Gap Opening in Graphene-Based 2D Heterostructures: The Interplay of Spin-Orbit Coupling, Hybridization, and Symmetry

Intercalating a Pb monolayer between graphene and SiC(0001) creates a densely packed metallic layer in close proximity to graphene. Using low-temperature four-point-probe scanning tunneling microscopy and density functional theory, we correlate the local conductivity of this two-dimensional heterostructure with spatially resolved spectroscopy. By varying the tunneling gap, we distinguish the density-of-states contributions of the decoupled graphene sheet and the buried Pb interface layer. At large tip-sample separations, the spectra resemble those of charge-neutral, quasi-freestanding graphene with a small contribution of the metallic Pb layer beneath. This separation confirms the presence of a 5 meV energy gap in graphene, primarily arising from symmetry breaking induced by the epitaxial Pb layer. A proximity-induced intrinsic spin-orbit coupling appears negligible or is compensated by Rashba-type interactions.