Collapse of the coherence gap in Kondo semiconductors

Abstract

Kondo semiconductors exhibit a gap in their low-temperature excitation spectra, and can be modeled by the Anderson lattice model at half filling which shows a hybridization gap. The mixed-valent Kondo semiconductor ${\mathrm{SmB}}_{6}$ also shows in-gap magnetic excitations that exhibit the temperature dependence and dispersion that are expected from magnetic excitons. The gaps measured in the Kondo semiconductors are extremely sensitive to doping in which the f ions are substituted by non-$f$ ions. Recent measurements on ${\mathrm{YbB}}_{12}$ shows a feature similar to that of the magnetic exciton in ${\mathrm{SmB}}_{6}.$ Furthermore, the concentration dependence of the features observed in inelastic neutron-scattering spectra of doped ${\mathrm{YbB}}_{12}$ is different from the concentration dependence found in measurements of thermodynamic and transport properties. The impurity concentration dependence of the gap and the spin-exciton features are investigated. The spectroscopic properties are calculated for the Anderson lattice model at half filling, in which the disorder is treated in the coherent potential approximation. The results are compared with the recent experiments.