29,239 research outputs found
Superheating systematics of crystalline solids
Systematics of superheating (theta= T/Tm–1) of crystalline solids as a function of heating rate (Q) are established as beta= A(Q)(theta+ 1)theta2, where the normalized energy barrier for homogeneous nucleation is beta= 16pigammasl3/(3kTmDeltaHm2), T is temperature, Tm melting temperature, A a Q-dependent parameter, gammasl interfacial energy, DeltaHm heat of fusion, and k Boltzmann's constant. For all elements and compounds investigated, beta varies between 0.2 and 8.2. At 1 and 10^12 K/s, A = 60 and 31, theta= 0.05–0.35 and 0.06–0.45, respectively. Significant superheating is achievable via ultrafast heating. We demonstrate that the degree of superheating achieved in shock-wave loading and intense laser irradiation as well as in molecular dynamics simulations (Q~10^12 K/s) agrees with the theta–beta–Q systematics
Exclusive Decay of Quarkonia and Meson into a Lepton Pair Combined with Two Pions
We study the exclusive decay of , and into a lepton
pair combined with two pions in the two kinematic regions. One is specified by
the two pions having large momenta, but a small invariant mass. The other is
specified by the two pions having small momenta. In both cases we find that in
the heavy quark limit the decay amplitude takes a factorized form, in which the
nonperturbative effect related to heavy meson is represented by a NRQCD matrix
element. The nonperturbative effects related to the two pions are represented
by some universal functions characterizing the conversion of gluons into the
pions. Using models for these universal functions and chiral perturbative
theory we are able to obtain numerical predictions for the decay widths. Our
numerical results show that the decay of \jpsi is at order of with
reasonable cuts and can be observed at BES II and the proposed BES III and
CLEO-C. For other decays the branching ratio may be too small to be measured.Comment: 19 pages, Latex 2e file, 12 EPS figures (included). Replaced with
version to appear in Eur. Phys. J. C,published online: 8 May 200
Quantum phonon transport of molecular junctions amide-linked with carbon nanotubes: a first-principle study
Quantum phonon transport through benzene and alkane chains amide-linked with
single wall carbon nanotubes (SWCNTs) is studied within the level of density
functional theory. The force constant matrices are obtained from standard
quantum chemistry software. The phonon transmission and thermal conductance are
from the nonequilibrium Green's function and the mode-matching method. We find
that the ballistic thermal conductance is not sensitive to the compression or
stretching of the molecular junction. The terminating groups of the SWCNTs at
the cutting edges only influence the thermal conductance quantitatively. The
conductance of the benzene and alkane chains shows large difference. Analysis
of the transmission spectrum shows that (i) the low temperature thermal
conductance is mainly contributed by the SWCNT transverse acoustic modes, (ii)
the degenerate phonon modes show different transmission probability due to the
presence of molecular junction, (iii) the SWCNT twisting mode can hardly be
transmitted by the alkane chain. As a result, the ballistic thermal conductance
of alkane chains is larger than that of benzene chains below 38 K, while it is
smaller at higher temperature.Comment: 5 pages, 5 figure
NARX-based nonlinear system identification using orthogonal least squares basis hunting
An orthogonal least squares technique for basis hunting (OLS-BH) is proposed to construct sparse radial basis function (RBF) models for NARX-type nonlinear systems. Unlike most of the existing RBF or kernel modelling methods, whichplaces the RBF or kernel centers at the training input data points and use a fixed common variance for all the regressors, the proposed OLS-BH technique tunes the RBF center and diagonal covariance matrix of individual regressor by minimizing the training mean square error. An efficient optimization method isadopted for this basis hunting to select regressors in an orthogonal forward selection procedure. Experimental results obtained using this OLS-BH technique demonstrate that it offers a state-of-the-art method for constructing parsimonious RBF models with excellent generalization performance
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Elastic net prefiltering for two class classification
A two-stage linear-in-the-parameter model construction algorithm is proposed aimed at noisy two-class classification problems. The purpose of the first stage is to produce a prefiltered signal that is used as the desired output for the second stage which constructs a sparse linear-in-the-parameter classifier. The prefiltering stage is a two-level process aimed at maximizing a model’s generalization capability, in which a new elastic-net model identification algorithm using singular value decomposition is employed at the lower level, and then, two regularization parameters are optimized using a particle-swarm-optimization algorithm at the upper level by minimizing the leave-one-out (LOO) misclassification rate. It is shown that the LOO misclassification rate based on the resultant prefiltered signal can be analytically computed without splitting the data set, and the associated computational cost is minimal due to orthogonality. The second stage of sparse classifier construction is based on orthogonal forward regression with the D-optimality algorithm. Extensive simulations of this approach for noisy data sets illustrate the competitiveness of this approach to classification of noisy data problems
Direct shock wave loading of Stishovite to 235 GPa: Implications for perovskite stability relative to an oxide assemblage at lower mantle conditions
Pure stishovite and coesite samples with zero porosity and dimensions appropriate for planar shock wave experiments have been synthesized with multi-anvil high-pressure techniques. The equation of state of stishovite is obtained by direct shock wave loading up to 235 GPa: K_(0T) = 306 ± 5 GPa and K'_(0T) = 5.0 ± 0.2 where K_(0T) and K'_(0T) are ambient bulk modulus and its pressure derivative, respectively. The Hugoniots (shock equations of state) for stishovite, coesite and quartz achieve widely differing internal energy states at equal volume and therefore allow us to determine the Gruneisen parameter of stishovite. On the basis of the resulting P-V-T equation of state for stishovite and previous studies on other phases on the MgO-SiO_2 binary, the breakdown reaction of MgSiO_3-perovskite to MgO and SiO_2 was calculated. Our calculations show that perovskite is thermodynamically stable relative to the stishovite and periclase assemblage at lower mantle conditions. We obtain similar results for a range of models, despite the appreciable differences among these experiment-based thermodynamic parameters
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