On the Origin of Superconductivity at Nickel-Bismuth Interfaces

TL;DR

This study investigates the emergence of superconductivity at Ni/Bi interfaces, revealing that annealing induces NiBi3 formation, which is responsible for superconductivity, complicating the analysis of intrinsic interface properties.

Contribution

It demonstrates that superconductivity in Bi/Ni bilayers arises from NiBi3 formation during annealing, highlighting the importance of interface stability in such systems.

Findings

Superconductivity appears after about 14 days at room temperature.

Annealing at 70°C promotes NiBi3 formation, leading to superconductivity.

The activation energy for NiBi3 formation is approximately 0.86 eV.

Abstract

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 $70\,^\circ$C. We show that the onset of superconductivity in our samples is coincident with the formation of ordered NiBi$_3$ intermetallic alloy, a known $s$-wave superconductor. We calculate that the annealing process has an activation energy of $(0.86\pm 0.06)$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.