Field-Induced Gap inS=1/2Antiferromagnetic Chains

Abstract
In a recent neutron-scattering experiment on the quasi-one-dimensional $S=1/2$ antiferromagnet Cu Benzoate, a gap was induced by an applied magnetic field. We argue that the primary mechanism of the gap formation is an effective staggered field due to both the alternating $g$-tensor and the Dzyaloshinskii-Moriya interaction. We explain the dependence of the gap on the applied field, as well as identify several peaks in the structure factor $S(q,\omega)$.Comment: 5 pages, REVTEX, one figure in eps fil
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