Long range order and two-fluid behavior in heavy electron materials

TL;DR

This paper investigates the behavior of heavy electron materials using NMR Knight shift measurements, revealing how the Kondo liquid coexists with local moments and how different ordered states emerge from the hybridized state.

Contribution

It extends Knight shift measurements to new materials, demonstrating the relationship between Kondo liquid relocalization and magnetic order, and clarifies the origin of hidden order versus antiferromagnetism.

Findings

Antiferromagnetic order in CeRhIn$_5$ is preceded by relocalization of the Kondo liquid.

Hidden order in URu$_2$Si$_2$ emerges directly from the Kondo liquid.

Results support a strong coupling between ground state and hybridization in the Kondo lattice.

Abstract

The heavy electron Kondo liquid is an emergent state of condensed matter that displays universal behavior independent of material details. Properties of the heavy electron liquid are best probed by NMR Knight shift measurements, which provide a direct measure of the behavior of the heavy electron liquid that emerges below the Kondo lattice coherence temperature as the lattice of local moments hybridizes with the background conduction electrons. Because the transfer of spectral weight between the localized and itinerant electronic degrees of freedom is gradual, the Kondo liquid typically coexists with the local moment component until the material orders at low temperatures. The two-fluid formula captures this behavior in a broad range of materials in the paramagnetic state. In order to investigate two-fluid behavior and the onset and physical origin of different long range ordered ground states in heavy electron materials, we have extended Knight shift measurements to URu$_2$Si$_2$, CeIrIn$_5$ and CeRhIn$_5$. In CeRhIn$_5$ we find that the antiferromagnetic order is preceded by a relocalization of the Kondo liquid, providing independent evidence for a local moment origin of antiferromagnetism. In URu$_2$Si$_2$ the hidden order is shown to emerge directly from the Kondo liquid and so is not associated with local moment physics. Our results imply that the nature of the ground state is strongly coupled with the hybridization in the Kondo lattice in agreement with phase diagram proposed by Yang and Pines.