Observation of Chiral Fermions with a Large Topological Charge and and Associated Fermi-Arc Surface States in CoSi

TL;DR

This study experimentally confirms the existence of chiral topological fermions with large topological charge in CoSi, revealing associated Fermi-arc surface states and expanding the understanding of quantum states in topological semimetals.

Contribution

First experimental demonstration of chiral topological fermions with large topological charge in a transition-metal silicide, specifically CoSi, using angle-resolved photoemission spectroscopy.

Findings

Identification of spin-1 chiral fermion and double Weyl fermion in CoSi

Observation of Fermi-arc surface states connecting bulk Fermi surfaces

Bulk Fermi surfaces composed of bands related to these exotic fermions

Abstract

Topological semimetals materialize a new state of quantum matter where massless fermions protected by a specific crystal symmetry host exotic quantum phenomena. Distinct from well-known Dirac and Weyl fermions, structurally-chiral topological semimetals are predicted to host new types of massless fermions characterized by a large topological charge, whereas such exotic fermions are yet to be experimentally established. Here, by using angle-resolved photoemission spectroscopy, we experimentally demonstrate that a transition-metal silicide CoSi hosts two types of chiral topological fermions, spin-1 chiral fermion and double Weyl fermion, in the center and corner of the bulk Brillouin zone, respectively. Intriguingly, we found that the bulk Fermi surfaces are purely composed of the energy bands related to these fermions. We also find the surface states connecting the Fermi surfaces associated with these fermions, suggesting the existence of the predicted Fermi-arc surface states. Our result provides the first experimental evidence for the chiral topological fermions beyond Dirac and Weyl fermions in condensed-matter systems, and paves the pathway toward realizing exotic electronic properties associated with unconventional chiral fermions.