EFFECTS OF SPIN VACANCIES ON THE CORRELATED SPIN DYNAMICS IN LA2CU1-XZNXO4 FROM CU-63 NUCLEAR-QUADRUPOLE RESONANCE RELAXATION

Cu-63 nuclear quadrupole resonance (NQR) relaxation measurements in La 2CuO4 doped Zn are used in order to investigate the temperature depend ence of the in-plane magnetic correlation length xi(2D), and the effec ts associated to spin vacancies in two dimensional quantum Heisenberg antiferromagnets (QHAF). The relaxation rates T-1(-1) and T-2(-1) have been related to the static generalized susceptibility chi(q,0) and to the decay rate Gamma(q) of the normal excitations. By using scaling a rguments for chi(4,0) and Gamma(q), the relaxation rates have been exp ressed in close form in terms of xi(2D)(x,T) and its dependence on tem perature and spin doping x thus extracted. The experimental findings a re analyzed in light of the renormalized classical (RC) and quantum cr itical (QC) behaviors predicted for xi(2D), by recent theories for S=1 /2 HAF in square lattices. It is first shown that in pure La2CuO4, xi( 2D) is consistent with a RC regime up to about 900 K, with tendency to ward the QC regime above. The spin vacancies reduce the Ndel temperatu re according to the law T-N(x) approximate to T-N(0)(1-3 5x). From the temperature dependence of Cu-63 NQR relaxation rate T-1(-1), T-2(-1) and from the composition dependence of T-N it is consistently proved t hat the effect on xi(2D) can be accounted for by the modification of t he spin stiffness in a simple dilutionlike model, the system still rem aining in the RC regime, at least for T less than or equal to 900 K. ( C) 1997 American Institute of Physics.