Dendritic and homogeneous regimes of flux penetration into YBCO films

Abstract

Dendritic flux patterns in superconducting YBCO films are studied on a nanosecond time-scale. It is found that dendrites only develop for certain values of the external field and temperature. It is well-known that superconductors carrying high critical current densities tend to develop instabilities in their magnetic flux patterns which manifest themselves as macroscopic flux jumps [1]. A combination of magneto-optics and pulsed laser irradiation can be used to investigate the flux propagation with high spatial and temporal resolution in the micrometer and nanosecond range. Using this technique it was found that for thin superconducting films the flux jump instability can give rise to dendritic magnetic flux avalanches propagating with tip velocities as high as 50 km/s [2,3].In this work we report on a systematic magneto-optic study of various types of dynamic flux patterns in thin films. Depending on temperature T and magnetic field B we observe two completely different regimes of flux penetration: either in the form of a smooth flux front, or as a dendritic flux avalanche.In our experiments we used YBCO films with thicknesses 100, 200, 330, and 690 nm, grown on a SrTiO 3 substrate by thermal reactive co-evaporation (the c-axis is perpendicular to the substrate) [3]. The superconducting transition temperatures are between 86 and 90 K and critical current densities are between 2 Â 10 6 and 4 Â 10 6 A/m 2 at 77 K. To visualize the magnetic c-component of the field a Bi-doped yttrium iron garnet (YIG) film on gadolinium gallium garnet with in-plane anisotropy is used as magneto-optic layer [4], which is mounted on top of the superconductor [5]. In order to distinguish between positive and negative direction of the field the polarizer and analyzer in our polarization microscope were not crossed completely and fields between )50 and +180 mT could be observed. The YIG film allows to obtain a time resolution of about 200 ps (see [6]).Each experiment is done as follows. The sample is zero field cooled below T c . After reaching a stable temperature a magnetic field B a perpendicular to the sample surface is applied, and then the laser pulse is triggered. The thin film geometry enhances the field at the edges and the magnetic flux penetrates into the superconducting film from there. The first part of the laser pulse is now used to change the local magnetic field. This is done by focusing the laser pulse with a cylindrical lens onto the film from the substrate side of the sample. A stripe of 50 lm width is heated above T c , dividing the quadratic sample into two regions of equal size. Magnetic flux fills the non-superconducting stripe and the T/s). Now the magnetic flux starts to penetrate perpendicular to the stripe into the superconductor. Fig. 1 shows the flux profile at 30 K after a time delay of 67.8 ns (top) and the final state (bottom). We found that this homogeneous flux penetration is restricted to a region in the T =T c versus B a =j c diagram as shown in Fig. 2. Above a certain magnetic field which de...

Ray optics behavior of flux avalanche propagation in superconducting films

“…[18] for a review. It is also found experimentally that the propagation speed of such avalanches can be amazingly high, up to 160 km/s in their very early stage [19,20].…”

Ultrafast magnetic flux dendrite propagation into thin superconducting films

“…A wealth of recent experiments convincingly demonstrate that a propagating dendritic flux pattern driven by the flux jumping instability is a general phenomenon typical for Bean's-type critical state [13,14,15,22,23,24,25]. Indeed, the flux dendrites were observed under a wide variety of conditions in superconducting films of Nb [13,14,22], YBa 2 Cu 3 O 7−δ [13,15,23], Nb 3 Sn [24], and MgB 2 [25].…”

“…Indeed, the flux dendrites were observed under a wide variety of conditions in superconducting films of Nb [13,14,22], YBa 2 Cu 3 O 7−δ [13,15,23], Nb 3 Sn [24], and MgB 2 [25].…”

Dynamics of the dendritic flux instability in YBa 2 Cu 3 O 7 − δ films

“…In these measurements we applied a slightly different technique, using a line focus instead of a point focus to nucleate the dendrites [17]. The line was placed lengthwise across the quadratic sample to divide it into two rectangles of equal size.…”