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