2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/131842We consider the conductivity $σ_{xx}$ 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 $σ$-model including a topological term with $θ=π$. 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 $σ_{xx}$ acquires the value characteristic for the quantum Hall transition.4 pages, 1 figureMesoscale and Nanoscale PhysicsDisordered Systems and Neural Networkstext