Signature of Fermi arc surface states in Andreev reflection (Vol. 49 No.5-6)
Weyl semimetals are conductors which are characterized by topologically protected conducting surface states. In contrast to three-dimensional topological insulators described by Z2 invariant, Weyl surface states inherit the chiral property of the Chern insulator edge states, similarly to the quantum Hall effect regime. To observe this difference in symmetry, we experimentally investigate charge transport through the junction between a niobium superconductor and a three-dimensional WTe2 Weyl semimetal. In addition to classical Andreev reflection, we observe sharp non-periodic subgap resistance resonances. From an analysis of their positions, magnetic field and temperature dependencies, we can interpret them as an analog of Tomasch geometrical oscillations for transport along the topological surface state across the region of proximity-induced superconductivity at the Nb-WTe2 interface. The crucial point is that observation of distinct geometrical resonances implies a specific transmission direction for carriers, which is impossible for trivial two-dimensional surface states in planar junctions without strict axial symmetry. In contrast, for Weyl chiral surface states the preferable direction is present, forming a specific transmission direction for surface carriers. Thus, observation of distinct geometrical resonances is a hallmark of the Fermi arc Weyl surface states.
A. Kononov, O. O. Shvetsov, S. V. Egorov, A. V. Timonina, N.N.Kolesnikov and E. V. Deviatov, Signature of Fermi arc surface states in Andreev reflection at the WTe2 Weyl semimetal surface,