Andreev Spectroscopic Imaging STM of the Superconducting Topological Surface State of UTe₂

Although nodal spin-triplet topological superconductivity appears probable in UTe₂, its bulk 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. Classically, all odd-parity superconductors should exhibit zero-energy surface Andreev bound states - (ABS) which are generated by the universal π-phase-shift during Andreev reflections from the odd-parity pair potential Δ(k). For typical Fermi surface geometries this ABS is also a superconductive topological surface state. Our theoretic analysis shows that specific ABS 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 to discover a powerful zero-energy ABS signature at the (0-11) crystal termination. As the tunnel barrier is reduced, the zero-energy Andreev conductance peak evolves into two finite-energy particle-hole symmetric conductance maxima, signifying that UTe₂ superconductivity is non-chiral B_3u symmetry.