Singlet Magnetism in Heavy Fermions

TL;DR

This paper explores singlet magnetism in heavy fermion compounds UPt$_3$ and URu$_2$Si$_2$, proposing that broken time-reversal symmetry in their intermediate valence states influences their magnetic properties.

Contribution

It introduces a model where singlet magnetism arises from specific valence configurations and explains the differing magnetic behaviors of UPt$_3$ and URu$_2$Si$_2$.

Findings

Magnetic moment vanishes in UPt$_3$ with dominant 5f^1 configuration.

URu$_2$Si$_2$ exhibits non-magnetic transition with small admixture of half-integer spin.

Broken time-reversal symmetry affects the magnetic states in these heavy fermion systems.

Abstract

We consider singlet magnetism for the uranium ions in UPt$_3$ and URu$_2$Si$_2$ assuming that time-reversal symmetry is broken for the {\em coherent state of intermediate valence}. The relative weight of the two involved configurations should be different for UPt$_3$ and URu$_2$Si$_2$. If in UPt$_3$ the configuration $5f^1$ on the U-ion prevails in the coherent state below the magnetic transition, the magnetic moment would vanish for the particular choice of the {\em ionic} wave function. In case of URu$_2$Si$_2$, the phase transition is non-magnetic in the first approximation -- the magnetic moment arises from a small admixture of a half-integer spin configuration.