We consider the conductivity $\sigma$ of graphene with negligible intervalley scattering at half filling. We derive the effective field theory, which, for the case of a potential disorder, is a symplectic-class sigma model including a topological term with $\theta =\pi$. As a consequence, the system is at a quantum critical point with a universal value of the conductivity of the order of $e^2/h$. When the effective time-reversal symmetry is broken, the symmetry class becomes unitary, and $\sigma$ acquires the value characteristic for the quantum Hall transition.

• Received 5 February 2007

DOI:https://doi.org/10.1103/PhysRevLett.98.256801