New classes of chiral topological nodes with non-contractible surface Fermi arcs in CoSi

TL;DR

This study reports the first experimental observation of new chiral topological nodes, including spin-1 and charge-2 Dirac nodes, in CoSi, with non-contractible surface Fermi arcs connecting their projections across the entire surface Brillouin zone.

Contribution

The paper provides the first experimental evidence of chiral topological nodes beyond Weyl nodes, revealing non-contractible Fermi arcs in CoSi enforced by crystalline symmetries.

Findings

Discovery of spin-1 and charge-2 Dirac nodes in CoSi.

Observation of non-contractible surface Fermi arcs.

Topologically protected surface states connecting the nodes.

Abstract

In condensed matter systems, chiral topological nodes are robust band crossing points in momentum space that carry nonzero Chern numbers. The chirality is manifested by the presence of surface Fermi arcs connecting the projections of nodes with opposite Chern numbers. In addition to the well-known Weyl nodes, theorists have proposed several other types of chiral topological nodes in condensed matter systems, but the direct experimental evidence of their existence is still lacking. Here, using angle-resolved photoemission spectroscopy, we reveal two types of new chiral nodes, namely the spin-1 nodes and charge-2 Dirac nodes, at the band crossing points near the Fermi level in CoSi, the projections of which on the (001) surface are connected by topologically protected surface Fermi arcs. As these chiral nodes in CoSi are enforced at the Brillouin zone (BZ) center and corner by the crystalline symmetries, the surface Fermi arcs connecting their projections form a non-contractible path traversing the entire (001) surface BZ, in sharp contrast to pairs of Weyl nodes with small separation. Our work marks the first experimental observation of chiral topological nodes beyond the Weyl nodes both in the bulk and on the surface in condensed matter systems.