330 research outputs found
Probing Disordered Substrates by Imaging the Adsorbate in its Fluid Phase
Several recent imaging experiments access the equilibrium density profiles of
interacting particles confined to a two-dimensional substrate. When these
particles are in a fluid phase, we show that such data yields precise
information regarding substrate disorder as reflected in one-point functions
and two-point correlations of the fluid. Using Monte Carlo simulations and
replica generalizations of liquid state theories, we extract unusual two-point
correlations of time-averaged density inhomogeneities induced by disorder.
Distribution functions such as these have not hitherto been measured but should
be experimentally accessible.Comment: 10 pages revtex 4 figure
Onset of dendritic flux avalanches in superconducting films
We report a detailed comparison of experimental data and theoretical
predictions for the dendritic flux instability, believed to be a generic
behavior of type-II superconducting films. It is shown that a thermo-magnetic
model published very recently [Phys. Rev. B 73, 014512 (2006)] gives an
excellent quantitative description of key features like the instability onset
(first dendrite appearance) magnetic field, and how the onset field depends on
both temperature and sample size. The measurements were made using
magneto-optical imaging on a series of different strip-shaped samples of MgB2.
Excellent agreement is also obtained by reanalyzing data previously published
for Nb.Comment: 4 pages, 5 figure
Low field vortex matter in YBCO: an atomic beam magnetic resonance study
We report measurements of the low field structure of the magnetic vortex
lattice in an untwinned YBCO single-crystal platelet. Measurements were carried
out using a novel atomic beam magnetic resonance (ABMR) technique. For a 10.7 G
field applied parallel to the c-axis of the sample, we find a triangular
lattice with orientational order extending across the entire sample. We find
the triangular lattice to be weakly distorted by the a-b anisotropy of the
material and measure a distortion factor, f = 1.16. Model-experiment
comparisons determine a penetration depth, lambda_ab = 140 (+-20) nm. The paper
includes the first detailed description of the ABMR technique. We discuss both
technical details of the experiment and the modeling used to interpret the
measurements.Comment: 44 pages, 13 figures, submitted to Phys. Rev. B Revision includes
Postscript wrapped figures + minor typo
Experiments in vortex avalanches
Avalanche dynamics is found in many phenomena spanning from earthquakes to
the evolution of species. It can be also found in vortex matter when a type II
superconductor is externally driven, for example, by increasing the magnetic
field. Vortex avalanches associated with thermal instabilities can be an
undesirable effect for applications, but "dynamically driven" avalanches
emerging from the competition between intervortex interactions and quenched
disorder constitute an interesting scenario to test theoretical ideas related
with non-equilibrium dynamics. However, differently from the equilibrium phases
of vortex matter in type II superconductors, the study of the corresponding
dynamical phases - in which avalanches can play a role - is still in its
infancy. In this paper we critically review relevant experiments performed in
the last decade or so, emphasizing the ability of different experimental
techniques to establish the nature and statistical properties of the observed
avalanche behavior.Comment: To be published in Reviews of Modern Physics April 2004. 17 page
‘Albania: €1’ or the story of ‘big policies, small outcomes’: how Albania constructs and engages its diaspora
Since the fall of the communist regime in the early 1990s, Albania has experienced one of the most significant emigrations in the world as a share of its population. By 2010 almost half of its resident population was estimated to be living abroad – primarily in neighbouring Greece and Italy, but also in the UK and North America. This chapter discusses the emergence and establishment of the Albanian diaspora, its temporal and geographical diversity, and not least its involvement with Albania itself. Albania’s policymaking and key institutions are considered, with a focus on matters of citizenship; voting rights; the debate on migration and development; and not least the complex ways in which kin-state minority policies – related to ethnic Albanians living in Kosovo, Montenegro, southern Serbia, Macedonia and Greece – are interwoven with Albania’s emigration policies
Interplay of dendritic avalanches and gradual flux penetration in superconducting MgB2 films
Magneto-optical imaging was used to study a zero-field-cooled MgB2 film at
9.6K where in a slowly increasing field the flux penetrates by abrupt formation
of large dendritic structures. Simultaneously, a gradual flux penetration takes
place, eventually covering the dendrites, and a detailed analysis of this
process is reported. We find an anomalously high gradient of the flux density
across a dendrite branch, and a peak value that decreases as the applied field
goes up. This unexpected behaviour is reproduced by flux creep simulations
based on the non-local field-current relation in the perpendicular geometry.
The simulations also provide indirect evidence that flux dendrites are formed
at an elevated local temperature, consistent with a thermo-magnetic mechanism
of the instabilityComment: 5 pages, 5 figures, submitted to Supercond. Sci. Techno
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