  As heavy analog of graphene, plumbene is a two-dimensional material with
strong spin-orbit coupling effects. Using scanning tunneling microscopy (STM),
we observe that Pb forms a flat honeycomb lattice on an Fe monolayer on
Ir(111). In contrast, without the Fe layer, a c(2x4) structure of Pb on Ir(111)
is found. We use density functional theory (DFT) calculations to rationalize
these findings and analyze the impact of the hybridization on the plumbene band
structure. In the unoccupied states the splitting of the Dirac cone by
spin-orbit interaction is clearly observed while in the occupied states of the
freestanding plumbene we find a band inversion that leads to the formation of a
topologically non-trivial gap. Exchange splitting as mediated by the strong
hybridization with the Fe layer drives a quantum spin Hall to quantum anomalous
Hall state transition.
