  In this Letter, we derive an effective theory of graphene on a hexagonal
Boron Nitride (h-BN) substrate. We show that the h-BN substrate generically
opens a spectral gap in graphene despite the lattice mismatch. The origin of
that gap is particularly intuitive in the regime of strong coupling between
graphene and its substrate, when the low-energy physics is determined by the
topology of a network of zero energy modes. For twisted graphene bilayers,
where inversion symmetry is present, this network percolates through the system
and the spectrum is gapless. The breaking of that symmetry by h-BN causes the
zero energy modes to close into rings. The eigenstates of these rings hybridize
into flat bands with gaps in between. The size of this band gap can be tuned by
a gate voltage and it can reach the order of magnitude needed to confine
electrons at room temperature.
