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Weyl semimetal
Weyl fermions are massless chiral fermions that play an important role in quantum field theory and the standard model. They may be thought of as a building block for fermions in quantum field theory. For example, one-half of a charged Dirac fermion of a definite chirality is a Weyl fermion. They have not been observed as a fundamental particle in nature. Weyl fermions may be realized as emergent quasiparticles in a low-energy condensed matter system.
==Experimental discovery==
A Weyl semimetal is a solid state crystal whose low energy excitations are Weyl fermions. A Weyl semimetal enables the first-ever realization of Weyl fermions. It is a topologically nontrivial phase of matter that broadens the topological classification beyond topological insulators.〔 The Weyl fermions at zero energy correspond to points of bulk band degeneracy, the Weyl nodes that are separated in momentum space. Weyl fermions have distinct chiralities, either left handed or right handed. In a Weyl semimetal crystal, the chiralities associated with the Weyl nodes can be understood as topological charges, leading to monopoles and anti-monopoles of Berry curvature in momentum space, which (the splitting) serve as the topological invariant of this phase.〔 Comparing to the Dirac fermions in graphene or on the surface of topological insulators, Weyl fermions in a Weyl semimetal are the most robust electrons and do not depend on symmetries except the translation symmetry of the crystal lattice. Hence the Weyl fermion quasiparticles in a Weyl semimetal possess a high degree of mobility. Due to the nontrivial topology, a Weyl semimetal is expected to demonstrate Fermi arc electron states on its surface.〔〔 These arcs are discontinuous or disjoint segments of a two dimensional Fermi contour, which are terminated onto the projections of the Weyl fermion nodes on the surface.
On July 16, 2015 the first experimental observations of Weyl fermion semimetal in an inversion symmetry-breaking single crystal material tantalum arsenide (TaAs) were made.〔 Both Weyl fermions and Fermi arc surface states were observed, which established its topological character.〔 This discovery was built upon previous theoretical predictions proposed in November 2014. Weyl points were also observed in a non-fermionic system, a photonic crystal.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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