Origin of Lattice Spin in Graphitic Systems

Lattice spin, in planar condensed matter system with emergent Dirac dispersion, is shown to emerge from the inherent SU(2) symmetry, arising through Schwingers angular momentum construction from anti-commuting Heisenberg operators of the sub-lattices. The presence of a mass term in the emergent Dirac dispersion is essential for the existence of this spin. The usual hopping term, that entangles the two sub-lattices, leads to the orbital counterpart. Relative sub-lattice displacements, that couple to the effective Dirac fermions like U(1) gauge fields, do not effect the lattice spin.