Observation of \textit{p}-wave superconductivity in epitaxial Bi/Ni bilayers*

In a singlet superconductor, the two electrons within a Cooper pair have opposite spins whereas the two electrons have the same spin orientation in a triplet superconductor. Most superconductors to date are singlet superconductors with an $s$-wave (e.g., Pb, Nb) isotropic gap, except the high $T_{C}$ cuprates with a $d$-wave gap with nodes. However, $p$-wave triplet superconductivity, although theoretically expected, was rarely observed. He$^{\mathrm{3}}$ is the only triplet superfluid in nature, and Sr$_{\mathrm{2}}$RuO$_{\mathrm{4}}$ has been suspected to be a triplet superconductor. We report the observation of $p$-wave triplet superconductivity in epitaxial Bi/Ni bilayers with $T_{C}$ up to 4 K in multiple thickness ranges. Using ballistic injection of unpolarized and especially highly spin-polarized electrons with energies within the superconducting gap, Andreev reflection spectroscopy directly accesses the pairing symmetry within the superconducting gap. We obtain conclusive evidence of $p$-wave superconductivity.