Exotic fermions in condensed-matter systems

Condensed-matter systems can host quasiparticle excitations that are analogues to elementary particles such as Majorana, Weyl, and Dirac fermions. Recent advances in band theory have expanded the classification of fermions in crystals, and revealed crystal symmetry-protected electron excitations that have no high-energy counterparts. The first classes of crystal symmetry -protected electron excitations without high-energy counterparts are characterized by three-, six-, or eightfold degenerate points at the Fermi level and named as ‘new fermions’ by Bradlyn et al. While the second types are characterized by one dimensional nodal lines such as nodal-link,nodal-chain and nodal-net fermions in which more than two nodal lines are connected with each other. Here, using angle-resolved photoemission spectroscopy, we demonstrate the existence of three-component fermions and nodal-net fermions in solids where the quasiparticle excitations are beyond the Majorana-Weyl-Dirac classification. Our experimental discovery opens up a way of exploring the new physics of unconventional fermions in condensed-matter systems.