The Haldane model (1988) on a honeycomb lattice. Berry curvature Ω(k) = ∇k × A(k) acts like a magnetic field in k-space. Chern number C = (1/2π)∫∫Ω d²k ∈ ℤ.
C ≠ 0 implies topological phase with chiral edge states (bulk-boundary correspondence). σxy = Ce²/h.