# Planar tunneling spectroscopy of the topological Kondo insulator SmB$_6$

## TL;DR

This study improves planar tunneling spectroscopy of SmB$_6$ by using plasma oxidation to create high-quality barriers, revealing surface Dirac fermions and their limited topological protection due to interactions with bulk excitations.

## Contribution

It demonstrates a reliable method for tunneling spectroscopy of SmB$_6$ and provides detailed insights into the surface states and their interaction with bulk excitations.

## Key findings

- High-quality tunnel barriers achieved via plasma oxidation.
- Reproducible conductance spectra showing surface Dirac fermions.
- Surface states interact with spin excitons, limiting topological protection.

## Abstract

Several technical issues and challenges are identified and investigated for the planar tunneling spectroscopy of the topological Kondo insulator SmB$_6$. Contrasting behaviors of the tunnel junctions prepared in two different ways are analyzed and explained in detail. The conventional approach based on an AlO$_\text{x}$ tunnel barrier results in unsatisfactory results due to the inter-diffusion between SmB$_6$ and deposited Al. On the contrary, plasma oxidation of SmB$_6$ crystals produces high-quality tunnel barriers on both (001) and (011) surfaces. Resultant conductance spectra are highly reproducible with clear signatures for the predicted surface Dirac fermions and the bulk hybridization gap as well. The surface states are identified to reside on two or one {\it distinguishable} Dirac cone(s) on the (001) and (011) surface, respectively, in good agreement with the recent literature. However, their topological protection is found to be limited within the low energy region due to their inevitable interaction with the bulk excitations, called spin excitons, consistent with a recent theoretical prediction. Implications of our findings on other physical properties in SmB$_6$ and also other correlated topological materials are remarked.