Vortex dynamics and instabilities in layered and homogeneous Ta/Ge superconductors

We present measurements of a dynamically induced instability in the vortex state of an amorphous multilayer and a single alloy layer of Ta/Ge, The critical vortex velocity shows quantitative agreement with the predictions of Larkin and Ovchinnikov calculated using parameters determined independently from the normal state resistivity data. Both samples show a weak field dependence in the critical velocity implying a velocity dependent pinning force in the vortex solid state and a broadening of the transition resulting from the velocity distribution in the liquid state.