Correlation functions of a Heisenberg antiferromagnet

The correlation functions of a Heisenberg antiferromagnet are investigated. The method employs a drone-fermion representation for spin-half and a classification of terms with respect to a high density parameter 1/z. One- and two-particle correlation functions are evaluated up to order (1/z) 1 and their connection with the physical properties of an antiferromagnet discussed. This enables results with a determined range of validity to be obtained for the spin wave energy and damping, assumed small, as functions of momentum and temperature. The damping is found to increase with temperature, but is small for any temperature below T N for sufficiently long wavelengths. This is similar to the result obtained for the ferromagnet by Vaks et al. The spin wave energy and damping can be deduced from neutron scattering experiments, and the theoretical predictions are found to be in good qualitative agreement with measurements obtained for CoF 2 and NiF 2 .