Thermally assisted flux flow in superconductor

Abstract

We have theoretically studied the case of the linear response (thermally assisted flux flow (TAFF) regime) in which the ac field amplitude applied is sufficiently low. The general equations that describe both the linear and the nonlinear responses of the superconducting system are 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 for our (HgPb-1234) superconductor have been studied in detail. It is found that the crossover ac field amplitude is mT under a dc field of 40 mT for the HgPb-1234 sample. This finding seems to be nicely in agreement with the theoretical prediction for the crossover due to the transition from TAFF to flux creep regime. Moreover, is found to depend on different samples or more generally on the flux pinning properties of the superconductors and also on the strength of the dc magnetic field. When the applied ac field amplitude > 0.05 mT, the system 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 with .