We have analyzed the electromagnetic properties of small and long YBa 2 Cu 3 O 7Ϫx grain-boundary Josephson junctions. The Swihart velocity increases with junction width ͑w͒ while the ratio of the relative dielectric constant to the barrier thickness (/t) decreases. We found that the product wϫ/t is approximately constant. These results have been explained in the framework of the filamentary model, where the barrier can be regarded as a disordered dielectric medium with a high density of superconducting filaments. Experiments demonstrate that a controllable variation of these parameters can be achieved by helium irradiation at 80 keV. We give examples of an enhancement of weak-link properties of junctions for doses in the range of 10 13 cm
Ϫ2. Raising the dose we can sweep the modification of the weak-link properties from an increase of the junction critical current of about 10% to a severe degradation of the coupling energy of the barrier, although the superconducting properties of the electrodes always worsen gradually.
Thin films of CuGaSe2 have been produced by rf sputtering. Compositional, structural, electrical, and optical properties are strongly influenced by growth temperature. At substrate temperatures lower than 300 °C amorphous or poorly crystalline Se-excess films are obtained, showing high resistivity (≊103 Ω cm) and optical transitions at 1.62, 1.80, and 2.4 eV (values lower than the single-crystal counterparts). At the higher growth temperatures, polycrystalline films are obtained (average grain size 0.7 μm) with lower values of resistivity (1 Ω cm), and optical transitions at 1.68, 1.90, and 2.55 eV (very close to the single-crystal values). A hopping conduction mechanism has been detected at the lower measuring temperature (T<150 K), and a grain boundary limited conduction process at the higher measurements temperature (T>150 K). Structural and compositional characteristics are used to explain the behavior observed in the electrical and optical properties.
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