1zExpansion for the Heisenberg Antiferromagnet at Low Temperatures

Abstract

It is shown that the expansion in density of spin deviations for the Heisenberg antiferromagnet at low temperatures leads to a $\frac{1}{z}$ expansion for the free energy. In contrast to the $\frac{1}{S}$ expansion, the kinematic properties of spin operators are realized within each order of the expansion. The spin-wave energy at long wavelength is also calculated.