We study under which general conditions a pair of Dirac points in the electronic spectrum of a two-dimensional crystal may merge into a single one. The merging signals a topological transition between a semi-metallic phase and a band insulator. We derive a universal Hamiltonian that describes the physical properties of the transition, which is controlled by a single parameter, and analyze the Landau-level spectrum in its vicinity. This merging may be observed in the organic salt alpha-(BEDT-TTF)_2 I_3 or in an optical lattice of cold atoms simulating deformed graphene.
@article{arxiv.0904.2117,
title = {Merging of Dirac points in a two-dimensional crystal},
author = {G. Montambaux and F. Piechon and J. -N. Fuchs and M. O. Goerbig},
journal= {arXiv preprint arXiv:0904.2117},
year = {2009}
}