  The origin of AC resistivity in the high Tc superconductors is not addressed
adequately in literature. We found out, Josephson Junction (JJ) decoupling is
the main origin of the AC resistivity in high Tc superconductors. We have
measured the AC resistivity in the superconductors in the low frequency range
by measuring the complex AC impedance of superconducting YBa2Cu3O7 (YBCO)
polycrystalline samples. Our data shows that under certain conditions when the
number density of Josephson Junctions (JJ) present in the sample and the JJ
critical current crosses a threshold value, AC resistivity in the
superconducting state keeps on increasing with lowering temperature. The
underlying mechanism is an interesting interplay of JJ coupling energy,
amplitude of the supply AC voltage and the current applied to the
superconducting sample. The effect of the applied AC current of different
frequencies and the variation of temperature were studied in detail. To find
out the exact relation between the JJ coupling energy, JJ number density,
applied AC frequency, the amplitude of AC current and the AC resistivity in the
superconductors, we have studied samples of different grain sizes, pressurized
with different pressure and sintered at different physical situations. These
results have important implications for the understanding of the origin of AC
resistivity and characterization of superconducting samples. In this paper we
also extend the capability of the AC impedance studies in superconductors for
the characterization of materials for device applications.
