Spatial variation of a giant spin-orbit effect induces electron confinement in graphene on Pb islands

The electronic band structure of a material can acquire interesting topological properties in the presence of a magnetic field or as a result of the spin-orbit coupling(1-3). We study graphene on Ir, with Pb monolayer islands intercalated between the graphene sheet and the Ir surface. Although the graphene layer is structurally unaffected by the presence of the Pb islands, its electronic properties change markedly, with regularly spaced resonances appearing. We interpret these resonances as the effect of a strong and spatially modulated spin-orbit coupling, induced in graphene by the Pb monolayer. As well as confined electronic states, the electronic spectrum has a series of gaps with non-trivial topological properties, resembling a realization of the quantum spin Hall effect proposed by Bernevig and Zhang(4).