  A variety of novel quantum phases have been achieved in twist bilayer
graphene (tBLG) and other moire superlattices recently, including correlated
insulators, superconductivity, magnetism, and topological states. These
phenomena are very sensitive to the moire superlattices, which can hardly be
changed rapidly or intensely. Here, we report the experimental realization of a
high-order moire pattern (a high-order interference pattern) in
graphene-monolayer xenon heterostructure (G/mXe), with moire period in-situ
tuned from few nanometers to infinity by changing the lattice constant of Xe
through different annealing temperatures and pressures. We use angle-resolved
photoemission spectroscopy to directly observe that replicas of graphene Dirac
cone emerge and move close to each other in momentum-space as moire pattern
continuously expands in real-space. When the moire period approaches infinity,
the replicas finally overlap with each other and an energy gap is observed at
the Dirac point induced by intervalley coupling, which is a manifestation of
Kekule distortion. We construct a continuum moire Hamiltonian, which can
explain the experimental results well. The form of moire Hamiltonian in G/mXe
is similar to that in tBLG, and moire band with narrow bandwidth is predicted
in G/mXe. However, the moire Hamiltonian couples Dirac fermions from different
valleys in G/mXe, instead of ones from different layers in tBLG. Our work
demonstrates a novel platform to study the continuous evolution of moire
pattern and its modulation effect on electronic structure, and provides an
unprecedented approach for tailoring Dirac fermions with tunable intervalley
coupling.
