NMR evidence for localized spins on Cu(2) sites from Cu NMR in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ and ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.75}$ single crystals

We report ${}_{}{}^{63}{}_{}{}^{}$,65Cu NMR spectra and nuclear spin-lattice relaxation data for ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$(${\mathrm{T}}_{\mathrm{c}}$=91 K) and ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6.75}$(${\mathrm{T}}_{\mathrm{c}}$ ${=5\mathrm{}}_7$ K) single crystals between 45 and 300 K. An analysis of the spin-lattice relaxation rates gives evidence for localized electronic spins on the planar Cu(2) sites. In ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6.75}$ both fourfold and twofold oxygen-coordinated Cu(1) are observed; the latter experience the same quadrupolar coupling (${\nu }_Q$≃31.5 MHz) as the Cu(2) sites but a nearly zero magnetic hyperfine shift and a much slower relaxation rate ($d^{10}$ configuration).