SPIN DOPING VIS-A-VIS TO CHARGE DOPING IN THE PARAMAGNETIC PHASE OF LA2CUO4 FROM CU-63 NQR RELAXATION

Cu-63 NQR spin-lattice and spin-spin relaxation measurements are used to compare the effects of charge doping (such as in La2-ySryCuO4) with the ones due to the spin vacancies induced by the Zn2+ S = 0 for Cu2 S = 1/2 substitution in La2CuO4. In particular the possible role of i tinerant and/or localized singlets in driving this typical quantum Hei senberg 2D antiferromagnet in the quantum disordered regime is address ed. The in-plane magnetic correlation length xi(2D) in the paramagneti c phase is quantitatively extracted and its dependence on charge and s pin doping is derived. It is found that xi(2D)(x, T) in La2Cu1-xZnxO4, as well as the drop in the Neel temperature T-N(x), can consistently be explained by taking into account the effect of the spin vacancies o n the spin stiffness rho(s)(x), in the framework of a conventional dil ution-like model. When the singlets ape itinerant (as in La2-ySryCuO4) xi(2D)(y, T) and correspondingly rho(s)(y) are markedly more affected . Furthermore the dramatic drop in T-N(y) cannot longer be explained b y a conventional mean field description for the interplane coupling.