Weyl semimetals

Definition

Weyl semimetals are a class of materials that host Weyl fermions as low-energy excitations, characterized by the presence of Weyl points in their band structure. These points are created when conduction and valence bands touch, leading to a nontrivial topological phase. Weyl semimetals exhibit unique electronic properties, such as high mobility and chiral anomaly, making them an exciting area of study in condensed matter physics.

5 Must Know Facts For Your Next Test

1. Weyl semimetals possess two types of Weyl points: Type I, which exhibit linear dispersion, and Type II, which show a tilted dispersion relation.
2. The presence of Weyl points gives rise to robust surface states known as Fermi arcs, connecting projections of Weyl points on the surface Brillouin zone.
3. Weyl semimetals can be classified based on their symmetry properties, with some materials requiring inversion symmetry breaking to create Weyl points.
4. The chiral anomaly in Weyl semimetals leads to unusual transport phenomena, such as enhanced conductivity under applied magnetic fields.
5. Examples of Weyl semimetals include TaAs, NbP, and MoTe$_2$, which have been experimentally confirmed to exhibit Weyl behavior.

Review Questions

• How do Weyl semimetals differ from Dirac semimetals in terms of their band structure and associated excitations?
  • Weyl semimetals differ from Dirac semimetals primarily in the nature of their band structures and excitations. While both types feature massless fermionic excitations, Weyl semimetals have Weyl points where conduction and valence bands touch and can exist as pairs with opposite chirality. In contrast, Dirac semimetals possess Dirac points where four bands meet at a single point in momentum space. The presence of Weyl points leads to unique topological properties in Weyl semimetals that aren't present in Dirac semimetals.
• Discuss the implications of the chiral anomaly in Weyl semimetals for their electronic transport properties.
  • The chiral anomaly has significant implications for the electronic transport properties of Weyl semimetals. It causes the non-conservation of chiral charge, which can result in enhanced conductivity when a magnetic field is applied. This behavior manifests as negative magnetoresistance, where the resistance decreases under a magnetic field due to an imbalance between left-handed and right-handed Weyl fermions. This unusual response highlights the rich physics associated with Weyl semimetals and sets them apart from conventional materials.
• Evaluate the potential applications of Weyl semimetals in modern technology and material science.
  • Weyl semimetals hold significant potential for various applications in modern technology and material science due to their unique electronic properties. For instance, their high mobility and robustness against scattering make them ideal candidates for next-generation electronic devices, potentially improving performance in transistors and sensors. Additionally, the unusual transport phenomena driven by the chiral anomaly could lead to innovative applications in spintronics or quantum computing. As research continues to uncover new materials and functionalities within this class, Weyl semimetals may pave the way for advancements in both fundamental physics and practical technologies.

Related terms

Topological insulators: Materials that have insulating bulk states but conductive surface states due to their nontrivial topological order.

Dirac semimetals: A type of semimetal that features Dirac cones in their electronic band structure, hosting massless Dirac fermions as excitations.

Chiral anomaly: A phenomenon where the number of left-handed and right-handed Weyl fermions is not conserved in certain conditions, leading to observable effects like negative magnetoresistance.