811 research outputs found
A fundamental limit to the efficiency of spin-exchange optical pumping of 3He nuclei
We establish the existence of a fundamental limit to the efficiency of
spin-exchange optical pumping of 3He nuclei by collisions with spin-polarized
alkali-metal atoms. Using accurate ab initio calculations of molecular
interactions and scattering properties, we show that the maximum 3He spin
polarization that can be achieved in spin-exchange collisions with potassium
(39K) and silver (107Ag) atoms is limited by the anisotropic hyperfine
interaction. We find that spin exchange in Ag-He collisions occurs much faster
than in K-He collisions, suggesting the possibility of using Ag in
spin-exchange optical pumping experiments to increase the production rate of
hyperpolarized 3He. Our analysis indicates that measurements of trap loss rates
of 2S atoms in the presence of cold 3He gas may be used to probe anisotropic
spin-exchange interactions in atom-He collisions.Comment: 5 pages, 4 figure
Study of the correlation effects in Yb^+ and implications for parity violation
Calculation of the energies, magnetic dipole hyperfine structure constants,
E1 transition amplitudes between the low-lying states, and nuclear
spin-dependent parity-nonconserving amplitudes for the ^2S_{1/2} -
^2D_{3/2,5/2} transitions in ^{171}Yb^+ ion is performed using two different
approaches. First, we carried out many-body perturbation theory calculation
considering Yb^+ as a monovalent system. Additional all-order calculations are
carried out for selected properties. Second, we carried out configuration
interaction calculation considering Yb as a 15-electron system and compared the
results obtained by two methods. The accuracy of different methods is
evaluated. We find that the monovalent description is inadequate for evaluation
of some atomic properties due to significant mixing of the one-particle and the
hole-two-particle configurations. Performing the calculation by such different
approaches allowed us to establish the importance of various correlation
effects for Yb^+ atomic properties for future improvement of theoretical
precision in this complicated system.Comment: 11 pages;v2: minor changes and one reference adde
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Mechanical Properties and Biological Responses of Bioactive Glass Ceramics Processed using Indirect SLS
This paper will report on research which aims to generate bone replacement components by
processing bioactive glass-ceramic powders using indirect selective laser sintering. The indirect
SLS route has been chosen as it offers the ability to tailor the shape of the implant to the
implantation site, and two bioactive glass ceramic materials have been processed through this
route: apatite-mullite and apatite-wollostanite. The results of bend tests, to investigate
mechanical properties, and in vitro and in vivo experiments to investigate biological responses of
the materials will be reported, and the suitability of completed components for implant will be
assessed.Mechanical Engineerin
Van der Waals Interactions in DFT using Wannier Functions: improved and coefficients by a new approach
A new implementation is proposed for including van der Waals interactions in
Density Functional Theory using the Maximally-Localized Wannier functions. With
respect to the previous DFT/vdW-WF method, the present DFT/vdW-WF2 approach,
which is based on the simpler London expression and takes into account the
intrafragment overlap of the localized Wannier functions, leads to a
considerable improvement in the evaluation of the van der Waals
coefficients, as shown by the application to a set of selected dimers.
Preliminary results on Ar on graphite and Ne on the Cu(111) metal surface
suggest that also the coefficients, characterizing molecule-surfaces van
der Waals interactions are better estimated with the new scheme.Comment: 5 pages, 2 table
Power-law carrier dynamics in semiconductor nanocrystals at nanosecond time scales
We report the observation of power law dynamics on nanosecond to microsecond
time scales in the fluorescence decay from semiconductor nanocrystals, and draw
a comparison between this behavior and power-law fluorescence blinking from
single nanocrystals. The link is supported by comparison of blinking and
lifetime data measured simultaneously from the same nanocrystal. Our results
reveal that the power law coefficient changes little over the nine decades in
time from 10 ns to 10 s, in contrast with the predictions of some diffusion
based models of power law behavior.Comment: 3 pages, 2 figures, compressed for submission to Applied Physics
Letter
Coulomb interactions in single, charged self-assembled quantum dots: radiative lifetime and recombination energy
We present results on the charge dependence of the radiative recombination
lifetime, Tau, and the emission energy of excitons confined to single
self-assembled InGaAs quantum dots. There are significant dot-to-dot
fluctuations in the lifetimes for a particular emission energy. To reach
general conclusions, we present the statistical behavior by analyzing data
recorded on a large number of individual quantum dots. Exciton charge is
controlled with extremely high fidelity through an n-type field effect
structure, providing access to the neutral exciton (X0), the biexciton (2X0)
and the positively (X1+) and negatively (X1-) charged excitons. We find
significant differences in the recombination lifetime of each exciton such
that, on average, Tau(X1-) / Tau(X0) = 1.25, Tau(X1+) / Tau(X0) = 1.58 and
Tau(2X0) / Tau(X0) = 0.65. We attribute the change in lifetime to significant
changes in the single particle hole wave function on charging the dot, an
effect more pronounced on charging X0 with a single hole than with a single
electron. We verify this interpretation by recasting the experimental data on
exciton energies in terms of Coulomb energies. We show directly that the
electron-hole Coulomb energy is charge dependent, reducing in value by 5-10% in
the presence of an additional electron, and that the electron-electron and
hole-hole Coulomb energies are almost equal.Comment: 8 pages, 7 figures, submitted to Phys. Rev.
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