  We show that the interaction between flexural phonons, when corrected by the
exchange of electron-hole excitations, may place the graphene sheet very close
to a quantum critical point characterized by the strong suppression of the
bending rigidity of the membrane. Ripples arise then due to spontaneous
symmetry breaking, following a mechanism similar to that responsible for the
condensation of the Higgs field in relativistic field theories. In the presence
of membrane tensions, ripple condensation may be reinforced or suppressed
depending on the sign of the tension, following a zero-temperature buckling
transition in which the order parameter is given essentially by the square of
the gradient of the flexural phonon field.
