Pseudo spin gap in high‐Tc oxides observed by NMR

From the temperature dependence of the ⁶³Cu nuclear spin‐lattice relaxation rate, 1/T₁T, in the planar Cu(2) sites, it is now well established that a highly enhanced and strongly temperature dependent relaxation 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 superconducting 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. This revised version was published online in August 2006 with corrections to the Cover Date.