Consequences of Fermi-surface order on Weyl-semimetal boundary arcs

Determine the phases and physical consequences produced when correlated ordering develops on the boundary Fermi arcs of a Weyl semimetal rather than at its bulk Weyl nodes, in the absence of an established theoretical framework for correlated order on a topological Fermi surface.

Background

The paper situates the CoSi (001) surface CDW within the broader problem of how electronic correlations interact with topological boundary states. In a Weyl semimetal, bulk Weyl nodes can be gapped by a charge-density wave, potentially producing an axion-insulating phase. The authors emphasize that an analogous instability occurring on surface Fermi arcs may generate qualitatively different physics because the arcs are open, topologically connected boundary states rather than conventional closed Fermi surfaces.

Fermi-arc nesting has been proposed as a mechanism for surface charge-density-wave order, and prior STM work on CoSi provides experimental evidence for such an instability. However, the general consequences of correlated ordering on topological boundary Fermi arcs remain unresolved; the CoSi measurements establish an experimental foothold but do not supply a general theoretical framework or complete characterization of the resulting phase.