Optical Properties of Kondo Insulators

This talk will discuss the optical properties of Kondo insulators, including the dielectric constant and magnetic permeability. Using a large-N Abrikosov fermion representation in a treatment of a Kondo lattice model, we show that the dielectric constant is determined by the squared ratio of the plasma frequency to the direct gap, predicting dielectric constants of order 1000, diffractive indices of order 30. By treating the magnetic correlations of the f-electrons as a gapless spin-liquid, described by dispersing f-electrons coupled to a gauge field we derive an effective model for the gauge fields which includes a continuum of excitations as well as a hybridization between the external electro-magnetic gauge field and the emergent gauge field. The gradient expansion of the effective model predicts a massless `light’ mode and a massive Higgs mode. The observation of this Higgs mode would provide an interesting opportunity to establish the presence of spin fractionalization and emergent gauge fields in Kondo insulators.