1,544 research outputs found
Matrix effects in ion-induced emission as observed in Ne collisions with Cu-Mg and Cu-Al alloys
Ion induced Auger electron emission is used to study the surfaces of Al, Mg, Cu - 10 at. % Al, Cu - 19.6 at. % Al, and Cu - 7.4 at. % Mg. A neon (Ne) ion beam whose energy is varied from 0.5 to 3 keV is directed at the surface. Excitation of the lighter Ne occurs by the promotion mechanism of Barat and Lichten in asymmetric collisions with Al or Mg atoms. Two principal Auger peaks are observed in the Ne spectrum: one at 22 eV and one at 25 eV. Strong matrix effects are observed in the alloys as a function of energy in which the population of the second peak is greatly enhanced relative to the first over the pure materials. For the pure material over this energy range this ratio is 1.0. For the alloys it can rise to the electronic structure of alloys and to other surface tools such as secondary ion mass spectroscopy
Variation of the hopping exponent in disordered silicon MOSFETs
We observe a complex change in the hopping exponent value from 1/2 to 1/3 as
a function of disorder strength and electron density in a sodium-doped silicon
MOSFET. The disorder was varied by applying a gate voltage and thermally
drifting the ions to different positions in the oxide. The same gate was then
used at low temperature to modify the carrier concentration.
Magnetoconductivity measurements are compatible with a change in transport
mechanisms when either the disorder or the electron density is modified
suggesting a possible transition from a Mott insulator to an Anderson insulator
in these systems.Comment: 6 pages, 5 figure
Metal-insulator transition at B=0 in a dilute two dimensional GaAs-AlGaAs hole gas
We report the observation of a metal insulator transition at B=0 in a high
mobility two dimensional hole gas in a GaAs-AlGaAs heterostructure. A clear
critical point separates the insulating phase from the metallic phase,
demonstrating the existence of a well defined minimum metallic conductivity
sigma(min)=2e/h. The sigma(T) data either side of the transition can be
`scaled' on to one curve with a single parameter (To). The application of a
parallel magnetic field increases sigma(min) and broadens the transition. We
argue that strong electron-electron interactions (rs = 10) destroy phase
coherence, removing quantum intereference corrections to the conductivity.Comment: 4 pages RevTex + 4 figures. Submitted to PRL. Originally posted 22
September 1997. Revised 12 October 1997 - minor changes to referencing,
figure cations and figure
Parallel quantized charge pumping
Two quantized charge pumps are operated in parallel. The total current
generated is shown to be far more accurate than the current produced with just
one pump operating at a higher frequency. With the application of a
perpendicular magnetic field the accuracy of quantization is shown to be 20
ppm for a current of pA. The scheme for parallel pumping presented in
this work has applications in quantum information processing, the generation of
single photons in pairs and bunches, neural networking and the development of a
quantum standard for electrical current. All these applications will benefit
greatly from the increase in output current without the characteristic decrease
in accuracy as a result of high-frequency operation
MOBSTER – III. HD 62658: a magnetic Bp star in an eclipsing binary with a non-magnetic ‘identical twin’
HD 62658 (B9p V) is a little-studied chemically peculiar star. Light curves obtained by the Kilodegree Extremely Little Telescope (KELT) and Transiting Exoplanet Survey Satellite (TESS) show clear eclipses with a period of about 4.75 d, as well as out-of-eclipse brightness modulation with the same 4.75 d period, consistent with synchronized rotational modulation of surface chemical spots. High-resolution ESPaDOnS circular spectropolarimetry shows a clear Zeeman signature in the line profile of the primary; there is no indication of a magnetic field in the secondary. PHOEBE modelling of the light curve and radial velocities indicates that the two components have almost identical masses of about 3 M_⊙. The primary’s longitudinal magnetic field〈B_z〉 varies between about +100 and −250 G, suggesting a surface magnetic dipole strength B_d = 850 G. Bayesian analysis of the Stokes V profiles indicates B_d = 650 G for the primary and B_d < 110 G for the secondary. The primary’s line profiles are highly variable, consistent with the hypothesis that the out-of-eclipse brightness modulation is a consequence of rotational modulation of that star’s chemical spots. We also detect a residual signal in the light curve after removal of the orbital and rotational modulations, which might be pulsational in origin; this could be consistent with the weak line profile variability of the secondary. This system represents an excellent opportunity to examine the consequences of magnetic fields for stellar structure via comparison of two stars that are essentially identical with the exception that one is magnetic. The existence of such a system furthermore suggests that purely environmental explanations for the origin of fossil magnetic fields are incomplete
In Situ, On-Demand Lubrication System Developed for Space Mechanisms
Many moving mechanical assemblies (MMA) for space mechanisms rely on liquid lubricants to provide reliable, long-term performance. The proper performance of the MMA is critical in assuring a successful mission. Historically, mission lifetimes were short and MMA duty cycles were minimal. As mission lifetimes were extended, other components, such as batteries and computers, failed before lubricated systems. However, improvements in these ancillary systems over the last decade have left the tribological systems of the MMAs as the limiting factor in determining spacecraft reliability. Typically, MMAs are initially lubricated with a very small charge that is supposed to last the entire mission lifetime, often well in excess of 5 years. In many cases, the premature failure of a lubricated component can result in mission failure
Evidence for multiple impurity bands in sodium-doped silicon MOSFETs
We report measurements of the temperature-dependent conductivity in a silicon
metal-oxide-semiconductor field-effect transistor that contains sodium
impurities in the oxide layer. We explain the variation of conductivity in
terms of Coulomb interactions that are partially screened by the proximity of
the metal gate. The study of the conductivity exponential prefactor and the
localization length as a function of gate voltage have allowed us to determine
the electronic density of states and has provided arguments for the presence of
two distinct bands and a soft gap at low temperature.Comment: 4 pages; 5 figures; Published in PRB Rapid-Communication
Spiral Orbit Tribometer
The spiral orbit tribometer (SOT) bridges the gap between full-scale life testing and typically unrealistic accelerated life testing of ball-bearing lubricants in conjunction with bearing ball and race materials. The SOT operates under realistic conditions and quickly produces results, thereby providing information that can guide the selection of lubricant, ball, and race materials early in a design process. The SOT is based upon a simplified, retainerless thrust bearing comprising one ball between flat races (see figure). The SOT measures lubricant consumption and degradation rates and friction coefficients in boundary lubricated rolling and pivoting contacts. The ball is pressed between the lower and upper races with a controlled force and the lower plate is rotated. The combination of load and rotation causes the ball to move in a nearly circular orbit that is, more precisely, an opening spiral. The spiral s pitch is directly related to the friction coefficient. At the end of the orbit, the ball contacts the guide plate, restoring the orbit to its original radius. The orbit is repeatable throughout the entire test. A force transducer, mounted in-line with the guide plate, measures the force between the ball and the guide plate, which directly relates to the friction coefficient. The SOT, shown in the figure, can operate in under ultra-high vacuum (10(exp -9) Torr) or in a variety of gases at atmospheric pressure. The load force can be adjusted between 45 and 450 N. By varying the load force and ball diameter, mean Hertzian stresses between 0.5 and 5.0 GPa can be obtained. The ball s orbital speed range is between 1 and 100 rpm
A Photometric Survey for Variables and Transits in the Field of Praesepe with KELT
The Kilodegree Extremely Little Telescope (KELT) project is a small aperture,
wide-angle search for planetary transits of solar-type stars. In this paper, we
present the results of a commissioning campaign with the KELT telescope to
observe the open cluster Praesepe for 34 nights in early 2005. Lightcurves were
obtained for 69,337 stars, out of which we identify 58 long period variables
and 152 periodic variables. Sixteen of these are previously known as variable,
yielding 194 newly discovered variable stars for which we provide properties
and lightcurves. We also searched for planetary-like transits, finding four
transit candidates. Follow-up observations indicate that two of the candidates
are astrophysical false positives, with two candidates remaining as potential
planetary transits.Comment: 45 pages, 16 figures. Submitted to AJ. PDF version with full
resolution figures located at
http://www.astronomy.ohio-state.edu/~pepper/kelt.pd
Finite size scaling for quantum criticality using the finite-element method
Finite size scaling for the Schr\"{o}dinger equation is a systematic approach
to calculate the quantum critical parameters for a given Hamiltonian. This
approach has been shown to give very accurate results for critical parameters
by using a systematic expansion with global basis-type functions. Recently, the
finite element method was shown to be a powerful numerical method for ab initio
electronic structure calculations with a variable real-space resolution. In
this work, we demonstrate how to obtain quantum critical parameters by
combining the finite element method (FEM) with finite size scaling (FSS) using
different ab initio approximations and exact formulations. The critical
parameters could be atomic nuclear charges, internuclear distances, electron
density, disorder, lattice structure, and external fields for stability of
atomic, molecular systems and quantum phase transitions of extended systems. To
illustrate the effectiveness of this approach we provide detailed calculations
of applying FEM to approximate solutions for the two-electron atom with varying
nuclear charge; these include Hartree-Fock, density functional theory under the
local density approximation, and an "exact"' formulation using FEM. We then use
the FSS approach to determine its critical nuclear charge for stability; here,
the size of the system is related to the number of elements used in the
calculations. Results prove to be in good agreement with previous Slater-basis
set calculations and demonstrate that it is possible to combine finite size
scaling with the finite-element method by using ab initio calculations to
obtain quantum critical parameters. The combined approach provides a promising
first-principles approach to describe quantum phase transitions for materials
and extended systems.Comment: 15 pages, 19 figures, revision based on suggestions by referee,
accepted in Phys. Rev.
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