Electronic properties of Pb islands on graphene: Consequences of a weak interface coupling from a combined STM and ab initio study
Résumé
By means of scanning tunneling microscopy and spectroscopy, we investigate the electronic properties of
lead islands (width 5–100 nm, thickness 5–25 monolayers) deposited by molecular beam epitaxy on twisted
graphene layers grown on SiC(000-1). We find that elastic scattering processes govern the local density of
states probed at the surface of the Pb islands, inducing (i) the well-known quantum well states due to electron
confinement in the direction perpendicular to the surface and (ii) spatial in-plane periodic modulations related
to quasiparticle interferences off the island edges. Through a quantitative analysis of these effects, compared
with ab initio calculations for a two-dimensional Pb slab, we conclude that the lead islands grown on the surface
of graphene can be considered as freestanding from the point of view of their electronic structure, leaving the
surrounding graphene layer unperturbed. Accordingly, low bias tunneling spectra show evidence of a sizable
interface resistance. Nevertheless, we suggest that the transparency of the interface, which can be estimated from
its resistance, is good enough to induce superconductivity within the underlying graphene layer by proximity
effect with the Pb islands.