Weyl fermion
A massless quasiparticle and fermion, which carries electric charge, and cannot move backwards, that may exist in Weyl semimetals, where an electron can be split into two Weyl fermions which move in opposite directions, which can form an electron by combining two Weyl fermions.
Weyl fermion: a one-way quasiparticle in exotic metals
A Weyl fermion is a quasiparticle that emerges in certain crystalline materials called Weyl semimetals, where the normal rules of electron motion break down. Unlike ordinary electrons, which can move in any direction through a crystal lattice, a Weyl fermion moves in only one direction, determined by its spin orientation. This directional constraint is what makes it genuinely useful: in a Weyl semimetal, a single electron can split into two Weyl fermions traveling opposite ways, and two Weyl fermions can recombine into one electron.
The key property is that Weyl fermions are massless, meaning they have zero or negligible effective mass within the crystal. This allows them to move at extremely high velocities, close to the Fermi velocity of the material, which can be on the order of 106 meters per second. Because they carry electric charge and move with exceptional mobility, they conduct electricity and respond to magnetic fields in ways that ordinary electrons cannot. The one-directional nature means that each Weyl fermion chirality (left-handed or right-handed spin alignment) permits motion along a specific axis only.
In practical materials, Weyl semimetals are typically transition metal compounds or materials with strong spin-orbit coupling and broken time-reversal or inversion symmetry. Tungsten ditelluride and tantalum arsenide are among the first materials where Weyl fermions were experimentally observed. The electronic structure of these materials contains crossing points in the band diagram (called Weyl points) where the energy bands touch at a single point in momentum space, creating the conditions for Weyl fermion behavior.
Industrial relevance and measurement
Weyl fermions are relevant to emerging device concepts in electronics and sensing. Their high mobility and directional transport could enable ultra-low-loss conductors or new types of magnetic sensors that exploit the anomalous Hall effect. Detection typically requires angle-resolved photoemission spectroscopy (ARPES) to map the band structure directly, or magnetotransport measurements that reveal anomalies characteristic of Weyl fermion populations.
The phenomenon is named after Hermann Weyl, who described massless solutions to the Dirac equation in 1929. In condensed matter physics, the term refers specifically to the quasiparticle excitations that obey this Weyl equation within the crystal. This is distinct from Dirac fermions, which have both left and right chiralities and can move in all directions. Understanding Weyl fermions remains largely in the research phase, with commercial applications still being explored in fields such as topological electronics and quantum sensing.