Weyl semimetals are
semimetals or metals whose quasiparticle excitation is the Weyl
fermion, a particle that played a crucial role in quantum field theory but has not been observed as a fundamental particle in vacuum.[1]
In these materials, electrons have a linear dispersion relation, making them a solid-state analogue of relativistic massless particles.[2]
Weyl fermions may be realized as emergent
quasiparticles in a low-energy condensed matter system. This prediction was first proposed by
Conyers Herring in 1937, in the context of
electronic band structures of solid state systems such as electronic crystals.[5][6] Topological materials in the vicinity of band inversion transition became a primary target in search of topologically protected bulk electronic band crossings.[7]
The first (non-electronic) liquid state which is suggested, has similarly emergent but neutral excitation and theoretically interpreted
superfluid's
chiral anomaly as observation of
Fermi points is in
Helium-3 A superfluid phase.[8][non-primary source needed] Crystalline tantalum arsenide (TaAs) is the first discovered topological Weyl fermion semimetal which exhibits topological surface
Fermi arcs where Weyl fermion is electrically charged along the line of original suggestion by Herring.[6][9] An electronic Weyl fermion is not only charged but stable at room temperature where there is no such superfluid or liquid state known.[citation needed]
Experimental observation
A Weyl semimetal is a solid state
crystal whose low energy excitations are Weyl fermions that carry electrical charge even at room temperatures.[11][12][13] A Weyl semimetal enables realization of Weyl fermions in electronic systems.[9] It is a topologically nontrivial phase of matter, together with Helium-3 A superfluid phase, that broadens the topological classification beyond topological insulators.[14] The Weyl fermions at zero energy correspond to points of bulk band degeneracy, the Weyl nodes (or Fermi points) 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 (Fermi points) 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.[11] Comparable 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.[9][11] 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. A 2012 theoretical investigation of superfluid Helium-3[15] suggested Fermi arcs in neutral superfluids.
On 16 July 2015 the first experimental observations of Weyl fermion semimetal and topological Fermi arcs in an inversion symmetry-breaking single crystal material
tantalum arsenide (TaAs) were made.[9] Both Weyl fermions and Fermi arc surface states were observed using direct electronic imaging using
ARPES, which established its topological character for the first time.[9] This discovery was built upon previous theoretical predictions proposed in November 2014 by a team led by Bangladeshi scientist
M Zahid Hasan.[16][17]
Weyl points (Fermi points) were also observed in non-electronic systems such as photonic crystals, in fact even before their experimental observation in electronic systems[18][19][20] and Helium-3 superfluid quasiparticle spectrum (neutral fermions).[21] Note that while these systems are different from electronic condensed matter systems, the basic physics is very similar.
Crystal growth, structure and morphology
TaAs is the first discovered Weyl semimetal (conductor). Large-size (~1 cm), high-quality TaAs single crystals[22] can be obtained by
chemical vapor transport method using iodine as the transport agent.
TaAs crystallizes in a
body-centered tetragonal unit cell with
lattice constants a = 3.44 Å and c = 11.64 Å and
space group I41md (No. 109). Ta and As atoms are six coordinated to each other. This structure lacks a horizontal mirror plane and thus inversion symmetry, which is essential to realize Weyl semimetal.
TaAs single crystals have shiny facets, which can be divided into three groups: the two truncated surfaces are {001}, the trapezoid or isosceles triangular surfaces are {101}, and the rectangular ones {112}. TaAs belongs to
point group 4mm, the equivalent {101} and {112} planes should form a ditetragonal appearance. The observed morphology can be vary of degenerated cases of the ideal form.
Beside the initial discovery of TaAs as Weyl semimetal, many other materials such as Co2TiGe, MoTe2, WTe2, LaAlGe and PrAlGe have been identified to exhibit Weyl semimetallic behavior.[23][24]
Applications
The Weyl fermions in the bulk and the Fermi arcs on the surfaces of Weyl semimetals are of interest in physics and materials technology.[3][25] The high mobility of charged Weyl fermions may find use in electronics and computing.
In 2017,[26] a research team from Vienna University of Technology carrying out experimental work to develop new materials, and a team at Rice University carrying out theoretical work, have produced material which they term Weyl–Kondo semimetals.[27]
A group of international researchers led by a team from Boston College discovered in 2019 that the Weyl semimetal Tantalum Arsenide delivers the largest intrinsic conversion of light to electricity of any material, more than ten times larger than previously achieved.[28]
^Bevan, T. D. C.; Manninen, A. J.; Cook, J. B.; Hook, J. R.; Hall, H. E.; Vachaspati, T.; Volovik, G. E. (1997-04-17). "Momentum creation by vortices in superfluid 3He as a model of primordial baryogenesis". Nature. 386 (6626): 689–692.
arXiv:cond-mat/9611164.
Bibcode:
1997Natur.386..689B.
doi:
10.1038/386689a0.
S2CID4315194.