Direct Determination of the Power Threshold Value of Vortex Avalanche in YBa2Cu3O7-x Thin Films Triggered by a Laser Pulse

Vortex stability is critically important for applications of type-II superconductors. If some vortices are detached from their pinning sites by external perturbations, the concomitant Joule heat will facilitate further motion of other vortices. This is a positive feedback process, which leads to a flux avalanche in the superconductor. So far, the existing research has not addressed the problem of the power threshold value (PVT) for activating a flux avalanche. The primary objective is to study the influences of temperatures, applied magnetic fields and laser activating sites on the PTV of YBa2Cu3O7-x (YBCO) thin films, and to find the relationship between avalanche velocities and laser activating sites. We constructed two systems including the classical and high speed magneto-optical imaging (MOI) system. Both systems include two optical branches: one serves as an illuminating system, and the other is used to trigger the dendritic flux avalanche in superconducting film. The PTV was obtained by the classical MOI system while avalanche velocities were acquired via the high speed MOI system. The PTVs were experimentally obtained at different given temperatures, applied magnetic fields and activating sites, as well as the dependence of avalanche velocity on activating sites. Theoretically, two models are provided to depict the dependence of PTV on temperature and avalanche velocity on activating sites, respectively. The calculated results according to the present models are in reasonable agreement with the experiments. The experimental results and constructed theoretical models help us to understand the characteristics of flux avalanche in YBCO films and are of significance to evaluate the flux stability for practical applications of high temperature superconducting films.