Spin correlations in a Heisenberg system within the paramagnetic region

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Abstract
The two-spin correlation function for a Heisenberg system is investigated, in the paramagnetic region, using a new approach through which one is able to reveal the dynamics more clearly than previously. The conventional mode-mode coupling theory is obtained as a lowest-order approximation; this entails ignoring all third- and higher-order irreducible spin correlations. It is argued that the mechanism for sloppy spin waves is lost in the mode-mode theory and that this is the reason for some of the most noticable discrepancies between theory and experiments.