Band structure reconstruction in the topological semimetal PrAlSi

The interplay between nontrivial topology, magnetism, and strong correlation has generated considerable research interest in condensed matter physics. The topological $R\text{Al}X$ ($R$ = rare earth; $X$ = Si and Ge) family has provided an excellent platform for exploring these complex interactions. Here, we performed infrared spectroscopy measurements on the ferromagnetic (FM) topological semimetal PrAlSi, in order to investigate the impact of FM orderings on the topological band structure. We find that the optical conductivity associated with the Dirac/Weyl cones exhibits two segments of linearly increasing parts in the normal state, connected by a kink feature at around 1960 ${\mathrm{cm}}^{\ensuremath{-}1}$. By entering the FM state, however, an additional linear-growing segment shows up in between the original ones, suggesting that the band structure is reconstructed. We propose that these observations can be effectively explained by a scenario where the Dirac/Weyl nodes are split into pairs of Weyl nodes with lower degeneracy, due to the time reversal symmetry breaking induced by the FM ordering. This band structure reconstruction also leads to a sudden enhancement of the itinerant carrier density. In addition, the effective mass of the itinerant carriers are estimated to be three orders of magnitude smaller than the free electron mass, providing a rare case where nearly all the free carriers exhibit behaviors characteristic of relativistic Dirac or Weyl fermions. Our results demonstrate a compelling example of the strong interaction between magnetic order and topological band structures, which opens up new avenues for exploring novel topological materials and their potential applications.