Abstract
The spectrum of excitations of a spin-(1/2 Heisenberg chain in the region of anisotropy 1>Δ>0 (where the model is critical and has no bound states) exhibits a ‘‘tower structure’’ in the energy-momentum plane as predicted by the hypothesis of conformal invariance. For a given magnetization the apices of the towers are located on a parabola. The spectrum shows remarkable degeneracies at some discrete values of Δ given by the Tutte-Behara numbers.

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