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When electrons are subject to a large external magnetic field, theconventional charge quantum Hall effect \cite{Klitzing,Tsui} dictates that anelectronic excitation gap is generated in the sample bulk, but metallicconduction is permitted at the boundary. Recent theoretical models suggest thatcertain bulk insulators with large spin-orbit interactions may also naturallysupport conducting topological boundary states in the extreme quantum limit,which opens up the possibility for studying unusual quantum Hall-like phenomenain zero external magnetic field. Bulk Bi$_{1-x}$Sb$_x$ single crystals areexpected to be prime candidates for one such unusual Hall phase of matter knownas the topological insulator. The hallmark of a topological insulator is theexistence of metallic surface states that are higher dimensional analogues ofthe edge states that characterize a spin Hall insulator. In addition to itsinteresting boundary states, the bulk of Bi$_{1-x}$Sb$_x$ is predicted toexhibit three-dimensional Dirac particles, another topic of heightened currentinterest. Here, using incident-photon-energy-modulated (IPEM-ARPES), we reportthe first direct observation of massive Dirac particles in the bulk ofBi$_{0.9}$Sb$_{0.1}$, locate the Kramers' points at the sample's boundary andprovide a comprehensive mapping of the topological Dirac insulator's gaplesssurface modes. These findings taken together suggest that the observed surfacestate on the boundary of the bulk insulator is a realization of the much soughtexotic "topological metal". They also suggest that this material has potentialapplication in developing next-generation quantum computing devices.

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