466 research outputs found
Coaxial Atomic Force Microscope Tweezers
We demonstrate coaxial atomic force microscope (AFM) tweezers that can trap
and place small objects using dielectrophoresis (DEP). An attractive force is
generated at the tip of a coaxial AFM probe by applying a radio frequency
voltage between the center conductor and a grounded shield; the origin of the
force is found to be DEP by measuring the pull-off force vs. applied voltage.
We show that the coaxial AFM tweezers (CAT) can perform three dimensional
assembly by picking up a specified silica microsphere, imaging with the
microsphere at the end of the tip, and placing it at a target destination.Comment: 9 pages, 3 figures, in review at Applied Physics Letter
Coulomb scattering cross-section in a 2D electron gas and production of entangled electrons
We calculate the Coulomb scattering amplitude for two electrons injected with
opposite momenta in an interacting 2DEG. We include the effect of the Fermi
liquid background by solving the 2D Bethe-Salpeter equation for the
two-particle Green function vertex, in the ladder and random phase
approximations. This result is used to discuss the feasibility of producing
spin EPR pairs in a 2DEG by collecting electrons emerging from collisions at a
pi/2 scattering angle, where only the entangled spin-singlets avoid the
destructive interference resulting from quantum indistinguishability.
Furthermore, we study the effective 2D electron-electron interaction due to the
exchange of virtual acoustic and optical phonons, and compare it to the Coulomb
interaction. Finally, we show that the 2D Kohn-Luttinger pairing instability
for the scattering electrons is negligible in a GaAs 2DEG.Comment: 19 pages, 10 figure
Formation and decay of electron-hole droplets in diamond
We study the formation and decay of electron-hole droplets in diamonds at
both low and high temperatures under different excitations by master equations.
The calculation reveals that at low temperature the kinetics of the system
behaves as in direct-gap semiconductors, whereas at high temperature it shows
metastability as in traditional indirect-gap semiconductors. Our results at low
temperature are consistent with the experimental findings by Nagai {\em et al.}
[Phys. Rev. B {\bf 68}, 081202 (R) (2003)]. The kinetics of the e-h system in
diamonds at high temperature under both low and high excitations is also
predicted.Comment: 7 pages, 8 figures, revised with some modifications in physics
discussion, to be published in PR
Scaling of transverse nuclear magnetic relaxation due to magnetic nanoparticle aggregation
The aggregation of superparamagnetic iron oxide (SPIO) nanoparticles
decreases the transverse nuclear magnetic resonance (NMR) relaxation time T2 of
adjacent water molecules measured by a Carr-Purcell-Meiboom-Gill (CPMG)
pulse-echo sequence. This effect is commonly used to measure the concentrations
of a variety of small molecules. We perform extensive Monte Carlo simulations
of water diffusing around SPIO nanoparticle aggregates to determine the
relationship between T2 and details of the aggregate. We find that in the
motional averaging regime T2 scales as a power law with the number N of
nanoparticles in an aggregate. The specific scaling is dependent on the fractal
dimension d of the aggregates. We find T2 N^{-0.44} for aggregates with d=2.2,
a value typical of diffusion limited aggregation. We also find that in
two-nanoparticle systems, T2 is strongly dependent on the orientation of the
two nanoparticles relative to the external magnetic field, which implies that
it may be possible to sense the orientation of a two-nanoparticle aggregate. To
optimize the sensitivity of SPIO nanoparticle sensors, we propose that it is
best to have aggregates with few nanoparticles, close together, measured with
long pulse-echo times.Comment: 20 pages, 3 figures, submitted to Journal of Magnetism and Magnetic
Material
Effect of primary organic sea spray emissions on cloud condensation nuclei concentrations
This work estimates the primary marine organic aerosol global emission source and its impact on cloud condensation nuclei (CCN) concentrations by implementing an organic sea spray source function into a series of global aerosol simulations. The source function assumes that a fraction of the sea spray emissions, depending on the local chlorophyll concentration, is organic matter in place of sea salt. Effect on CCN concentrations (at 0.2% supersaturation) is modeled using the Two-Moment Aerosol Sectional (TOMAS) microphysics algorithm coupled to the GISS II-prime general circulation model. The presence of organics affects CCN activity in competing ways: by reducing the amount of solute available in the particle and decreasing surface tension of CCN. To model surfactant effects, surface tension depression data from seawater samples taken near the Georgia coast were applied as a function of carbon concentrations. A global marine organic aerosol emission rate of 17.7 Tg C yr<sup>−1</sup> is estimated from the simulations. Marine organics exert a localized influence on CCN(0.2%) concentrations, decreasing regional concentrations by no more than 5% and by less than 0.5% over most of the globe, assuming direct replacement of sea salt aerosol with organic aerosol. The decrease in CCN concentrations results from the fact that the decrease in particle solute concentration outweighs the organic surfactant effects. The low sensitivity of CCN(0.2%) to the marine organic emissions is likely due to the small compositional changes: the mass fraction of OA in accumulation mode aerosol increases by only ~15% in a biologically active region of the Southern Ocean. To test the sensitivity to uncertainty in the sea spray emissions process, we relax the assumption that sea spray aerosol number and mass remain fixed and instead can add to sea spray emissions rather than replace existing sea salt. In these simulations, we find that marine organic aerosol can increase CCN by up to 50% in the Southern Ocean and 3.7% globally during the austral summer. This vast difference in CCN impact highlights the need for further observational exploration of the sea spray aerosol emission process as well as evaluation and development of model parameterizations
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