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Chi-Cheng Lee

Bio: Chi-Cheng Lee is an academic researcher from University of Tokyo. The author has contributed to research in topics: Weyl semimetal & Fermion. The author has an hindex of 33, co-authored 84 publications receiving 12251 citations. Previous affiliations of Chi-Cheng Lee include Academia Sinica & Brookhaven National Laboratory.


Papers
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Journal ArticleDOI
TL;DR: The experimental discovery of a Weyl semimetal, tantalum arsenide (TaAs), using photoemission spectroscopy, which finds that Fermi arcs terminate on the Weyl fermion nodes, consistent with their topological character.

2,789 citations

Journal ArticleDOI
07 Aug 2015-Science
TL;DR: In this article, the experimental discovery of a Weyl semimetal, tantalum arsenide (TaAs), was reported, which is a new state of matter that hosts Weyl fermions as emergent quasiparticles and admits a topological classification that protects Fermi arcs on the boundary of a bulk sample.
Abstract: A Weyl semimetal is a new state of matter that hosts Weyl fermions as emergent quasiparticles and admits a topological classification that protects Fermi arc surface states on the boundary of a bulk sample. This unusual electronic structure has deep analogies with particle physics and leads to unique topological properties. We report the experimental discovery of a Weyl semimetal, tantalum arsenide (TaAs). Using photoemission spectroscopy, we directly observe Fermi arcs on the surface, as well as the Weyl fermion cones and Weyl nodes in the bulk of TaAs single crystals. We find that Fermi arcs terminate on the Weyl fermion nodes, consistent with their topological character. Our work opens the field for the experimental study of Weyl fermions in physics and materials science.

2,272 citations

Journal ArticleDOI
TL;DR: The results show that in the TaAs-type materials the WeylSemimetal state does not depend on fine-tuning of chemical composition or magnetic order, which opens the door for the experimental realization of Weyl semimetals and Fermi arc surface states in real materials.
Abstract: Weyl fermions are massless chiral fermions that play an important role in quantum field theory but have never been observed as fundamental particles. A Weyl semimetal is an unusual crystal that hosts Weyl fermions as quasiparticle excitations and features Fermi arcs on its surface. Such a semimetal not only provides a condensed matter realization of the anomalies in quantum field theories but also demonstrates the topological classification beyond the gapped topological insulators. Here, we identify a topological Weyl semimetal state in the transition metal monopnictide materials class. Our first-principles calculations on TaAs reveal its bulk Weyl fermion cones and surface Fermi arcs. Our results show that in the TaAs-type materials the Weyl semimetal state does not depend on fine-tuning of chemical composition or magnetic order, which opens the door for the experimental realization of Weyl semimetals and Fermi arc surface states in real materials. Proposals for the realization of Weyl semimetals, topologically non-trivial materials which host Weyl fermion quasiparticles, have faced demanding experimental requirements. Here, the authors predict such a state in stoichiometric TaAs, arising due to the breaking of inversion symmetry.

1,375 citations

Journal ArticleDOI
TL;DR: In this article, the first Weyl semimetal was identified in a class of stoichiometric materials, including TaAs, TaP, NbAs, and NbP, which break crystalline inversion symmetry.
Abstract: The recent discoveries of Dirac fermions in graphene and on the surface of topological insulators have ignited worldwide interest in physics and materials science. A Weyl semimetal is an unusual crystal where electrons also behave as massless quasi-particles but interestingly they are not Dirac fermions. These massless particles, Weyl fermions, were originally considered in massless quantum electrodynamics but have not been observed as a fundamental particle in nature. A Weyl semimetal provides a condensed matter realization of Weyl fermions, leading to unique transport properties with novel device applications. Here, we THEORETICALLY identify the first Weyl semimetal in a class of stoichiometric materials (TaAs, NbAs, NbP, TaP), which break crystalline inversion symmetry, including TaAs, TaP, NbAs and NbP. Our first-principles calculation-based predictions on TaAs reveal the spin-polarized Weyl cones and Fermi arc surface states in this compound. We also observe pairs of Weyl points with the same chiral charge which project onto the same point in the surface Brillouin zone, giving rise to multiple Fermi arcs connecting to a given Weyl point. Our results show that TaAs is the first topological semimetal identified which does not depend on fine-tuning of chemical composition or magnetic order, greatly facilitating an exploration of Weyl physics in real materials. (Note added: This theoretical prediction of November 2014 (see paper in Nature Communications) was the basis for the first experimental discovery of Weyl Fermions and topological Fermi arcs in TaAs recently published in Science (2015) at this http URL)

948 citations

Journal ArticleDOI
TL;DR: In this article, the Weyl semimetal state in an inversion-symmetry-breaking single-crystalline solid, Niobium arsenide (NbAs), has been discovered.
Abstract: Three types of fermions play a fundamental role in our understanding of nature: Dirac, Majorana and Weyl. Whereas Dirac fermions have been known for decades, the latter two have not been observed as any fundamental particle in high-energy physics, and have emerged as a much-sought-out treasure in condensed matter physics. A Weyl semimetal is a novel crystal whose low-energy electronic excitations behave as Weyl fermions. It has received worldwide interest and is believed to open the next era of condensed matter physics after graphene and three-dimensional topological insulators. However, experimental research has been held back because Weyl semimetals are extremely rare in nature. Here, we present the experimental discovery of the Weyl semimetal state in an inversion-symmetry-breaking single-crystalline solid, niobium arsenide (NbAs). Utilizing the combination of soft X-ray and ultraviolet photoemission spectroscopy, we systematically study both the surface and bulk electronic structure of NbAs. We experimentally observe both the Weyl cones in the bulk and the Fermi arcs on the surface of this system. Our ARPES data, in agreement with our theoretical band structure calculations, identify the Weyl semimetal state in NbAs, which provides a real platform to test the potential of Weyltronics. Experiments show that niobium arsenide is a Weyl semimetal.

754 citations


Cited by
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Journal ArticleDOI
TL;DR: Weyl and Dirac semimetals as discussed by the authors are three-dimensional phases of matter with gapless electronic excitations that are protected by topology and symmetry, and they have generated much recent interest.
Abstract: Weyl and Dirac semimetals are three-dimensional phases of matter with gapless electronic excitations that are protected by topology and symmetry. As three-dimensional analogs of graphene, they have generated much recent interest. Deep connections exist with particle physics models of relativistic chiral fermions, and, despite their gaplessness, to solid-state topological and Chern insulators. Their characteristic electronic properties lead to protected surface states and novel responses to applied electric and magnetic fields. The theoretical foundations of these phases, their proposed realizations in solid-state systems, and recent experiments on candidate materials as well as their relation to other states of matter are reviewed.

3,407 citations

Journal ArticleDOI
TL;DR: The experimental discovery of a Weyl semimetal, tantalum arsenide (TaAs), using photoemission spectroscopy, which finds that Fermi arcs terminate on the Weyl fermion nodes, consistent with their topological character.

2,789 citations

Journal ArticleDOI
07 Aug 2015-Science
TL;DR: In this article, the experimental discovery of a Weyl semimetal, tantalum arsenide (TaAs), was reported, which is a new state of matter that hosts Weyl fermions as emergent quasiparticles and admits a topological classification that protects Fermi arcs on the boundary of a bulk sample.
Abstract: A Weyl semimetal is a new state of matter that hosts Weyl fermions as emergent quasiparticles and admits a topological classification that protects Fermi arc surface states on the boundary of a bulk sample. This unusual electronic structure has deep analogies with particle physics and leads to unique topological properties. We report the experimental discovery of a Weyl semimetal, tantalum arsenide (TaAs). Using photoemission spectroscopy, we directly observe Fermi arcs on the surface, as well as the Weyl fermion cones and Weyl nodes in the bulk of TaAs single crystals. We find that Fermi arcs terminate on the Weyl fermion nodes, consistent with their topological character. Our work opens the field for the experimental study of Weyl fermions in physics and materials science.

2,272 citations

Journal ArticleDOI
TL;DR: In this paper, the authors present a survey of the use of Wannier functions in the context of electronic-structure theory, including their applications in analyzing the nature of chemical bonding, or as a local probe of phenomena related to electric polarization and orbital magnetization.
Abstract: The electronic ground state of a periodic system is usually described in terms of extended Bloch orbitals, but an alternative representation in terms of localized "Wannier functions" was introduced by Gregory Wannier in 1937. The connection between the Bloch and Wannier representations is realized by families of transformations in a continuous space of unitary matrices, carrying a large degree of arbitrariness. Since 1997, methods have been developed that allow one to iteratively transform the extended Bloch orbitals of a first-principles calculation into a unique set of maximally localized Wannier functions, accomplishing the solid-state equivalent of constructing localized molecular orbitals, or "Boys orbitals" as previously known from the chemistry literature. These developments are reviewed here, and a survey of the applications of these methods is presented. This latter includes a description of their use in analyzing the nature of chemical bonding, or as a local probe of phenomena related to electric polarization and orbital magnetization. Wannier interpolation schemes are also reviewed, by which quantities computed on a coarse reciprocal-space mesh can be used to interpolate onto much finer meshes at low cost, and applications in which Wannier functions are used as efficient basis functions are discussed. Finally the construction and use of Wannier functions outside the context of electronic-structure theory is presented, for cases that include phonon excitations, photonic crystals, and cold-atom optical lattices.

2,217 citations