Zn-substituted YBa2Cu4O8 high-temperature superconductors have been probed by 89Y
nuclear magnetic resonance (NMR), susceptibility, and thermopower measurements. With
89Y 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 Tc with a negligible decrease in the spin-gap energy and an inhomogeneous filling
in of the gap in spin density of states.