Spin dynamics and magnetic correlation length in two-dimensional quantum Heisenberg antiferromagnets

The correlated spin dynamics and temperature dependence of the correlation length xi(T) in two-dimensional quantum (S = 1/2) Heisenberg antiferromagne ts (2DQHAF) on a square lattice are discussed in light of experimental resu lts of proton spin lattice relaxation in copper formate tetradeuterate. In this compound the exchange constant is much smaller than the one in recentl y studied 2DQHAF, such as La2CuO4 and Sr2CuO2Cl2. Thus the spin dynamics ca n be probed in detail over a wider temperature range. The NMR relaxation ra tes turn out to be in excellent agreement with a theoretical mode-coupling calculation. The deduced temperature behavior of xi(T) is in agreement with high-temperature expansions, quantum Monte Carlo simulations, and the pure quantum self-consistent harmonic approximation. Contrary to the prediction s of the theories based on the nonlinear a model, no evidence of crossover between different quantum regimes is observed.