  Lateral superlattices have attracted major interest as this may allow one to
modify spectra of two dimensional electron systems and, ultimately, create
materials with tailored electronic properties. Previously, it proved difficult
to realize superlattices with sufficiently short periodicity and weak disorder,
and most of the observed features could be explained in terms of commensurate
cyclotron orbits. Evidence for the formation of superlattice minibands (so
called Hofstadter's butterfly) has been limited to the observation of new
low-field oscillations and an internal structure within Landau levels. Here we
report transport properties of graphene placed on a boron nitride substrate and
accurately aligned along its crystallographic directions. The substrate's moire
potential leads to profound changes in graphene's electronic spectrum.
Second-generation Dirac points appear as pronounced peaks in resistivity
accompanied by reversal of the Hall effect. The latter indicates that the sign
of the effective mass changes within graphene's conduction and valence bands.
Quantizing magnetic fields lead to Zak-type cloning of the third generation of
Dirac points that are observed as numerous neutrality points in fields where a
unit fraction of the flux quantum pierces the superlattice unit cell. Graphene
superlattices open a venue to study the rich physics expected for
incommensurable quantum systems and illustrate the possibility to controllably
modify electronic spectra of 2D atomic crystals by using their crystallographic
alignment within van der Waals heterostuctures.
