Order Parameter in Quantum Antiferromagnets

Abstract

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 \sqrt3 and 1 for the ratio r of the mean z-component m of the staggered spin per particle m_s^z in an infinitesimal staggered field B, to the r.m.s. value m₀ of m_s^z with B=0, in the thermodynamic limit. We give arguments which put doubt on both values. Also we prove r≥1, given m₀≠0.