Scanning SQUID microscope study of vortex polygons and shells in weak-pinning disks of an amorphous superconducting film

Abstract

Direct observation of vortices by the scanning SQUID microscopy was made on large mesoscopic disks of an amorphous MoGe thin film. Owing to the weak pinning nature of the amorphous film, vortices are able to form geometry induced, (quasi-)symmetric configurations of polygons and concentric shells in the large disks. Systematic measurements made on selected disks allow us to trace not only how the vortex pattern evolves with magnetic field, but also how the vortex polygons change in size and rotate with respect to the disk center. The results are in good agreement with theoretical considerations for mesoscopic disks with sufficiently large diameter. A series of vortex images obtained in a disk with a pinning site reveals a unique line symmetry in vortex configurations, resulting in modifications of the shell filling rule and the magic number.

Creating nanostructured superconductors on demand by local current annealing

“…5, we observed a one dimensional vortex distribution on an annulus, and found that the vortex spacing (a) did not follow the Abrikosov lattice derived from the GL equation, 2 √3 ⁄ / for a close-packed hexagonal array; this equation leads to sparse vortices at low fields (H < 0.7 T) and dense vortices at high fields (H > 0.7 T). In previous measurements of mesoscopic superconducting disks [4,16,45], the first vortex is typically observed in the center of the disk in contrast to the results in Fig. 5.…”

Direct observation of condensate and vortex confinement in nanostructured superconductors

“…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 ).…”

Vortex shells in mesoscopic triangles of amorphous superconducting thin films

“…In this study, we extend the previous work not only to study vortex states in equilateral triangle dots for L more than 11 (up to 15), but also to understand whether vortex arrangements can be interpreted in terms of vortex shells, as seen in circle and square dots [7,12].…”

“…The sample stage has three stepper motors which allow us to approach and scan the samples in scales of µm with respect to the magnetic sensor. The vortex images presented in this study were taken on equilibrium vortex states prepared by the field-cool procedure, in which the magnetic field was applied in some temperatures (∼ 15 K) above T c , followed by cooling the samples in the magnetic field to temperatures of 3-4 K (< T c ) for imaging [7]. 3,12).…”

“…For large vorticities, a piece(s) of the triangular vortex lattice remains near the disk center because of densely packed vortices [3,5], whereas for small vorticities, vortices are arranged themselves into regular polygons situated on concentric circular rings from the disk center [1,2,4]. This is called "vortex shells", of which configurations have been visualized by some experimental studies on mesoscopic superconducting disks of Nb and amorphuous MoGe thin films [6,7].…”