Vortex dynamics in superconducting microbridges with low-vortex pinning forces and superconducting flux flow transistors

We present a mean-field model for the mixed state of superconducting microbridges with low pinning forces which accounts for the I-V characteristics measured on superconducting flux flow transistors (SFFTs) as well as the improvement of the performances of these devices when they include several microbridges. From the model it turns out that the vortex motion is not mostly driven by the bias current as is assumed in classical flux flow descriptions but by a diffusion process. Moreover, it is shown that due to the magnetic coupling between bridges, the critical current of a device with n bridges is larger than n times the critical current of a single bridge. Expressions for this increase in the critical current and the current gain of these devices are given. The model also accounts for the dynamic resistance modulation of SFFTs when an external magnetic field is applied. (C) 1995 American Institute of Physics.