Critical phenomenon of vortex motion in superconductors: vortex instability and flux pinning

We have studied vortex dynamics in superconducting materials at very high vortex velocities as a function of the applied magnetic field. High velocity vortex dynamics can become critical, so that an instability occurs, leading the system to quench abruptly to the normal state. The presence of pinning mechanisms in all superconductors not only is able to foster high critical currents but it can strongly in-fluence vortex flow, thus determining a different behavior of the critical vortex velocity v*. The magnetic field dependence of v* is extremely sensitive to the type of material pinning, and this is crucial for an applicative point of view, since vortex motion approaching v* means a dissipative flux flow state which will probably end with a flux flow instability. If it is possible to predict these critical parameters, than it will be easier to control those critical phenomena. Although a fully theoretical model of flux flow instability in the presence of pinning is still lacking, a phenomenological approach has been recently proposed for the hot-electron vortex flow instability. Here we present a successful example of perfect correspondence between experiment and theoretical approach in the case of Mo₃Ge thin films with and without geometrical pinning barriers. (author)