Scanning tunneling microscope study of the unconventional superconducting state in UTe₂

Superconductivity in UTe₂ (T_c = 1.6 - 2 K) with extremely high upper critical field, far exceeding the Pauli limit, as well as its re-entrant behavior at even higher magnetic field is a subject of great recent interest. It has been proposed that the superconducting state in UTe₂ has a triplet chiral p-wave pairing symmetry with a non-unitary order parameter. Here we use a dilution refrigerator scanning tunneling microscope (STM) to measure the spatial variation of the superconducting gap on both (001) and (011) surfaces of UTe₂ cleaved in ultra-high vacuum. We observe in-gap states on step edges and defects, similar to those reported previously [1], which implies that the pairing symmetry in UTe₂ is likely not s-wave. We also probe the response of superconducting state to adatoms defects deposited on the UTe₂ surface resolving Yu-Shiba-Rusinov (YSR) states near individual impurities. Similarly to the in-gap states near step edges, the YSR states show abnormally asymmetric spatial distribution, which may shed new light to the unconventional pairing symmetry in this system. Finally, we have explored imaging the local density of states in a magnetic field to the UTe₂ sample to probe its vortex lattice.

[1] Jiao, Lin, et al. Nature 579.7800 (2020): 523-527.