Breakdown of the Kondo insulating state in SmB6 by introducing Sm vacancies

TL;DR

This study investigates how introducing Sm vacancies affects the electronic and vibrational properties of SmB6, revealing that vacancies fill the hybridization gap and quench in-gap excitations, impacting its topological insulating state.

Contribution

It demonstrates that even less than 1% Sm vacancies significantly alter the hybridization gap and in-gap excitations in SmB6, providing insights into the stoichiometry requirements for topological Kondo insulators.

Findings

Sm vacancies are detectable below 1% via Raman scattering.

Increased vacancies fill the hybridization gap with electronic states.

Vacancies quench in-gap exciton excitations at about 1% concentration.

Abstract

SmB6 is a proposed topological Kondo insulator where the presence of topological nontriviality can be tuned by variations in the Sm valence. Experimentally, Sm valence can be changed by tuning stoichiometry of SmB6. We show that Raman scattering can detect vacancies lower than 1% of Sm sites in SmB6 crystal by probing the intensity of defect-induced scattering of the acoustic phonon branch at 10~meV. In the electronic Raman spectra of SmB6 at temperatures below 130~K, we observe features developing in A$_{1g}$ and E$_g$ symmetries at 100 and 41~meV which we assign to excitations between hybridized bands, and depressed spectral weight below 20~meV associated with the hybridization gap. With the increased number of Sm vacancies up to 1% we observe an increase of spectral weight below 20~meV showing that the gap is filling in with electronic states. For the sample with the lowest number of vacancies the in-gap exciton excitations with long lifetimes protected by hybridization gap are observed at 16-18~meV in E$_g$ and T$_{2g}$ symmetries. These excitations broaden as a decrease in the lifetime with increasing number of vacancies and are quenched by the presence of in-gap states at concentration of Sm vacancies of about 1%. Based on this study we suggest that only the most stoichiometric SmB6 samples have a bulk gap necessary for topological Kondo insulators.