Dynamics of gapped spin systems in high magnetic field

The subject of the talk is the dynamics of gapped spin systems in presence of an external magnetic field sufficiently high to close the energy gap. The focus will be on anisotropic S = 1 Haldane chain, although applications to weakly coupled dimer system of the TlCuCl3 type and spin ladders with four-spin (ring) interaction will be briefly considered. Existing effective field-theoretical descriptions [1, 2, 3] of anisotropic S = 1 Haldane chain in applied field are reviewed and their predictions are compared with the results of recent inelastic neu-tron scattering experiments on the Haldane chain material NDMAP [4]. It is shown that none of the known theories can provide a consistent description of the experimental data. A new effective theory, based on the dimer field description for alternated S = 1 2 chain, is proposed. In the isotropic case it coincides with the theory of Affleck [1] but differs from it considerably when a finite anisotropy is present. It is shown that the theory is capable of a quantitative description of the data on NDMAP with a satisfactory accuracy, in the whole range of the applied fields both below and above the critical field [4, 5, 6]