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 has not yet been established. If spin-triplet, it should have odd parity so that 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 <i>s</i>-wave superconductor distinguish between chiral and non-chiral. To search for such phenomena in UTe₂, we employ <i>s</i>-wave superconductive scan-tip imaging 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 nodes aligned with the crystal <i>a</i>-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 <i>a</i>-axis, and splitting of Andreev conductance maximum due to <i>s</i>-wave proximity, categorizes the superconductive of a D_2h-symmetry crystal as the odd-parity non-chiral B_3u state.