42,275 research outputs found
A Solid State Pulsed Coagulating Diathermy Instrument
Solid state pulsed coagulating diathermy instrumen
Fast and Robust Techniques for the Euclidean p-Median Problem with Uniform Weights
We discuss new solution techniques for the p-median problem, with the goal being to improve the solution time and quality of current techniques. In particular, we hybridize the discrete Lloyd algorithm and the vertex substitution heuristic. We also compare three starting point techniques and present a new solution method that provides consistently good results when appropriately initialized
Validating Sample Average Approximation Solutions with Negatively Dependent Batches
Sample-average approximations (SAA) are a practical means of finding
approximate solutions of stochastic programming problems involving an extremely
large (or infinite) number of scenarios. SAA can also be used to find estimates
of a lower bound on the optimal objective value of the true problem which, when
coupled with an upper bound, provides confidence intervals for the true optimal
objective value and valuable information about the quality of the approximate
solutions. Specifically, the lower bound can be estimated by solving multiple
SAA problems (each obtained using a particular sampling method) and averaging
the obtained objective values. State-of-the-art methods for lower-bound
estimation generate batches of scenarios for the SAA problems independently. In
this paper, we describe sampling methods that produce negatively dependent
batches, thus reducing the variance of the sample-averaged lower bound
estimator and increasing its usefulness in defining a confidence interval for
the optimal objective value. We provide conditions under which the new sampling
methods can reduce the variance of the lower bound estimator, and present
computational results to verify that our scheme can reduce the variance
significantly, by comparison with the traditional Latin hypercube approach
Optical studies of carrier and phonon dynamics in Ga_{1-x}Mn_{x}As
We present a time-resolved optical study of the dynamics of carriers and
phonons in Ga_{1-x}Mn_{x}As layers for a series of Mn and hole concentrations.
While band filling is the dominant effect in transient optical absorption in
low-temperature-grown (LT) GaAs, band gap renormalization effects become
important with increasing Mn concentration in Ga_{1-x}Mn_{x}As, as inferred
from the sign of the absorption change. We also report direct observation on
lattice vibrations in Ga1-xMnxAs layers via reflective electro-optic sampling
technique. The data show increasingly fast dephasing of LO phonon oscillations
for samples with increasing Mn and hole concentration, which can be understood
in term of phonon scattering by the holes.Comment: 13 pages, 3 figures replaced Fig.1 after finding a mistake in
previous versio
Resonant Coherent Phonon Spectroscopy of Single-Walled Carbon Nanotubes
Using femtosecond pump-probe spectroscopy with pulse shaping techniques, one
can generate and detect coherent phonons in chirality-specific semiconducting
single-walled carbon nanotubes. The signals are resonantly enhanced when the
pump photon energy coincides with an interband exciton resonance, and analysis
of such data provides a wealth of information on the chirality-dependence of
light absorption, phonon generation, and phonon-induced band structure
modulations. To explain our experimental results, we have developed a
microscopic theory for the generation and detection of coherent phonons in
single-walled carbon nanotubes using a tight-binding model for the electronic
states and a valence force field model for the phonons. We find that the
coherent phonon amplitudes satisfy a driven oscillator equation with the
driving term depending on photoexcited carrier density. We compared our
theoretical results with experimental results on mod 2 nanotubes and found that
our model provides satisfactory overall trends in the relative strengths of the
coherent phonon signal both within and between different mod 2 families. We
also find that the coherent phonon intensities are considerably weaker in mod 1
nanotubes in comparison with mod~2 nanotubes, which is also in excellent
agreement with experiment.Comment: 21 pages, 22 figure
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