Fermi surface symmetry and evolution of the electronic structure across the paramagnetic-helimagnetic transition in MnSi/Si(111)

MnSi has been extensively studied for five decades; nonetheless detailed information on the Fermi surface (FS) symmetry is still lacking. This missed information prevents a comprehensive understanding of the nature of the magnetic interaction in this material. Here, by performing angle-resolved photoemission spectroscopy on high-quality MnSi films epitaxially grown on Si(111), we unveil the FS symmetry and the evolution of the electronic structure across the paramagnetic-helimagnetic transition at T-C similar to 40 K, along with the appearance of sharp quasiparticle emission below T-C. The shape of the resulting FS is found to fulfill robust nesting effects. These effects can be at the origin of strong magnetic fluctuations not accounted for by the state-of-the-art quasiparticle self-consistent GW approximation. From this perspective, the unforeseen quasiparticle damping detected in the paramagnetic phase and relaxing only below T-C, along with the persistence of the d-band splitting well above T-C, at odds with a simple Stoner model for itinerant magnetism, opens the search for exotic magnetic interactions favored by FS nesting and affecting the quasiparticle lifetime.