Excitation spectra of gapped 1D spin systems in strong magnetic field

Abstract It is shown that in systems with axial symmetry the spectrum above the critical field H c contains, in addition to the low-energy spinon continuum, edge-type singularities replacing two upper magnon modes. If the axial symmetry is broken, the singularities collapse into three well-defined magnon modes. A field-theoretical approach capable of quantitative description of experimental data on the S =1 Haldane material NDMAP is proposed.