Absence of metallic character at certain Cu sites in Y${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.7}$

We report ${}_{}{}^{63}{}_{}{}^{}\mathrm{Cu}$ nuclear-quadrupolar-resonance spectra and nuclear-spin relaxation data for the oxygen-deficient phase Y${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6.7}$. Compared with Y${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7.0}$, the spin-lattice relaxation rates are reduced by more than three orders of magnitude for a class of sites identified as nonmetallic Cu(1) "chain" sites with twofold oxygen coordination. Generation of these $3d^{10}$ ${\mathrm{Cu}}^{+}$ sites by ordering of oxygen vacancies onto "empty" chains increases the carrier concentration and may help maintain the observed 60-K plateau in the variation of $T_c$ with oxygen concentration.