Understanding of hopping matrix for 2D materials taking 2D honeycomb and   square lattices as study cases
  In this work, a trial understanding for the physics underling the construction of exchange (hopping) matrix $\mathbf{E}$ in Heisenberg model (tight binding model) for 2D materials is done. It is found that the $\mathbf{E}$ matrix describes the particles exchange flow under short range (nearest neighbor) hopping interaction which is effected by the lattice geometry. This understanding is then used to explain the dispersion relations for the 2D honeycomb lattice with zigzag and armchair edges obtained for graphene nanoribbons and magnetic stripes. It is found that the particle flow by hopping in the zigzag nanoribbons is a translation flow and shows $\mathbf{\cos^2}(q_xa)$ dependance while it is a rotational flow in the armchair nanoribbons. At $q_xa/\pi=0.5$, the particles flow in the edge sites of zigzag nanoribbons with dependance of $\mathbf{\cos^2}(q_xa)$ is equal to zero. At the same time there is no vertical hopping in those edge sites which lead to the appearance of peculiar zigzag flat localized edge states. 
Maher Ahmed
cond-mat.mes-hall cond-mat.mtrl-sci cond-mat.other