Nonlinear Excitations in Quasi-One-Dimensional Triangular Antiferromagnets

Recent neutron experiments have revealed that the excitation spectrum in quasi-one-dimensional ABX 3 -type antiferromagnets cannot be explained within the conventional linear-spin-wave theory even in the ordered phase. To explain this result, we take into account two-magnon excitation processes in the spin-wave expansion and apply it to CsNiCl 3 at T =0. We find that in addition to modifying the energy and intensity of the one-magnon spectrum, these processes can result in large peaks due to two-magnon continuum. The coupling between longitudinal and transverse fluctuations is found to be important as previously suggested by phenomenological Landau-Ginzburg models. Our results agree reasonably well with available experiments. Similar effects can be expected in other quasi-one-dimensional systems having non-collinear spin structures.