Vortex Pinning in Niobium covered by a thin polycrystalline Gold

Abstract

Owing to its superconducting properties, Niobium (Nb) is an excellent candidate material for superconducting electronics and applications in quantum technology. Here we perform scanning tunneling microscopy and spectroscopy experiments on Nb films covered by a thin gold (Au) film. We investigate the minigap structure of the proximitized region and provide evidence for a highly transparent interface between Nb and Au, beneficial for device applications. Imaging of Abrikosov vortices in presence of a perpendicular magnetic field is reported. The data show vortex pinning by the granular structure of the polycrystalline Au film. Our results show robust and homogeneous superconducting properties of thin Nb film in the presence of a gold capping layer. The Au film not only protects the Nb from surface oxidation but also preserves its excellent superconducting properties.

Figures (3)

• Figure 1: Normalized differential tunneling conductance of the Nb film for $B = 0$. The data was normalized to the conductance at high bias ($\pm 2.5$ mV). A fit of the data to the BCS-Dynes model is shown, as well (red line), and the corresponding residual (orange line).
• Figure 2: Vortex imaging at $B = 120$ mT. (a) STM topography of the surface of the Au/Nb film. Scan parameters $U = 3$ mV, $I = 200$ pA. (b-d) Differential conductance maps of the same area as shown in (a) for a DC bias of (b) 0 mV, (c) 0.85 mV, and (d) 3 mV. (a-d) Image size $400$ nm $\times$$400$ nm. (e) Normalized differential conductance of the film as a function of distance from the vortex core. The vertical dashed lines indicate the bias values at which the data in (b) to (d) was measured. (f) Normalized zero bias differential conductance (ZBDC) as a function of distance from the vortex core. A fit of the data is also shown, from which the superconducting coherence length $\xi$ is obtained.
• Figure 3: (a) Variation of normalized differential conductance, measured away from the vortices, with magnetic field, applied perpendicularly to the sample plane. (b) Variation of normalized zero bias differential conductance (ZBDC) with error bars, measured away from the vortices with magnetic fields applied perpendicularly to the sample plane.