EVIDENCE FOR TEMPERATURE-DEPENDENT ANTIFERROMAGNETIC CORRELATION LENGTH IN HIGH-T-C CU OXIDES

We report the temperature(T)-dependent anisotropy r(= W-63(ab)/(63) W-cc) Of the planar Cu-63 nuclear spin-lattice relaxation rates W-63(a) (alpha=ab, cc) in the high-T-c Cu oxides, YBa2Cu4O8, YBa2Cu3O6.96 and Tl2Ba2CuO6+delta, from around T-c up to about 300 K. We discuss within the spin-fluctuation model that the anisotropy r is dependent only on the antiferromagnetic correlation length xi and that its T-dependence for the lightly-doped material is different from that of the over-doped ones. We provide the T-dependent anisotropy r as direct evidence of the T-dependent xi.