Research on novel vortex states in mesoscopic superconductors

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For superconductors with sizes comparable to the superconducting coherence length and/or the magnetic penetration depth, it has been theoretically predicted for a long time that the response to magnetic field is significantly different from that of bulk superconductors, leading to formation of novel vortex states such as giant vortex states (GVSs) and multivortex states (MVSs): While in bulk superconductors vortices form Abrikosov triangular lattice, in mesoscopic thin films, vortex arrangement reflects the sample shape, or multiples vortices combine into one, forming a giant vortex. In 2004, we experimentally verified the existence of these vortex states for the first time by using our original technique, multiple-small-tunnel-junction (MSTJ) method. In the present research started in 2005, we have obtained the following results: (1) We found a general method for distinction between GVSs and MVSs using the temperature dependence of vortex expulsion fields, (2) We experimentally confir … More med the basic properties of novel vortex states such as sample-shape and temperature dependence of vortex states, relationship between stability of vortices and sample geometry and effect of defects, (3) We verified the existence of one-dimensional (1D) vortex in mesoscopic rings, which has been predicted for more than 20 years but never confirmed, and showed that the nonuniform cross section is crucial for the stabilization of the 1D vortex, (4) We found a new effect of superconducting fluctuation which depends on the applied flux in rings, (5) We searched for the theoretically-predicted antivortices, and determined the parameter range for the existence of antivortices, (6) We developed a method to determine the position of vortices in a superconductor with 2x2 nanoholes, and (7) We developed a method to control the vortex states by external parameters. We made clear the background physics of these phenomena by the close collaboration with theoretical groups of Akita Univ. and Univ. Antwerp. Our successful results not only give a better understanding of the physics of quantum systems confined in small area, but also open the way for the future device application based on nanoscale superconductivity engineering. Less