Field-induced gap in Cu benzoate and other S=1/2 antiferromagnetic chains

Recent experiments on the S=1/2 antiferromagnetic chain compound, Cu benzoa te, discovered an unexpected gap scaling as approximately the 2/3 power of an applied magnetic field. A theory of this gap, based on an effective stag gered field, orthogonal to the applied uniform field, resulting from a stag gered gyromagnetic tensor and a Dzyaloshinskii-Moriya interaction, leading to a sine-Gordon quantum field theory, has been developed. Here we discuss many aspects of this subject in considerable detail, including a review of the S=1/2 chain in a uniform field, a spin-wave theory analysis of the unif orm plus staggered field problem, exact amplitudes for the scaling of gap, staggered susceptibility and staggered magnetization with field or temperat ure, intensities of soliton and breather peaks in the structure function, a nd field and temperature dependence of the total susceptibility. [S0163-182 9(99)03226-9].