[Paper Review] New classes of chiral topological nodes with non-contractible surface Fermi arcs in CoSi

Zhicheng Rao, Hongcheng Li+19 more|arXiv (Cornell University)|Jan 10, 2019

Topological Materials and Phenomena34 references96 citations

TL;DR

The paper reports the experimental discovery of spin-1 and charge-2 Dirac chiral nodes in CoSi, with non-contractible surface Fermi arcs connected across the surface Brillouin zone, observed via ARPES.

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.

Motivation & Objective

• Motivate the search for chiral topological nodes beyond Weyl nodes in condensed matter systems.
• Demonstrate the existence and surface connectivity of new node types (spin-1 and charge-2 Dirac) in CoSi.
• Show that crystalline symmetries enforce these nodes at high-symmetry points in the Brillouin zone.

Proposed method

• Use angle-resolved photoemission spectroscopy (ARPES) to probe bulk band crossings near the Fermi level.
• Identify and characterize spin-1 and charge-2 Dirac nodes at Brillouin zone center and corner.
• Map surface Fermi arcs on the (001) surface and establish their non-contractible topology across the surface BZ.
• Leverage crystalline symmetry constraints to explain node enforcement and surface state connectivity.

Experimental results

Research questions

• RQ1 Do spin-1 and charge-2 Dirac-like chiral nodes exist near the Fermi level in CoSi?
• RQ2 Can the surface Fermi arcs connect node projections in a non-contractible path across the (001) surface BZ?
• RQ3 Are these nodes symmetry-enforced at specific Brillouin zone points (center and corner) in CoSi?

Key findings

• Two new chiral node types are observed: spin-1 nodes and charge-2 Dirac nodes near the Fermi level in CoSi.
• The projections of these nodes on the (001) surface are connected by topologically protected surface Fermi arcs.
• The surface Fermi arcs form a non-contractible path traversing the entire (001) surface Brillouin zone.
• The nodes are enforced by crystalline symmetries at the Brillouin zone center and corner.
• This constitutes the first experimental observation of chiral topological nodes beyond Weyl nodes in condensed matter systems.