  Umklapp processes play a fundamental role as the only intrinsic mechanism
that allows electrons to transfer momentum to the crystal lattice and,
therefore, provide a finite electrical resistance in pure metals. However,
umklapp scattering has proven to be elusive in experiment as it is easily
obscured by other dissipation mechanisms. Here we show that electron-electron
umklapp scattering dominates the transport properties of
graphene-on-boron-nitride superlattices over a wide range of temperatures and
carrier densities. The umklapp processes cause giant excess resistivity that
rapidly increases with increasing the superlattice period and are responsible
for deterioration of the room-temperature mobility by more than an order of
magnitude as compared to standard, non-superlattice graphene devices. The
umklapp scattering exhibits a quadratic temperature dependence accompanied by a
pronounced electron-hole asymmetry with the effect being much stronger for
holes rather than electrons. Aside from fundamental interest, our results have
direct implications for design of possible electronic devices based on
heterostructures featuring superlattices.
