1,293 research outputs found
Magnetic relaxation of type II superconductors in a mixed state of entrapped and shielded flux
The magnetic relaxation has been investigated in type II superconductors when
the initial magnetic state is realized with entrapped and shielded flux (ESF)
contemporarily. This flux state is produced by an inversion in the magnetic
field ramp rate due to for example a magnetic field overshoot. The
investigation has been faced both numerically and by measuring the magnetic
relaxation in BSCCO tapes. Numerical computations have been performed in the
case of an infinite thick strip and of an infinite slab, showing a quickly
relaxing magnetization in the first seconds. As verified experimentally, the
effects of the overshoot cannot be neglected simply by cutting the first 10-100
seconds in the magnetic relaxation. On the other hand, at very long times, the
magnetic states relax toward those corresponding to field profiles with only
shielded flux or only entrapped flux, depending on the amplitude of the field
change with respect to the full penetration field of the considered
superconducting samples. In addition, we have performed numerical simulations
in order to reproduce the relaxation curves measured on the BSCCO(2223) tapes;
this allowed us to interpret correctly also the first seconds of the
curves.Comment: 9 pages, 12 figures submit to PR
Harmonics of the AC susceptibility as probes to differentiate the various creep models
We measured the temperature dependence of the 1st and the 3rd harmonics of
the AC magnetic susceptibility on some type II superconducting samples at
different AC field amplitudes, hAC. In order to interpret the measurements, we
computed the harmonics of the AC susceptibility as function of the temperature
T, by integrating the non-linear diffusion equation for the magnetic field with
different creep models, namely the vortex glass-collective creep
(single-vortex, small bundle and large bundle) and Kim-Anderson model. We also
computed them by using a non-linear phenomenological I-V characteristics,
including a power law dependence of the pinning potential on hAC. Our
experimental results were compared with the numerically computed ones, by the
analysis of the Cole-Cole plots. This method results more sensitive than the
separate component analysis, giving the possibility to obtain detailed
information about the contribution of the flux dynamic regimes in the magnetic
response of the analysed samples.Comment: 9 pages, 6 figures, submitted to Physica
Spin Coulomb drag beyond the random phase approximation
We study the spin Coulomb drag in a quasi-two-dimensional electron gas beyond
the random phase approximation (RPA). We find that the finite transverse width
of the electron gas causes a significant reduction of the spin Coulomb drag.
This reduction, however, is largely compensated by the enhancement coming from
the inclusion of many-body local field effects beyond the RPA, thereby
restoring good agreement with the experimental observations by C. P. Weber
\textit{et al.}, Nature, \textbf{437}, 1330 (2005).Comment: 3 figures, accepted for publication in Phys. Rev. Let
Effects of thickness on the spin susceptibility of the 2D electron gas
Using available quantum Monte Carlo predictions for a strictly 2D electron
gas, we have estimated the spin susceptibility of electrons in actual devices
taking into account the effect of the finite transverse thickness and finding a
very good agreement with experiments. A weak disorder, as found in very clean
devices and/or at densities not too low, just brings about a minor enhancement
of the susceptibility.Comment: 4 pages, 3 figure
Hand-draw sketching for image retrieval through fuzzy clustering techniques
Nowadays, the growing of digital media such as images represents an important issue for niultimedia mining applications. Since the traditional information retrieval techniques developed for textual documents do not support adequately these media, new approaches for indexing and retrieval of images are needed. In this paper, we propose an approach for retrieving image by hand-drawn object sketch. For this purpose. we address the classification of images based on shape recognition. The classification is based on the combined use of geometrical and moments features extracted by a given collection of images and achieves shape-based classification through fuzzy clustering techniques. Then, the retrieval is obtained using a hand-draw shape that becomes a query to submit to the system and get ranked similar images
Exchange and correlation effects on the plasmon dispersions and the Coulomb drag in low-density electron bilayers
We investigate the effect of exchange and correlation (xc) on the plasmon
spectrum and the Coulomb drag between spatially separated low-density
two-dimensional electron layers. We adopt a new approach, which employs dynamic
xc kernels in the calculation of the bi-layer plasmon spectra and of the
plasmon-mediated drag, and static many-body local field factors in the
calculation of the particle-hole contribution to the drag. The spectrum of
bi-layer plasmons and the drag resistivity are calculated in a broad range of
temperatures taking into account both intra- and inter-layer correlation
effects. We observe that both plasmon modes are strongly affected by xc
corrections. After the inclusion of the complex dynamic xc kernels, a decrease
of the electron density induces shifts of the plasmon branches in opposite
directions. And this is in stark contrast to the tendency obtained within the
RPA that both optical and acoustical plasmons move away from the boundary of
the particle-hole continuum with a decrease in the electron density. We find
that the introduction of xc corrections results in a significant enhancement of
the transresistivity and qualitative changes in its temperature dependence. In
particular, the large high-temperature plasmon peak that is present in the
random phase approximation is found to disappear when the xc corrections are
included. Our numerical results at low temperatures are in good agreement with
the results of recent experiments by M. Kellogg {\it et al.}, Solid State
Commun. \textbf{123}, 515 (2002).Comment: 28 pages, 15 figure
Electron Correlation and Charge Transfer Instability in Bilayered Two Dimensional Electron Gas
We prove that the predicted charge transfer state in symmetric bilayers of
two dimensional electron gases is always unstable at zero bias voltage, due to
interlayer correlation and/or tunneling. This is most easily seen by resorting
to a pseudospin formalism and considering coherent states obtained from the
charge transfer state through rotations of the pseudospins. Evidently, the
charge transfer state is stabilized by a sufficiently strong gate voltage, as
found in recent experiments. We show that a simple model, in which the layers
are strictly two dimensional, is able to account quantitatively for such
experimental findings, when correlation is properly included.Comment: 5 pages, 3 figures. Subm. to Europhys. Let
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