Orateur
Description
We report superconductivity in sputter-deposited Ni–Bi bilayer thin films with varying thicknesses [Bi (40–80 nm)/Ni (5–10 nm)]. While as-deposited films show no superconductivity, post-deposition annealing induces the formation of the intermetallic NiBi3 phase, leading to the emergence of superconductivity. The Bi (80 nm)/Ni (10 nm) film, corresponding closely to NiBi₃ stoichiometry, exhibits the sharpest transition with a maximum critical temperature ~4.16 K. The formation of the NiBi₃ phase is confirmed via XRD and EDX, establishing a direct correlation between superconductivity and phase formation. Transport measurements using four-probe resistivity and low-frequency two-coil mutual inductance techniques reveal sharp superconducting transitions.
Scanning tunneling microscopy and spectroscopy (STM/STS) were performed to investigate the microscopic nature of the superconducting state. Tunneling conductance spectra directly probing the quasiparticle density of states reveal a V-shaped gap structure with suppressed coherence peaks. The spectra are well described by anisotropic s-wave gap. The extracted gap value at 0.35mK is ∆(meV)=0.68 +0.3 cos(2θ), giving the maximum gap value to be 0.98meV. The corresponding gap ratio 2\Delta_{\mathrm{max}}/k_BT_c\approx5.5 is significantly larger than the weak-coupling BCS value of 3.53, demonstrating strong-coupling superconductivity. The temperature dependence of the superconducting gap Δ(T), extracted from tunneling spectroscopy, clearly exhibits strong-coupling behavior. Zero-bias conductance maps under applied magnetic fields provide direct visualization of vortex states and their spatial distribution. From spatially resolved spectroscopy on the vortex core, the coherence length is estimated to be 23nm. We do not observe any signature of zero bias conductance peaks (ZBCP). Penetration depth measurements were performed in a 3He cryostat using a two-coil mutual inductance technique operating at 30 kHz. We obtain we get λ(T=0)≈515 nm for Bi 80 nm/Ni 10 nm film. The temperature variation of 1/\lambda^2 also indicates a strongly coupled anisotropic superconductor.
Our study establishes NiBi3 as an anisotropic s-wave superconductor in the extremely strong coupling limit.