Instability of quadratic band degeneracies and the emergence of Dirac points

Abstract: Consider the Schr\"{o}dinger operator $H = -\Delta + V$, where the potential $V$ is real, $\mathbb{Z}^2$-periodic, and additionally invariant under the symmetry group of the square. We show that, under typical small linear deformations of $V$, the quadratic band degeneracy points occurring over the high-symmetry quasimomentum $\boldsymbol{M}$ (see [27, 28]) each split into two separated degeneracies over perturbed quasimomenta $\boldsymbol{D}^+$ and $\boldsymbol{D}^-$, 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 $\boldsymbol{D}^+$ or $\boldsymbol{D}^-$ are governed by a system of Dirac equations with an advection term. Symmetry-breaking perturbations and induced band topology are also discussed.