Inelastic neutron scattering was used to measure the magnetic field dependence of spin excitations in the antiferromagnetic S=1/2 chain CuCl_2 2(dimethylsulfoxide) (CDC) in the presence of uniform and staggered fields. Dispersive bound states emerge from a zero-field two-spinon continuum with different finite energy minima at wave numbers q=pi and q_i approx pi (1-2<S_z>). The ratios of the field dependent excitation energies are in excellent agreement with predictions for breather and soliton solutions to the quantum sine-Gordon model, the proposed low-energy theory for S=1/2 chains in a staggered field. The data are also consistent with the predicted soliton and n=1,2 breather polarizations and scattering cross sections.
@article{arxiv.cond-mat/0305476,
title = {Bound spinons in an antiferromagnetic S=1/2 chain with a staggered field},
author = {M. Kenzelmann and Y. Chen and C. Broholm and D. H. Reich and Y. Qiu},
journal= {arXiv preprint arXiv:cond-mat/0305476},
year = {2009}
}