Mechanism of flux flow activation in c-axis oriented Bi2Sr2CaCu2O8+δ thin films

Abstract

The study of vortex line dynamics and its relationship with different kinds of defects is one of the major concepts tackled in the search for understanding the vortex matter of high critical temperature superconductors (HTSC). The pinning centers may be point defects, columnar or planar disorders and grain boundary or intra/intergranular nonsuperconducting regions. In this paper, the transport measurements done gave insights into the macroscopic motion of the vortices. This, in turn, gives us an idea of the pinning sites predominating in a certain external magnetic field and temperature regime. This work is motivated by the fact that knowing the mechanism of flux flow activation allows for possibly controlling the dissipative and noise properties of devices based on HTSC films.
Flux motion is the mechanism leading to power dissipation and flux flow resistance in the superconducting mixed state. This motion is due to the Lorentz force induced by the applied transport current. Above a certain value, the critical current, the vortices are 'de-pinned' and start to move with a velocity proportional to the electric field strength that develops along the sample. In addition to the Lorentz force, motion of the vortices may be due to thermal activation/fluctuations. Furthermore, effects on the film electrical characteristics are expected even in the absence of an externally applied field. The pinning sites, inherent in the film, influences the motion of the vortices produced by the self-magnetic field of the bias current. The dissipative properties of these sites may also be affected by thermal fluctuations.
In this study, the mechanism of flux flow activation in c-axis oriented Bi₂Sr₂CaCu₂O_8+δ was determined in different temperature regimes with no external field and at different applied magnetic fields below 1.0 T when the film is in its transition region. From the theoretical fitting of our I-V data, we find that flux flow and flux creep contribute largely to the activation of vortex motion in these films. In addition, our measurements with and without an external magnetic field show that pinning due to point defects is not a factor for these films at currents below 20 mA and temperatures below the transition temperature.