1. Joseph Henry Laboratories of Physics, Department of Physics,
   
2. Department of Chemistry,
   
3. Princeton Center for Complex Materials, Princeton Institute for the Science and Technology of Materials, Princeton University, Princeton, New Jersey 08544, USA

Correspondence to: M. Z. Hasan^1,3 Correspondence and requests for materials should be addressed to M.Z.H. (Email: mzhasan@princeton.edu).

When electrons are subject to a large external magnetic field, the conventional charge quantum Hall effect^{1, 2} 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 quantum limit^{3, 4, 5}, which opens up the possibility for studying unusual quantum Hall-like phenomena in zero external magnetic fields⁶. Bulk Bi_1-xSb_x single crystals are predicted to be prime candidates^{7, 8} for one such unusual Hall phase of matter known as the topological insulator^{9, 10, 11}. 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 quantum spin Hall insulator^{3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13}. In addition to its interesting boundary states, the bulk of Bi_1-xSb_x is predicted to exhibit three-dimensional Dirac particles^{14, 15, 16, 17}, another topic of heightened current interest following the new findings in two-dimensional graphene^{18, 19, 20} and charge quantum Hall fractionalization observed in pure bismuth²¹. However, despite numerous transport and magnetic measurements on the Bi_1-xSb_x family since the 1960s¹⁷, no direct evidence of either topological Hall states or bulk Dirac particles has been found. Here, using incident-photon-energy-modulated angle-resolved photoemission spectroscopy (IPEM-ARPES), we report the direct observation of massive Dirac particles in the bulk of Bi_0.9Sb_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'^{9, 10, 11}. They also suggest that this material has potential application in developing next-generation quantum computing devices that may incorporate 'light-like' bulk carriers and spin-textured surface currents.