Rectification of vortex motion in Superconducting Nb thin Films with spacing-graded array of pinning sites

• Main Authors: Tian-Chiuan Wu, 吳添全
• Other Authors: Tzong-Jer Yang
• Format: Others
• Language: en_US
• Published: 2007
• Online Access: http://ndltd.ncl.edu.tw/handle/14826365750359214194

博士 === 國立交通大學 === 電子物理系所 === 95 === The asymmetric potentials can provide ways to control the motion of particles in devices. The dc rectification of an ac-driven particle is known as the ratchet effect. For superconductor, we can take advantage of anisotropic pinning in the pinning sites to control the vortex motion. The rectified vortex motion in superconductor is induced by an ac applied force in an asymmetric pinning potential. In this thesis asymmetrically modulated spatial distribution of symmetric pinning sites were studied. The pinning centers are prepared by electron beam lithography with spacing-graded arrays of submicrometer-scaled holes in Nb superconducting thin films. Two different gradient of the hole have been fabricated. The lattice-constant variation results in the change of the pinning site’s density. The gradient structure breaks the symmetry of the vortex pinning potential. For small gradient, at least four matching fields are found in Magnetoresistance (MR) curves. For large gradient, it is found that the commensuration effects were eliminated by introducing graded concentration of hole distributions. MR curves show a kink around the first matching field and no higher-order matching field is observed. The transport properties are carried out using an ac current through the Nb superconducting film along the x-axis with magnetic field perpendicular to the film plane. The dc voltage drop Vdc is recorded along the x-axis by a dc nanovoltmeter. The measurements revealed pronounced rectified voltage which is mainly characterized by vortex-vortex interaction. The asymmetric pinning potentials are crucial to produce the ratchet behavior. Vortex-vortex interaction changes the effective pinning landscape of vortices and asymmetric potential is formed. Vortices depin easily from high concentration to low concentration of pinning sites. For small gradient sample, the rectified voltage is larger at matching fields than that at intermediate fields. Another remarkable situation is that vortices start to repin near the matching fields. Thus the rectified voltage varies periodically with the number of vortex per pinning center. For large gradient sample, the effects of the vortex-vortex interaction are enhanced. The rectified voltage can be found in every selected magnetic fields and the rectified voltage is about 10 times larger than the small gradient sample. In addition, a drastic change of the rectified voltages appear for magnetic field above/below the first matching field. The interstitial vortices are formed in the film above the first matching field. A reversible vortex motion is induced by the interstitial vortices for the field above the first matching field. These results demonstrated that the rectification can be characterized by the spacing-graded of pinning sites.