Dynamics of the dendritic flux instability in YBa2Cu3O7-δ films

– We have studied the dynamics of magnetic-flux avalanches in superconducting YBa2Cu3O7−δ films by means of a fast magneto-optic pump-probe technique. Two regimes of propagation are found: in regime I, directly after the nucleation of the avalanches by a femtosecond laser pulse, the velocity v of the flux dendrites depends strongly on the magnetic field, and values of v up to 180 km/s are observed. Within some ten nanoseconds after nucleation regime II is reached, where the propagation velocity has dropped by one order of magnitude and is nearly independent of sample temperature and magnetic field. Comparison with previous experiments on flux jumps in superconductors shows good qualitative agreement. Introduction. – When a magnetic field is applied to a superconductor, the resulting flux pattern in the material does in general not correspond to the equilibrium distribution, but rather to a metastable state due to magnetic pinning. The system is therefore prone to avalanche-like flux jumps, i.e. a redistribution of the flux which is closer to the equilibrium state, resulting from an external perturbation or some intrinsic fluctuation in the superconducting sample. The first spatially resolved observation of such flux jumps was already reported in 1967 by Wertheimer et al. [1], who investigated superconducting Nb discs with a thickness of several tenths of a millimeter by means of a magneto-optic technique. These authors found finger-like flux fronts with a typical width of 1mm which propagated with a typical speed between 5 and 100m/s, depending on the sample thickness, the thinnest samples displaying the highest flux front velocity. Later investigations of thin superconducting Nb and YBa2Cu3O7−δ films showed a similar flux jump phenomenon, most often resulting in a dendritic flux pattern, both when the avalanche developed spontaneously [2–4] and when it was triggered by local heating of the sample with a short laser pulse [5,6]. For YBa2Cu3O7−δ with a film thickness of 300 nm, the width of the flux branches was about 0.1mm, and their speed of propagation was determined to be on the order of 5 · 10 m/s [6]. That dendritic flux avalanches are a rather general phenomenon in thin superconductors was corroborated recently by similar observations in MgB2 films [7]. (∗) E-mail: [email protected]