Frequent Josephson junction decoupling is the main origin of ac losses in the superconducting state

Abstract

The origins of ac losses in the high Tc superconductors are not addressed adequately in the literature. We found out that frequent Josephson junction (JJ) decoupling (both intergranular and interlayer) due to the flow of ac is one of the main origins of the ac losses in high Tc superconductors. We have determined the ac losses in superconductors in the rf range by measuring the absolute value of nonresonant rf power absorbed by the samples. Our data show that under certain conditions when both the number density of JJs present in the sample and the JJ critical current cross a threshold value, ac losses in the superconducting state keep on increasing with decreasing temperature below Tc. The underlying mechanism is an interesting interplay of JJ coupling energy and the amplitude of rf voltage applied to the sample. The effects of an applied magnetic field, variation of rf, and temperature were studied in detail. To find out the exact relation among the JJ coupling energy, JJ number density, applied ac frequency, the amplitude of ac, and the ac losses in superconductors, we have studied samples with different crystalline properties, different grain sizes, pressurized at different pressures, and sintered under different physical and chemical conditions. These results have important implications for the understanding of the origin of ac losses and the characterization of superconducting samples. In this paper we also extend the capability of the ac loss studies in superconductors for the characterization of materials for device applications.