CORRELATED SPIN DYNAMICS IN 2-D QUANTUM HEISENBERG ANTIFERROMAGNETS FROM NMR RELAXATION IN COPPER FORMIATE TETRADEUTERATE

Proton nuclear magnetic resonance (NMR) T-1 in a single crystal of copper formiate tetradeuterate is used to study the correlated Cu2+ spin dynamics and to derive the temperature behavior of the in-plane magnetic correlation length. The results are compared with the predictions of recent theoretical models for the spin dynamics in planar quantum Heisenberg antiferromagnets in a wide temperature range (from the Ni el temperature up to a reduced temperature T/J similar to 1.4, with J in-plane exchange integral). In particular, it is shown that, in contrast to the predictions of the nonlinear sigma model, no crossover to a quantum critical regime occurs and that the experimental findings are well reproduced by deriving the NMR relaxation rate in the framework of the standard mode-mode coupling theory.