SPIN DYNAMICS IN A 2-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 temperat ure 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+ corr elated spin dynamics in the paramagnetic phase of the S=1/2 two-dimens ional (2D) Heisenberg (H) antiferromagnets (AF), and into the disorder effects associated with the spin vacancy due to Zn2+ (S=0) for Cu2+ s ubstitution. In particular, by using scaling arguments for the static generalized susceptibility, chi(-->(q),0), and for the decay rate, Gam ma-->(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 temperatur e and Zn doping, x, is extracted. The experimental findings are analyz ed in light of the quantum critical and renormalized classical behavio rs for xi(2D) predicted by recent theories for S=1/2 HAF on square lat tices. It is shown that up to T similar or equal to 900 K, xi(2D) is c onsistent with the assumption of a renormalized classical regime, in a greement with recent neutron scattering results and at variance with p revious interpretations of the NQR data. It is discussed how Zn affect s xi(2D) through the modification in the spin stiffness and comparison with the disorder induced by itinerant extra holes is made.