Comparison of the 1-magnon dispersion energy and spectral weight A1(k) for CFTD (solid points) and our series results.

The Heisenberg model, described by the Hamiltonian

has served, for many years, as a generic model for understanding and describing antiferromagnetic order in solids. Until relatively recently the low energy properties of this model could only be calculated approximately, via spin-wave theory. In such a situation it is never clear whether discrepancies between theory and experiment are due to the shortcomings of the approximate calculation or the inadequacy of the model.
Quantities that can be most readily compared between theory and experiment are the energies of low energy quasiparticle excitations (magnon dispersion curves), which can be measured via inelastic neutron scattering, and the dynamical or integrated structure factors, which reflect the underlying dynamics of the system and are proportional to the total scattering intensity.

In recent years our group, and others, have developed powerful series expansion techniques which are able to compute dispersion relations and structure factors to high accuracy. This allows, for the first time, a reliable comparison between the model and real materials. Figure (a) shows the recently measured magnon dispersion curve for the quasi two-dimensional material (CuDCOO)₂·4D₂O (CFTD) and our series results, with one fitting parameter J=6.13meV. We have also computed the 1-magnon spectral weight, and Figure (b) compares our results with experimental measurements for CFTD. In both cases the agreement is excellent.
With the building of new and more powerful neutron scattering facilities, including the new Lucas Heights research reactor, more precise data, including measurements of multiparticle states, are becoming available and allowing ever more detailed testing of theoretical models.

Weihong Zheng, Chris Hamer and Jaan Oitmaa