Probing pairing symmetry in UTe₂ using phase-sensitive measurements

Newly discovered heavy fermion superconductor UTe₂ featuring an exceedingly large upper critical field has attracted a great deal of attention. Even though it is widely believed that UTe₂ possesses an odd-parity, spin-triplet pairing symmetry, direct evidence for it is lacking, especially at zero or low magnetic fields. We prepared planar tunnel junctions of In (an s-wave superconductor) and UTe₂ and measured orientation-dependent Josephson coupling between them. We found a finite Josephson coupling in junctions prepared on (001) but not in those prepared on (1-10) surface, a selection rule in the Josephson effect. The size of the Josephson current is consistent with that of the first-order coupling. Our observations suggest that, among all point-group-symmetry-allowed pairing states, the odd-parity, spin-triplet state of B_1u is adopted in UTe₂. We also observed features in the single-particle tunneling spectra on (1-10) surface attributable to the formation of Andreev surface bound states.