| Weyl semimetal | |
|---|---|
| Name | Weyl semimetal |
| Type | Topological quantum material |
| Discovered | 2015 (experimental confirmations) |
| First theory | Theoretical proposals and subsequent work by Qi, Shen, and Vishwanath |
| Key people | Haldane, Ashvin Vishwanath, Shoucheng Zhang, N. P. Armitage, Shengjun Wu |
| Applications | Low-dissipation electronics, spintronics, quantum sensing |
Weyl semimetal
A Weyl semimetal is a class of three-dimensional crystalline material in which low-energy electronic excitations behave as massless Weyl fermions, leading to topologically protected band crossings called Weyl nodes. These materials realize concepts from high-energy particle physics in condensed matter, provide platforms to study anomalous transport such as the chiral anomaly in solids, and are central to modern research in topological phases and quantum materials.
The concept of Weyl semimetals emerged from the intersection of condensed matter physics and quantum field theory. Theoretical predictions in the early 2010s built on earlier work on Dirac semimetals and on breakthroughs in understanding topological insulators by researchers such as Kane and Zhang. Experimental confirmations began around 2015 with angle-resolved photoemission spectroscopy (ARPES) studies on materials like TaAs that identified Weyl nodes and associated surface states. Weyl semimetals advanced the program of realizing relativistic quasiparticles in solids following analogies to Weyl's 1929 solution of the Weyl equation in particle physics.
A Weyl semimetal is characterized by isolated linear band crossings in momentum space where two nondegenerate bands meet at nodes described by the Weyl Hamiltonian, a two-band variant of the Dirac equation. Each Weyl node carries a chirality or topological charge equivalent to a monopole of Berry curvature in the Brillouin zone. The existence of Weyl nodes requires either broken time-reversal symmetry (e.g., in magnets such as Mn3Sn) or broken inversion symmetry (e.g., in noncentrosymmetric crystals like TaAs). The separation of nodes in momentum or energy makes the system robust to weak perturbations; annihilation requires nodes of opposite chirality to merge. Topological invariants such as the Chern number computed on two-dimensional slices of the Brillouin zone quantify the topology and predict protected surface states called Fermi arcs.
Materials hosting Weyl semimetal phases include transition-metal pnictides and chalcogenides. Prototypical inversion-breaking Weyl semimetals include TaAs, NbAs, TaP, and NbP. Magnetic Weyl semimetals with broken time-reversal symmetry are realized in compounds like Co3Sn2S2, Mn3Sn, and certain Heusler alloys studied at institutions such as Max Planck Institute for Chemical Physics of Solids and Brookhaven National Laboratory. Engineered realizations appear in photonic crystals, cold-atom lattices (e.g., work at MIT and Stanford University groups), and metamaterials that emulate Weyl band structure. Materials growth techniques include molecular beam epitaxy (MBE) and chemical vapor transport developed by laboratories at Argonne National Laboratory and university groups.
Key experimental probes include angle-resolved photoemission spectroscopy (ARPES), scanning tunneling microscopy (STM), magnetotransport, and optical spectroscopy. ARPES directly images bulk Weyl nodes and surface Fermi arcs; landmark ARPES studies were reported by groups led by N. P. Armitage and collaborators. Transport measurements reveal large and anisotropic magnetoresistance and signatures of the chiral anomaly such as negative longitudinal magnetoresistance when electric and magnetic fields are parallel. Quantum oscillation experiments (Shubnikov–de Haas, de Haas–van Alphen) probe Fermi surface topology, while terahertz and infrared spectroscopy detect anomalous Hall and optical gyrotropic responses tied to Berry curvature. Neutron and muon experiments have been used to characterize magnetic Weyl candidates at facilities like ISIS Neutron and Muon Source.
Weyl semimetals display electromagnetic responses derived from topological band structure. The chiral anomaly — nonconservation of chiral charge under parallel electric and magnetic fields — manifests as enhanced conductivity along the field direction and is related to semiclassical Berry-curvature corrections described in works by Dmitri Pesin and others. Intrinsic anomalous Hall effects arise in ferromagnetic Weyl systems and are quantized on two-dimensional slices of the Brillouin zone. Optical phenomena such as the circular photogalvanic effect (CPGE) and nonlinear Hall responses have been proposed and observed, connecting to theoretical proposals by groups at Harvard University and Princeton University.
Prospective applications exploit high mobility, strong spin–orbit coupling, and topologically protected transport. Proposed devices include low-dissipation interconnects, terahertz detectors, spintronic elements leveraging large intrinsic anomalous Hall conductivities, and sensors exploiting chiral magnetic responses. Photonic and acoustic analogues enable robust waveguides and disorder-tolerant devices in engineered platforms; research on device integration involves collaborations between academic groups and companies investing in quantum materials, including work at IBM Research and Google Quantum AI on materials characterization.
Outstanding challenges include understanding disorder and interaction effects on Weyl physics, the role of correlation-driven instabilities (such as density waves or superconductivity), and controlling node positions by strain or heterostructure engineering. The interplay with strong correlations in heavy-fermion or kagome lattices (e.g., Co3Sn2S2) remains an active area. Theoretical extensions explore higher-order Weyl points, multifold fermions, non-Hermitian Weyl physics relevant for driven systems, and Weyl phenomena in moiré materials—topics pursued in many theoretical groups worldwide, including at the Perimeter Institute and major universities. Continued interplay of theory, materials synthesis, and advanced spectroscopy will determine technological prospects and deepen links between condensed matter and high-energy physics.
Category:Topological materials Category:Quantum materials Category:Condensed matter physics