Ultimate Vortex Confinement Studied by Scanning Tunneling Spectroscopy

Abstract

We report a detailed scanning tunneling microscopy study of a superconductor in a strong vortex confinement regime. This is achieved in a thin nanoisland of Pb having a size d about 3 times the coherence length, and a thickness h such that h<<d<<lambda, where lambda is the London penetration depth. In this geometry the magnetic field evolution of local tunneling spectra reveals only two superconducting configurations to exist: zero and single vorticity. The normal state is reached at mu(0)H(c)=0.46 T, about 6 times the critical field of bulk Pb, with no higher order vorticity observed. Basing on our STS data, we discuss the role of local supercurrents in both configurations.

Direct observation of condensate and vortex confinement in nanostructured superconductors

“…Figure 1 Therefore, the 5 nm Au layer protects the sample against oxidation, but also allows us to explore the superconducting gap underneath the Au layer [20]. From these data we also obtained the zero bias conductance (ZBC) giving a direct measure for the strength of the superconducting condensate at the tip position [12][13][14]. Figure 1(d) presents the distribution of the ZBC across the nanostructure at a temperature of 0.5 K and zero applied magnetic field.…”

“…This can be tackled using scanning tunneling microscopy (STM), as shown recently on very small in situ grown superconducting Pb islands [12][13][14][15]. These experiments indicate the high spatial resolution of STM to visualize both the superconducting condensate and the supercurrents.…”

“…The observed configurations are the results of the imposed boundary conditions and the repulsive magnetic interactions between vortices. In the strong confinement regime (i.e., nanostructure size ∼ξ λ), the distribution of the superconducting condensate is governed by the kinetic energy of the supercurrent distribution and magnetic interactions can be neglected [12]. Since most superconducting nanodevices work in this regime, it is of utmost importance to explore the strong confinement region with the appropriate, high resolution, tools.…”

“…2(b). The transitions between different vortex states are visualized by a change in the ZBC conductance while sweeping the magnetic field up [12,13,23]. In order to reduce the noise level, an average over 16 CITS curves is taken, generating the curve shown in Fig.…”

Temperature dependence of the superconducting proximity effect quantified by scanning tunneling spectroscopy

“…17 From this analysis we extrapolate an energy gap at 0 K of ∆ island (0) = 1.24 ± 0.03 meV and a critical temperature of T C = 6.4 ± 0.1 K. These values are in good agreement with previous studies on superconducting Pb islands. 7,8,18,20,21 To study the temperature dependence of the superconducting proximity effect, we measure differential conductance spectra as a function of position from the Pb island into the WL for different temperatures from 0.38 to 6 K. Spectra taken with the superconducting Nb-tip show [ Fig. 1(h)] a wider gap and two sharp and intense peaks at bias V peak = ±2.6 meV.…”