On the behavior of a Heisenberg antiferromagnet at very low temperatures
  We present an analytical result for the ratio of the physical correlation length in a 2D Heisenberg antiferromagnet on a square lattice, and the one which is actually computed in numerical simulations. This last correlation length is deduced from the second moment of the structure factor at the antiferromagnetic momentum Q. We show that the ratio is very close to one in agreement with previously obtained numerical result of the 1/N expansion. 
Andrey V. Chubukov and Oleg A. Starykh