27,231 research outputs found
Giant magnetoresistance of multiwall carbon nanotubes: modeling the tube/ferromagnetic-electrode burying contact
We report on the giant magnetoresistance (GMR) of multiwall carbon nanotubes
with ultra small diameters. In particular, we consider the effect of the
inter-wall interactions and the lead/nanotube coupling. Comparative studies
have been performed to show that in the case when all walls are well coupled to
the electrodes, the so-called inverse GMR can appear. The tendency towards a
negative GMR depends on the inter-wall interaction and on the nanotube le ngth.
If, however, the inner nanotubes are out of contact with one of the electrodes,
the GMR remains positive even for relatively strong inter-wall interactions
regardless of the outer nanotube length. These results shed additional light on
recently reported experimental data, where an inverse GMR was found in some
multiwall carbon nanotube samples.Comment: 5 pages, 5 figure
Isotopic dependence of the giant monopole resonance in the even-A ^{112-124}Sn isotopes and the asymmetry term in nuclear incompressibility
The strength distributions of the giant monopole resonance (GMR) have been
measured in the even-A Sn isotopes (A=112--124) with inelastic scattering of
400-MeV particles in the angular range
--. We find that the experimentally-observed GMR energies
of the Sn isotopes are lower than the values predicted by theoretical
calculations that reproduce the GMR energies in Pb and Zr very
well. From the GMR data, a value of MeV is obtained
for the asymmetry-term in the nuclear incompressibility.Comment: Submitted to Physical Review Letters. 10 pages; 4 figure
The Giant Monopole Resonance in Pb isotopes
The extraction of the nuclear incompressibility from the isoscalar giant
monopole resonance (GMR) measurements is analysed. Both pairing and mutually
enhanced magicity (MEM) effects play a role in the shift of the GMR energy
between the doubly closed shell Pb nucleus and other Pb isotopes.
Pairing effects are microscopically predicted whereas the MEM effect is
phenomenologically evaluated. Accurate measurements of the GMR in open-shell Pb
isotopes are called for.Comment: 4 page
A nonparametric Bayesian approach toward robot learning by demonstration
In the past years, many authors have considered application of machine learning methodologies to effect robot learning by demonstration. Gaussian mixture regression (GMR) is one of the most successful methodologies used for this purpose. A major limitation of GMR models concerns automatic selection of the proper number of model states, i.e., the number of model component densities. Existing methods, including likelihood- or entropy-based criteria, usually tend to yield noisy model size estimates while imposing heavy computational requirements. Recently, Dirichlet process (infinite) mixture models have emerged in the cornerstone of nonparametric Bayesian statistics as promising candidates for clustering applications where the number of clusters is unknown a priori. Under this motivation, to resolve the aforementioned issues of GMR-based methods for robot learning by demonstration, in this paper we introduce a nonparametric Bayesian formulation for the GMR model, the Dirichlet process GMR model. We derive an efficient variational Bayesian inference algorithm for the proposed model, and we experimentally investigate its efficacy as a robot learning by demonstration methodology, considering a number of demanding robot learning by demonstration scenarios
Ab-initio-calculations of the GMR-effect in Fe/V multilayers
In a self-consistent semi-empirical numerical approach based on
ab-initio-calculations for small samples, we evaluate the GMR effect for
disordered (001)-(3--Fe/3--V) multilayers by means of a Kubo
formalism. We consider four different types of disorder arrangements: In case
(i) and (ii), the disorder consists in the random interchange of some Fe and V
atoms, respectively, at interface layers; in case (iii) in the formation of
small groups of three substitutional Fe atoms in a V interface layer and a
similar V group in a Fe layer at a different interface; and for case (iv) in
the substitution of some V atoms in the innermost V layers by Fe. For cases (i)
and (ii), depending on the distribution of the impurities, the GMR effect is
enhanced or reduced by increasing disorder, in case (iii) the GMR effect is
highest, whereas finally, in case (iv), a negative GMR is obtained (''inverse
GMR'').Comment: LaTex, 30 pages, including 16 drawings; to appear in JMM
Calculation of Giant Magnetoresistance in Laterally Confined Multilayers
We have studied the Giant Magnetoresistance (GMR) for laterally confined
multilayers, e.g., layers of wires, using the classical Boltzmann equation in
the current-in-plane (CIP) geometry. For spin-independent specularity factors
at the sides of the wires we find that the GMR due to bulk and surface
scattering decreases with lateral confinement. The length scale at which this
occurs is of order the film thickness and the mean free paths. The precise
prefactor depends on the relative importance of surface and bulk scattering
anisotropies. For spin-dependent specularity factors at the sides of the wires
the GMR can increase in some cases with decreasing width. The origin of the
change in the GMR in both cases can be understood in terms of lateral
confinement changing the effective mean free paths within the layers.Comment: 18 pages, 7 figure
Magnetoresistance and structural study of electrodeposited Ni-Cu/Cu multilayers
Electrodeposition was used to produce Ni Cu/Cu multilayers by two-pulse plating (galvanostatic/potentiostatic control) from a single sulfate/sulfamate electrolyte at an optimized Cu deposition potential for the first time. Magnetoresistance measurements were carried out at room temperature for the Ni Cu/Cu multilayers as a function of the Ni Cu and Cu layer thicknesses and the electrolyte Cu2+ ion concentration. Multilayers with Cu layer thicknesses above 2 nm exhibited a giant magnetoresistance (GMR) effect with a dominating ferromagnetic contribution and with low saturation fields (below 1 kOe). A significant contribution from superparamagnetic (SPM) regions with high saturation fields occurred only for very small nominal magnetic layer thicknesses (around 1 nm). The presence of SPM regions was concluded from the GMR data also for thick magnetic layers with high Cu contents. This hints at a significant phase-separation in Ni-Cu alloys at low-temperature processing, in agreement with previous theoretical modeling and experiments. Low-temperature measurements performed on a selected multilayer down to 18 K indicated a strong increase of the GMR as compared to the room-temperature GMR. Structural studies of some multilayer deposits exhibiting GMR were performed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The XRD patterns of Ni Cu/Cu multilayers exhibited in most cases clear satellite peaks, indicating a superlattice structure which was confirmed also by cross-sectional TEM. The deterioration of the multilayer structure revealed by XRD for high Cu-contents in the magnetic layer confirmed the phase-separation concluded from the GMR data
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