Radio-frequency magnetoabsorption in YBCO - Ag and BSCCO superconductors: The cross-over between two-level and one-level critical state models

Abstract

We have studied the virgin curve and hysteresis of radio-frequency magnetoabsorption in YBCO - Ag(YBa₂Cu₃O_7-δ :Ag_1.2) and BSCCO (Bi₂Sr₂CaCu₂O_8+δ) samples at 77 K in a magnetic field up to a value of 2 kOe. The hysteresis of YBCO - Ag is in good agreement with the prediction of the two-level critical state model. However, in the case of BSCCO the agreement is there only when the maximum field does not exceed 67.5 Oe for a frequency of 1.5 MHz. For higher fields the results resemble those of the one-level critical state model. This is due to the fact that at fields greater than 67.5 Oe the superconducting grain boundaries become normal. The cross-over point shifts to 55 Oe when the frequency is increased to 12 MHz. A qualitative explanation of this shift is given on the basis of the disordered model.