Citation of the 2017 (the 5th) Nishina Asia Award

Hongming Weng
(Professor, Institute of Physics, Chinese Academy of Science)

For his theoretical contributions to the discovery of Weyl semimetals
Weyl semimetal is a new topological state of matter that differs from topological insulators. Its
remarkable property provides an opportunity to realize the so-called Weyl fermions as low-energy
quasi-particles — a long sought massless chiral particle proposed as the building block of the Standard
Model, but is yet to be observed in particle physics experiments. In recent years, we have witnessed
the rapid development of this field, stimulated by the discovery of real Weyl semimetal materials, towhich Weng has made crucial contributions. Weng’s work in collaboration with his colleagues, using
the first-principles calculations, predicted the unique topological electronic structure of TaAs family
compounds [1], which had been known. Weng et al. has put the material in new perspective, leading
to the discovery of Weyl semimetals [2] and Weyl fermions [3]. This work constitutes a good example
that computational predictions can drive fundamental discovery and materials design.
In 1928, Paul Dirac proposed the 4-component Dirac equation to describe the motion of relativistic
electrons. In the following year, in 1929, Hermann Weyl found that Dirac equation can be further
simplified if electrons are massless. This led to the Weyl equation with 2-component chiral Weyl
fermions as its low energy particles. Since then, the simplest Weyl representation has been widely
accepted in quantum field theory and used as the building block of the Standard Model for particle
physics. Unfortunately, all fundamental particles found up to now are massive (including neutrinos)
due to symmetry breaking, therefore none of them are true Weyl fermions.
In contrast, in condensed matter physics, the progress in studying topological states has opened up
the possibility of finding Weyl fermions as low-energy quasi-particles of a new topological state,called Weyl semimetals. In 2011, a couple of candidates were theoretically proposed as Weylsemimetals, although they are difficult to realize. In the following years, 2012-2014, further progress
was made. Topological Dirac semimetals, where two Weyl nodes with opposite chirality coexist andoverlap in the momentum space, were theoretically predicted and experimentally demonstrated. Weng
has made important contributions in the studies of both kinds of materials.
The true challenge has been: how to find a realistic compound with separated Weyl nodes at theFermi level? For that purpose one needs to break either time reversal or inversion symmetry, and thelatter should be a better choice for experimental measurements. First-principles calculations shouldplay an important role towards that end. After trying numerous compounds, the breakthrough wasmade by Weng et al. in 2014 [1]. They theoretically predicted that TaAs family compounds are noncentrosymmetric (with an inversion-symmetry broken crystal structure) Weyl semimetals. Soon after1

the theoretical prediction, Weng et al., collaborating closely with experimental groups, finallyobserved the Weyl nodes in the bulk and related Fermi arcs on the surface of TaAs in 2015 [2, 3]. This
series of work leads to the discovery of Weyl semimetals and Weyl fermions. The other members ofTaAs family compounds, such as NbAs, TaP and NbP, have also been all confirmed to be Weyl
semimetals by recent experiments. It should be mentioned that this field is highly competitive, and asimilar experimental discovery has been made almost simultaneously by a few groups worldwide.
Nevertheless, the predictive role played by Weng et al. is crucial in this endeavor. For this reason,
Weng well deserves the Nishina Asia Award.

References:
1. Weyl Semimetal Phase in Noncentrosymmetric Transition-Metal Monophosphides,
Hongming Weng*, Chen Fang, Zhong Fang, B. A. Bernevig, Xi. Dai,Phys. Rev. X 5, 011029 (2015);
2. Experimental discovery of Weyl semimetal TaAs,
B.-Q. Lv*, Hongming Weng*, B. B. Fu, X. P. Wang, H. Miao, J. Ma, P. Richard, X. C. Huang, L.
X. Zhao, G. F. Chen, Z. Fang, X. Dai, T. Qian, and H. Ding,
Phys. Rev. X 5, 031013 (2015);
3. Observation of Weyl nodes in TaAs,
B.-Q. Lv*, N. Xu*, Hongming Weng*, J. Z. Ma, P. Richard, X. C. Huang, L. X. Zhao, G. F. Chen,C. E. Matt, F. Bisti, V. N. Strocov, J. Mesot, Z. Fang, X. Dai, T. Qian, M. Shi,
Nature Physics 11, 724-727 (2015).

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