Phase-sensitive determination of the odd-parity pairing in UTe₂

UTe₂ possesses a space inversion symmetry. Consequently, its symmetry properties of the orbital- and spin-parts of the superconducting order parameter (OP) are locked - UTe₂ can be either an even-parity, spin-singlet or odd-parity, spin-triplet superconductor. Even though the remarkable observation of exceedingly large upper critical field and the intriguing reentrant behavior makes it difficult to envision a simple spin-singlet pairing state in UTe₂, direct evidence for odd-parity, spin-triplet superconductivity in UTe₂ is still lacking, calling for phase-sensitive experiments based on the orientation dependence of the Josephson coupling between an s-wave superconductor and UTe₂, which provides direct information on the pairing symmetry of an unconventional superconductor. In this talk we report our work on the phase-sensitive determination of the odd-parity pairing in UTe₂. Nonzero Josephson coupling was observed in In/UTe₂ junctions prepared on the (001) and (01-1) surfaces, while no coupling was detected on the (1-10) surface (in zero or low magnetic fields). This phenomenon known as the selection rule in the Josephson effect of unconventional superconductors suggest that, among all symmetry-allowed pairing states for UTe₂, the odd-parity, spin-triplet state of B_1u is adopted. We also performed quasiparticle tunneling measurements on (1-10) surface and obtained tunneling spectra with features indicative of the formation of Andreev surface bound states, which are expected when quasiparticles traversing the semiclassical path of Andreev reflection experiences a sign change in the superconducting OP. Ongoing and future work on quasiparticle tunneling in high magnetic fields will also be discussed. The work is done in collaboration with Camilla M. Moir, Nathan J. McKee, Eric Lee-Wong, Ryan E. Baumbach, M. Brian Maple, and Ying Liu and supported at Penn State by NSF under grant DMR 2312899.