  Nonlinearities are inherent to the dynamics of two-dimensional materials.
Phenomena like intermodal coupling already arise at amplitudes of only a few
nanometers, and a range of unexplored effects still awaits to be harnessed.
Here, we demonstrate a route for generating mechanical frequency combs in
graphene resonators undergoing symmetry-breaking forces. We use electrostatic
force to break the membrane's out-of-plane symmetry and tune its resonance
frequency towards a two-to-one internal resonance, thus achieving strong
coupling between two of its mechanical modes. When increasing the drive level,
we observe splitting of the fundamental resonance peak, followed by the
emergence of a frequency comb regime. We attribute the observed physics to a
non-symmetric restoring potential, and show that the frequency comb regime is
mediated by a Neimark bifurcation of the periodic solution. These results
demonstrate that mechanical frequency combs and chaotic dynamics in 2D material
resonators can emerge near internal resonances due to symmetry-breaking.
