Abstract
The transport characteristics of YyPr1−yBa2Cu3O7−x thin film systems were systematically investigated as a function of the Y concentration and film thickness. The films in the superconducting state changed their transport characteristics to those of the nonsuperconducting state when the film thickness was decreased beyond the value at which the film sheet resistance is equal to the quantized sheet resistance h/4e2. This phenomenon is a kind of superconductor-insulator transition. The resistance-temperature characteristics in the insulator phase are a result of the variable-range-hopping conduction between localized states. The electric-field effect was measured especially in the insulator phase. The resistance modulation ratio ΔR/R in the insulator phase was much greater than the carrier density modulation ratio ΔN/N. The ratio of ΔR/R to ΔN/N increased as the Y concentration increased. In particular, the (ΔR/R)/(ΔN/N) value reached as high as 50 for a Y0.9Pr0.1Ba2Cu3O7−x film in the insulator phase. Higher (ΔR/R)/(ΔN/N) values effectively enhance the performance of superconducting field effect transistors.
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