Bruno Nachtergaele
Bounds on the mass gap of the ferromagnetic XXZ chain
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ABSTRACT.  We prove rigorous lower and upper bounds for the mass gap 
of the ferromagnetic spin 1/2 XXZ chain. The two bounds coincide
asymptotically in the Ising limit $\Delta\to\infty$. Near the
isotropic point, $\Delta=1$, the estimates are good enough to
determine the critical behaviour of the mass gap unambiguously.
The derivation does not rely on exact solutions.