Numerical Evidence for the Haldane Conjecture

The Haldane conjecture, when applied to the Heisenberg O(3) model with a \theta term in two dimensions, states that the correlation length \xi diverges when \theta approaches \pi. To verify this conjecture we have numerically simulated the model at imaginary \theta and then analytically continued the results to real \theta. We have obtained that the value where the model should become critical is \theta=3.10(5) in agreement with the expectation.