Understanding Weyl Semimetals: A Gateway to Novel Quantum Phenomena

Definition: Weyl semimetals are a class of quantum materials that host massless Weyl fermions as their electronic excitations. These materials exhibit a range of exotic physical properties, including high mobility of charge carriers, unusual magnetic responses, and the presence of topologically protected surface states.


Definition and Principle of Operation

Weyl semimetals are characterized by their electronic structure, where conduction and valence bands touch at points called Weyl nodes. These nodes occur in pairs with opposite chirality, leading to nontrivial topological properties. The existence of Weyl fermions in these materials was predicted on theoretical grounds and has been confirmed through various experimental techniques, including angle-resolved photoemission spectroscopy (ARPES).

How Weyl Semimetals Work

The distinctive feature of Weyl semimetals is the linear dispersion of electronic states near the Weyl nodes, resembling the dispersion relation of relativistic particles. This results in massless charge carriers with high mobility. The separation of Weyl nodes in momentum space or energy leads to unique electromagnetic responses, such as the chiral anomaly, which manifests as an imbalance in the population of Weyl fermions with different chiralities under certain conditions.
Weyl semimetals also exhibit surface states that form Fermi arcs, which are incomplete Fermi surfaces connecting the projections of the bulk Weyl nodes onto the surface Brillouin zone. These surface states are topologically protected and contribute to the unusual transport properties observed in these materials.

Key Features of Weyl Semimetals

Weyl semimetals stand out due to several key features:
  • Massless Weyl Fermions: The presence of massless charge carriers leads to exceptional electronic properties, including high conductivity and unusual magnetic responses.
  • Topological Surface States: The topologically protected surface states, or Fermi arcs, are a hallmark of Weyl semimetals, enabling unique surface conductivity and potential applications in quantum computing and spintronics.
  • Chiral Anomaly: The phenomenon where an applied electric field along a magnetic field can cause an imbalance in the number of left- and right-handed fermions, leading to novel transport phenomena.
  • Magnetic and Optical Properties: Weyl semimetals exhibit unusual magnetic and optical responses, such as the anomalous Hall effect and negative magnetoresistance, which are of interest for next-generation electronic and photonic devices.

Applications and Future Directions

The unique properties of Weyl semimetals open up new possibilities for applications in various technology sectors:
  • Topological Quantum Computing: The topologically protected states in Weyl semimetals could be harnessed for qubits, offering a platform for robust quantum computing.
  • Spintronics: The high mobility of charge carriers and the spin-momentum locking in Weyl semimetals make them promising candidates for spintronic devices, which exploit the spin of electrons for data storage and transfer.
  • Photonic Devices: The unusual optical properties of Weyl semimetals, including their response to light and magnetic fields, suggest potential applications in designing novel photonic devices for telecommunications, sensing and optical computing.
  • Energy Technologies: The efficiency and high conductivity of Weyl semimetals may lead to their use in energy conversion and storage technologies, potentially enhancing the performance of solar cells and batteries.

Further Reading

Annual Review of Condensed Matter Physics, Topological Materials: Weyl Semimetals