Cu63(2) nuclear quadrupole and nuclear magnetic resonance studies of YBa2Cu4O8in the normal and superconducting states

We report nuclear quadrupole resonance (NQR) and nuclear magnetic resonance (NMR) measurements of the spin lattice and Gaussian spin-spin relaxation times, $T_1$ and $T_{2G}$, respectively, in an unaligned (NQR) and aligned (NMR) powder of Y${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_4$ ${\mathrm{O}}_8$. Experimental details, critical for accurate NQR measurements in this system, are discussed. The normal-state data are analyzed using several models. Evidence for the existence of a spin gap is presented. The normal-state data agree well with an analysis using scaling theory predictions for $z=1\mathrm{}$ quantum critical and quantum disordered regimes. The superconducting state data agree with recent calculations for $d$-wave superconductivity.