Superconducting proximity effect in a Rashba-type surface state of Pb/Ge(111)

The Rashba superconductor, in which spin-splitting bands become superconducting, is fascinating as a novel superconducting system in low dimensional systems. Here, we present the results of $\textit{in-situ}$ transport measurements on a Rashba-type surface state of the striped incommensurate (SIC) phase of a Pb atomic layer on Ge(111) surface with additional Pb islands/clusters on it. We found that two-step superconducting transitions at around 7 K and 3 K occurred. The latter superconducting transition is suggested to be induced at the non-superconducting Rashba SIC area because of the lateral proximity effect caused by the superconducting Pb clusters. Our results propose a new type of Rashba superconductor, which is a new platform to understand the Rashba superconducting systems.