THERMALLY ASSISTED FLUX-FLOW IN HG0.69PB0.31BA2CA3CU4O10+DELTA SUPERCONDUCTOR

We have theoretically studied the case of the linear response (thermal ly assisted flux flow (TAFF) regime) in which the ac field amplitude a pplied is sufficiently low. The general equations that describe both t he linear and the nonlinear responses of the superconducting system ar e similar but with different forms of effective attempt time. By using the ac susceptibility technique, the ac field amplitude and frequency dependence of the imaginary part of the ac susceptibility chi'' for o ur Hg0.69Pb0.31Ba2Ca3Cu4O10+delta (HgPb-1234) superconductor have been studied in detail. It is found that the crossover ac field amplitude mu(0)h(0,crossover) is similar to 0.05 mT under a dc field of 40 mT fo r the HgPb-1234 sample. This finding seems to be nicely in agreement w ith the theoretical prediction for the crossover due to the transition from TAFF to flux creep regime. Moreover, mu(0)h(0,crossover) is foun d to depend on different samples or more generally on the flux pinning properties of the superconductors and also on ?he strength of the do magnetic field. When the applied ac field amplitude >0.05 mT, the syst em is brought into the flux creep regime. In this regime, it is found that the effective pinning potential for our HgPb-1234 superconductor is proportional to the inverse power of the current density, that is U -eff alpha J(-mu) with mu = 0.5.