Quantum critical scattering in uranium non-Fermi liquid compounds

Our recent observation of universal scaling in ω/T of the magnetic response of the non-Fermi liquid compound UCu5-xPdx provides evidence of single-impurity quantum critical fluctuations of the uranium 5f-electrons. The scaling is observed over a wide range of frequencies from ∝1 to 25 meV, and a wide range of temperatures from ∝10 to 300 K, with evidence of a cross-over to more conventional moment fluctuations above 25 meV. We now have evidence from measurements with full polarization analysis of the development of antiferromagnetic correlations below 10 K in UCu4Pd, but not in UCu3.5Pd1.5, indicating the cross-over from the quantum critical regime to a low-temperature quantum-disordered regime as predicted by phenomenological theories of quantum phase transitions. (orig.)