Singlet superconductivity in a single-crystal NiBi₃ superconductor

Andreev reflection spectroscopy with unpolarized and highly spin-polarized currents has been utilized to study an intermetallic single-crystal superconductor 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 2Δ/k _B T, gap symmetry and its temperature dependence, combined with the antiparallel spin state show that the bulk NiBi₃ is a singlet s-wave superconductor.