Heavy Fermion scaling: Uranium versus Cerium and Ytterbium compounds

TL;DR

This paper investigates the applicability of scaling laws linking neutron spectra, specific heat, and susceptibility in heavy Fermion compounds, highlighting differences between uranium and rare-earth-based materials and proposing corrections for better understanding.

Contribution

It identifies reasons for the failure of scaling laws in uranium compounds and suggests corrections, advancing the understanding of 5f electron itineracy effects in heavy Fermion systems.

Findings

Scaling laws work well for rare earth compounds.

Scaling laws fail for many uranium compounds due to AF fluctuations and unknown degeneracy.

Correcting for these effects improves scaling in some uranium compounds.

Abstract

In an effort to explore the differences between rare-earth-based and uranium-based heavy Fermion (HF) compounds that reflect the underlying difference between local 4$f$ moments and itinerant 5$f$ moments we analyze scaling laws that relate the low temperature neutron spectra of the primary ("Kondo-esque") spin fluctuation to the specific heat and susceptibility. While the scaling appears to work very well for the rare earth intermediate valence compounds, for a number of key uranium compounds the scaling laws fail badly. There are two main reasons for this failure. First, the presence of antiferromagnetic (AF) fluctuations, which contribute significantly to the specific heat, alters the scaling ratios. Second, the scaling laws require knowledge of the high temperature moment degeneracy, which is often undetermined for itinerant 5$f$ electrons. By making plausible corrections for both effects, better scaling ratios are obtained for some uranium compounds. We point out that while both the uranium HF compounds and the rare earth intermediate valence (IV) compounds have spin fluctuation characteristic energies of order 5 - 25 meV, they differ in that the AF fluctuations that are usually seen in the U compounds are never seen in the rare earth IV compounds. This suggests that the 5f itineracy increases the f-f exchange relative to the rare earth case.