TOPOLOGICAL PROTECTION OF WEYL FERMIONS VISUALIZED ON THE ATOMIC SCALE*

Topological electronic materials host exotic boundary modes, that cannot be realized as standalone states, but only at the boundaries of a topologically classified bulk. Topological Weyl semimetals, whose bulk electrons exhibit chiral Weyl-like dispersion, host Fermi-arc states on their surfaces. The Fermi-arc surface bands disperse along open momentum contours terminating at the surface projections of bulk Weyl nodes with opposite chirality. Such reduction of the surface degrees of freedom by their segregation to opposite surfaces of the sample, that reoccurs in all topological states of matter and even exhibited by topological defects [1], provides topological protection from their surface elimination. We have confirmed the Weyl topological classification of both the inversion symmetry broken compound TaAs [2] and the time reversal symmetry broken Co3Sn2S2 [3] by spectroscopic visualization of their Fermi-arc surface states through the interference patterns those electrons embed in the local density of states. This has allowed us to examine their unique nature and level of protection against perturbations. In TaAs the Fermi arc bands are found to be much less affected by the surface potential compared to trivial bands that also exist on its surfaces. In contrast, in Co3Sn2S2 the dispersion of the topological Fermi-arc bands, and even their inter-Weyl node connectivity, are found to vary with the surface termination. A possible resolution of this discrepancy will be discussed.

[1] Abhay Kumar Nayak et al, “Resolving the Topological Classification of Bismuth with Topological Defects” Science Advances 5, eaax6996 (2019)
[2] Rajib Batabyal et al, “Visualizing weakly bound surface Fermi arcs and their correspondence to bulk Weyl fermions” Science Advances 2, e1600709 (2016)
[3] Noam Morali et al, “Fermi-arc diversity on surface terminations of the magnetic Weyl semimetal Co3Sn2S2”Science 365, 1286 (2019)