Abstract
The electrical resistivity and I-V characteristics of high-T c YBa c Cu 3O 7− δ films prepared by r.f. magnetron sputtering have been measured in detail. The sample deposited onyo a water-cooled substrate showed particular behaviour as follows: (i) the resistance began to decrease gradually from a very high temperature T = 155 K in a different manner from the metallic behaviour before the conventional transition; (ii) the I-V characteristics due to a superconducting weak link could be clearly observed at low temperature. Moreover, its modified characteristics continued to a higher temperature than the usual on-set temperature T c-on; (iii) the differential resistance dV/dl in the normal state derived from the I-V curves in the region of I⪢ I c (critical current) did not decrease before the conventional transition but increased gradually with decreasing temperature. In consideration of these experimental results, these resistive phenomena above T c-on are strongly suggested to be due to the very large superconducting fluctuation effect or the influence of some new effect.
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