Normal state spin susceptibility in YBa2Cu3O6.92 single crystal from Cu-63and Y-89 nuclear magnetic resonance

We present an extensive nuclear magnetic resonance (NMR) and nuclear quadru pole resonance (NQR) study of the normal state (T > T-c) of an YBa1.93Sr0.0 7Cu3O6.92 single crystal (T-c = 90 K). The NMR data is interpreted using a model for the imaginary part of the dynamical electron spin susceptibility chi"(q,omega) that is consistent with the inelastic neutron scattering (INS ) experiments (carried out on the same sample investigated here): the q-dep endence is assumed to be Gaussian (commensurate), with short and T independ ent coherence length xi. This model enabled us to perform a quantitative an alysis of the planar copper spin-lattice and spin-spin relaxation rates whi ch evidenced that a spin pseudogap starts to open at T* approximate to 130 K, confirming the INS results. Moreover, we show that the AF fluctuations c ontribution to yttrium relaxation is not negligible due to the dipolar coup ling of the yttrium with the neighboring copper spins. The main signature o f this feature is that a relation Kalpha alphaT1T = const does not hold for the yttrium site at higher temperatures. Our main conclusion is that, in o rder to explain the yttrium (and the planar oxygen) relaxation coherently w ith the INS results and the widely accepted one-band picture, the character istic energy Gamma(0) of the spin dynamics near q = 0 must be strongly T de pendent, otherwise the one-band description is not feasible. All the NMR re sults are interpreted within the ionic model of the hyperfine Hamiltonian t aking into account the anisotropy of the g-factor and the static spin susce ptibility chi(s). (C) 1999 Published by Elsevier Science B.V. All rights re served.