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I'm looking for the critical exponent ν (i.e. the one of the correlation length) of the Heisenberg (i.e. equal coupling in all directions) antiferromagnetic spin-1/2 model in 1D...

Furthermore, do you know to which universality class it belongs? Is it true that it's the Kosterlitz-Thouless class? Do you know a good (review-) article about this topic? I couldn't find anything useful neither in Sachdev's book nor in Takahashi's one, but maybe I didn't check carfully enough...

One more question: there are two possible ways to move away from criticality in the case of the Heisenberg model, namely altering one of the coulpling s.t. you end up with the XXZ-model, or adding an external homogenous field, say in Z-direction. Do they both yield the same critical behaviour?

Many thanks!