Kokubo, Nobuhito*; Miyahara, Hajime*; Okayasu, Satoru; Nojima, Tsutomu*
Journal of the Physical Society of Japan, 84(4), p.043704_1 - 043704_4, 2015/04
We report on the direct observation of vortex states confined in equilateral and isosceles triangular dots of weak pinning amorphous superconducting thin films with a scanning superconducting quantum interference device microscope. The observed images illustrate not only pieces of a triangular vortex lattice as commensurate vortex states, but also incommensurate vortex states including metastable ones. We comparatively analyze vortex configurations found in different sample geometries and discuss the symmetry and stability of commensurate and incommensurate vortex configurations against deformations of the sample shape.
Kokubo, Nobuhito*; Okayasu, Satoru; Nojima, Tsutomu*; Tamochi, Hirotaka*; Shinozaki, Bunju*
Journal of the Physical Society of Japan, 83(8), p.083704_1 - 083704_5, 2014/08
Kokubo, Nobuhito*; Okayasu, Satoru; Kanda, Akinobu*; Shinozaki, Bunju*
Physical Review B, 82(1), p.014501_1 - 014501_7, 2010/07
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 of symmetry in vortex configurations, resulting in modifications of the shell filling rule and the magic number.
Kokubo, Nobuhito*; Okayasu, Satoru; Shinozaki, Bunju*
Physica C, 468(15-20), p.1316 - 1318, 2008/09
We have investigated spatial distributions of vortices induced in small superconducting disks of amorphous MoGe. Scanning SQUID microcopy allows us to observe directly magnetic distributions near the surface of the disks. Results on disks with diameter 0.05 mm demonstrate that quasi-symmetric distributions imposed by the circle geometry appear for vorticity up to 5. Meanwhile, for vorticity more than 6, vortices form an unique shell structure, in which one vortex is located in the center and others form a circle around. The critical vorticity for this structural transition is consistent with results on Nb disks imaged by the magnetic decoration experiments.
Nishio, Taichiro*; Okayasu, Satoru; Suzuki, Junichi; Kokubo, Nobuhito*; Kadowaki, Kazuo*
Physical Review B, 77(5), p.052503_1 - 052503_4, 2008/02
We have observed the magnetic field distribution of a vortex generated in amorphous ()-MoGe thin films with various thicknesses () (, where is the penetration depth) with a scanning SQUID microscope. From analyses of the field distribution as functions of the film thickness and temperature, it is found that an effective in-plane penetration depth () extends with descreasing d, in accordance with the Pearl's prediction =2/. Temperature dependence of is also consistent with the two-fluid model involving the Pearl's prediction.
Okayasu, Satoru; Kokubo, Nobuhito*; Kakeya, Itsuhiro*
Physica C, 463-465, p.294 - 296, 2007/10
A scanning SQUID microsope is a powerful tool for direct observations of vortices. Magnetic sensitivity is high enough, but spacial resolution is limited by the size of pickup coils. To improve the spacial resolution, a trial usage of microprobe on the pickupcoil was achieved. Preliminary results show the improvement of sensitivity, not spacial resolution.
Kokubo, Nobuhito*; Okayasu, Satoru; Tamochi, Hirotaka*; Shinozaki, Bunju*
no journal, ,
In a mesoscopic superconductor, characteristic configurations of quantum vortices, such as polybonal structures or shell structures, can be observed. We have been investigating the votices configurations in amorphous MoGe mesoscopic diskform superconductors using a scanning SQUID microscope. In this study, we expand our research objects to 80m 80m square-shaped superconductors. The vortices configurations appear with the different symmetries from the circular case.
Kitano, Kohei*; Okayasu, Satoru; Nojima, Tsutomu*; Sasaki, Takahiko*; Kokubo, Nobuhito*
no journal, ,
We try to observe a new vortex state including anti-vortex by introducing small bores or hollows in microscopic superconductors. We made direct observations of vortex states of microscopic square superconducting samples with 5 bores, located the center of the sample and four corners, by using a scanning SQUID microscope. A unique vortex alignment at low vorticity (L5) is observed in comparison with the previous measurements on the samples with no bore. It becomes clear that the vortex alignment is determined by the distance of bores and the interaction among the vortices.