Nucleation and growth of a flux instability in superconducting ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ films

Using a high speed magneto-optic technique we have investigated the dynamics of a new flux instability in thin superconducting ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ films exposed to an external magnetic field of several ${10}^{\mathrm{-}2}$ T. The instability was nucleated by a 5 ns laser pulse, which heated a small spot of the sample in a region of high shielding currents. Two subsequent regimes in the development of the instability are discovered, which give rise to strikingly different flux distributions. The formation of flux branches, which are characteristic for the second stage, occurs on a time scale of a hundred nanoseconds.