Abstract
Weyl semimetals are a class of materials that can be regarded as three-dimensional analogs of graphene upon breaking time-reversal or inversion symmetry. Electrons in a Weyl semimetal behave as Weyl fermions, which have many exotic properties, such as chiral anomaly and magnetic monopoles in the crystal momentum space. The surface state of a Weyl semimetal displays pairs of entangled Fermi arcs at two opposite surfaces. However, the existence of Weyl semimetals has not yet been proved experimentally. Here, we report the experimental realization of a Weyl semimetal in TaAs by observing Fermi arcs formed by its surface states using angle-resolved photoemission spectroscopy. Our first-principles calculations, which match remarkably well with the experimental results, further confirm that TaAs is a Weyl semimetal.
- Received 15 July 2015
DOI:https://doi.org/10.1103/PhysRevX.5.031013
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Published by the American Physical Society
Viewpoint
Where the Weyl Things Are
Published 8 September 2015
Analogs to massless fermions predicted by particle physicists 80 years ago have been found in a crystalline metal and in a photonic crystal.
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Popular Summary
Hermann Weyl proposed in 1929 that a massless solution of the Dirac equation can represent a new type of particle—the Weyl fermion, which apparently breaks left-right symmetry. However, the Weyl fermion has remained elusive in particle physics for more than 80 years; the neutrino was regarded as a Weyl fermion until it was found to have mass. Recently, significant advances in topological materials have provided an alternative way to realize Weyl fermions in condensed matter as an emergent phenomenon. Weyl semimetals are predicted to be a class of topological materials that can be thought of as three-dimensional analogs of graphene upon breaking time-reversal or inversion symmetry. Some electrons in a Weyl semimetal can behave exactly like Weyl fermions. When these Weyl fermions are projected onto a surface, they will exhibit Fermi arcs connecting two Weyl fermions with opposite chirality. We use angle-resolved photoemission spectroscopy measurements to clearly observe Fermi arcs on the (001) surface of noncentrosymmetric crystal TaAs; we also study the shape and connectivity of these Fermi arcs.
We develop a mathematically rigorous method to confirm the existence of Fermi arcs, namely, by counting the crossing number of Fermi surfaces through an arbitrary closed momentum loop in the two-dimensional surface Brillouin zone. Furthermore, based on first-principles calculations, we are able to clearly resolve the pattern of these Fermi arcs connecting the projected Weyl fermions; separated Weyl nodes are a characteristic of a Weyl semimetal. Observations of Fermi arcs are a direct and distinguished exhibition of Weyl fermions or magnetic monopoles in a crystal, which is one of the hallmarks of a Weyl semimetal. We have accordingly shown that TaAs is a Weyl semimetal, which has stimulated intensive studies. The negative magnetoresistance of the semimetal due to the chiral anomaly has also been reported, and the Weyl nodes in the bulk states have been observed via soft x-ray photoemission.
Possible superconductivity in Weyl semimetals is currently being pursued for realizing exotic topological superconductivity and Majorana fermions.