Chiral anomaly factory: Creating Weyl fermions with a magnetic field

The Weyl fermion -- originally proposed to describe neutrinos -- arises in a topological semimetallic phase in condensed matter systems, exhibiting such novel properties as surface Fermi arcs and a chiral anomaly. Although Weyl fermions have been observed, it is challenging to find materials that exhibit them near the Fermi level. The authors prove that Weyl fermions can be created in band-inverted materials in a large class of crystal systems by applying a magnetic field along various symmetry axes of the crystal. As the field direction is changed, the Weyl points move in momentum space, during which time pairs are created and annihilated. However, in the highly symmetric T₃ point group, the Weyl points cannot completely disappear: at least one pair must remain for any direction of the magnetic field. Furthermore, a semiclassical analysis shows that the magnetoresistance scales differently for Weyl fermions created by a magnetic field compared to intrinsic Weyl points. The ability to create Weyl fermions will lead to new material candidates in these crystal systems; controlling their positions opens the possibility to track and manipulate Fermi arcs.