Polarity-driven surface metallicity in SmB$_6$

TL;DR

This study reveals that the surface metallicity in SmB$_6$ is driven by surface polarity, specifically due to boron dangling bonds, providing new insights into its residual conductivity and topological properties.

Contribution

It demonstrates that surface polarity causes metallic surface states in SmB$_6$, a novel mechanism not previously understood in topological Kondo insulators.

Findings

Surface states are polarity-driven and not from bulk band structure.

Boron dangling bonds on polar surfaces induce metallicity.

Surface metallicity explains residual conductivity in SmB$_6$.

Abstract

By a combined angle-resolved photoemission spectroscopy and density functional theory study, we discover that the surface metallicity is polarity-driven in SmB$_6$. Two surface states, not accounted for by the bulk band structure, are reproduced by slab calculations for coexisting B$_6$ and Sm surface terminations. Our analysis reveals that a metallic surface state stems from an unusual property, generic to the (001) termination of all hexaborides: the presence of boron $2p$ dangling bonds, on a polar surface. The discovery of polarity-driven surface metallicity sheds new light on the 40-year old conundrum of the low-temperature residual conductivity of SmB$_6$, and raises a fundamental question in the field of topological Kondo insulators regarding the interplay between polarity and nontrivial topological properties.