Computer simulations of dynamics of vortex lines in superconducting flux flow transistors

We study the dynamics of vortex lines in superconducting flux flow transistors (SFFTs) made of High T/sub c/ superconductors, YBa/sub 2/Cu/sub 3/O/sub x/. By computer simulations we have found that the density of vortex lines in SFFTs has close relation to device I-V characteristics rather than the velocity of vortex lines and that we have to increase the density of vortex lines to get ones with available gain. To be more concrete, in order to increase the density of vortex lines we have to lower surface barriers at the edge of weak-links and heighten the magnetic field near the edge of weak links. Especially, it is very important to heighten the magnetic field at the both sides of a link in a SFFT. Under the control of the control current, vortex lines in a SFFT are driven by the transport current (the bias current) mainly, but it is important to think of the other forces such as the force by screening current, the force induced by the other vortices and the force induced by the image vortices. Based on the local stochastic equation of motion for vortex lines, computer simulations for some conditions are performed to discuss the dynamic behavior of vortex lines in a SFFT. We report the effect of the external magnetic field induced by the control current on the motion of vortex lines in SFFTs.<>