BLOCH OSCILLATIONS OF MAGNETIC SOLITONS IN ANISOTROPIC SPIN-1 2 CHAINS/

We study the quantum dynamics of solitonlike domain walls in anisotrop ic spin-1/2 chains in the presence of magnetic fields. In the absence of fields, domain walls form a Bloch band of delocalized quantum state s while a static field applied along the easy axis localizes them into Wannier wave packets and causes them to execute Bloch oscillations, i .e., the domain walls oscillate along the chain with a finite Bloch fr equency and amplitude. In the presence of the field, the Bloch band, w ith a continuum of extended states, breaks up into the Wannier-Zeeman ladder-a discrete set of equally spaced energy levels. We calculate th e dynamical structure factor S-ZZ(q,omega) in the one-soliton sector a t finite frequency, wave vector, and temperature, and find sharp peaks at frequencies which are integer multiples of the Bloch frequency. We further calculate the uniform magnetic susceptibility and find that i t too exhibits peaks at the Bloch frequency. We identify several candi date materials where these Bloch oscillations should be observable, fo r example, via neutron-scattering measurements. For the particular com pound CoCl2.2H(2)O we estimate the Bloch amplitude to be on the order of a few lattice constants, and the Bloch frequency on the order of 10 0 GHz for magnetic fields in the Tesla range and at temperatures of ab out 18 K.