The antiferromagnetic Heisenberg model on a chain with nearest and next nearest neighbor couplings is mapped onto the SO(3) nonlinear sigma model in the continuum limit. In one spatial dimension this model is always in its disordered phase and a gap opens to excited states. The latter form a doubly degenerate spin-1 branch at all orders in 1/N. We argue that this feature should be present in th...

By using the concept of negativity, we investigate entanglement in (1/2,1) mixed-spin Heisenberg systems. We obtain the analytical results of entanglement in small isotropic Heisenberg clusters with only nearest-neighbor (NN) interactions up to four spins and in the four-spin Heisenberg model with both NN and next-nearest-neighbor (NNN) interactions. For more spins, we numerically study effects...

J 1 − J 2 quantum Heisenberg antiferromagnet on the triangular lattice: a group symmetry analysis of order by disorder. Abstract On the triangular lattice, for J 2 /J 1 between 1/8 and 1, the classical Heisen-berg model with first and second neighbor interactions presents four-sublattice ordered ground-states. Spin-wave calculations of Chubukov and Jolicoeur [9] and Korshunov [10] suggest that ...