Planar ${}^{17}\mathrm{O}$ NMR study of ${\mathrm{Pr}}_y{\mathrm{Y}}_{1-y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$

We report the planar ${}^{17}\mathrm{O}$ NMR shift in Pr-substituted ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+x},$ which at $x=1\mathrm{}$ exhibits a characteristic pseudogap temperature dependence, confirming that Pr reduces the concentration of mobile holes in the ${\mathrm{CuO}}_2$ planes. Our estimate of the rate of this counterdoping effect, obtained by comparison with the shift in pure samples with reduced oxygen content, is found insufficient to explain the observed reduction of $T_c.$ From the temperature-dependent magnetic broadening of the ${}^{17}\mathrm{O}$ NMR we conclude that the Pr moment and the local magnetic defect induced in the ${\mathrm{CuO}}_2$ planes produce a long-range spin polarization in the planes, which is likely associated with the extra reduction of $T_c.$ We find a qualitatively different behavior in the oxygen-depleted ${\mathrm{Pr}}_y{\mathrm{Y}}_{1-y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.6},$ i.e., the suppression of $T_c$ is nearly the same, but the magnetic broadening of the ${}^{17}\mathrm{O}$ NMR appears weaker. This difference may signal a weaker coupling of the Pr to the planes in the underdoped compound, which might be linked with the larger Pr to ${\mathrm{CuO}}_2$ plane distance and correspondingly weaker hybridization.