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

Abstract

With a view to investigating the nature of weak links in granular superconductors at very dilute level, we have studied the low-field microwave absorption signal (below about 50 G) in the electron paramagnetic resonance mode of detection in samples of Tl₂BaCa₂Cu₂O_8+δ and YBa₂Cu³O_7-δ at different dilution levels in the Al²O₃ matrix. The signal has been monitored as a function of temperature as well as concentration. The temperature dependence of the signal intensity has been understood by applying a model in which microwave absorption is assumed to be caused by the field- and tempera ture-dependent currents in the Josephson links. It is shown that the parameter ηo quantifying the strength of coupling between superconducting grains 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 maximum. Using the ηo value so obtained, the fitting for I _pp(T) fhroughout 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 found to predominate, whereas the relative contribution due to S-N-S junctions predominates on dilution. In extremely diluted samples, the surviving junctions were found to have only S-N-S character.