Non-trivial surface states of samarium hexaboride at the (111) surface

TL;DR

This study reveals non-trivial surface states on the (111) surface of samarium hexaboride, showing topologically protected metallic states with helical spin polarization, advancing understanding of its electronic structure and topological nature.

Contribution

It provides detailed ARPES measurements of the SmB$_6$ (111) surface, demonstrating topologically non-trivial surface states and clarifying previous controversies about its classification.

Findings

Metallic two-dimensional state disperses across the bulk Kondo gap

Surface states exhibit helical in-plane spin polarization

Supports topological classification of SmB$_6$ as non-trivial

Abstract

The peculiar metallic electronic states observed in the Kondo insulator, samarium hexaboride (SmB$_6$), has stimulated considerable attention among those studying non-trivial electronic phenomena. However, experimental studies of these states have led to controversial conclusions mainly to the difficulty and inhomogeneity of the SmB$_6$ crystal surface. Here, we show the detailed electronic structure of SmB$_6$ with angle-resolved photoelectron spectroscopy measurements of the three-fold (111) surface where only two inequivalent time-reversal-invariant momenta (TRIM) exist. We observe the metallic two-dimensional state was dispersed across the bulk Kondo gap. Its helical in-plane spin polarisation around the surface TRIM suggests that SmB$_6$ is topologically non-trivial, according to the topological classification theory for weakly correlated systems. Based on these results, we propose a simple picture of the controversial topological classification of SmB$_6$.