Theory of Spin Exciton in the Kondo Semiconductor $Yb B_{12}$

TL;DR

This paper presents a theoretical model explaining the dispersive spin excitations observed in the Kondo semiconductor YbB12, linking them to hybridized quasiparticle bands and small differences in CEF hybridization.

Contribution

It introduces a theoretical framework based on the periodic Anderson model and RPA to explain magnetic excitations in YbB12, matching experimental observations.

Findings

Two dispersive spin resonance excitations at the continuum threshold.

Maximum intensity at the antiferromagnetic zone boundary.

Upward dispersion merging with the continuum.

Abstract

The Kondo semiconductor $Yb B_{12}$ exhibits a spin and charge gap of approximately 15 meV. Close to the gap energy narrow dispersive collective excitations were identified by previous inelastic neutron scattering experiments. We present a theoretical analysis of these excitations. Starting from a periodic Anderson model for crystalline electric field (CEF) split 4f states we derive the hybridized quasiparticle bands in slave boson mean-field approximation and calculate the momentum dependent dynamical susceptibility in random phase approximation (RPA). We show that a small difference in the hybridization of the two CEF (quasi-) quartets leads to the appearance of two dispersive spin resonance excitations at the continuum threshold. Their intensity is largest at the antiferromagnetic (AF) zone boundary point and they have an upward dispersion which merges with the continuum less than halfway into the Brillouin zone. Our theoretical analysis explains the most salient features of previously unexplained experiments on the magnetic excitations of $Yb B_{12}$.