Problem of Two Spin Deviations in a Linear Chain with Next-Nearest-Neighbor Interactions

The problem of two spin deviations from the fully aligned state is studied in a linear chain for the Hamiltonian [J(i,j) > 0]: H=-1/2 ∑ (nn)J(i,j)S_i.S_j-1/2;α ∑ (nnn)J(i,j)S_i.S_j, where (nn) and (nnn) mean nearest- and next-nearest-neighbor interactions, respectively. The behavior of the bound state, which is found to exist for α ≥ 0 only, is discussed.