Basic study of supemanducting device with asymmetric flux pinning centers

第二类超导体的临界电流密度Jc由磁通线晶格和钉扎中心之间的钉扎相互作用决定。Jc属性是由人工钉扎中心（APC）的引入。目前，人工钉扎中心已经通过拉制工艺引入到金属超导线材中以实现高Jc。另一方面，提出了一种利用微加工技术在超导薄膜中引入人工钉扎中心的新方法。本论文主要研究了非对称人工钉扎中心的引入、它们在洛仑兹力作用下的行为以及引入非对称人工钉扎中心的新型器件。数值分析表明，当外加电流极性改变时，由于钉扎中心形状的不对称所引起的钉扎势的不对称会导致磁通钉扎力和临界电流密度分布的显著不对称.为了研究Nb薄膜的非对称性，采用光刻技术在Al_2O_3（1102）单晶衬底上蒸镀了0.5 μ m厚的Nb薄膜，并在薄膜中引入了非对称人工钉扎中心（APC）。图案具有4.0-5.0 μm的周期性间隔。另外，代替临界电流密度的分布，使用磁光成像方法直接观察样品中的磁通密度的分布。通过使用磁光图像比较磁通密度最低的位置和磁通密度最高的位置之间的偏移，观察到由APC的不对称形状引起的磁通密度的不对称分布。将研究一种具有非对称临界电流特性的新器件。

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.