29 research outputs found
Avalanches and Self-Organized Criticality in Superconductors
We review the use of superconductors as a playground for the experimental
study of front roughening and avalanches. Using the magneto-optical technique,
the spatial distribution of the vortex density in the sample is monitored as a
function of time. The roughness and growth exponents corresponding to the
vortex landscape are determined and compared to the exponents that characterize
the avalanches in the framework of Self-Organized Criticality. For those
situations where a thermo-magnetic instability arises, an analytical non-linear
and non-local model is discussed, which is found to be consistent to great
detail with the experimental results. On anisotropic substrates, the anisotropy
regularizes the avalanches
Dendritic flux avalanches and nonlocal electrodynamics in thin superconducting films
We present numerical and analytical studies of coupled nonlinear Maxwell and
thermal diffusion equations which describe nonisothermal dendritic flux
penetration in superconducting films. We show that spontaneous branching of
propagating flux filaments occurs due to nonlocal magnetic flux diffusion and
positive feedback between flux motion and Joule heat generation. The branching
is triggered by a thermomagnetic edge instability which causes stratification
of the critical state. The resulting distribution of magnetic microavalanches
depends on a spatial distribution of defects. Our results are in good agreement
with experiments performed on Nb films.Comment: 4 pages, 3 figures, see http://mti.msd.anl.gov/aran_h1.htm for
extensive collection of movies of dendritic flux and temperature pattern
Preventing probe induced topography correlated artifacts in Kelvin Probe Force Microscopy
Kelvin Probe Force Microscopy (KPFM) on samples with rough surface topography can be hindered by topography correlated artifacts. We show that, with the proper experimental configuration and using homogeneously metal coated probes, we are able to obtain amplitude modulation (AM) KPFM results on a gold coated sample with rough topography that are free from such artifacts. By inducing tip inhomogeneity through contact with the sample, clear potential variations appear in the KPFM image, which correlate with the surface topography and, thus, are probe induced artifacts. We find that switching to frequency modulation (FM) KPFM with such altered probes does not remove these artifacts. We also find that the induced tip inhomogeneity causes a lift height dependence of the KPFM measurement, which can therefore be used as a check for the presence of probe induced topography correlated artifacts. We attribute the observed effects to a work function difference between the tip and the rest of the probe and describe a model for such inhomogeneous probes that predicts lift height dependence and topography correlated artifacts for both AM and FM-KPFM methods. This work demonstrates that using a probe with a homogeneous work function and preventing tip changes is essential for KPFM on non-flat samples. From the three investigated probe coatings, PtIr, Au and TiN, the latter appears to be the most suitable, because of its better resistance against coating damage