Hasan Research Group

Laboratory for Topological Quantum Matter & Advanced Spectroscopy

Topology of the electronic structure of a crystal is manifested in its surface states. In topological insulators B_1-xSb_x and Bi₂Se₃ or topological crystalline insulators such as the Pb_1-xSn_xTe(Se), the bulk has a full insulating energy gap, whereas the surface possesses an odd or even number of spin polarized surface or edge states. It is the 2D surface state or 1D edge state properties that carry the signature of topology, which is the key focus of experiments on all topological states of matter. Very recently, the possibility of realizing new topological states beyond insulators, such as metals or semimetals, has attracted much attention. Unlike insulators, semimetals are materials, whose bulk conduction and valence bands have small but finite overlap. Thus there does not exist a full band gap irrespective of the choice of the chemical potential. Since the definition of bulk topological number for topological insulators strictly requires a full bulk energy gap, a topological state that might exist in a semimetal should be fundamentally distinct from the topological states studied in insulating materials.

Recently, a distinct topological state has been proposed in metals or semimetals whose spin-orbit band structure features three-dimensional Dirac quasiparticles. We use angle-resolved photoemission spectroscopy to experimentally observe a pair of spin-polarized Fermi arc surface states on the surface of the Dirac semimetal Na₃Bi at its native chemical potential. Our systematic results collectively identify a topological phase in a gapless material. The observed Fermi arc surface states open research frontiers in fundamental physics and possibly in spintronics.