ONE-DIMENSIONAL ANISOTROPIC HEISENBERG MODEL IN THE TRANSVERSE MAGNETIC FIELD
Dmitriev D.V., Krivnov V.Ya., Ovchinnikov A.A., Langari A.

Received: March 29, 2002

PACS: 75.10.Jm

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The one-dimensional spin-1/2 XXZ model in a transverse magnetic field is studied. It is shown that the field induces a gap in the spectrum of the model with the easy-plain anisotropy. Using conformal invariance, the field dependence of the gap is found at small fields. The ground state phase diagram is obtained. It contains four phases with the long-range order of different types and a disordered phase. These phases are separated by critical lines, where the gap and the long-range order vanish. Using scaling estimates, the mean-field approach, and numerical calculations in the vicinity of all critical lines, we find the critical exponents of the gap and the long-range order. It is shown that the transition line between the ordered and disordered phases belongs to the universality class of the transverse Ising model.