99 research outputs found
First principle study of intrinsic defects in hexagonal tungsten carbide
The characteristics of intrinsic defects are important for the understanding
of self-diffusion processes, mechanical strength, brittleness, and plasticity
of tungsten carbide, which present in the divertor of fusion reactors. Here, we
use first-principles calculations to investigate the stability of point defects
and their complexes in WC. Our calculation results confirm that the formation
energies of carbon defects are much lower than that of tungsten defects. The
outward relaxations around vacancy are found. Both interstitial carbon and
interstitial tungsten atom prefer to occupy the carbon basal plane projection
of octahedral interstitial site. The results of isolated carbon defect
diffusion show that the carbon vacancy stay for a wide range of temperature
because of extremely high diffusion barriers, while carbon interstitial
migration is activated at lower temperatures for its considerable lower
activation energy. These results provide evidence for the presumption that the
800K stage is attributed by the annealing out of carbon vacancies by long-range
migration.Comment: Submitted to Journal of Nuclear Material
Quantization of adiabatic pumped charge in the presence of superconducting lead
We investigate the parametric electron pumping of a double barrier structure
in the presence of a superconducting lead. The parametric pumping is
facilitated by cyclic variation of the barrier heights and of the
barriers. In the weak coupling regime, there exists a resonance line in the
parameter space so that the energy of the quasi-bound state is in
line with the incoming Fermi energy. Levinson et al found recently that the
pumped charge for each pumping cycle is quantized with for normal
structure when the pumping contour encircles the resonance line. In the
presence of a superconducting lead, we find that the pumped charge is quantized
with the value
Optimal quantum pump in the presence of a superconducting lead
We investigate the parametric pumping of a hybrid structure consisting of a
normal quantum dot, a normal lead and a superconducting lead. Using the time
dependent scattering matrix theory, we have derived a general expression for
the pumped electric current and heat current. We have also derived the
relationship among the instantaneous pumped heat current, electric current, and
shot noise. This gives a lower bound for the pumped heat current in the hybrid
system similar to that of the normal case obtained by Avron et al
Silicon 1s Near Edge X-ray Absorption Fine Structure Spectroscopy of Functionalized Silicon Nanocrystals
NSERCPeer ReviewedSilicon 1s Near Edge X-ray Absorption Fine Structure (NEXAFS) spectra of silicon nanocrystals have been examined as a function of nanocrystal size (3 – 100 nm), varying surface functionalization (hydrogen or 1-pentyl termination), or embedded in oxide. The NEXAFS spectra are characterized as a function of nanocrystal size and surface functionalization. Clear spectroscopic evidence for long range order is observed Si-NCs that are 5-8 nm in diameter or larger. Energy shifts in the silicon 1s NEXAFS spectra of covalently functionalized silicon nanocrystals with changing size are attributed to surface chemical shifts and not to quantum confinement effects
Parametric quantum spin pump
We investigate a non-adiabatic parametric quantum pump consists of a
nonmagnetic scattering region connected by two ferromagnetic leads. The
presence of ferromagnetic leads allows electrons with different spins to
experience different potential landscape. Using this effect we propose a
quantum spin pump that drives spin-up electrons to flow in one direction and
spin-down electrons to flow in opposite direction. As a result, the spin pump
can deliver a spin current with vanishing charge current
Non-adiabatic charge pump: an exact solution
We derived a general and exact expression of current for quantum parametric
charge pumps in the non-adiabatic regime at finite pumping frequency and finite
driving amplitude. The non-perturbative theory predicts a remarkable plateau
structure in the pumped current due to multi-photon assisted processes in a
double-barrier quantum well pump involving only a {\it single} pumping
potential. It also predicts a current reversal as the resonant level of the
pump crosses the Fermi energy of the leads
Measurements of the observed cross sections for exclusive light hadron production in e^+e^- annihilation at \sqrt{s}= 3.773 and 3.650 GeV
By analyzing the data sets of 17.3 pb taken at GeV
and 6.5 pb taken at GeV with the BESII detector at the
BEPC collider, we have measured the observed cross sections for 12 exclusive
light hadron final states produced in annihilation at the two energy
points. We have also set the upper limits on the observed cross sections and
the branching fractions for decay to these final states at 90%
C.L.Comment: 8 pages, 5 figur
Search for the Rare Decays J/Psi --> Ds- e+ nu_e, J/Psi --> D- e+ nu_e, and J/Psi --> D0bar e+ e-
We report on a search for the decays J/Psi --> Ds- e+ nu_e + c.c., J/Psi -->
D- e+ nu_e + c.c., and J/Psi --> D0bar e+ e- + c.c. in a sample of 5.8 * 10^7
J/Psi events collected with the BESII detector at the BEPC. No excess of signal
above background is observed, and 90% confidence level upper limits on the
branching fractions are set: B(J/Psi --> Ds- e+ nu_e + c.c.)<4.8*10^-5, B(J/Psi
--> D- e+ nu_e + c.c.) D0bar e+ e- + c.c.)<1.1*10^-5Comment: 10 pages, 4 figure
Measurements of psi(2S) decays to octet baryon-antibaryon pairs
With a sample of 14 million psi(2S) events collected by the BESII detector at
the Beijing Electron Positron Collider (BEPC), the decay channels psi(2S)->p
p-bar, Lambda Lambda-bar, Sigma0 Sigma0-bar, Xi Xi-bar are measured, and their
branching ratios are determined to be (3.36+-0.09+-0.24)*10E-4,
(3.39+-0.20+-0.32)*10E-4, (2.35+-0.36+-0.32)*10E-4, (3.03+-0.40+-0.32)*10E-4,
respectively. In the decay psi(2S)->p p-bar, the angular distribution parameter
alpha is determined to be 0.82+-0.17+-0.04.Comment: 8 pages, 8 figure
Study of J/psi decays to Lambda Lambdabar and Sigma0 Sigma0bar
The branching ratios and Angular distributions for J/psi decays to Lambda
Lambdabar and Sigma0 Sigma0bar are measured using BESII 58 million J/psi.Comment: 11 pages, 5 figure
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