  The interface between two-dimensional (2D) crystals often forms a Moire
superstructure that imposes a new periodicity, which is a key element in
realizing complex electronic phases as evidenced in twisted bilayer graphene. A
combined angle resolved photoemission spectroscopy measurements and
first-principles calculations reveal the formation of a Moire superstructure
between a 2D Dirac semi-metallic crystal, graphene, and a 2D insulating crystal
of noble gas, xenon. Incommensurate diffraction pattern and folded Dirac cones
around the Brillouin zone center imply the formation of hexagonal crystalline
array of xenon atoms. The velocity of Dirac fermions increases upon the
formation of the 2D xenon crystal on top of graphene due to the enhanced
dielectric screening by the xenon over-layer. These findings not only provide a
novel method to produce a Moire superstructure from the adsorption of noble gas
on 2D materials, but also to control the physical properties of graphene by the
formation of a graphene-noble gas interface.
