Ultrafast dephasing of coherent optical phonons in atomically controlled GeTe/Sb2_{2}Te3_{3} superlattices

Femtosecond dynamics of coherent optical phonons in GeTe/Sb$_{2}$Te$_{3}$ superlattices (SLs), a new class of semiconductor SLs with three different states, have been investigated by using a reflection-type pump-probe technique at various lattice temperatures. The time-resolved transient reflectivity (TR) obtained in as-grown SLs exhibits the coherent A$_{1}$ optical modes at 5.10 THz and 3.78 THz, while only the single A$_{1}$ mode at 3.68 THz is observed in annealed SLs. The decay rate of the A$_{1}$ mode in annealed SLs is strongly temperature dependent, while that in as-grown SLs is not temperature dependent. This result indicates that the damping of the coherent A$_{1}$ phonons in amorphous SLs is governed by the phonon-defect (vacancy) scattering rather than the anharmonic phonon-phonon coupling.Comment: 5 pages, 5 figure

Mitochondrial Genome Polymorphism in Lolium perenne

The restriction fragment length polymorphisms (RFLPs) of mitochondrial DNA (mtDNA) of perennial ryegrass (Lolium perenne L.) were investigated to elucidate the genetic relatedness among the 128 cultivars including diploid and tetraploid. Many patterns of RFLPs were observed and allowed assigning of the cultivars into the main eight haplotypes of mitochondrial genome relatedness. The American cultivars were classified into haplotype I and VIII which were remote at the mitochondrial genome from each other, the European ones were distributed to all haplotypes and the tetraploid ones were mostly assigned into the haplotype V. The assessment of mtDNA RFLPs may be a valuable method in analyzing a cytoplasmic differentiation among the perennial ryegrass cultivars. Further investigations are required to elucidate mtDNA diversity in relation with the maternal effects on the agronomic traits of perennial ryegrass

Dynamical Susceptibility in KDP-type Crysals above and below Tc II

The path probability method (PPM) in the tetrahedron-cactus approximation is applied to the Slater-Takagi model with dipole-dipole interaction for KH2PO4-type hydrogen-bonded ferroelectric crystals in order to derive a small dip structure in the real part of dynamical susceptibility observed at the transition temperature Tc. The dip structure can be ascribed to finite relaxation times of electric dipole moments responsible for the first order transition with contrast to the critical slowing down in the second order transition. The light scattering intensity which is related to the imaginary part of dynamical susceptibility is also calculated above and below the transition temperature and the obtained central peak structure is consistent with polarization fluctuation modes in Raman scattering experiments.Comment: 8 pages, 11 figure

Numerical Simulation of Dispersion around an Isolated Cubic Building: Model Evaluation of RANS and LES. Building and Environment.

Several studies have been carried out on CFD prediction based on a RANS (Reynolds Averaged Navier–Stokes equations) model for dispersion around buildings, but it was reported that a RANS computation often provides extremely high concentration, which are not observed in usual measurements. These results suggest that transient simulations such as the large-eddy simulation (LES) might be required to achieve more accurate results. Nevertheless, very few studies have evaluated the basic performance of LES in modeling the dispersion field for a simple configuration in comparison with the RANS model. Therefore, relative performance of these simulation methods for dispersion problem around buildings should be clarified in order to make it possible to choose a suitable numerical method for its purpose. The purpose of this study is to confirm the accuracy of LES in modeling plume dispersion near and around a simple building model and to clarify the mechanism for the discrepancy in relation to the RANS computation. Simple LES modeling gives better results than RNG modeling of the distribution of concentration, although the difference for mean velocity is not so large. The horizontal diffusion of concentration is well reproduced by LES. This tendency is closely related to the reproduction of unsteady periodic fluctuation around cubical forms in LES

CFD Modeling of Pollution Dispersion in a Street Canyon: Comparison between LES and RANS

CFD modeling using RANS and LES of pollutant dispersion in a three-dimensional street canyon is investigated by comparison with measurements. The purpose of this study is to confirm the accuracy of LES in modeling plume dispersion in a simple street canyon model and to clarify the mechanism of the discrepancy in relation to RANS computation. Simple LES modeling is shown by comparison with wind tunnel experiments to give better results than conventional RANS computation (RNG) modeling of the distribution of mean concentration. The horizontal diffusion of concentration is well reproduced by LES, mainly due to the reproduction of unsteady concentration fluctuations in the street canyon

CFD Simulation of Near-Field Pollutant Dispersion in the Urban Environment: A Review of Current Modeling Techniques

Near-field pollutant dispersion in the urban environment involves the interaction of a plume and the flow field perturbed by building obstacles. In the past two decades, micro-scale Computational Fluid Dynamics (CFD) simulation of pollutant dispersion around buildings and in urban areas has been widely used, sometimes in lieu of wind tunnel testing. This paper reviews current modeling techniques in CFD simulation of near-field pollutant dispersion in urban environments and discusses the findings to give insight into future applications. Key features of near-field pollutant dispersion around buildings from previous studies, i.e., three-dimensionality of mean flow, unsteadiness of large-scale flow structure, and anisotropy of turbulent scalar fluxes, are identified and discussed. This review highlights that it is important to choose appropriate numerical models and boundary conditions by understanding their inherent strengths and limitations. Furthermore, the importance of model evaluation was emphasized. Because pollutant concentrations around buildings can vary by orders of magnitudes in time and space, the model evaluation should be performed carefully, while paying attention to their uncertainty. Although CFD has significant potential, it is important to understand the underlying theory and limitations of a model in order to appropriately investigate the dispersion phenomena in question

CFD Modeling of Pollutant Dispersion in Building Array: Evaluation of Turbulent Scalar Flux Modeling in RANS Model Using LES Results

Modeling of turbulent scalar flux in CFD (Computational Fluid Dynamics) for near-field dispersion around buildings is examined by investigating both velocity and concentration fields obtained by two modeling approaches, i.e., RANS RNG k-ε and LES. A building array model with a point source located in between the central buildings is adopted as a target configuration. First, the prediction accuracy of LES is confirmed by comparing with the RNGcomputation and the results from an experiment conducted by the authors. LES gives better results than RNG, in terms of time-averaged velocity and concentration distribution in comparison with the wind tunnel experimental results. Next, the eddy viscosity and the eddy diffusivity are determined by LES data using a least square approach as suggested in the dynamic sub-grid scale model. Large differences can be observed between the distributions of the estimated eddy viscosity by using LES data and the eddy diffusivity obtained by RNG, since the eddy diffusivity is not always proportional to the eddy viscosity

Supernova Nucleosynthesis and Extremely Metal-Poor Stars

We investigate hydrodynamical and nucleosynthetic properties of the jet-induced explosion of a population III $40M_\odot$ star and compare the abundance patterns of the yields with those of the metal-poor stars. We conclude that (1) the ejection of Fe-peak products and the fallback of unprocessed materials can account for the abundance patterns of the extremely metal-poor (EMP) stars and that (2) the jet-induced explosion with different energy deposition rates can explain the diversity of the abundance patterns of the metal-poor stars. Furthermore, the abundance distribution after the explosion and the angular dependence of the yield are shown for the models with high and low energy deposition rates $\dot{E}_{\rm dep}=120\times10^{51} {\rm ergs s^{-1}}$ and $1.5\times10^{51} {\rm ergs s^{-1}}$. We also find that the peculiar abundance pattern of a Si-deficient metal-poor star HE 1424--0241 can be reproduced by the angle-delimited yield for $\theta=30^\circ-35^\circ$ of the model with $\dot{E}_{\rm dep}=120\times10^{51} {\rm ergs s^{-1}}$.Comment: 6 pages, 3 figures. To appear in "ORIGIN OF MATTER AND EVOLUTION OF GALAXIES: From the Dawn of Universe to the Formation of Solar System", AIP Conf. Proc. 1016 (December 2007, Sapporo), eds. T. Suda, T. Nozawa, et al. (Melville: AIP