Singlet superconductivity in single-crystal NiBi₃ superconductor

Verification of triplet superconductors (SCs), especially those that can carries spin angular momentum, is essential for understanding fundamental mechanism of unconventional superconductivity as well as important applications in quantum computing and spintronics. But singlet and triplet SCs have similar macroscopic properties, an ideal experiment would be to directly measure the spin states of Cooper pairs. In this work, Andreev reflection spectroscopy with unpolarized and highly spin-polarized currents has been utilized to study an intermetallic single-crystal SC NiBi₃. Magnetoresistance at zero bias voltage of point contacts shows the occurrence and suppression of Andreev reflection by unpolarized and polarized current, respectively. The gap value, its symmetry and temperature dependence have been determined using an unpolarized current. The spin state in the NiBi₃ sample is determined to be antiparallel using a highly spin-polarized current. The gap value 2D/k_BT, gap symmetry and its temperature dependence, combined with the antiparallel spin state show that the bulk NiBi₃ is a singlet s-wave superconductor.