In-gap collective mode spectrum of the Topological Kondo Insulator SmB6

TL;DR

This paper models the collective excitation spectrum of SmB6, a topological Kondo insulator, using a slave boson approach to connect its band topology with strong electron correlations, and compares the results with experimental data.

Contribution

It introduces a perturbative slave boson method applied to a realistic Anderson model to accurately reproduce the collective mode spectrum of SmB6, linking band topology with electron interactions.

Findings

Good quantitative match of the energy spectrum with experiments

Qualitative agreement of spectral weight distribution

Identifies the collective mode as a magnetically active exciton

Abstract

Samarium hexaboride (SmB$_6$) is the first strongly correlated material with a recognized non-trivial band-structure topology. Its electron correlations are seen by inelastic neutron scattering as a coherent collective excitation at the energy of 14 meV. Here we calculate the spectrum of this mode using a perturbative slave boson method. Our starting point is the recently constructed Anderson model that properly captures the band-structure topology of SmB$_6$. Most self-consistent renormalization effects are captured by a few phenomenological parameters whose values are fitted to match the calculated and experimentally measured mode spectrum in the first Brillouin zone. A simple band-structure of low-energy quasiparticles in SmB$_6$ is also modeled through this fitting procedure, because the important renormalization effects due to Coulomb interactions are hard to calculate by ab-initio methods. Despite involving uncontrolled approximations, the slave boson calculation is capable of producing a fairly good quantitative match of the energy spectrum, and a qualitative match of the spectral weight throughout the first Brillouin zone. We find that the "fitted" band-structure required for this match indeed puts SmB$_6$ in the class of strong topological insulators. Our analysis thus provides a detailed physical picture of how the SmB$_6$ band topology arises from strong electron interactions, and paints the collective mode as magnetically active exciton.