Abstract:
Topological superconductors (SCs) with spin-triplet pairing and spin-orbit coupling provide an ideal platform for exploring unconventional superconductivity and spin current phenomena. However, spin-triplet SCs are rare and extremely hard to identify. The pairing symmetry of the SCs is dictated by parity, with centrosymmetric SCs hosting either a spin-singlet or a spin-triplet pairing, while noncentrosymmetric SCs exhibit singlet/triplet admixtures. In this talk, we demonstrate two key methods, kink-point upper critical field and half-quantum flux, to distinguish spin-triplet and spin-singlet pairing in β-Bi₂Pd and BiPd. Notably, BiPd exists in both centrosymmetric γ-BiPd and noncentrosymmetric a-BiPd, allowing us to investigate how crystal symmetry influences pairing states. This marks the first time both methods have been used to identify pairing symmetries in materials with identical compositions but different structures. Beyond superconductivity, the exotic band structure and heavy element composition of β-Bi₂Pd contribute significantly to the spin Hall effect, enabling efficient spin-orbit torque generation. We explore various spin current phenomena in β-Bi₂Pd. Using harmonic Hall and spin Hall magnetoresistance measurements, we reveal substantial, reliable, and robust charge-to-spin conversion in β-Bi₂Pd. Notably, both superconductivity and the spin current phenomena strongly depend on the crystallinity of β-Bi₂Pd, highlighting the critical role of crystal integrity in shaping its topological states. Furthermore, these superior spin current phenomena can be realized in energy-efficient spin-orbit torque switching of ferromagnetic layers with perpendicular anisotropy in new β-Bi₂Pd-based spintronic devices. These findings establish β-Bi₂Pd as a promising platform for spin current transport and topological superconductivity, with potential applications in spintronics and quantum technologies.