Multipolar order and excitations in rare-earth boride Kondo systems

TL;DR

This paper reviews the complex electronic states and multipolar orders in rare-earth boride Kondo systems, highlighting experimental observations of excitations and the interplay between localized and itinerant electron behaviors.

Contribution

It provides a comprehensive overview of multipolar orders, excitations, and the dual localized-itinerant electron picture in rare-earth boride Kondo systems, including recent experimental findings.

Findings

Observation of multipolar waves via inelastic neutron scattering.

Detection of collective spin exciton modes in Kondo insulators.

Identification of YbB12 and SmB6 as topological Kondo insulators.

Abstract

We review the cubic rare-earth boride series which displays diverse electronic states like localized 4f electron multiplets split by the crystal electric field (CEF), itinerant heavy-fermion quasiparticle bands of the Kondo lattice as well as gapped Kondo insulator or mixed-valent semiconductor states. Fairly exotic states may appear due to the 'hidden' order of multipoles carried by degenerate CEF multiplets, in addition to magnetic order present in many RB6 (R = rare earth) compounds. Most prominent are CeB6 and its La-diluted alloys which exhibit antiferro-quadrupolar and octupolar ordering enabled by the cubic G8 quartet state. The associated collective excitations are multipolar waves with a dispersion characteristic for the underlying order and accessible by inelastic neutron scattering. This localized multipolar-moment picture of R-borides is complemented by the itinerant Kondo-lattice approach. Collective spin exciton modes inside the hybridization gap around symmetry points of the Brillouin zone may appear. This has been observed in heavy-fermion metal CeB6 and in particular in the Kondo insulators YbB12 and SmB6. The latter is also the prime candidate for a strongly correlated topological insulator.