Frequent JJ decoupling is the main origin of AC losses in the   superconducting state
  The origins of AC losses in the high Tc superconductors are not addressed adequately in literature. We found out, frequent Josephson Junction (JJ) decoupling (both intergranular and the interlayer) due to the flow of AC current 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 non-resonant rf power absorbed by the samples. Our data shows 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 keeps 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 effect of an applied magnetic field, variation of rf frequency and temperature were studied in detail. To find out the exact relation between the JJ coupling energy, JJ number density, applied AC frequency, the amplitude of AC current and the AC losses in superconductors, we have studied samples of different crystalline properties, different grain sizes, pressurized with different pressure and sintered at different physical and chemical situations. The implementations of these results are discussed. These results have important implications for the understanding of the origin of AC losses and characterization of superconducting samples. In this paper we also extend the capability of the AC losses studies in superconductors for the characterization of materials for device applications. 
S. Sarangi, S. P. Chockalingam S. V. Bhat