Visualizing the Odd-Parity Superconducting Order Parameter and Its Quasiparticle Surface Band in UTe₂

A distinctive identifier of nodal intrinsic topological superconductivity (ITS) would the appearance of an Andreev bound state on crystal surfaces parallel to the nodal axis, in the form of a topological quasiparticle surface band (QSB) appearing only for T < TC. Moreover, the theory shows that specific QSB characteristics observable in tunneling to an s-wave superconductor can distinguish between chiral and non-chiral ITS order parameter Δ_k. To search for such phenomena in UTe₂, s-wave superconductive scan-tip scanning tunneling microscopy (STM) imaging was employed. It reveals an intense zero-energy Andreev conductance maximum at the UTe2 (0–11) crystal termination. The development of the zero-energy Andreev conductance peak into two finite-energy particle-hole symmetric conductance maxima as the tunnel barrier is reduced and then signifies that UTe₂ superconductivity is non-chiral. Quasiparticle interference imaging (QPI) for an ITS material should be dominated by the QSB for energies within the superconductive energy gap |E| ≤ Δ, so that bulk Δk characteristics of the ITS can only be detected excursively. Again using a superconducting scan-tip, the in-gap quasiparticle interference patterns of the QSB of UTe₂ were visualized. Specifically, a band of Bogoliubov quasiparticles appears as a characteristic sextet qi ∶ i = 1 − 6 of interference wavevectors, showing that QSB dispersions k(E) occur only for energies |E| ≤ Δ_max and only within the range of Fermi momenta projected onto the (0–11) crystal surface. In combination, these phenomena are consistent with a bulk Δk exhibiting spin-triplet, time-reversal conserving, odd-parity, a-axis nodal, B3u symmetry in UTe2.