Dirac Cone

Physics: Dirac cone: E(k) = ±ℏv_F·k, with v_F ≈ 10⁶ m/s in graphene (c/300). Pseudospin = sublattice degree of freedom, locked to momentum direction. Klein tunneling: massless Dirac electrons pass through a potential barrier with T=1 at normal incidence — the electron converts to a hole inside the barrier, and transmission is perfect regardless of barrier height. This is unique to Dirac fermions; Schrödinger electrons always have T→0 for high barriers. Berry phase of π around the Dirac point causes anomalous quantum Hall effect σ_xy = ±4(N+½)e²/h. A gap opens when sublattice symmetry is broken (mass term m·v²).