CeSb: Evidence for Weyl fermions in a magnetic Kondo system

Materials where the electronic bands have unusual topologies both allow for the realization of novel physics and have a wide range of potential applications. When two electronic bands with linear dispersions intersect, the excitations can often be described as Weyl fermions which are massless particles with a particular chirality. Upon breaking time reversal or inversion symmetry, Weyl points of opposite chirality separate in momentum space, leading to a range of remarkable properties. Evidence for Weyl fermions has recently been realized in compounds lacking inversion symmetry, as well as weakly correlated, non-magnetic materials in an applied magnetic field. Here we report the presence of Weyl fermions in the ferromagnetic state of the low-carrier density, strongly correlated Kondo lattice system CeSb, from electronic structure calculations and angle-dependent magnetoresistance measurements. When the applied magnetic field is parallel to the electric current, a pronounced negative magnetoresistance is observed within the ferromagnetic state, which is destroyed upon slightly rotating the field away from the current direction. These results suggest that CeSb belongs to a new class of compounds with both massless Weyl fermions caused by broken time reversal symmetry in the ferromagnetic state, and heavy electron bands. This may provide a platform for studying the interplay between topology, magnetism and strong electron-electron interactions.