  Silicene, analogous to graphene, is a one-atom-thick two-dimensional crystal
of silicon which is expected to share many of the remarkable properties of
graphene. The buckled honeycomb structure of silicene, along with its enhanced
spin-orbit coupling, endows silicene with considerable advantages over graphene
in that the spin-split states in silicene are tunable with external fields.
Although the low-energy Dirac cone states lie at the heart of all novel quantum
phenomena in a pristine sheet of silicene, the question of whether or not these
key states can survive when silicene is grown or supported on a substrate
remains hotly debated. Here we report our direct observation of Dirac cones in
monolayer silicene grown on a Ag(111) substrate. By performing angle-resolved
photoemission measurements on silicene(3x3)/Ag(111), we reveal the presence of
six pairs of Dirac cones on the edges of the first Brillouin zone of Ag(111),
other than expected six Dirac cones at the K points of the primary
silicene(1x1) Brillouin zone. Our result shows clearly that the unusual Dirac
cone structure originates not from the pristine silicene alone but from the
combined effect of silicene(3x3) and the Ag(111) substrate. This study
identifies the first case of a new type of Dirac Fermion generated through the
interaction of two different constituents. Our observation of Dirac cones in
silicene/Ag(111) opens a new materials platform for investigating unusual
quantum phenomena and novel applications based on two-dimensional silicon
systems.
