Waves in Honeycomb Structures
  We review recent work of the authors on the non-relativistic Schr\"odinger equation with a honeycomb lattice potential, $V$. In particular, we summarize results on (i) the existence of Dirac points, conical singularities in dispersion surfaces of $H_V=-\Delta+V$ and (ii) the two-dimensional Dirac equations, as a large, but finite time, effective description of $e^{-iH_Vt}\psi_0$, for data $\psi_0$, which is spectrally localized at a Dirac point. We conclude with a formal derivation and discussion of the effective large time evolution for the nonlinear Schr\"odinger - Gross Pitaevskii equation for small amplitude initial conditions, $\psi_0$. The effective dynamics are governed by a nonlinear Dirac system. 
Charles L. Fefferman and Michael I. Weinstein
nlin.PS cond-mat.mes-hall math-ph math.AP math.MP quant-ph