Critical Phenomena of a Quantum Heisenberg-like Antiferromagnet

Ordering of two-dimensional Heisenberg (2DH) spin systems has been a most interesting problem from the viewpoint of cooperativity. Any phase transition associated with long range order was predicted not to occur down to 0 K[1], So the phase transition with spontaneous magnetization found in the actual 2DH-like magnets examined so far has been attributed to a non-negligible interplane interaction and/or Ising(I)-type anisotropy. Apart from the ideal behaviour, the phase transition of 2DH-like antiferromagnets, especially of S=l/2 if any, is interesting because the critical phenomena could be modified by large quantum fluctuations[2] and a new aspect of cooperativity may be found.