PSEUDO SPIN GAP IN HIGH-T-C OXIDES OBSERVED BY NMR

From the temperature dependence of the Cu-63 nuclear spin-lattice rela xation rate, 1/T1T, in the planar Cu(2) sites, it is now well establis hed that a highly enhanced and strongly temperature dependent relaxati on process due to antiferromagnetic Cu spin fluctuations exists in all of the high-T-c's. The data also exhibit the opening of a gap in the low-lying magnetic excitations with an energy comparable to the superc onducting gap, particularly for the so-called low doping regime. It is also found that, irrespective of the system, the temperature at which the spin-gap opens, T-sg, determined as the peak of 1/T1T vs. T, has a linear decrease with increasing of the doping concentration. A spin- gap phase diagram is proposed.