Date of Original Version
We present a new approach to the low temperature dynamics of a quantum Heisenberg antiferromagnetic chain, which employs a combination of techniques: exact finite chain calculations; exact Bethe Ansatz calculations; and exact sum rules and selection rules. A striking property of the selection rules is that the contribution of several classes of states to the dynamics in non-zero field for finite systems is shown to vanish in the thermodynamic limit. Many novel quantum field-dependent effects appear such as soft modes, and multiple peak structure in the structure factor, which should be experimentally observable.
Gerhard Müller, Harry Thomas, Hans Beck and Jill C. Bonner. Magnetic field effects on the spin dynamics of the linear spin-1/2 Heisenberg antiferromagnet. Solid State Commun. 38 (1981), 1-5.