大象传媒

Synopsis

Since the discovery of superconductivity in UTe₂ in 2019, this material has attracted significant attention as a strong candidate for a rare odd-parity spin-triplet superconductor with potential non-trivial topological properties. Although UTe₂ has been extensively investigated in recent years, two fundamental issues remain unresolved: the nature of its superconducting order parameter and the microscopic mechanism responsible for spin-triplet pairing. Some of the early characterization of the magnetic and electronic properties of UTe₂ were influenced by extrinsic sample effects, but recent improvements in sample growth methods have made it possible for many of the applicable experimental techniques to clearly access the intrinsic properties of this material.

This thesis presents the results of zero-field (ZF) and transverse-field (TF) muon spin rotation/relaxation (渭SR) studies of chemical-vapor transport (CVT) grown and higher-quality molten-salt flux (MSF) grown UTe₂ single crystals aimed at shedding light on the two outstanding issues mentioned above. Our ZF-渭SR measurements in the bulk of MSF-grown UTe₂ suggest that the superconducting order parameter preserves time-reversal symmetry (TRS) and hence has a single-component which limits the pairing symmetry to four possibilities. Our TF-渭SR measurements of the muon (渭⁺) Knight shift in the normal state of CVT-grown and MSF-grown samples detect the formation of a heavy-Fermi liquid state due to the development of Kondo lattice coherence but also reveal an unexpected gradual relocalization of U 5f moments prior to the onset of superconductivity. This finding suggests that the Kondo effect in UTe₂ is orbital selective, which may significantly impact the electronic structure and superconducting pairing.

Keywords: UTe₂; Muon spin rotation/relaxation; Muon Knight shift; Time-reversal symmetry; Unconventional superconductivity