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Abstract
Spin dynamics at planar sites in oxygen-deficient ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathrm{x}}$ have been investigated by nuclear spin-lattice relaxation measurements using $^{63}$,65Cu nuclear quadrupole resonance and $^{89}\mathrm{Y}$ NMR. A sharp decrease of $^{63}\mathrm{Cu}$ nuclear relaxation below 100 K in ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6.7}$ suggests the possible onset of spin pairing in individual planes well above the transition to three-dimensional bulk superconductivity. Comparison of the relaxation rates for the 60-K superconductor ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6.7}$ with those of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7.0}$ in the range 100--300 K shows that the planar Cu(2) spin dynamics are strongly affected by changes in carrier concentration.
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