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Temperature dependence studies of persistent currents in superconductors Knittel, Anton

Abstract

A study has been made of some energy and momentum properties associated with the electrons of superconducting indium. In the experiments, an electric current was induced in the indium sample and measurements were made to detect any effects of an increasing super-state electron density on the current. By lowering the sample's temperature appropriately, the electron density could be controlled. The indium sample studied was in the form of a thin film, constructed by vacuum evaporation onto a glass substrate. Two indium wires connected the ends of the thin film to a copper wafer, forming a complete electric circuit. This circuit was electrically isolated and measurements of the current through the thin film were made with a search-coil coupled to the current's magnetic field. A relationship between the current changes observed and some momentum properties of superstate electrons was then established. The assumption that all "virtual pairs" in a superconductor have a common momentum proved consistent with the experimental results. Theoretical calculations are given which suggest that only those normal electrons with a preferred momentum can take part in increasing the superconductive electron density. This implies that a superstate electron system may effect the transformation of internal energy into work. However, the experiments carried out have not been sufficiently sensitive to show this conclusively. The inference stated above is based on the experimental results in conjunction with the required agreement of theoretical calculations with accepted theory. The indium-copper junction resistance had a resistance value which was markedly temperature dependent in the region below 3.4°K (the transition temperature of pure indium). A variation of resistance between the extremes 9.4 x 10⁻¹º Ω at 3.1°K and 8 x 10⁻¹º Ω at 1.2°K was found. The cause of this large resistance change is ascribed to various effects but probably the most important one is the diffusion of copper impurity into the indium. Arguments in favor of this explanation are given. A number of suggestions are also included which may be helpful to the design of experiments similar to the one reported in this thesis.

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