COLLOQUIUM 2024

Abstract

The interplay of superconductivity and topology stands at the frontier of moden quantum matter physics. Intrinsic topological superconductivity is important because its electronic properties are topologically protected and robust against local perturbations. Although topological superconductivity appears probable in UTe₂, its superconductive order parameter Δ_k has not yet been established. If spin-triplet, it should have odd parity so that Δ_-k=-Δ_k and, in addition, may break time-reversal symmetry. A distinctive identifier of such nodal spin-triplet superconductors is the appearance of an Andreev bound state (ABS) on surfaces parallel to a nodal axis, in the form of a topological surface band (TSB). Moreover, theory shows that specific TSB characteristics observable in tunneling to an s-wave superconductor distinguish between chiral and non-chiral Δ_k. To search for such phenomena in UTe₂, we employ s-wave superconductive scan-tip imaging [1] and discover a distinct TSB signature, an intense zero-energy Andreev conductance maximum at the (0-11) crystal termination. Its imaging yields quasiparticle scattering interference evidence for two Δ_k nodes aligned with the crystal a-axis. Most critically, development of the zero-energy Andreev conductance peak into two finite-energy particle-hole symmetric conductance maxima as the tunnel barrier is reduced, signifies that UTe₂ superconductivity is non-chiral. Overall, this combination of a zero-energy Andreev conductance maximum at the UTe₂ (0-11) surface, internodal scattering along the a-axis, and splitting of Andreev conductance maximum due to s-wave proximity, categorizes the superconductive Δ_k of UTe₂ as the odd-parity non-chiral topological state [2].

[1] Nature 618, 921 (2023).
[2] Gu, Wang et al., Science in Review (2024).