ABSTRACT
The cubic rare-earth boride series 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. Furthermore, at low temperatures fairly exotic ordered states may appear due to the “hidden” order of multipoles carried by degenerate CEF multiplets, in addition to common (dipolar) magnetic order present in many RB6 (R = rareearth) systems. Most prominent are CeB6 and its La diluted alloys which exhibit quadrupolar and octupolar ordering enabled by the cubic Г8 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 RB6 has to be complemented by the itinerant Kondo lattice approach. Due to the presence of hybridization and collective ordering gaps, a singular magnetic response can lead to the appearance of collective spin exciton modes inside the gap around symmetry points of the Brillouin zone (BZ). This has been observed in heavy-fermion metal CeB6 and in particular in the Kondo insulators YbB12 and SmB6. The latter,which has no Landau-type local symmetry breaking is also the prime candidate for a strongly correlated insulator with topological order, caused by odd number of band crossings of 4f and 5d bands in the BZ. The signature of topological order is the existence of massless Dirac surface states with helical spin polarization, a topic of intense investigation in SmB6.
