Superconductivity in a single atomic plane – RODESIS

While anticipated theoretically long ago by V. Ginzburg, superconductivity in a true two-dimensional (2D) electronic system was discovered only recently in 2010 in monolayers of lead (Pb) and indium (In) grown on Si(111). Achieving such remarkable state of matter for a one atom-thick film requires the the molecular beam epitaxy as growth method. Fascinating electronic properties were further discovered in the field of monolayer superconductors. In particular it was shown in the team of the PI of this proposal that crystalline monolayer superconductors, like Pb/Si(111), present emergent granular superconducting properties on length scales much shorter than the superconducting coherence length ?0. Such a phenomenon was never reported before in any other ultrathin film made out of conventional superconducting material. This emergent spatial granularity is characterized by two striking uncorrelated phenomena:
i) There are large spatial fluctuations of the height of the superconducting coherence peaks over a length l << ?0.
ii) There are electronic states induced inside the superconducting energy gap and this gap filling spatially fluctuates over a length l << ?0.
Both features cannot at present be rationalized theoretically and it is established by a recent experimental/theoretical work done by the team of the applicants that both features cannot be caused by magnetic impurities. The purpose of this proposal is to reveal and model two independent mechanisms at play in atomically thin films hosting 2D electrons. Using the deposition of neutral and donor atoms on such films our first objective is to engineer controlled point-like and extended defects to show that feature i) is a characteristic signature emerging from the peculiar spatial distribution of the non-magnetic disorder. Using an innovative method developed in the team, cross-correlations between the spatial changes occurring in the superconducting properties and spatial changes developing in the local disorder potential will be systematically searched for. The completion of the controlled disorder tuning and its revealed impact on the local superconducting properties will allow further advanced theoretical modelling by the theoretician involved in the project. Our second objective is to demonstrate that feature ii) is induced by a very strong Rashba spin-orbit coupling present in surface monolayer superconductors made of heavy atoms. To this end, we will tune the Rashba spin-orbit coupling using various substrates, by replacing Pb with In atoms, and by alloying Pb with lighter atoms. Then, by further engineering for each system the density and extension of non-magnetic defects we aim at showing that the quasiparticle states induced in the gap of the Pb/Si(111) monolayer originate from the scattering by non-magnetic defects of the spin-triplet part of the order parameter, in the very same way magnetic impurities are known to induce Shiba states locally in a pure s-wave singlet superconductor. The experimental work will be carried out on a state-of-the-art 300mK scanning tunneling microscope (STM) equipped with high perpendicular magnetic field (8T) and operating under ultrahigh vacuum conditions. The consortium gathered for this 3-years proposal has a strong cohesion over the past years with many top-level common publications. The team members present very high experimental and theoretical skills in the field of mesoscopic superconductivity and growth techniques to fulfill this project. The outcome of this work will furnish a new paradigm of experimental and theoretical features characterizing 2D-electron-based superconductors with strong Rashba spin-orbit coupling. The output will have a strong impact on other recently discovered monolayer superconductors like the dichalcogenides, iron selenide and superconductors at oxide interfaces. We will also give a new controlled platform to fabricate topological superconductivity by depositing magnetic elements.