Basic study of supemanducting device with asymmetric flux pinning centers

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The critical current density Jc of type II superconductor is determined by the pinning interaction between flux line lattices and pinning centers. Jc properties are governed by the introduction of artificial pinning centers (APC). Currently, artificial pinning centers have been introduced into metallic superconducting wires through the drawing process to achieve high Jc. On the other hand, a new method using microfabrication to introduce artificial pinning centers into a superconducting film was proposed. In this study, the introduction of asymmetric artificial pinning centers, their behavior under Lorentz force and a new device introduced asymmetric artificial pinning centers were investigated. Using numerical analysis, it was shown that the asymmetric pinning potentials caused by the asymmetric shape of the pinning centers could induce significant asymmetries in the magnetic flux pinning force and the distribution of the critical current density when the polarity of the applied current was changed. In order to investigate the asymmetric properties, step-shaped patterns were introduced as asymmetric artificial pinning centers (APCs) in a 0.5-μm-thick Nb film evaporated on an Al_2O_3 (1102) single crystalline substrate by using a lithographic technique. The patterns had periodic intervals of 4.0-5.0 μm. In addition, instead of the distribution of the critical current density, the distribution of the magnetic flux density in the specimen was directly observed using the magneto-optical imaging method. The asymmetric distribution of the magnetic flux density caused by the asymmetric shape of the APCs was observed by comparing the shift between the position where the magnetic flux density is the lowest and the position where it is the highest using the magneto-optical images. A new device with asymmetric critical current properties would be studied.