Cu Spin Dynamics and Possible Planar Superconductivity Above T sub c in YBa sub 2 Cu sub 3 O sub (6.7).

Spin dynamics at the planar Cu(2) sites in oxygen deficient YBa sub 2 Cu sub 3 O sub (6+x) have been investigated by sup (63,65) Cu nuclear quadrupole resonance and spin- lattice relaxation measurements. Comparison of the relaxation rates for the 60 K superconductor YBa sub 2 Cu sub 3 O sub (6.7) with those of YBa sub 2 Cu sub 3 O sub (7.0) in the range 100-300 K shows that the Cu(2) spin dynamics are strongly affected by changes in carrier concentration. A sharp decrease of nuclear relaxation below 100 K in YBa sub 2 Cu sub 3 O sub (6.7) suggests the development of superconducting fluctuations in individual planes well above the onset of three-dimensional superconductivity.