NdAlSi, a type II magnetic Weyl semimetal*

The combination of type-II Weyl semimetal and the coexistence of broken time-reversal symmetry and inversion symmetry is highly unusual. RAlX materials (R = rare earth, X = Si, Ge) have recently attracted much attention as rare examples of such materials. We focus on NdAlSi due to its interesting magnetic and electronic properties. It shows multiple magnetic phase transitions mediated by RKKY-type interactions with ferromagnetism at 7.2 K and ferrimagnetism (up-up-down Nd spin ordering) at 3.3 K. Using first-principles density-functional theory, we find 20 pairs of Weyl nodes in the paramagnetic phase, 28 pairs of Weyl nodes in the ferromagnetic phase, and 26 pairs of Weyl nodes in the antiferromagnetic phase. We also focus on the Fermi surfaces associated with the Weyl nodes hosting a nesting vector consistent with the experimentally observed magnetic ordering at q ~ [2/3,2/3,0]. Our work exposing the connections between the Weyl node structure and the magnetic ordering should be applicable to this class of materials more broadly, stimulating further interest in their remarkable properties.