pb corresponding to 3 and 10 monolayers, respectively. We found that both samples are two-dimensional superconductors because the coherence length is much longer than the film thickness. We also found that even though the surface is not entirely covered by Pb(111) islands, the samples exhibit superconductivity which indicates that superconductivity is induced in the wetting layer by the proximity effect from the Pb islands. For understanding two-dimensional superconductivity, it should be discussed by combining the structure analysis.
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