Analysis of NMR Spin-Lattice Relaxation Rates in Cuprates
  We investigate nuclear spin-lattice relaxation data in the normal state of optimally doped YBaCuO_7 by analyzing the contributions to the relaxation rate of the copper, planar oxygen and yttrium along the directions perpendicular to the applied field. In this new picture there is no contrasting temperature dependence of the copper and oxygen relaxation. We use the model of fluctuating fields to express the rates in terms of hyperfine interaction energies and an effective correlation time tau_eff characterizing the dynamics of the spin fluid. The former contain the effects of the antiferromagnetic static spin correlations, which cause the hyperfine field constants to be added coherently at low temperature and incoherently at high temperature. The degree of coherency is therefore controlled by the spin-spin correlations. The model is used to determine the temperature-dependent correlation lengths. The temperature-dependent effective correlation time is found to be made up of a linear and a constant contribution that can be related to scattering and spin fluctuations of localized moments respectively. The extrapolation of our calculation at higher temperature fits the data also very well at those temperatures. The underdoped compounds YBaCuO_6.63 and YBaCuO_8 are studied in the limit of the data available with some success by modifying the effective correlation time with a gap parameter. The copper data of the LaSrCuO series are then discussed in terms of the interplay between the two contributions to tau_eff. 
A. Uldry and P. F. Meier