Frequent JJ decoupling is the main origin of AC losses in the superconducting state

摘要

The origins of AC losses in the high Tc superconductors are not addressedadequately in literature. We found out, frequent Josephson Junction (JJ)decoupling (both intergranular and the interlayer) due to the flow of ACcurrent 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 bymeasuring the absolute value of non-resonant rf power absorbed by the samples.Our data shows that under certain conditions when both the number density ofJJs present in the sample and the JJ critical current cross a threshold value,AC losses in the superconducting state keeps on increasing with decreasingtemperature below Tc. The underlying mechanism is an interesting interplay ofJJ coupling energy and the amplitude of rf voltage applied to the sample. Theeffect of an applied magnetic field, variation of rf frequency and temperaturewere studied in detail. To find out the exact relation between the JJ couplingenergy, JJ number density, applied AC frequency, the amplitude of AC currentand the AC losses in superconductors, we have studied samples of differentcrystalline properties, different grain sizes, pressurized with differentpressure and sintered at different physical and chemical situations. Theimplementations of these results are discussed. These results have importantimplications for the understanding of the origin of AC losses andcharacterization of superconducting samples. In this paper we also extend thecapability of the AC losses studies in superconductors for the characterizationof materials for device applications.