  Breakdown of the quantum Hall effect (QHE) is commonly associated with an
electric field approaching the inter Landau-level (LL) Zener field, ratio of
the Landau gap and cyclotron radius. Eluded in semiconducting heterostructures,
in spite of extensive investigation, the intrinsic Zener limit is reported here
using high-mobility bilayer graphene and high-frequency current noise. We show
that collective excitations arising from electron-electron interactions are
essential. Beyond a noiseless ballistic QHE regime a large superpoissonian shot
noise signals the breakdown via inter-LL scattering. The breakdown is
ultimately limited by collective excitations in a regime where phonon and
impurity scattering are quenched. The breakdown mechanism can be described by a
Landau critical velocity as it bears strong similarities with the roton
mechanism of superfluids.
