Highly dissipative state of YBa2Cu3O7 thin films at very large current densities

Flux flow instabilities at high current densities have been recently report ed to occur in high temperature superconductors. Such flux flow, instabilit ies were already predicted by Larkin and Ouchinnikov. However, other mechan isms may possibly also account for this behaviour, as for instance hot spot s or thermal instabilities. To investigate the mechanism at the origin of t hese instabilities, rue have carried out detailed I-V experiments, using sh ort current pulses, on YBCO thin, films grown, on sapphire substrates. When the current density exceeds 2-3 times the conventional critical current de nsity, we observe an extremely fast transition from a lour dissipative stat e into a highly dissipative state. We present some indications that this tr ansition does not originate from temperature instabilities but is driven by the current.