Majorana fermions in honeycomb lattices

We study the formation of Majorana fermions in honeycomb-lattice structures in the presence of a Zeeman field, Rashba spin-orbit coupling, and in the proximity of an s-wave superconductor. We show that an exact mapping exists between an anisotropic hexagonal-lattice nanoribbon at k = 0 and a one-dimensional chain, for which the existence of Majorana fermions has been extensively discussed. Consequently we can find the conditions for the emergence of Majorana fermions in such a ribbon for particular values of the chemical potential such as the top or the bottom of the band, and the Van Hove singularities, and relate the existence of Majoranas to a band inversion in the bulk band structure. Moreover we find that similar situations arise in anisotropic lattices and we give some examples which show the formation of Majorana fermions in these structures.