Tuning through the quantum critical point in UCu5-xNix:: Rapid variations in the specific heat

Using specific heat down to 0.05 K, we determine the variation of the non-Fermi liquid behavior in strongly disordered UCu5-xNix, 0.5 <= x <= 1.2, as the Ni doping is varied through the quantum critical concentration, x(QC), that suppresses the 16.5 K antiferromagnetism in pure UCu5. Contrary to the case of Pd-doped UCu5, where the Pd experiences partial sublattice ordering, Ni-which unlike Pd is smaller than Cu-goes equally on the four equivalent smaller Cu sites in UCu5 at a concentration of about 25% at x(QC)=1. This ensures strong lattice disorder. This disorder is confirmed by large high angle x-ray linewidths and large measured residual resistivity values of >= 700 mu Omega cm. C/T approximate to log T over the whole temperature range of measurement in the well-disordered UCu5-xNix only within 5% of x(QC). By x=1.1, a temperature dependence for C/T sets in below 0.3 K that is more divergent than either the log T or T-1+lambda fits to the higher temperature data, similar to behavior recently observed and analyzed in YbRh2Si2. This progression of the temperature dependence of the specific heat in UCu5-xNix around x(QC)approximate to 1 as a function of x is rapid compared to other known doped non-Fermi liquid systems.