Transport and NMR studies of the effect of Ni substitution on superconductivity and the normal-state pseudogap in YBa2Cu4O8

Resistance, thermopower, and ⁸⁹Y nuclear magnetic resonance (NMR) measurements have been carried out on YBa₂(Cu_1-xNi_x)₄O₈ superconductors. Nickel substitution reduces both T_c and the pseudogap energy at almost the same rate as does Zn substitution, but unlike Zn substitution, there is no second peak in the ⁸⁹Y NMR spectra that can be attributed to Y atoms sited near a local moment. We show that this discrepancy is consistent with there being no local suppression of the normal-state pseudogap about the Ni impurity contrary to the case for Zn.