Y89NMR study of the effect of Zn substitution on the spin dynamics ofYBa2Cu4O8

Zn-substituted ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_4$ ${\mathrm{O}}_8$ high-temperature superconductors have been probed by ${}_{}{}^{89}{}_{}{}^{}\mathrm{Y}$ nuclear magnetic resonance (NMR), susceptibility, and thermopower measurements. With ${}_{}{}^{89}{}_{}{}^{}\mathrm{Y}$ NMR we have been able to study the gap in the spin density of states using the real part of the spin susceptibility determined from the Knight shift as Zn is progressively substituted onto the plane copper site. Zn substitution results in a rapid decrease in ${\mathit{T}}_{\mathit{c}}$ with a negligible decrease in the spin-gap energy and an inhomogeneous filling in of the gap in spin density of states.