We report on microwave and low-frequency properties of composites consisting of YBa₂{Cu}₃O_7-x (YBCO) or Bi₂{Sr}₂CaCu₂{O}ₓ$ grains embedded in a polymer matrix with strong emphasis on microwave transmission experiments through YBCO-polymer composites. The low-frequency conductivity vs the YBCO volume fraction f can be described by a three-dimensional percolation model with a percolation threshold of fₚ{}0.2. The microwave phase velocity and the transmission coefficient are significantly larger in the superconducting state of the grains compared to their normal-state values. These parameters are sensitive to the external magnetic field in the superconducting state. The increase of the phase velocity in the superconducting state is attributed to the change of the effective magnetic permeability which is in conflict with previous theoretical predictions.
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Golosovsky et al. (1992) studied this question.
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