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Dirac Semimetals

Graphene is an example of 2D Dirac semimetal. Another example of 2D Dirac Semimetal is the 2D surface states of 3D topological insulators while Fermi level is tuned to lie at the Dirac point inside the bulk gap of the semiconductor. In 3D there can be many different types of Dirac Semimetals: Nature Commun. 5, 4786 (2014)
Science 347, 294 (2015)
Phys. Rev. B 91, 241114(R) (2015)
Phys. Rev. B 92, 075115 (2015)

Topology of the electronic structure of a crystal is manifested in its surface states. In topological insulators B1-xSbx and Bi2Se3 or topological crystalline insulators such as the Pb1-xSnxTe(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.


Symmetry-broken three-dimensional (3D) topological Dirac semimetal systems with strong spin-orbit coupling can host many exotic Hall-like phenomena and Weyl fermion quantum transport. Using high-resolution angle-resolved photoemission spectroscopy, we performed systematic electronic structure studies on Cd3As2 and Na3Bi which have been predicted to be the parent material, from which many unusual topological phases can be derived. We experimentally verified their 3D nature. Our discovery of the Dirac-like bulk topological semimetal phase in Cd3As2 and Na3Bi opens the door for exploring higher dimensional spin-orbit Dirac physics in a real material.


Observation of Fermi Arc Surface States in a Topological Metal.
Published in S.-Y. Xu, C. Liu, S. Kushwaha, et al., Science 347, 294 (2014).

Observation of a three-dimensional topological Dirac semimetal phase in high-mobility Cd3As2.
Published in M. Neupane, S.-Y. Xu, R. Sankar, et al., Nature Commun. 5, 4786 (2014).

Observation of a bulk 3D Dirac multiplet, Lifshitz transition, and nestled spin states in Na3Bi.
Published in S.-Y. Xu, C. Liu, S. K. Kushwaha, et al., arXiv:1312.7624 (2013).

Lifshitz transition and van Hove singularity in a 3D Topological Dirac Semimetal.
Published in S.-Y. Xu, C. Liu, I. Belopolski, et al., arXiv:1502.06917 (2015).

Bulk crystal growth and electronic characterization of the 3D Dirac Semimetal Na3Bi.
Published in S. K. Kushwaha, J. W. Krizan, B. E. Feldman, et al., Phys. Rev. B 92, 075115 (2015).

Ultraquantum magnetoresistance in single-crystalline Ag2Se.
Published in C. Zhang, H. Li, T.-R. Chang, et al., arXiv:1502.02324 (2015).

Surface Versus Bulk Dirac States Tuning in a Three-Dimensional Topological Dirac Semimetal.
Published in M. Neupane, S.-Y. Xu, N. Alidoust, et al., Phys. Rev. B 91, 241114(R) (2015).