Direct observation of condensate and vortex confinement in nanostructured superconductors

Abstract

In this work we report a scanning tunneling microscopy investigation of lithographically defined superconducting nanosquares. The obtained spectroscopic maps reveal the spatial evolution of both the superconducting condensate and the screening currents as a function of the applied magnetic field. The symmetry of the nanostructure is imposed on the condensate and it controls the distribution of the vortices inside the nanosquare. Our local study allows exploring the impact of small structural defects, omnipresent in these kind of structures, on both the supercurrent and vortex distribution. As a result, direct experimental evidence of vortex pinning and current crowding at the nanoscale has been obtained. DOI: 10.1103/PhysRevB.93.054514 Confinement effects play an important role in different physical phenomena especially in quantum systems like Bose-Einstein condensates, superconductors, and superfluids. For example, the ability to structure superconducting devices at length scales comparable to the characteristic sizes (penetration dept λ and coherence length ξ ) of the condensate revealed a vast world of possibilities to explore quantum phenomena (e.g., creation of artificial atoms [1], induction of quantum phase slip lines [2], confinement effects [3], to name a few). In the latter example, theoretical modeling of these systems solving the Ginzburg-Landau (G-L) [4,5] or Bogoliubov-deGennes (B-dG) [6] equations for single and/or multiband mesoscopic superconductors has been done. All these simulations unveil the importance of confinement in mesoscopic superconducting systems. For example, the symmetry of the nanostructure will compete with the vortex-vortex interaction resulting in different vortex configurations compared to the triangular Abrikosov lattice, found in bulk superconductors [3][4][5][6].These intriguing effects were experimentally investigated, using low temperature transport measurements, by probing the influence of nanostructuring on the superconductor/normal phase boundary [3]. Although these measurements proved the importance of size and shape they do not give sufficient local information about the spatial distribution of the superconducting condensate. Moreover, a different approach is needed to explore the condensate in the nondissipative (zero voltage) state. In order to tackle these issues a second set of experiments probes the magnetic field profiles, generated by the superconducting currents, by using magnetic field sensitive probes (e.g., Hall probe [7,8], scanning Hall probe microscopy [9], Bitter decoration [10], and scanning SQUID microscopy [11]). These techniques indeed visualized the symmetry-induced vortex configurations in superconducting nanostructures within the low confinement regime (i.e., nanostructure size ∼λ ξ ). 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 gov...

Pure odd-frequency superconductivity at the cores of proximity vortices

“…These bound states yield a peak in the LDOS of a vortex core at the Fermi level [33] that was first observed in Ref. 35 and then followed up by numerous theoretical and experimental works [36][37][38][39][40][41][42][43][44]. These low-energy states reflect relevant details of the bulk gap structure of the superconducting state [45].…”

Electronic structure of a mesoscopic superconducting disk: Quasiparticle tunneling between the giant vortex core and the disk edge

“…The transitions between different vortex states while sweeping the magnetic field up, are visualized by abrupt changes (or jumps) in ZBC [19,21,22], which are determined by the LDOS N (ε, r, φ) at the Fermi level ε = 0. Let us consider a certain point (T = T s , H = H s ) at the phase diagram Fig.…”

Stability of degenerate vortex states and multi-quanta confinement effects in a nanostructured superconductor with Kagome lattice of elongated antidots

“…can form vortex-antivortex configurations [3][4][5][6], vortex fusion and giant vortex states [9][10][11][12][13][14][15][16][17], which strongly depend on the shape and the size of the samples [18,19] (i.e., shape and finite-size effects). Moreover, the theory and experiments revealed that the interactions between vortices and pinning sites should also be considered [20][21][22][23][24][25]. For example, the vortex clusters and the giant vortices can be observed in the presence of the strong disorder [21] and the void structures [23].…”

“…For example, the vortex clusters and the giant vortices can be observed in the presence of the strong disorder [21] and the void structures [23]. In addition, very recent experiments have indicated that weak pinning has a clear impact on the vortex distributions in nanostructured superconductors [22,24,25]. The confinement of vortices, studied in these literatures, results from the screening current flowing along the sample edges [22], which is a continuous function with magnetic field.…”