Cu nuclear quadrupole resonance ofYBa2Cu3Oxwith varying oxygen content

Abstract

${}_{}{}^{63}{}_{}{}^{}\mathrm{Cu}$ and ${}_{}{}^{65}{}_{}{}^{}\mathrm{Cu}$ nuclear quadrupole resonance (NQR) results of eleven $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_x$ samples with $x$ ranging from 7.00 to 6.00 are reported. All measurements were done at room temperature. An attempt was made to quantify integrated signal intensities. The spin-lattice relaxation times ($T_1$) were either shorter than 0.3 ms or longer than 60 ms. Because the short relaxation times are interpreted as arising from the interaction with conduction electrons, the $T_1\text{'}\mathrm{s}$ could be used to distinguish between metallic and nonconducting substructures. In the $7.0<x<\mathrm{}6.9\mathrm{}$ range where $T_c=90\mathrm{}$ K, the shapes of the NQR spectra were independent of $x$, but their overall intensity decreased with decreasing oxygen content. In the intermediate range, $6.8<x<\mathrm{}6.4\mathrm{}$ where $T_c=55\mathrm{}$ K, narrow peaks due to nonconducting substructures were observed together with a distinctive short-$T_1$ line shape, indicating that the (super)conducting state with this oxygen stoichiometry is essentially different from that at $x=7\mathrm{}$. This short-$T_1$ signal was more pronounced in low-temperature annealed samples for which a $T_c$ of 55 K was observed over a larger $x$ range. It is shown to be consistent with a superordered structure of oxygen vacancies in the conducting substructures. No metallic sites were found for $x<\mathrm{}6.3\mathrm{}$ where $T_c\sim 0$. A total of six Cu resonance frequencies were identified between 20 and 32 MHz, some of which could be assigned to specific local crystal structures. In several instances, when the oxygen content was lowered, an NQR peak due to metallic Cu was replaced by a nonmetallic signal at the same frequency. The overall integrated NQR intensity gradually decreased from a value corresponding to three Cu atoms per formula unit for $x=7\mathrm{} \mathrm{to} 1$ Cu for $x=6\mathrm{}$. This is thought to be caused by antiferromagnetic interactions of the electron spins.