Interplay of magnetic order and Weyl nodal structure in NdAlSi

Weyl semimetals are topologically non-trivial phases of matter that sustain low-energy excitations in the massless Weyl fermions. It is necessary to break either inversion or time-reversal symmetry to establish a Weyl semimetal. A rare occasion arises, however, if a material breaks both symmetries. After an introduction to the physics of Weyl semimetals, I will focus on NdAlSi as a paradigmatic example of a noncentrosymmetric magnetic Weyl semimetal. This remarkable system has nodal touchings even if magnetism and spin-orbit coupling are absent, but their populations and locations are strongly affected as these are restored. Using first-principles density functional calculations, we determined the evolution of these Weyl points and their associated electron and hole pockets in both magnetic and nonmagnetic phases, with and without spin-orbit coupling. The possible role of the Weyl structure and associated nesting wavevectors in mediating RKKY interactions and determining the observed form of the magnetic order will also be discussed.