1,449 research outputs found
Flux Noise in MgB2 Thin Films
We have performed flux noise and AC-susceptibility measurements on two 400 nm
thick MgB films. Both measurement techniques give information about the
vortex dynamics in the sample, and hence the superconducting transition, and
can be linked to each other through the fluctuation-dissipation-theorem. The
transition widths for the two films are 0.3 and 0.8 K, respectively, and the
transitions show a multi step-like behavior in the AC-susceptibility
measurements. The same phenomenon is observed in the flux noise measurements
through a change in the frequency dependence of the spectral density at each
step in the transition. The results are discussed and interpreted in terms of
vortices carrying an arbitrary fraction of a flux quantum as well as in terms
of different macroscopic regions in the films having slightly different
compositions, and hence, different critical temperatures.Comment: 8 pages, 4 figures, conference contribution to "Fluctuations and
Noise", Santa Fe, New mexico 1-4 june 200
Growth of superconducting MgB2 thin films via postannealing techniques
We report the effect of annealing on the superconductivity of MgB2 thin films
as functions of the postannealing temperature in the range from 700 C to 950 C
and of the postannealing time in the range from 30 min to 120 min. On annealing
at 900 C for 30 min, we obtained the best-quality MgB2 films with a transition
temperature of 39 K and a critical current density of ~ 10^7 A/cm^2. Using the
scanning electron microscopy, we also investigated the film growth mechanism.
The samples annealed at higher temperatures showed the larger grain sizes,
well-aligned crystal structures with preferential orientations along the
c-axis, and smooth surface morphologies. However, a longer annealing time
prevented the alignment of grains and reduced the superconductivity, indicating
a strong interfacial reaction between the substrate and the MgB2 film.Comment: 7 pages, 4 figures include
Instability of defensive alliances in the predator-prey model on complex networks
A model of six-species food web is studied in the viewpoint of spatial
interaction structures. Each species has two predators and two preys, and it
was previously known that the defensive alliances of three cyclically predating
species self-organize in two-dimensions. The alliance-breaking transition
occurs as either the mutation rate is increased or interaction topology is
randomized in the scheme of the Watts-Strogatz model. In the former case of
temporal disorder, via the finite-size scaling analysis the transition is
clearly shown to belong to the two-dimensional Ising universality class. In
contrast, the geometric or spatial randomness for the latter case yields a
discontinuous phase transition. The mean-field limit of the model is
analytically solved and then compared with numerical results. The dynamic
universality and the temporally periodic behaviors are also discussed.Comment: 5 page
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