Superconducting proximity effect on a Rashba-split Pb/Ge(111)-β√3 × √3 surface

Abstract

Using a cryogenic scanning tunneling microscope we investigated the superconducting proximity effect on a Pb/Ge(111)-β√3 × √3 reconstructed surface, where Rashba-split surface states are found at the Fermi energy, particularly focusing on the roles of surface steps in its propagation. Spatial modulation in the induced superconducting properties, which was predicted for the spin-split states, was not observed. We found that the surface steps affect the proximity effect much less than the case of a striped-incommensurate Pb/Si(111) surface, where the steps disrupt the propagation. From a comparison with simulated proximity-induced dip profiles, the electrical conductivity through a single atomic-height surface step was estimated. It is found that the obtained step conductivity on the Pb/Ge(111) surface is more than 10 times larger than that of the Pb/Si surface. These results indicate a significant contribution of subsurface states to the proximity effect and the electrical transport through the surface layers of the Pb/Ge(111) structure.