Introduction to Weyl Semimetals

Weyl semimetals are a class of topological materials characterized by low energy excitations of Weyl fermions. These fermions are analogous to Dirac fermions, but they exhibit unique topological properties in three-dimensional space. Theoretical predictions and recent experimental observations have confirmed the existence of these novel materials, opening up new avenues for both fundamental research and potential applications in spintronics, quantum computing, and advanced electronic devices.

Understanding Weyl Nodes

Definition: Weyl nodes are points in momentum space where the conduction and valence bands touch, leading to a distinctive topological band structure. Weyl nodes are the three-dimensional analogs of Dirac points in graphene, but they carry a topological charge or chirality, which can be either positive or negative.

Properties: Each Weyl node has a topological charge or chirality, which leads to fascinating physical phenomena. For example, the presence of surface states and unique properties like spin-momentum locking, where the spin of the surface states is correlated with their momentum.

Exploring Fermi Arcs

Definition: Fermi arcs are surface states connecting the projections of Weyl nodes on the surface Brillouin zone. Unlike conventional surface states, which are closed loops, Fermi arcs are open, connecting Weyl nodes with opposite chirality.

Properties: The existence of Fermi arcs is a direct result of the topological nature of Weyl semimetals. They exhibit unique properties, such as spin-momentum locking, where the spin of the surface states is correlated with their momentum. This phenomenon is crucial for understanding the electrical and magnetic properties of Weyl semimetals.

Physical Significance and Topological Protection

Topological Protection: Both Weyl nodes and Fermi arcs are robust against certain types of perturbations, making them topologically protected. This means they can survive in the presence of defects and disorder in the material, ensuring their stability and consistency.

Applications: The unique electronic properties of Weyl semimetals, including the presence of Fermi arcs, make them promising candidates for applications in spintronics, quantum computing, and novel electronic devices. These materials offer a high degree of controllability and mobility, which is critical for integrating them into advanced technologies.

Experimental Observations and Theoretical Predictions

On July 16, 2015, the first experimental observations of Weyl fermions and their associated Fermi arc surface states were made in an inversion symmetry-breaking single crystal material, tantalum arsenide (TaAs). This discovery was built upon previous theoretical predictions proposed in November 2014. The existence of Weyl points was also observed in a non-fermionic system, a photonic crystal, further validating the theoretical framework.

Images and detector signals can effectively indicate the presence of Weyl fermion nodes and Fermi arc states. These observations have significant implications for the broader field of condensed matter physics and material science, promising both fundamental insights and practical applications.