| Weyl semimetal | |
|---|---|
| Name | Weyl semimetal |
| Caption | Schematic of Weyl nodes and Fermi arcs |
| Type | Quantum material |
| Discovered | 2015 (experimental) |
| Notable | Weyl fermions, chiral anomaly, Fermi arc surface states |
Weyl semimetal
A Weyl semimetal is a class of topological phase of electronic matter in which low-energy quasiparticles behave as massless Weyl fermions, exhibiting linear dispersion near discrete band-touching points called Weyl points. These materials provide a condensed-matter realization of concepts from quantum field theory and high-energy physics and matter to quantum materials research because they enable tabletop studies of relativistic phenomena such as the chiral anomaly and surface Fermi arc states.
Weyl semimetals occupy a central place in modern condensed matter physics and the study of topological phases due to their robust band topology and symmetry-protected electronic structure. Building on theoretical predictions by Haldane, Qi, and others, the Weyl semimetal concept unites ideas from Berry phase theory, band theory and quantum electrodynamics analogies. The discovery of candidate materials such as TaAs and experiments at facilities like Stanford University and Max Planck institutes made Weyl physics experimentally tractable, linking fundamental symmetries (time-reversal, inversion) with observable transport signatures.
In band-structure terms, a Weyl semimetal features pairs of nondegenerate band crossings (Weyl points) where the conduction and valence bands touch with linear dispersion in all three momentum directions. Each Weyl point acts as a monopole of Berry curvature in momentum space, characterized by an integer topological charge (chirality) determined by the integral of the Berry flux on a surrounding surface. Breaking either time reversal symmetry or inversion symmetry is necessary to split degenerate Dirac points into separate Weyl nodes, a mechanism clarified in theoretical work by Ashvin Vishwanath and others. Computationally, identification of Weyl points uses density functional theory (DFT) band calculations and symmetry analysis consistent with the crystal space group (e.g., in materials studied at Harvard University and LBNL).
The nontrivial topology of Weyl semimetals gives rise to open surface states known as Fermi arcs that connect projections of Weyl nodes of opposite chirality on the surface Brillouin zone. These arcs are protected by the bulk Weyl topology and contrast with closed Fermi surfaces in ordinary metals. Experimental mapping of Fermi arcs is often performed with angle-resolved photoemission spectroscopy (ARPES), pioneered at institutions including SLAC National Accelerator Laboratory and Paul Scherrer Institute. Theoretical descriptions relate Fermi arcs to surface Green's functions, topological invariants and bulk-boundary correspondence originally formalized in studies by Kane and Mele and Hasan and Kane.
Real material realizations include transition-metal monopnictides such as TaAs, NbAs, TaP, and engineered systems like photonic crystals and cold-atom lattices. Experimental detection relies on ARPES, scanning tunneling microscopy (STM), quantum oscillation measurements at places like Brookhaven National Laboratory and magnetotransport experiments demonstrating negative magnetoresistance. High-quality single crystals grown at university and national-lab facilities enabled direct observation of Weyl nodes and surface arcs; key experimental reports appeared from collaborations involving Princeton University and Institute of Physics, Chinese Academy of Sciences.
Weyl semimetals show distinctive transport signatures tied to their chiral charge, foremost the condensed-matter manifestation of the chiral anomaly: charge pumping between Weyl nodes under parallel electric and magnetic fields, producing negative longitudinal magnetoresistance. The effect has been measured in materials like TaAs and Cd3As2 and analyzed using semiclassical Boltzmann theory with Berry-curvature corrections and quantum field theoretic approaches. Related phenomena include the anomalous Hall effect in time-reversal-breaking Weyl systems and nonlocal transport mediated by chiral charge imbalance. Experimental interpretation often requires control of disorder, sample geometry, and chemical potential, with theoretical input from groups at ETH Zurich and University of Cambridge.
Minimal theoretical models include two-band lattice models exhibiting separated Weyl nodes, continuum Weyl Hamiltonians, and tight-binding constructions based on specific crystal symmetries. Analytical tools draw from quantum field theory (anomaly calculations), semiclassical wave-packet dynamics, and topological band theory. Computational methods include DFT for material prediction, maximally localized Wannier function construction for surface-state calculations, and Green's-function techniques for transport. Software packages and collaborative efforts at places like Argonne National Laboratory and Oak Ridge National Laboratory contribute to high-throughput searches for new Weyl materials.
Weyl semimetals are promising for applications leveraging robust surface conduction, high mobility, and unusual magneto-optical responses. Prospective technologies include low-dissipation interconnects, sensors exploiting large magnetoresistance, and platforms for testing topological quantum devices. Future directions emphasize materials discovery, heterostructures combining Weyl phases with superconductors to explore Majorana physics, and engineered platforms in photonics and cold atoms to simulate relativistic quantum phenomena. Sustained collaboration among universities, national laboratories, and industry—e.g., consortiums involving IBM and national research centers—will shape the translational path from fundamental Weyl physics to stable, scalable quantum technologies.
Category:Topological phases of matter Category:Quantum materials