Instability of quadratic band degeneracies and the emergence of Dirac points

Consider the Schr\"odinger operator H = - + V, where the potential V is Z²-periodic and invariant under spatial inversion, complex conjugation, and /2 rotation. We show that, under typical small linear deformations of V, the quadratic band degeneracy points, occurring over the high-symmetry quasimomentum M (see 24, 25) each split into two separated degeneracies over perturbed quasimomenta D^+ and D^-, and that these degeneracies are Dirac points. The local character of the degenerate dispersion surfaces about the emergent Dirac points are tilted, elliptical cones. Correspondingly, the dynamics of wavepackets spectrally localized near either D^+ or D^- are governed by a system of Dirac equations with an advection term. Generalizations are discussed.