Cu spin dynamics and superconducting precursor effects in planes above $T_c$ in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.7}$

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.