Superconducting pairing symmetry in UTe₂ probed by phase-sensitive experiment

Unconventional superconductivity in strongly correlated systems marked by non-s-wave pairing has been an important problem to explore in condensed matter physics for many years. The newly discovered heavy fermion superconductor UTe₂ is believed to be a non-s-wave, spin-triplet superconductor based primarily on the observation of the large upper critical field which greatly exceeds the Pauli limit, the reentrance of superconductivity at high magnetic field, and a small drop of Knight shift across the superconducting transition temperature. However, direct evidence for UTe₂ to be a spin-triplet superconductor is lacking, at least in low applied magnetic fields. Josephson effect-based phase-sensitive experiments are a powerful tool to probe the symmetry of the orbital part of the order parameter. We report our experiments on In-UTe₂ Josephson junctions prepared on differently oriented crystal surfaces. We observed crystal orientation-dependent Josephson coupling, which provides a good starting point for understanding the pairing symmetry of the order parameter in UTe₂.