34,280 research outputs found
Co-rich cobalt platinum nanowire arrays: effects of annealing
The effects of annealing on the crystal structure and magnetic properties of Co-rich cobalt platinum nanowire arrays embedded in anodic aluminium oxide membranes have been investigated. For this purpose, a rapid thermal annealing to temperatures of 300 °C to 800 °C has been used. Transmission electron microscopy and scanning electron microscopy show that the nanowires have a mean diameter of 14 nm and an estimated wire density of 7.8Ă—1010 cm-2. From x-ray diffraction patterns, we find that the nanowires are hcp and possess a preferred texture in which the c axis of the grains tends to lie along the major axis of the wire. Vibrating sample magnetometry measurements indicate that the easy axis is along the nanowire axis direction. Hysteresis loops, saturation magnetization, squareness ratio (Mr/Ms), and coercivity (perpendicular and parallel to the nanowire axis) have all been investigated as a function of the annealing temperature (TA). Coercivity parallel to the wire axis first increases with TA, attains a maximum at 600 °C (which is 150% of the as-deposited sample), and then decreases. By contrast there is relatively little change in the coercivity measured perpendicular to the wires. The saturation magnetization for the as-deposited sample is 1360 emu/cc and remains almost constant for annealing temperatures up to 500 °C: for TA>500 °C it decreases significantly. The maximum (Mr/Ms) ratio attained in this study is 0.99, the highest value reported thus far for cobalt platinum alloy nanowires. The data suggest that these materials are potential candidates for high-density magnetic recording media
Effect of pooling samples on the efficiency of comparative studies using microarrays
Many biomedical experiments are carried out by pooling individual biological
samples. However, pooling samples can potentially hide biological variance and
give false confidence concerning the data significance. In the context of
microarray experiments for detecting differentially expressed genes, recent
publications have addressed the problem of the efficiency of sample-pooling,
and some approximate formulas were provided for the power and sample size
calculations. It is desirable to have exact formulas for these calculations and
have the approximate results checked against the exact ones. We show that the
difference between the approximate and exact results can be large. In this
study, we have characterized quantitatively the effect of pooling samples on
the efficiency of microarray experiments for the detection of differential gene
expression between two classes. We present exact formulas for calculating the
power of microarray experimental designs involving sample pooling and technical
replications. The formulas can be used to determine the total numbers of arrays
and biological subjects required in an experiment to achieve the desired power
at a given significance level. The conditions under which pooled design becomes
preferable to non-pooled design can then be derived given the unit cost
associated with a microarray and that with a biological subject. This paper
thus serves to provide guidance on sample pooling and cost effectiveness. The
formulation in this paper is outlined in the context of performing microarray
comparative studies, but its applicability is not limited to microarray
experiments. It is also applicable to a wide range of biomedical comparative
studies where sample pooling may be involved.Comment: 8 pages, 1 figure, 2 tables; to appear in Bioinformatic
Random solids and random solidification: What can be learned by exploring systems obeying permanent random constraints?
In many interesting physical settings, such as the vulcanization of rubber,
the introduction of permanent random constraints between the constituents of a
homogeneous fluid can cause a phase transition to a random solid state. In this
random solid state, particles are permanently but randomly localized in space,
and a rigidity to shear deformations emerges. Owing to the permanence of the
random constraints, this phase transition is an equilibrium transition, which
confers on it a simplicity (at least relative to the conventional glass
transition) in the sense that it is amenable to established techniques of
equilibrium statistical mechanics. In this Paper I shall review recent
developments in the theory of random solidification for systems obeying
permanent random constraints, with the aim of bringing to the fore the
similarities and differences between such systems and those exhibiting the
conventional glass transition. I shall also report new results, obtained in
collaboration with Weiqun Peng, on equilibrium correlations and
susceptibilities that signal the approach of the random solidification
transition, discussing the physical interpretation and values of these
quantities both at the Gaussian level of approximation and, via a
renormalization-group approach, beyond.Comment: Paper presented at the "Unifying Concepts in Glass Physics" workshop,
International Centre for Theoretical Physics, Trieste, Italy (September
15-18, 1999
A discrete time-dependent method for metastable atoms in intense fields
The full-dimensional time-dependent Schrodinger equation for the electronic
dynamics of single-electron systems in intense external fields is solved
directly using a discrete method.
Our approach combines the finite-difference and Lagrange mesh methods. The
method is applied to calculate the quasienergies and ionization probabilities
of atomic and molecular systems in intense static and dynamic electric fields.
The gauge invariance and accuracy of the method is established. Applications to
multiphoton ionization of positronium and hydrogen atoms and molecules are
presented. At very high intensity above saturation threshold, we extend the
method using a scaling technique to estimate the quasienergies of metastable
states of the hydrogen molecular ion. The results are in good agreement with
recent experiments.Comment: 10 pages, 9 figure, 4 table
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Fine-Scale Variations in Eucritic Pyroxene FeO/MnO: Process vs. Provenance.
Most asteroidal igneous rocks are eucrite-like basalts and gabbros, composed mostly of ferroan low- and high-Ca pyroxenes and calcic plagioclase, plus smaller amounts of silica (most commonly tridymite), ilmenite, chromite, troilite, Ca-phosphate, metal and sometimes ferroan olivine. Eucrite-like mafic rocks are fragments of the crusts of differentiated asteroids, and most are likely from 4 Vesta
Variation of the Fine-Structure Constant from the de Sitter Invariant Special Relativity
There are obvious discrepancies among various experimental constraints on the
variation of the fine-structure constant, . We attempt to discuss the
issue in the framework of de Sitter invariant Special Relativity () and to present a possible solution to the disagreement. In
addition, on the basis of the observational data and the discussions presented
in this Letter, we derive a rough theoretical estimate of the radius of the
Universe.Comment: 8 pages, no figure
Single-particle-sensitive imaging of freely propagating ultracold atoms
We present a novel imaging system for ultracold quantum gases in expansion.
After release from a confining potential, atoms fall through a sheet of
resonant excitation laser light and the emitted fluorescence photons are imaged
onto an amplified CCD camera using a high numerical aperture optical system.
The imaging system reaches an extraordinary dynamic range, not attainable with
conventional absorption imaging. We demonstrate single-atom detection for
dilute atomic clouds with high efficiency where at the same time dense
Bose-Einstein condensates can be imaged without saturation or distortion. The
spatial resolution can reach the sampling limit as given by the 8 \mu m pixel
size in object space. Pulsed operation of the detector allows for slice images,
a first step toward a 3D tomography of the measured object. The scheme can
easily be implemented for any atomic species and all optical components are
situated outside the vacuum system. As a first application we perform
thermometry on rubidium Bose-Einstein condensates created on an atom chip.Comment: 24 pages, 10 figures. v2: as publishe
Dynamically generated electric charge distributions in Abelian projected SU(2) lattice gauge theories
We show in the maximal Abelian gauge the dynamical electric charge density
generated by the coset fields, gauge fixing and ghosts shows antiscreening as
in the case of the non-Abelian charge. We verify that with the completion of
the ghost term all contributions to flux are accounted for in an exact lattice
Ehrenfest relation.Comment: LATTICE98(confine
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The tarantula toxin GxTx detains K+ channel gating charges in their resting conformation.
Allosteric ligands modulate protein activity by altering the energy landscape of conformational space in ligand-protein complexes. Here we investigate how ligand binding to a K+ channel's voltage sensor allosterically modulates opening of its K+-conductive pore. The tarantula venom peptide guangxitoxin-1E (GxTx) binds to the voltage sensors of the rat voltage-gated K+ (Kv) channel Kv2.1 and acts as a partial inverse agonist. When bound to GxTx, Kv2.1 activates more slowly, deactivates more rapidly, and requires more positive voltage to reach the same K+-conductance as the unbound channel. Further, activation kinetics are more sigmoidal, indicating that multiple conformational changes coupled to opening are modulated. Single-channel current amplitudes reveal that each channel opens to full conductance when GxTx is bound. Inhibition of Kv2.1 channels by GxTx results from decreased open probability due to increased occurrence of long-lived closed states; the time constant of the final pore opening step itself is not impacted by GxTx. When intracellular potential is less than 0 mV, GxTx traps the gating charges on Kv2.1's voltage sensors in their most intracellular position. Gating charges translocate at positive voltages, however, indicating that GxTx stabilizes the most intracellular conformation of the voltage sensors (their resting conformation). Kinetic modeling suggests a modulatory mechanism: GxTx reduces the probability of voltage sensors activating, giving the pore opening step less frequent opportunities to occur. This mechanism results in K+-conductance activation kinetics that are voltage-dependent, even if pore opening (the rate-limiting step) has no inherent voltage dependence. We conclude that GxTx stabilizes voltage sensors in a resting conformation, and inhibits K+ currents by limiting opportunities for the channel pore to open, but has little, if any, direct effect on the microscopic kinetics of pore opening. The impact of GxTx on channel gating suggests that Kv2.1's pore opening step does not involve movement of its voltage sensors
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