Superconductivity of Pb Ultrathin Film on Ge(111) Surface

We have performed structure analysis and electrical conductivity measurements of Pb ultrathin films of different thicknesses grown on Ge (111) at low temperature by using electron diffraction, scanning tunneling microscopy, andin-situfour-point probe method in ultrahigh vacuum. Three samples with different deposition amounts of Pb corresponding to 1, 3 and 10 monolayer (ML) were revealed to have different structures. The 1 ML-Pb sample, having a wetting layer and tiny clusters on it, did not show superconductivity. The 10-ML-Pb sample, consisted of continuous Pb (111) thin film structure, showed thin-film superconductivity around 6 K. The 3-ML-Pb sample, consisted of the wetting layer with unconnected Pb (111) islands on it, also showed superconductivity around 4 K. This superconductivity is thought to be induced in the wetting layer by proximity effect from superconducting Pb (111) islands. Thus, it is important to study the detailed growth structures for understanding atomic-layer superconductivity.