Nanosecond magneto‐optic study of a new instability in thin YBa2Cu3O7−x films

We have investigated the dynamics of flux penetration into thin superconducting YBa2Cu3O7−x films using a magneto‐optic technique with nanosecond time resolution. For films carrying large shielding currents an instability in the magnetic flux distribution is discovered, which can be triggered by a local perturbation, e.g., by a focused laser pulse. The instability develops in the form of fine ramified flux structures, with a propagation speed of the flux front on the order of 5×104 m/s.