A topological Dirac insulator in a quantum spin Hall phase
Abstract
When electrons are subject to a large external magnetic field, the conventional charge quantum Hall effect 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 spinorbit interactions may also naturally support conducting topological boundary states in the quantum limit, which opens up the possibility for studying unusual quantum Halllike phenomena in zero external magnetic fields. Bulk Bi_{1x}Sb_{x} single crystals are predicted 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 higherdimensional analogues of the edge states that characterize a quantum spin Hall insulator. In addition to its interesting boundary states, the bulk of Bi_{1x}Sb_{x} is predicted to exhibit threedimensional Dirac particles, another topic of heightened current interest following the new findings in twodimensional graphene and charge quantum Hall fractionalization observed in pure bismuth. However, despite numerous transport and magnetic measurements on the Bi_{1x}Sb_{x} family since the 1960s, no direct evidence of either topological Hall states or bulk Dirac particles has been found. Here, using incidentphotonenergymodulated angleresolved photoemission spectroscopy (IPEMARPES), we report the 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 Dirac insulator's gapless surface electron bands. These findings taken together suggest that the observed surface state on the boundary of the bulk insulator is a realization of the `topological metal'. They also suggest that this material has potential application in developing nextgeneration quantum computing devices that may incorporate `lightlike' bulk carriers and spintextured surface currents.
 Publication:

Nature
 Pub Date:
 April 2008
 DOI:
 10.1038/nature06843
 arXiv:
 arXiv:0902.1356
 Bibcode:
 2008Natur.452..970H
 Keywords:

 Condensed Matter  Mesoscale and Nanoscale Physics;
 Condensed Matter  Disordered Systems and Neural Networks;
 Condensed Matter  Superconductivity
 EPrint:
 16 pages, 3 Figures. Submitted to NATURE on 25th November(2007)