Pair Wavefunction Symmetry in UTe₂ from Zero-Energy Surface State Visualization

Although nodal spin-triplet 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, due to the presence of a topological surface band (TSB). Moreover, theory 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 -1 1) crystal termination. Its imaging yields quasiparticle scattering interference signatures of 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 ABS, internodal scattering along the a-axis, and splitting of Andreev conductance maximum due to s-wave proximity, categorizes the superconductive Δ_k of a D_2h-symmetry crystal as the odd-parity non-chiral B_3u state.