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Journal Articles

Commensurate and incommensurate vortex states confined in mesoscopic triangles of weak pinning superconducting thin films

Kokubo, Nobuhito*; Miyahara, Hajime*; Okayasu, Satoru; Nojima, Tsutomu*

Journal of the Physical Society of Japan, 84(4), p.043704_1 - 043704_4, 2015/04

 Times Cited Count:4 Percentile:36.74(Physics, Multidisciplinary)

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.

Journal Articles

Direct imaging of vortex polygons and vortex shells in mesoscopic squares of a weak pinning superconducting thin film

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

 Times Cited Count:14 Percentile:67.22(Physics, Multidisciplinary)

Journal Articles

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

Kokubo, Nobuhito*; Okayasu, Satoru; Kanda, Akinobu*; Shinozaki, Bunju*

Physical Review B, 82(1), p.014501_1 - 014501_7, 2010/07

 Times Cited Count:28 Percentile:73.37(Materials Science, Multidisciplinary)

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.

Journal Articles

Shell structures of vortices in superconducting disks of an amorphous MoGe film

Kokubo, Nobuhito*; Okayasu, Satoru; Shinozaki, Bunju*

Physica C, 468(15-20), p.1316 - 1318, 2008/09

 Times Cited Count:6 Percentile:31(Physics, Applied)

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.

Journal Articles

Observation of an extended magnetic field penetration in amorphous superconducting MoGe films

Nishio, Taichiro*; Okayasu, Satoru; Suzuki, Junichi; Kokubo, Nobuhito*; Kadowaki, Kazuo*

Physical Review B, 77(5), p.052503_1 - 052503_4, 2008/02

 Times Cited Count:16 Percentile:57.51(Materials Science, Multidisciplinary)

We have observed the magnetic field distribution of a vortex generated in amorphous ($$alpha$$)-MoGe thin films with various thicknesses ($$d$$) ($$<$$$$lambda$$, where $$lambda$$ 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 ($$Lambda$$) extends with descreasing d, in accordance with the Pearl's prediction $$Lambda$$=2$$lambda$$$$^{2}$$/$$d$$. Temperature dependence of $$Lambda$$ is also consistent with the two-fluid model involving the Pearl's prediction.

Journal Articles

A Trial usage of microprobes for improvement of a scanning SQUID microscope

Okayasu, Satoru; Kokubo, Nobuhito*; Kakeya, Itsuhiro*

Physica C, 463-465, p.294 - 296, 2007/10

 Times Cited Count:0 Percentile:0(Physics, Applied)

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.

Oral presentation

Distributions of superconducting vortices confined in mesoscopic superconductors

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 80$$mu$$m $$times$$ 80$$mu$$m square-shaped superconductors. The vortices configurations appear with the different symmetries from the circular case.

Oral presentation

Observation of vortex states in small superconductors with artificial pinning centres

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 (L$$<$$5) 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.

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