Temperature-dependent phase reversal of nonresonant microwave and rf absorption in high-Tcsuperconductors

The phenomenon of temperature-dependent phase reversal of the nonresonant electromagnetic response of the high-${\mathit{T}}_{\mathit{c}}$ superconductors is studied in the rf and the microwave ranges. Phase reversal is the changeover from a maximum at zero magnetic field and the decrease with the increasing field to a minimum at zero field and the increase with the increasing field. The results are analyzed using an effective-medium theory, treating the sample as a percolating network consisting of the superconducting regions separated by normal metallic regions. The experimental observations are shown to follow naturally from the dependence of the superconducting fraction on the temperature and the magnetic field.