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作者 Dinner, Rafael Baruch
書名 Scanning Hall probe microscopy of supercurrents in YBCO films
國際標準書號 9780542983306
book jacket
說明 150 p
附註 Source: Dissertation Abstracts International, Volume: 67-11, Section: B, page: 6458
Adviser: M. R. Beasley
Thesis (Ph.D.)--Stanford University, 2006
High-temperature superconductors were discovered 20 years ago, inspiring dreams of levitating trains fed by superconducting power lines. The cuprates, particularly YBa2Cu3O7-delta (YBCO), still promise to fulfill such applications, but must be made to carry higher current density, Jc, which is limited by the rapid onset of dissipation. The dissipation arises from the movement of magnetic vortices in the material, driven by the magnetic field of the current. It is therefore natural to use magnetic imaging to understand these limits on the current
Initially, I fix a mesoscopic ring of YBCO to a micro-Hall sensor and demonstrate that the sensor is capable of detecting small numbers of vortices. I then proceed with magnetic imaging, constructing a cryogenic scanning Hall probe microscope that combines a 1 x 4 cm scan range with 200 nm positioning resolution by coupling stepper motors to high-resolution drivers and reducing gears. It enables me to image an entire sample, then zoom in on regions of interest, down to the level of an individual quantized vortex
Applying this capability to current-carrying YBCO strips, I generate magnetic movies of the materials' periodic response to applied ac currents. From the movies, I reconstruct current density by inverting the Biot-Savart law, and electric field by approximating dB/dt and using Faraday's law. I thereby obtain complete, space- and time-resolved characterizations of the materials, including maps of ac power losses
After demonstrating this analysis on a single-crystal film, I image two "coated conductors"---YBCO grown on metal tape. I find relatively homogeneous flux penetration in a film grown by pulsed laser deposition (PLD) on an ion beam assisted deposition (IBAD) substrate, which contrasts with the weak-link behavior of grain boundaries in a film grown by metalorganic deposition (MOD) on rolling assisted biaxially textured substrate (RABiTS). Nonetheless, the in-plane meandering of the MOD film's boundaries allows them to sustain high currents. This observation leads me to image individual boundaries with various in-plane tilts, where I conclude that control of grain boundary orientation offers a route to higher critical current density
School code: 0212
Host Item Dissertation Abstracts International 67-11B
主題 Physics, Electricity and Magnetism
Physics, Condensed Matter
Alt Author Stanford University
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