Microwave absorption studies of diluted high-temperature superconductors: delineation of superconductor-insulator-superconductor and superconductor-normal-superconductor junctions

With a view to investigating the nature of weak links in granular supercond uctors at very dilute level, we have studied the low-field microwave absorp tion signal (below about 50 G) in the electron paramagnetic resonance mode of detection in samples of Tl2BaCa2Cu2O8+delta, and YBa2Cu3O7-delta at diff erent dilution levels in the Al2O3 matrix. The signal has been monitored as a function of temperature as well as concentration. The temperature depend ence of the signal intensity has been understood by applying a model in whi ch microwave absorption is assumed to be caused by the field- and temperatu re-dependent currents in the Josephson links. It is shown that the paramete r eta (0) quantifying the strength of coupling between superconducting grai ns can be calculated from the value of temperature at which the intensity I -pp(T) (the subscript pp indicates a peak-to-peak value), goes through a ma ximum. Using the eta (0) value so obtained, the fitting for I-pp(T) through out the temperature range, was used to distinguish superconductor-insulator -superconductor (S-I-S) junctions from superconductor-normal-superconductor (S-N-S) junctions. In bulk samples the signal due to S-I-S junctions is fo und to predominate, whereas the relative contribution due to S-N-S junction s predominates on dilution. In extremely diluted samples, the surviving jun ctions were found to have only S-N-S character.