SPIN-GAP BEHAVIOR IN HIGH-T(C) OXIDES OBSERVED BY NUCLEAR-SPIN-LATTICE RELAXATION

From the temperature dependence of the nuclear spin-lattice relaxation rate, 1/T1T, in the planar Cu(2) sites, it is found that a highly enhanced and strongly temperature-dependent relaxation process due to-antiferromagnetic Cu spin fluctuations exists in all high-T(c) superconducting oxides. The data also exhibit the opening of a gap in the low-lying magnetic excitations with an energy comparable to the superconducting gap. In contrast to this T1 behavior, the temperature variations of the indirectly coupled nuclear spin-spin relaxation rate, 1/T2g, and the anisotropic Knight shifts K(alpha) have quite an independent signature; the former shows continuous increase toward T(c) and the latter in the low low-doping regime decreases from a much higher temperature than that at which the spin-gap is seen in 1/T1T. These features indicate that the generalized spin susceptibility, chi(q, omega), has very characteristic temperature variations for different q and omega regions.