  We propose an electrically tunable graphene-based metamaterial showing a
large nonlinear optical response at THz frequencies, which we calculate
analytically for the first time to our knowledge and arises from the intraband
current. The structure sustains a novel type of stable two-dimensional spatial
solitary wave, a relativistic version of the Townes soliton. These results can
be also applied to any material exhibiting a conical dispersion with massless
Dirac fermions.
