Magnetization, Maxwell's relations, and the local physics of Th _1-x U_x Ru₂ Si₂

The dilute Kondo compound, Th_1-x U_x Ru₂ Si₂, displays non-Fermi-liquid behavior but no zero-point entropy; it thus appears to elude description by known single-ion models. It may also provide a clue to the underlying local degrees of freedom in its dense counterpart, URu₂ Si₂. Here we use high-resolution magnetization studies to cross-check the thermodynamic consistency of previous experiments. Measurement of the field dependence of the temperature scale, T_F (H), associated with Fermi-liquid behavior probes the nature of the underlying impurity fixed point. We find that T_F (H) grows linearly with applied field, in contrast to the quadratic form expected for the two-channel Kondo model. We use a scaling argument to show that the observed behavior of T_F (H) is consistent with the absence of zero-point entropy, suggesting novel impurity behavior in this material. More generally, we suggest the field magnetization as a probe of single-ion physics and make predictions for its behavior in other actinide compounds.