6,494 research outputs found
Structural plasticity and associative memory in balanced neural networks with spike-time dependent inhibitory plasticity
Peer reviewe
A one-dimensional tunable magnetic metamaterial
We present experimental data on a one-dimensional superconducting
metamaterial that is tunable over a broad frequency band. The basic building
block of this magnetic thin-film medium is a single-junction (rf-)
superconducting quantum interference device (SQUID). Due to the nonlinear
inductance of such an element, its resonance frequency is tunable in situ by
applying a dc magnetic field. We demonstrate that this results in tunable
effective parameters of our metamaterial consisting of 54 SQUIDs. In order to
obtain the effective magnetic permeability from the measured data, we employ a
technique that uses only the complex transmission coefficient S21
Conservative and aggressive rough SVR modeling
AbstractSupport vector regression provides an alternative to the neural networks in modeling non-linear real-world patterns. Rough values, with a lower and upper bound, are needed whenever the variables under consideration cannot be represented by a single value. This paper describes two approaches for the modeling of rough values with support vector regression (SVR). One approach, by attempting to ensure that the predicted high value is not greater than the upper bound and that the predicted low value is not less than the lower bound, is conservative in nature. On the contrary, we also propose an aggressive approach seeking a predicted high which is not less than the upper bound and a predicted low which is not greater than the lower bound. The proposal is shown to use ϵ-insensitivity to provide a more flexible version of lower and upper possibilistic regression models. The usefulness of our work is realized by modeling the rough pattern of a stock market index, and can be taken advantage of by conservative and aggressive traders
Enhancement of Jc by Hf -Doping in the Superconductor MgB2: A Hyperfine Interaction Study
Measurements of the critical current density (Jc) by magnetization and the
upper critical field (Hc2) by magnetoresistance have been performed for
hafnium-doped MgB2. There has been a remarkable enhancement of Jc as compared
to that by ion irradiation without any appreciable decrease in Tc, which is
beneficial from the point of view of applications. The irreversibility line
extracted from Jc shows an upward shift. In addition, there has been an
increase in the upper critical field which indicates that Hf partially
substitutes for Mg. Hyperfine interaction parameters obtained from time
differential perturbed angular correlation (TDPAC) measurements revealed the
formation of HfB and HfB2 phases along with the substitution of Hf. A possible
explanation is given for the role of these species in the enhancement of Jc in
MgB2 superconductor
Experimental Evidence for Two-Dimensional Magnetic Order in Proton Bombarded Graphite
We have prepared magnetic graphite samples bombarded by protons at low
temperatures and low fluences to attenuate the large thermal annealing produced
during irradiation. An overall optimization of sample handling allowed us to
find Curie temperatures K at the used fluences. The
magnetization versus temperature shows unequivocally a linear dependence, which
can be interpreted as due to excitations of spin waves in a two dimensional
Heisenberg model with a weak uniaxial anisotropy.Comment: 4 pages, 3 figure
A multi-stable switchable metamaterial
The field of metamaterial research revolves around the idea of creating
artificial media that interact with light in a way unknown from naturally
occurring materials. This is commonly achieved by creating sub-wavelength
lattices of electronic or plasmonic structures, so-called meta-atoms, that
determine the interaction between light and metamaterial. One of the ultimate
goals for these tailored media is the ability to control their properties
in-situ which has led to a whole new branch of tunable and switchable
metamaterials. Many of the present realizations rely on introducing
microelectromechanical actuators or semiconductor elements into their meta-atom
structures. Here we show that superconducting quantum interference devices
(SQUIDs) can be used as fast, intrinsically switchable meta-atoms. We found
that their intrinsic nonlinearity leads to simultaneously stable dynamic
states, each of which is associated with a different value and sign of the
magnetic susceptibility in the microwave domain. Moreover, we demonstrate that
it is possible to switch between these states by applying a nanosecond long
pulse in addition to the microwave probe signal. Apart from potential
applications such as, for example, an all-optical metamaterial switch, these
results suggest that multi-stability, which is a common feature in many
nonlinear systems, can be utilized to create new types of meta-atoms
-Electron Ferromagnetism in Metal Free Carbon Probed by Soft X-Ray Dichroism
Elemental carbon represents a fundamental building block of matter and the
possibility of ferromagnetic order in carbon attracted widespread attention.
However, the origin of magnetic order in such a light element is only poorly
understood and has puzzled researchers. We present a spectromicroscopy study at
room temperature of proton irradiated metal free carbon using the elemental and
chemical specificity of x-ray magnetic circular dichroism (XMCD). We
demonstrate that the magnetic order in the investigated system originates only
from the carbon -electron system.Comment: 10 pages 3 color figure
TUBSAT, Low Cost Access to Space Technology
TUBSAT is a low cost platform for space technology experiments. Although its size is modest (micro-satellite class), it provides remarkable volume and space to the experiment. TUBSAT-A has been launched already and is not attitude controlled. TUBSAT-B and consecutive models will be provided with high accuracy (arc sec) attitude control to any desired direction via star sensor plus 3 reaction wheels. The launch of TUBSAT-B is presently scheduled for October 1993
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