Observation of a well-defined hybridization gap and in-gap states on the SmB6 (001) surface

TL;DR

This study uses ultra-low-temperature scanning tunneling microscopy to investigate SmB6, revealing a hybridization gap, in-gap states, and surface-dependent electronic variations, providing insights into its topological and correlated electronic properties.

Contribution

First detailed STM/STS analysis of SmB6 surface states showing hybridization gap and in-gap resonances, clarifying its electronic structure and surface variations.

Findings

Identification of an approximately 8 meV hybridization gap.

Detection of strong in-gap resonances below the Fermi level.

Observation of a split Kondo resonance at 100 mK.

Abstract

The rise of topology in condensed matter physics has generated strong interest in identifying novel quantum materials in which topological protection is driven by electronic correlations. Samarium hexaboride is a Kondo insulator for which it has been proposed that a band inversion between $5d$ and $4f$ bands gives rise to topologically protected surface states. However, unambiguous proof of the existence and topological nature of these surface states is still missing, and its low-energy electronic structure is still not fully established. Here we present a study of samarium hexaboride by ultra-low-temperature scanning tunneling microscopy and spectroscopy. We obtain clear atomically resolved topographic images of the sample surface. Our tunneling spectra reveal signatures of a hybridization gap with a size of about $8\ \mathrm{meV}$ and with a reduction of the differential conductance inside the gap by almost half, and surprisingly, several strong resonances below the Fermi level. The spatial variations of the energy of the resonances point towards a microscopic variation of the electronic states by the different surface terminations. High-resolution tunneling spectra acquired at $100\ \mathrm{mK}$ reveal a splitting of the Kondo resonance, possibly due to the crystal electric field.