Nonequilibrium transport was studied in superconducting Zn nanowires, connected with wide Zn electrodes. The wire exhibited drastically different behaviors when the electrodes were tuned from the superconducting state to the normal state, by applying magnetic fields. In the regime in which electrodes were superconducting, a suprising enhancement of superconductivity was observed upon the application of a small magnetic field. This enhancement was exhibited as an increase of the current or temperature at which the wire left its zero resistance state. Further experiments were carried out to study the dependence of the effect on various parameters such as magnetic field orientation, wire length and wire width. The results revealed that this enhancement is a result of nonequilibrium effect involving the boundary electrodes. In addition, it is more appropriate to treat it as a recovery of superconductivity, which was suppressed by the applied current. In the regime in which elecrodes were normal, we observed a nonzero residual resistance associated with the proxmity effect between the electrodes and the wire. Comparing the temperature and current dependence of this residual resistance revealed the breakdown of superconductivity at currents well below the depairing current. Further analysis suggests the breakdown of superconductivity in this situation does not originate from the usual supercurrent-depairing mechanism. Instead, it may be associated with the nonequilibrium distribution of quasiparticles. The breakdown is also charaterized by a critical voltage rather than a critical current.
University of Minnesota Ph.D. dissertation. December 2009. Major: Physics. Advisor: Allen M. Goldman. 1 computer file (PDF); xiv, 116 pages. Ill. (some col.)
Magnetic field tuned nonequilibrium transport in quasi-one dimensional Zn nanowires.
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