Exploration of vortex formation in niobium thin-film by using scanning SQUID microscope

• Main Authors: Fan, Sung Han, 范淞涵
• Other Authors: Chen, Jeng Chung
• Format: Others
• Language: zh-TW
• Published: 2016
• Online Access: http://ndltd.ncl.edu.tw/handle/kgggc4

碩士 === 國立清華大學 === 物理系 === 105 === This thesis aims to investigate the configurations of equilibrium vortices formed in superconducting niobium (Nb)-thin film wire after high current drive. The formation and distribution of vortices in superconducting thin film have strong implications not only for elucidating its breakdown mechanism as the external current applied exceeds the critical current density, but also for understanding its limit in practical applications. The vortex morphology of thin-film wire is studied with the variables of external magnetic fields, currents and temperatures and is observed by using a home-made Scanning Superconducting Quantum Interference Device microscope (SSQM). The content of this thesis is comprised by two parts. In the first part, we will describe our SSQM system. To enhance the capability of SSQM and to simplify its operation, we improve our original design. The primary improvements include the integration of the scanning scheme used in our custom-made scanning Hall Probe microscope, new cantilever design, and the low temperature tank-circuit design. We leave extensive technical details in the Appendixes. The main results and discussions are presented in the second part. The sample made by Nb thin-film is prepared by DC sputtering and is patterned into a zigzag geometry. In particular, we employ in-suite thin-film wires to introduce magnetic field to locally manipulate the vortex. We adopt a current heating procedure to transiently drive the superconducting wire into the normal state. We observe that the vortices tend to form in the disordered regime. These vortices are in general movable, and can be depopulated in the central area and tend to accumulated near the pattern boarders, by using alternating magnetic field and sending high currents. Our results suggest the importance of thin-film quality and the geometry for the applications of superconducting thin-film wires