Origin of superconductivity at nickel-bismuth interfaces

Unconventional superconductivity has been suggested to be present at the interface between bismuth and nickel in thin-film bilayers. In this work, we study the structural, magnetic, and superconducting properties of sputter deposited Bi/Ni bilayers. As-grown, our films do not display a superconducting transition; however, when stored at room temperature, after about 14 days our bilayers develop a superconducting transition up to 3.8 K. To systematically study the effect of low temperature annealing on our bilayers, we perform structural characterization with x-ray diffraction and polarized neutron reflectometry, along with magnetometry and low-temperature electrical transport measurements on samples annealed at 70∘C. We show that the onset of superconductivity in our samples is coincident with the formation of ordered NiBi3 intermetallic alloy, a known s-wave superconductor. We calculate that the annealing process has an activation energy of (0.86±0.06) eV. As a consequence, gentle heating of the bilayers will cause formation of the superconducting NiBi 3 at the Ni/Bi interface, which poses a challenge to studying any distinct properties of Bi/Ni bilayers without degrading that interface.