Emergence of Weyl Fermions by Ferrimagnetism in a Noncentrosymmetric Magnetic Weyl Semimetal*

Searching for angle-resolved photoemission spectroscopy (ARPES) signatures across magnetic phase transition is typically a difficult task, partly due to the formation of magnetic domains that are smaller than the typical beam spot size. In relation to the magnetic Weyl semimetal, many attempts have been made to study the shift of Weyl nodes across the magnetic transition. However, this is intrinsically difficult due to the smallness of the Zeeman energy. We have come up with a novel attempt, that is to look for evidence of Brillouin zone folding in a ferrimagnet. Here, using ARPES, we provide a new example of this by visualizing the electronic structure of a noncentrosymmetric magnetic Weyl semimetal candidate NdAlSi in both the paramagnetic and ferrimagnetic states. We observe surface Fermi arcs and bulk Weyl fermion dispersion as well as the regulation of Weyl fermions by ferrimagnetism. Our results establish NdAlSi as a magnetic Weyl semimetal and provide the first experimental observation of ferrimagnetic regulation of Weyl fermions in condensed matter.