Field-Induced Gap in $\mathit{S}=1\mathrm{}/2$ Antiferromagnetic Chains

In a recent neutron-scattering experiment on the quasi-one-dimensional $S=1\mathrm{}/2$ 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 $S(q,\omega )$.