Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session J1: Vortex Avalanches |
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Sponsoring Units: DCMP Chair: Ulrich Welp, Argonne National Laboratory Room: LACC 152 |
Tuesday, March 22, 2005 11:15AM - 11:51AM |
J1.00001: Self-organized criticality of vortices in superconducting films Invited Speaker: Magnetic flux dynamics in continuous and periodically patterned Nb films is studied using the magneto-optical imaging technique. Slow vortex penetration forming weak flux gradients and smooth flux fronts characterize the magnetization process at elevated temperatures. At low temperatures, microavalanches (irregular jumps of flux bundles with preferential orientation along field gradients) dominate the flux entry and exit and successive redistribution of the vortex density. Thus formed critical state is probed using scaling analysis of the correlation functions, lengths, width, and power spectra of fractal induction profiles in the samples. The resulting Housdorff and roughness exponents correspond to 1+1 dimensional nonlinear flux diffusion in systems with quenched disorder and long range correlations. The power spectra scaling confirms the self-affine character of the dynamically formed critical state. $\backslash $The nature of the matching effects in periodically patterned samples is reexamined. It is established that neither flat vortex distributions nor terraced states are realized at the fields corresponding to the integer number of vortices per hole. Rather, stronger flux gradients are formed at these fields indicating to the increased average pinning at matching conditions. Macroscopic thermo-magnetic avalanches (TMA) resulting in catastrophic magnetization jumps appear at T$<$4.5K. The sample structure is shown to be crucial for the development of thermally assisted flux instabilities, which follow the topography of the strongest pinning centers in the films. These observations will be analyzed using recent theoretical TMA models. This work was supported by the U.S. Department of Energy, Basic Energy Sciences, under Contract No. W-31-109-ENG-38. [Preview Abstract] |
Tuesday, March 22, 2005 11:51AM - 12:27PM |
J1.00002: Avalanches on vortex piles Invited Speaker: Rinke J. Wijngaarden The Bean state of pinned vortex matter very much resembles the slope of a pile of sand, as noted a long time ago by de Gennes. Recently, we discovered that this similarity goes much further: (i) avalanches occur on the slopes of both systems (ii) a close relation exists between the statistical properties of the (vortex) pile surface and those of the avalanches. We find that the punctuated behavior of the avalanches falls in the class of Self-Organized Criticality (SOC). The intriguing relation between the amount of disorder and the onset of SOC behavior was investigated in Niobium thin films, where disorder was introduced by adding interstitial hydrogen atoms, absorbed from the controlled surrounding gas. In Niobium deposited on R-plane sapphire we find that a minimum amount of disorder (created by absorbing hydrogen) is necessary for SOC to occur. In Niobium on A-plane sapphire, huge compact avalanches are observed. The behavior of these avalanches is compared to a recent model by Aranson et al., who e.g. predict a minimum amount of penetration before avalanches can occur, which is corroborated by experiment. [Preview Abstract] |
Tuesday, March 22, 2005 12:27PM - 1:03PM |
J1.00003: Dynamics of Magnetic Flux Avalanches in Superconducting Films Invited Speaker: Magnetic flux penetration into superconducting films can occur along two different scenarios: either in the form of homogeneously propagating flux fronts, or as a dendritic instability with branch-like flux avalanches propagating into the previously flux-free reagion of the superconductor. Since the relevant time scale for these processes in the case of thin films is in the nanosecond range, we have developed a fast pump-probe technique for magnetooptic imaging. The method is based on nucleating an event (e.g. the formation of a flux avalanche in a superconductor) by means of a femtosecond ``pump'' laser pulse, and taking a magnetooptic snapshot of the developing flux distribution by a delayed ``probe'' beam. The time resolution of this technique is given by the response time of the magnetooptic garnet films used, which in our experiment is about 100ps. Using this technique have investigated the dendritic instability for various film materials (e.g.YBa$_2$Cu$_3$O$_{7-d}$ and MgB$_2$) and have constructed a ``stability diagram'' which separates regions with homogeneous flux penetration from unstable ones. In addition we have studied systematically the influence of relevant parameters like film thickness and external magnetic field on the propagation characteristics of the flux dendrites. The experimental results are compared with a theoretical model for dendrite propagation, and good agreement is found. [Preview Abstract] |
Tuesday, March 22, 2005 1:03PM - 1:39PM |
J1.00004: Size distribution of flux avalanches in MgB2 films visualized by magneto-optical imaging Invited Speaker: We report on the quantitative and spatially resolved observation of flux avalanches in superconducting films. Magneto-optical imaging was used to visualize the flux penetration in MgB2 films subjected to a slowly varying perpendicular field. Below 10 K, flux avalanches with typical size around 20 microns and regular shape are found to occur at random locations along the flux front. The total number of vortices that participates in one avalanche is varying between 50 and 10000 [1]. An adiabatic model is proposed to calculate the flux jump size for a thin-strip superconductor. The flux density and temperature distributions in the final state after flux jump are calculated. The jump size is found to grow monotonously with applied field and this dependence is in a good agreement with experimental data. At larger applied fields we observe another kind of jumps: a much bigger dendritic and branching avalanches [2,3]. Their dimensions are limited only by the sample size, while their morphology can be described within a linear model based on Maxwell and thermal diffusion equations [4]. References: [1] A. V. Bobyl et al., Physica C 408-410, 508 (2004) [2] T. H. Johansen et al., Europhys. Lett. 59, No. 4, 599-605 (2002) [3] F. L. Barkov et al., Phys. Rev. B 67, 064513 (2003) [4] A. L. Rakhmanov et al., Phys. Rev. B 70, 224502 (2004) [Preview Abstract] |
Tuesday, March 22, 2005 1:39PM - 2:15PM |
J1.00005: Vortex nanoliquid in high-temperature superconductors Invited Speaker: Vortex matter is commonly considered as a homogenous glassy medium. Correlated disorder in the form of columnar defects (CDs) is shown to result in formation of new heterogeneous phases of vortex matter. We have developed a magneto-optical method that allows visualization of the distribution of small transport currents applied to BSCCO crystals irradiated through patterned masks [1]. When vortices outnumber CDs we identify two distinct populations: vortices residing on CDs are strongly pinned and form a rigid `porous' skeleton, whereas the excess vortices form weakly pinned ordered crystallites caged within the pores of the skeleton [2,3]. The melting process of this porous vortex matter is qualitatively different from melting of a homogenous system. The soft crystallites melt while the rigid skeleton remains in tact, forming a vortex nanoliquid in which intercalated liquid droplets of just few vortices are embedded in a porous solid matrix. The nanoliquid phase possesses unique properties and displays a high degree of correlation along the c-axis but no transverse critical current. The melting of heterogeneous vortex matter occurs in two steps resulting in a ``Y'' shaped phase diagram: first the soft crystallites undergo a melting transition forming a nanoliquid in which localized and delocalized vortices coexist, while a homogeneous liquid is formed at higher temperatures upon a delocalization transition of the skeleton from the CDs [1]. At lower fields the solid melts through a single first-order phase transition. [1] S. S. Banerjee, S. Goldberg, A. Soibel, Y. Myasoedov, M. Rappaport, E. Zeldov, F. de la Cruz, C. J. van der Beek, M. Konczykowski, T. Tamegai, and V. M. Vinokur, PRL 93, 097002 (2004). [2] S. S. Banerjee et al., PRL 90, 087004 (2003). [3] M. Menghini et al., PRL 90, 147001 (2003). [Preview Abstract] |
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