Flux flow velocity instability in wide superconducting films

To understand energy losses related to vortex high velocity we study the critical voltage in the current-voltage (I-V) characteristics above which in the flux flow regime a sudden voltage jump appears. Different mechanisms have been proposed to account for the existence of a critical vortex velocity corresponding to the observed instability, such as heating effects or intrinsic non equilibrium phenomena. Nevertheless experimental studies of flux flow instabilities in wide superconducting films have been less investigated. We report on critical voltage measurements in Nb wide superconducting strips. We perform I-V measurements as function of magnetic field by using different bias modes (sweeping, compensated long and short pulsing). The magnetic field dependence of the critical voltage shows different features in the three operation modes. We quantitatively estimate self-heating effects taking into account cryogenic stabilization criteria and thermal diffusion calculations, and we demonstrate that the observed dynamics instabilities are not triggered by Joule self-heating. The Larkin-Ovchinnikov (LO) theory of non linear effects in the vortex motion has been then applied to interpret the critical velocity results. The magnetic field dependence of the vortex critical velocity shows some discrepancies with the LO predicted behaviour. © 2008 IOP Publishing Ltd.