Research Publications Lab Facilities
Computation Platform
Internal Links
Fermi Arc

New Topological States of Matter (Inst. for Adv. Study Distinguished Lecture at HKUST)
Pedagogical Lecture Series

Discovery of Weyl Fermions, Topological Fermi Arcs and Topological Nodal-Line States of Matter
(Key Developments in Physics: 2Physics, 2016)

Science 347, 294 (2015)
Science 349, 613 (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.


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).
Discovery of Weyl Fermion Semimetal: TaAs, NbAs, SrSi2

Theoretical Prediction
: A Weyl Fermion semimetal with surface Fermi arcs in the transition metal monopnictide TaAs class
S.-M. Huang, S.-Y. Xu, I. Belopolski, C.-C. Lee, G. Chang, B. Wang, N. Alidoust, G. Bian, M. Neupane, C. Zhang, S. Jia, A. Bansil, H. Lin, M. Z. Hasan
Nature Commun. 6:7373 (2015) (submitted Nov. 2014)

Experimental Discovery: Discovery of a Weyl Fermion semimetal and topological Fermi arcs
S.-Y. Xu, I. Belopolski, N. Alidoust, M. Neupane, G. Bian, C. Zhang, R. Sankar, G. Chang, Z. Yuan, C.-C. Lee, S.-M. Huang, H. Zheng, J. Ma, D. S. Sanchez, B. Wang, A. Bansil, F. Chou, P. P. Shibayev, H. Lin, S. Jia, and M. Z. Hasan
Science 349, 613 (2015)

Discovery of a Weyl Fermion state with Fermi arcs in niobium arsenide
S.-Y. Xu, N. Alidoust, I. Belopolski, Z. Yuan, G. Bian, T.-R. Chang, H. Zheng, V. Strocov, D. S. Sanchez, G. Chang, C. Zhang, D. Mou, Y. Wu, L. Huang, C.-C. Lee, S.-M. Huang, B. Wang, A. Bansil, H.-T. Jeng, T. Neupert, A. Kaminski, H. Lin, S. Jia, and M. Z. Hasan
Nature Physics 11, 748 (2015)

Experimental discovery of a topological Weyl semimetal state in TaP
S.-Y. Xu, I. Belopolski, D. S. Sanchez, C. Guo, G. Chang, C. Zhang, G. Bian, Z. Yuan, H. Lu, Yi. Feng, T.-R. Chang, P. P. Shibayev, M. L. Prokopovych, N. Alidoust, H. Zheng, C.-C. Lee, S.-M. Huang, R. Sankar, F. Chou, C.-H. Hsu, H.-T. Jeng, A, Bansil, T. Neupert, V. N. Strocov, H. Lin, S. Jia, M. Z. Hasan
Science Advances 10, 1501092 (2015)

Transport Experiments: Tantalum Monoarsenide: an Exotic Compensated Semimetal
C. Zhang, Z. Yuan, S.-Y. Xu, Z. Lin, B. Tong, M. Z. Hasan, J. Wang, C. Zhang, S. Jia

Observation of the Adler-Bell-Jackiw chiral anomaly in a Weyl semimetal
C. Zhang, S.-Y. Xu, I. Belopolski, Z. Yuan, Z. Lin, B. Tong, N. Alidoust, C.-C. Lee, S.-M. Huang, H. Lin, M. Neupane, D. S. Sanchez, H. Zheng, G. Bian, J. Wang, C. Zhang, T. Neupert, M. Z. Hasan, S. Jia

A new type of Weyl semimetal with quadratic double Weyl fermions in SrSi2
S.-M. Huang, S.-Y. Xu, I. Belopolski, C.-C. Lee, G. Chang, B. Wang, N. Alidoust, M. Neupane, H. Zheng, D. Sanchez, A. Bansil, G. Bian, H. Lin, and M. Z. Hasan