Spin dynamics in a two-dimensional disordered S=1/2 Heisenberg paramagnet from Cu-63 NQR relaxation in Zn-doped La2CuO4

Cu-63 NQR T-1 and T-2 relaxation measurements in La2Cu1-xZnxO4, for 0 less than or equal to x less than or equal to 0.11 and in the temperature range T-N less than or equal to T less than or equal to 900 K, are presented. The results are used to derive insights into the Cu2+ correlated spin dynamics in the paramagnetic phase of the S=1/2 two-dimensional (2D) Heisenberg (H) antiferromagnets (AF), and into the disorder effects associated with the spin vacancy due to Zn2+ (S=0) for Cu2+ substitution. In particular, by using scaling arguments for the static generalized susceptibility, chi(-->(q),0), and for the decay rate, Gamma-->(q), of the normal excitations, T-2 and T-1 are related to the in-plane correlation length xi(2D)(x,T) and its dependence on temperature and Zn doping, x, is extracted. The experimental findings are analyzed in light of the quantum critical and renormalized classical behaviors for xi(2D) predicted by recent theories for S=1/2 HAF on square lattices. It is shown that up to T similar or equal to 900 K, xi(2D) is consistent with the assumption of a renormalized classical regime, in agreement with recent neutron scattering results and at variance with previous interpretations of the NQR data. It is discussed how Zn affects xi(2D) through the modification in the spin stiffness and comparison with the disorder induced by itinerant extra holes is made.