OBSERVABILITY OF BETHE-ANSATZ EXCITATIONS IN HEISENBERG-ISING SPIN SYSTEMS

The Hohenberg-Brinkman sum rule analysis is extended to Heisenberg-Ising chains of arbitrary anisotropy in uniform magnetic fields. It is found that when the system is gapless, Bethe-ansatz excitations dominate the weight function χzz′′(k,ω) at low k. They do not in general dominate χxx′′. When the excitation spectrum has a small gap, the mean energy of χzz′′(0,ω) is far larger than the gap energy single excitations no longer dominate, although they again become important as the gap gets very large. © 1979 The American Physical Society.