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 TC∼40K, along with the appearance of sharp quasiparticle emission below TC. 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 TC, along with the persistence of the d-band splitting well above TC, 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.