Single-spin fluid, spin gap, and d-wave pairing in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_4$${\mathrm{O}}_8$: A NMR and NQR study

We present results of ${}_{}{}^{17}{}_{}{}^{}\mathrm{O}$ and ${}_{}{}^{63,65}{}_{}{}^{}\mathrm{Cu}$ nuclear magnetic resonance (NMR) and nuclear quadrupolar resonance (NQR) studies in the normal and superconducting state of the 82-K superconductor ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_4$ ${\mathrm{O}}_8$. The various components of the Cu and O Knight-shift tensors show strong but similar temperature dependences over the temperature range from 8.5 to 300 K in both the ${\mathrm{CuO}}_2$ planes and the chains, supporting the picture that there is only one spin component in the planes and the chains, although with different susceptibilities. The oxygen data obey the Korringa relation. This may be interpreted as Fermi-liquid behavior of the electronic system far away from the antiferromagnetic wave vector. The temperature dependence of both the planar Cu and O shift tensors and the planar Cu spin-lattice relaxation rate suggest the opening of a pseudo-spin-gap well above ${\mathit{T}}_{\mathit{c}}$. The very different temperature dependence of 1/${\mathit{T}}_1$ at the planar O and Cu sites points to the reduced role of the antiferromagnetic correlated spin fluctuations at the O site. The data favor the conclusion that ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_4$ ${\mathrm{O}}_8$ is a d-wave superconductor. Evidence is provided by three data sets: the chain Knight shifts, the ratio of the planar copper and oxygen relaxation rates, and the individual low-temperature behavior of these rates.