A topological Dirac insulator in a quantum spin Hall phase : Experimental observation of first strong topological insulator

TL;DR

This paper reports the experimental discovery of a three-dimensional topological insulator in Bi$_{0.9}$Sb$_{0.1}$, revealing topologically protected surface states and massive Dirac particles in the bulk, advancing quantum materials research.

Contribution

It provides the first direct observation of massive Dirac particles and topological surface states in a bulk insulator, confirming theoretical predictions of topological insulators.

Findings

Observation of topological surface states in Bi$_{0.9}$Sb$_{0.1}$

Detection of massive Dirac particles in the bulk

Mapping of the topological Dirac insulator's gapless surface modes

Abstract

When electrons are subject to a large external magnetic field, the conventional charge quantum Hall effect \cite{Klitzing,Tsui} dictates that an electronic excitation gap is generated in the sample bulk, but metallic conduction is permitted at the boundary. Recent theoretical models suggest that certain bulk insulators with large spin-orbit interactions may also naturally support conducting topological boundary states in the extreme quantum limit, which opens up the possibility for studying unusual quantum Hall-like phenomena in zero external magnetic field. Bulk Bi$_{1-x}$Sb$_x$ single crystals are expected to be prime candidates for one such unusual Hall phase of matter known as the topological insulator. The hallmark of a topological insulator is the existence of metallic surface states that are higher dimensional analogues of the edge states that characterize a spin Hall insulator. In addition to its interesting boundary states, the bulk of Bi$_{1-x}$Sb$_x$ is predicted to exhibit three-dimensional Dirac particles, another topic of heightened current interest. Here, using incident-photon-energy-modulated (IPEM-ARPES), we report the first direct observation of massive Dirac particles in the bulk of Bi$_{0.9}$Sb$_{0.1}$, locate the Kramers' points at the sample's boundary and provide a comprehensive mapping of the topological Dirac insulator's gapless surface modes. These findings taken together suggest that the observed surface state on the boundary of the bulk insulator is a realization of the much sought exotic "topological metal". They also suggest that this material has potential application in developing next-generation quantum computing devices.