High-temperature ${}^{63}\mathrm{Cu}\left(2\right)\mathrm{}$ nuclear quadrupole and magnetic resonance measurements of ${\mathrm{YBa}}_2{\mathrm{Cu}}_4{\mathrm{O}}_8$

We report data on the Gaussian transverse relaxation rate ${1/T}_{2G},$ the Knight shift $K^s,$ and the spin-lattice relaxation time $T_1,$ in ${\mathrm{YBa}}_2{\mathrm{Cu}}_4{\mathrm{O}}_8$ from 100 K up to 715 K, extending the upper temperature of previous studies by 200 K. The $T_{2G}$ data are corrected for a small spin-lattice fluctuation contribution to the echo decay. Calculations of $T_1{T/T}_{2G}$ and $T_1{T/T}_{2G}^2$ show that there is a crossover in scaling behavior at a temperature $T_{\mathrm{cr}}\mathrm{\sim }500\mathrm{}\mathrm{K}.$ The Knight-shift data also exhibit a maximum at this temperature and decrease slowly with increasing temperature above this. Calculations using an independent form for the susceptibility due to Sokol, Singh, and Elstner suggest that the correlation length for antiferromagnetic fluctuations is on the order of one to two lattice spacings at this crossover temperature, in agreement with the proposal of Barzykin and Pines.