Evidence for Higher order topology in Bi and Bi$_{0.92}$Sb$_{0.08}$

TL;DR

This study provides experimental evidence that Bi and BiSb alloys are higher order topological insulators, showing localized hinge states on their surfaces, which confirms their topological nature and suggests a need to revisit their classification.

Contribution

The paper demonstrates that Bi and Bi$_{0.92}$Sb$_{0.08}$ are HOTIs through surface spectroscopy and hinge state observation, confirming their topological phase.

Findings

Bi and BiSb (110) films host hinge states consistent with HOTI theory

Surface analysis confirms the topological classification of these materials

Raises questions about the classification of Bi$_{x}$Sb$_{1-x}$ alloys

Abstract

Higher order topological insulators (HOTIs) are a new class of topological materials which host protected states at the corners or hinges of a crystal. HOTIs provide an intriguing alternative platform for helical and chiral edge states and Majorana modes, but there are very few known materials in this class. Recent studies have proposed Bi as a potential HOTI, however, its topological classification is not yet well accepted. In this work, we show that the (110) facets of Bi and BiSb alloys can be used to unequivocally establish the topology of these systems. Bi and Bi$_{0.92}$Sb$_{0.08}$ (110) films were grown on silicon substrates using molecular beam epitaxy and studied by scanning tunneling spectroscopy. The surfaces manifest rectangular islands which show localized hinge states on three out of the four edges, consistent with the theory for the HOTI phase. This establishes Bi and Bi$_{0.92}$Sb$_{0.08}$ as HOTIs, and raises questions about the topological classification of the full family of Bi$_{x}$Sb$_{1-x}$ alloys.