Order Parameter in Quantum Antiferromagnets

We discuss the nature of symmetry breaking in quantum antiferromagnets. We show that published arguments concerning the order parameter for the 2d s=1/2 Heisenberg antiferromagnet have to be reconsidered. These had led to √3 and 1 for the ratior of the mean z-component of the staggered spin per particle mzs in an infinitesimal staggered field B, to the r.m.s. value m0 of mzs with B=0, in the thermodynamic limit. We give arguments which put doubt on both values. Also we prove r ≧ 1, given m0 ≠ 0. © 1989, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.