Showing papers on "Saturation (graph theory) published in 2000"

Time dependent theory for random lasers

Abstract: A model to simulate the phenomenon of random lasing is presented. It couples Maxwell's equations with the rate equations of electronic population in a disordered system. Finite difference time domain methods are used to obtain the field pattern and the spectra of localized lasing modes inside the system. A critical pumping rate ${P}_{r}^{c}$ exists for the appearance of the lasing peaks. The number of lasing modes increases with the pumping rate and the length of the system. There is a lasing mode repulsion. This property leads to a saturation of the number of modes for a given size system and a relation between the localization length $\ensuremath{\xi}$ and average mode length ${L}_{m}$.

Damage Buildup in GaN under Ion Bombardment

Abstract: The damage buildup until amorphization in wurtzite GaN films under keV light ${(}^{12}\mathrm{C})$ and heavy ${(}^{197}\mathrm{Au})$ ion bombardment at room and liquid nitrogen $({\mathrm{LN}}_{2})$ temperatures is studied by Rutherford backscattering/channeling (RBS/C) spectrometry and transmission electron microscopy (TEM). The effect of beam flux on implantation damage in GaN is reported. A marked similarity between damage buildup for light and heavy ion bombardment regimes is observed. The results point to substantial dynamic annealing of irradiation defects even during heavy ion bombardment at ${\mathrm{LN}}_{2}$ temperature. Amorphization starts from the GaN surface with increasing ion dose for both ${\mathrm{LN}}_{2}$ and room-temperature bombardment with light or heavy ions. A strong surface defect peak, seen by RBS/C, arises from an amorphous layer at the GaN surface, as indicated by TEM. The origin of such an amorphous layer is attributed to the trapping of mobile point defects by the GaN surface, as suggested by the flux behavior. However, in the samples implanted with light ions to low doses $(1\ifmmode\times\else\texttimes\fi{}{10}^{15}{\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}),$ no amorphous layer on the GaN surface is revealed by TEM. Damage buildup is highly sigmodal for ${\mathrm{LN}}_{2}$ temperature irradiation with light or heavy ions. Formation of planar defects in the crystal bulk is assumed to provide a ``nucleation site'' for amorphization with increasing ion dose during irradiation at ${\mathrm{LN}}_{2}$ temperature. For room-temperature bombardment with heavy ions, the damage in the GaN bulk region saturates at a level lower than that of the amorphous phase, as measured by RBS/C, and amorphization proceeds from the GaN surface with increasing ion dose. For such a saturation regime at room temperature, implantation damage in the bulk consists of point-defect clusters and planar defects which are parallel to the basal plane of the GaN film. Various defect interaction processes in GaN during ion bombardment are proposed to explain the observed, somewhat unexpected behavior of disorder buildup.

Two-photon-excited luminescence and defect formation in SiO 2 nanoparticles induced by 6.4-eV ArF laser light

Abstract: The photoluminescence (PL) from 7- and 15-nm silica $({\mathrm{SiO}}_{2})$ nanoparticles induced both by ArF laser light $[{\ensuremath{\lambda}}_{\mathrm{exc}}=193\mathrm{nm}(6.4\mathrm{eV}),$ ${\ensuremath{\tau}}_{L}=15\mathrm{ns}]$ and by Nd:YAG (yttrium-aluminum-garnet) laser light $[{\ensuremath{\lambda}}_{\mathrm{exc}}=266\mathrm{nm}(4.66\mathrm{eV}),$ ${\ensuremath{\tau}}_{L}=8\mathrm{ns}]$ was studied. The laser light intensity dependencies of the PL yields reveal the two-photon (TP) process of the PL excitation in the case of ArF laser light. The PL results from the radiative relaxation of self-trapped excitons (STE- the blue band), also from the surface hydrogen-related species (the green band), and the bulk nonbridging oxygen hole centers (NBOHC's- the red band) excited by a radiationless relaxation of TP-produced free excitons (FE's). The main point is focused on the effect of the nanoparticle surface condition on the FE dynamics. The dynamics includes either an elastic scattering or quenching by the nanoparticle boundary, the laser heating of FE's up to energies in excess of the STE barrier, the FE energy transfer to the surface and bulk NBOHC's and hydrogen-related centers, the saturation of the FE density, and the biexciton process in the formation of Frenkel defects with their subsequent transformation into NBOHC's.

Segregation transitions in wet granular matter

Abstract: We report the effect of interstitial fluid on the extent of segregation by imaging the pile that results after bidisperse color-coded particles are poured into a silo. Segregation is sharply reduced and preferential clumping of small particles is observed when a small volume fraction of fluid ${V}_{f}$ is added. We find that viscous forces in addition to capillary forces have an important effect on the extent of segregation $s$ and the angle of repose $\ensuremath{\theta}$. We show that the sharp initial change and the subsequent saturation in $s$ and $\ensuremath{\theta}$ occurs over similar ${V}_{f}$. We also find that a transition back to segregation can occur when the particles are completely immersed in a fluid at low viscosities.

Cu K h α 1 , 2 hypersatellites: Suprathreshold evolution of a hollow-atom x-ray spectrum

Abstract: A high-resolution pure $\mathrm{Cu} {K}^{h}{\ensuremath{\alpha}}_{1,2}$ hypersatellite spectrum was measured by photoexcitation using synchrotron radiation. A shift ${E(K}^{h}{\ensuremath{\alpha}}_{2})\ensuremath{-}E(K{\ensuremath{\alpha}}_{1})=281.4\ifmmode\pm\else\textpm\fi{}0.3 \mathrm{eV}$ from the diagram lines, a splitting ${E(K}^{h}{\ensuremath{\alpha}}_{1})\ensuremath{-}{E(K}^{h}{\ensuremath{\alpha}}_{2})=23.6\ifmmode\pm\else\textpm\fi{}0.4 \mathrm{eV},$ and an intensity ratio ${R=I(K}^{h}{\ensuremath{\alpha}}_{1}{)/I(K}^{h}{\ensuremath{\alpha}}_{2})=0.29\ifmmode\pm\else\textpm\fi{}0.02$ are found for the two lines of the spectrum. Full-spectrum fits based on ab initio Dirac-Fock calculations agree well with the measured spectrum, when QED corrections and Breit interaction are included. A slightly higher calculated R may indicate that the intermediate coupling is not fully accounted for. The intensity's evolution from threshold (measured to be at $18.351\ifmmode\pm\else\textpm\fi{}0.015 \mathrm{keV})$ shows an unexpectedly long saturation range extending up to $\ensuremath{\sim}30 \mathrm{keV}.$ The intensity evolution deviates from the Thomas model, which should be valid in the adiabatic, near-threshold regime. The implications of our results for the Z variation of the coupling, correlations, and atomic interactions across the $3d$ transition elements are discussed.

ρ-ω mixing in asymmetric nuclear matter via a QCD sum rule approach

Abstract: We evaluate the operator product expansion (OPE) for a mixed correlator of the isovector and isoscalar vector currents in the background of the nucleon density with intrinsic isospin asymmetry (ie, excess of neutrons over protons) and match it with its imaginary part, given by resonances and continuum, via the dispersion relation The leading density-dependent contribution to {rho}-{omega} mixing is due to the scattering term, which turns out to be larger than any density dependent piece in the OPE We estimate that the asymmetric density of n{sub n}-n{sub p}{approx}25x10{sup -2}fm{sup -3} induces the amplitude of {rho}-{omega} mixing, equal in magnitude to the mixing amplitude in vacuum, with the constructive interference for positive and destructive for negative values of n{sub n}-n{sub p} We revisit sum rules for vector meson masses at finite nucleon density to point out the numerical importance of the screening term in the isoscalar channel, which turns out to be one order of magnitude larger than any density-dependent condensates over the Borel window This changes the conclusions about the density dependence of m{sub {omega}}, indicating {approx}40MeV increase at nuclear saturation density

Adsorption state of 1,4-cyclohexadiene on Si ( 100 ) ( 2 × 1 )

Abstract: The adsorption state of 1,4-cyclohexadiene on a $\mathrm{Si}(100)(2\ifmmode\times\else\texttimes\fi{}1)$ surface is studied using low-energy electron diffraction (LEED) and photoelectron spectroscopy (PES). LEED shows a sharp $2\ifmmode\times\else\texttimes\fi{}1$ pattern at saturation after the adsorption of 1,4-cyclohexadiene indicating that the Si dimer structure is maintained. Judging from the PES results, almost all the dangling bonds react on adsorption of 1,4-cyclohexadiene, and one of the two \ensuremath{\pi} bonds in 1,4-cyclohexadiene reacts with the Si dimer. Thus, the molecule adsorbs on a $\mathrm{Si}(100)(2\ifmmode\times\else\texttimes\fi{}1)$ dimer one to one with $di\ensuremath{-}\ensuremath{\sigma}$ bonding, and the ideal saturation coverage is expected to be 0.5 monolayer. It is concluded that the molecules are anisotropically aligned and the molecular plane is inclined from the surface normal, with the remaining \ensuremath{\pi} bond located at the vacuum side.

Atomic defects in hexagonal tungsten carbide studied by positron annihilation

Abstract: Vacancies on the two sublattices of hexagonal tungsten carbide were identified by means of positron lifetime studies after irradiation with electron of various energies. In the as-prepared state two predominant positron lifetime components occur indicating positron annihilation in the free state with a lifetime ${\ensuremath{\tau}}_{f}=124\ifmmode\pm\else\textpm\fi{}10$ ps according to the simple trapping model and positron trapping and annihilation in metal vacancies $({\ensuremath{\tau}}_{{V}_{W}}=175\ifmmode\pm\else\textpm\fi{}20$ ps) of low concentration. By means of low-energy electron irradiation at 1 MeV with an electron dose of $1.8\ifmmode\times\else\texttimes\fi{}{10}^{23} {\mathrm{m}}^{\ensuremath{-}2}$ exclusively carbon vacancies are introduced giving rise to saturation trapping and annihilation of positrons at carbon vacancies with a low lifetime of ${\ensuremath{\tau}}_{{V}_{C}}=136\ifmmode\pm\else\textpm\fi{}3$ ps. The isochronal annealing of irradiation induced vacancies reveals two annealing stages at 800 and 1200 K, which are ascribed to the annealing of carbon vacancies and the dissolving of small carbon vacancy clusters.

Dissociative adsorption of Si 2 H 6 on the Si(001) surface

Abstract: We present results of ab initio calculations, based on pseudopotentials and the density functional theory, for the dissociative adsorption of ${\mathrm{Si}}_{2}{\mathrm{H}}_{6}$ on the Si(001) surface. Various models are considered, containing H and the radicals ${\mathrm{SiH}}_{3}$ and ${\mathrm{SiH}}_{2}.$ Models based on the ${\mathrm{SiH}}_{2}$ radical were considered with three adsorption sites: (i) an on-dimer position, (ii) an intrarow position between two neighboring Si dimers in the same dimer row, and (iii) an inter-row position between adjacent Si dimer rows. The intrarow and bridge geometries are considered with and without the saturation of the Si dangling bonds with hydrogen. For the $2\ifmmode\times\else\texttimes\fi{}1$ surface reconstruction, the on-dimer geometry is energetically more favorable than the inter-row geometry. For the $2\ifmmode\times\else\texttimes\fi{}2$ reconstruction, without hydrogen passivation of the Si dangling bonds, the on-dimer and intrarow geometries leave the system fully passivated. With hydrogen passivation of the Si dangling bonds, the on-dimer geometry is more favorable than the intrarow geometry.

Ultrahigh B doping ({<=}10{sup 22} cm{sup -3}) during Si(001) gas-source molecular-beam epitaxy: B incorporation, electrical activation, and hole transport

Abstract: Si(001) layers doped with B concentrations C{sub B} between 1x10{sup 17} and 1.2x10{sup 22} cm{sup -3} (24 at %) were grown on Si(001)2x1 at temperatures T{sub s}=500-850 degree sign C by gas-source molecular-beam epitaxy from Si{sub 2}H{sub 6} and B{sub 2}H{sub 6}. C{sub B} increases linearly with the incident precursor flux ratio J{sub B{sub 2}}{sub H{sub 6}}/J{sub Si{sub 2}}{sub H{sub 6}} and B is incorporated into substitutional electrically active sites at concentrations up to C{sub B}{sup *}(T{sub s}) which, for T{sub s}=600 degree sign C, is 2.5x10{sup 20} cm{sup -3}. At higher B concentrations, C{sub B} increases faster than J{sub B{sub 2}}{sub H{sub 6}}/J{sub Si{sub 2}}{sub H{sub 6}} and there is a large and discontinuous decrease in the activated fraction of incorporated B. However, the total activated B concentration continues to increase and reaches a value of N{sub B}=1.3x10{sup 21} cm{sup -3} with C{sub B}=1.2x10{sup 22} cm{sup -3}. High-resolution x-ray diffraction (HR-XRD) and reciprocal space mapping measurements show that all films, irrespective of C{sub B} and T{sub s}, are fully strained. No B precipitates or misfit dislocations were detected by HR-XRD or transmission electron microscopy. The lattice constant in the film growth direction a{sub (perpendicular} {sub sign)} decreases linearly withmore » increasing C{sub B} up to the limit of full electrical activation and continues to decrease, but nonlinearly, with C{sub B}>C{sub B}{sup *}. Room-temperature resistivity and conductivity mobility values are in good agreement with theoretical values for B concentrations up to C{sub B}=2.5x10{sup 20} and 2x10{sup 21} cm{sup -3}, respectively. All results can be explained on the basis of a model which accounts for strong B surface segregation to the second-layer with a saturation coverage {theta}{sub B,sat} of 0.5 ML (corresponding to C{sub B}=C{sub B}{sup *}). At higher C{sub B} (i.e., {theta}{sub B}>{theta}{sub B,sat}), B accumulates in the upper layer as shown by thermally programmed desorption measurements, and a parallel incorporation channel becomes available in which B is incorporated into substitutional sites as B pairs that are electrically inactive but have a low charge-scattering cross section. (c) 2000 The American Physical Society.« less

Thermal-transport properties of CeNiSn

Abstract: We present thermal-conductivity $\ensuremath{\kappa}$ and thermopower S measurements on high-quality single crystalline CeNiSn along the three crystallographic axes $a, b,$ and c in the temperature range between 100 mK and 7 K and in magnetic fields up to 8 T applied along the a axis. Both $\ensuremath{\kappa}$ and S are highly anisotropic. However, characteristic features that may be attributed to the opening of a pseudogap in the electronic density of states (DOS) at the Fermi energy $\ensuremath{\eta}$ below 10 K are seen for all three crystallographic directions. These features are strongly suppressed by a magnetic field of 8 T applied along the a axis. At the lowest temperatures we have evidence for the presence of a residual metallic DOS at $\ensuremath{\eta},$ again for all three directions. The saturation of the reduced Lorenz number ${L/L}_{0}$ to a value distinctly higher than one is an interesting feature which deserves further investigation.

Electron-phonon interaction in a very low mobility GaAs/Ga 1-x Al x As δ-doped gated quantum well

Abstract: The energy relaxation rate for hot electrons in a gated $\mathrm{GaAs}/{\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x}\mathrm{As}$ $\ensuremath{\delta}$-doped quantum well has been measured over the temperature range 0.3--3 K. At higher temperatures the loss rate varies as ${T}^{5}$ and the magnitude agrees well with that predicted by the standard theory for piezoelectric electron-phonon scattering. At low temperatures the observed dependence changes to ${T}^{4},$ the crossover occuring near $\mathrm{ql}\ensuremath{\sim}0.35,$ where q is the average magnitude of the phonon wave vector and l the electron mean free path. This is in agreement with recent theoretical predictions for piezoelectric scattering in the dirty limit. The theory also predicts that the magnitude of the energy-loss rate should depend inversely on the conductivity of the sample. Good agreement is found at higher conductivities, but the measured values show saturation when the conductivity becomes very low.

Isotope shifts and hyperfine structure in near-ultraviolet transitions of Pb I by Doppler-free saturation spectroscopy

Abstract: Isotope shifts and hyperfine structures have been determined for the three near-ultraviolet transitions 405.8 nm ${(6s}^{2}{6p}^{2}{}^{3}{P}_{2}--{6s}^{2}6p7s{}^{3}{P}_{1}^{o}),$ 368.3 nm ${(6s}^{2}{6p}^{2}{}^{3}{P}_{1}--{6s}^{2}6p7s{}^{3}{P}_{0}^{o}),$ and 364.0 nm ${(6s}^{2}{6p}^{2}{}^{3}{P}_{1}--{6s}^{2}6p7s{}^{3}{P}_{1}^{o})$ in stable isotopes of Pb I using the technique of saturated absorption spectroscopy in a sputtered vapor. The isotope shifts for these transitions and the magnetic hyperfine interaction constants for the ${6s}^{2}{6p}^{2}{}^{3}{P}_{1,2}$ metastable levels are the first Doppler-free determinations for these systems reported in the literature to our knowledge. Combined with experimental data, previously given in the literature, and pseudorelativistic Hartree-Fock estimates, the field shifts (FS's) of seven relevant configurations are deduced: $\mathrm{FS}{(6s}^{2}6p7s)=2461\mathrm{MHz},$ $\mathrm{FS}{(6s}^{2}6p9p)=1838\mathrm{MHz},$ $\mathrm{FS}{(6s}^{2}6p6d)=1820\mathrm{MHz},$ $\mathrm{FS}{(6s}^{2}6p7d)=1668\mathrm{MHz},$ $\mathrm{FS}{(6s}^{2}6p8s)=2132\mathrm{MHz},$ $\mathrm{FS}{(6s}^{2}6p7p)=395\mathrm{MHz},$ and $\mathrm{FS}{(6s}^{2}6p5f)=\ensuremath{-}461\mathrm{MHz},$ referred to ${6s}^{2}{6p}^{2}.$ The hyperfine-structure integrals deduced from the experimental A factors for p electrons of the ground configuration ${6s}^{2}{6p}^{2}$ are in good agreement with results of ab initio calculations.

Photofission of actinide nuclei in the quasideuteron and lower part of the Δ energy region

Abstract: The total photofission cross sections for the actinide nuclei ${}^{232}\mathrm{Th},$ ${}^{233}\mathrm{U},$ ${}^{235}\mathrm{U},$ ${}^{238}\mathrm{U},$ and ${}^{237}\mathrm{Np}$ have been measured from 68 to 264 MeV using tagged photons at the Saskatchewan Accelerator Laboratory. The fission fragments were detected using parallel-plate avalanche detectors. The results show that the fission probability for ${}^{238}\mathrm{U}$ is 20% lower than that for ${}^{237}\mathrm{Np}$ and 40% higher than that for ${}^{232}\mathrm{Th}.$ Less significant differences were also found among the individual uranium isotopes. These results contradict the assumption that the fission probability for ${}^{238}\mathrm{U}$ is approximately equal to unity in this energy range. It has also been observed that the fission probability as a function of energy for all these isotopes is constant, with the exception of that for ${}^{232}\mathrm{Th},$ which increases with energy, although it seems to be reaching a saturation value. Comparison between the total photofission cross section for ${}^{237}\mathrm{Np}$ and the photoabsorption cross sections for lighter nuclei shows a behavior consistent with a broadening of the $\ensuremath{\Delta}$ resonance with increasing atomic mass.

A unitary model for structure functions and diffractive production at small x

Abstract: We propose a unified approach which describes both structure functions in the small-$x$ region and diffractive production in $\gamma^*p$-interactions. It is shown that the model, based on reggeon calculus and a quark-parton picture of the interaction, gives a good description of available experimental data in a broad region of $Q^2$ (including $Q^2 =0$) with a single Pomeron of intercept $\alpha_P(0) = 1.2$. Predictions for very small $x$ are given and the problem of saturation of parton densities is discussed.

High-pressure infrared spectroscopy of solid oxygen

Abstract: The infrared IR absorption spectra of molecular solid oxygen ${\mathrm{O}}_{2}$ have been studied to 92 GPa at room temperature. The IR-active vibron fundamental around $1500{\mathrm{cm}}^{\ensuremath{-}1}$ consisted of at least four absorption bands and their frequencies showed a turnover at 25 GPa with increasing pressure and then monotonically increased to 87 GPa after an initial decrease. The significant increase in the frequencies of the IR- and Raman-active vibrons has suggested a strengthening of the O-O intramolecular bond with pressure. An increase in the frequency difference between the IR and Raman vibrons with a pressure dependence similar to those of librons has also indicated an increase in the ${\mathrm{O}}_{2}\ensuremath{-}{\mathrm{O}}_{2}$ intermolecular bond. The absorbance saturation over all frequencies in the present observable region above 92 GPa was attributed to the insulator-metal transition due to a discontinuous band-gap closure. The structure of the \ensuremath{\varepsilon} phase has been examined based on the spectroscopic data.

Quantum kinetic theory of two-beam current injection in bulk semiconductors

Abstract: We develop a theory of current injection in bulk semiconductors by simultaneous excitation with two laser beams with frequencies $2{\ensuremath{\omega}}_{0},$ ${\ensuremath{\omega}}_{0}.$ Coherent mixing of the resulting one- and two-photon transitions generates an effective field ${A}_{\mathrm{eff}}(\mathbf{k})$ with different strengths at $\ifmmode\pm\else\textpm\fi{}\mathbf{k}$ points in momentum space. This asymmetry in carrier generation, producing the induced current, is controlled by the relative phase of the two fields. Quantum kinetic equations for the photogenerated carriers are derived from nonequilibrium Green functions. They are simplified here to the Boltzmann limit, and applied to a model of GaAs in the presence of LO phonons. Different forms of the conduction electron distributions result for generation from light- and heavy-hole bands, and give different saturation and relaxation rates for the induced current. Generation of THz radiation by the current is also discussed.

Atomic structural changes of a Br-chemisorbed Si ( 111 ) − 7 × 7 surface under 10–150 eV electron impact

Abstract: Electron-stimulated desorption from Br-chemisorbed $\mathrm{Si}(111)\ensuremath{-}7\ifmmode\times\else\texttimes\fi{}7$ surfaces was investigated by scanning tunneling microscopy. Irradiating these surfaces with electrons field emitted from the tip of the scanning tunneling microscope induced various desorption behaviors depending on the initial Br coverage and electron energy. At low Br coverage, Br atoms desorb but no atomic changes occur on the Si surface. At saturation coverage, Si adatoms as well as Br atoms desorb to a large extent. The cross section of Br atom desorption first increases near 15 eV and then increases by orders of magnitude with electron energy up to 150 eV. The cross section of adatom desorption, in contrast, is much less dependent on the electron energy in this energy region. These desorption behaviors are discussed from the viewpoint of electronic excitation at the Br-chemisorbed Si(111) surface.

Phase-coherence time saturation in mesoscopic systems: Wave-function collapse

Abstract: A finite phase-coherence time ${\ensuremath{\tau}}_{\ensuremath{\varphi}}^{\mathrm{meas}}$ emerges from a sequence of idealized measurements over a quantum system. For a rapid sequence, the phase-coherence time is found explicitly and, for the stationary charge conduction problem, is bounded. For an arbitrary time interval between measurements, we propose a general expression for ${\ensuremath{\tau}}_{\ensuremath{\varphi}}^{\mathrm{meas}}$ to all orders.

Thermal Chemistry of CH3 on Si/Cu(100)†

Abstract: Photoelectron spectroscopy (PES), thermal programmed desorption (TPD) studies, and scanning tunneling microscopy (STM) investigated the interaction and chemistry of CH{sub 3} (generated by the thermal cracking of azomethane) on Si/Cu(100). Si was deposited on Cu(100) by the thermal decomposition of SiH{sub 4} at 420 K. STM of adsorbate-free Si/Cu(100) at a less than saturation coverage of Si revealed a surface that contained large domains of a Cu{sub 2}Si structure. These Cu{sub 2}Si domains coexisted with regions that were believed to be lower in fractional Si coverage. TPD results showed that (CH{sub 3}){sub 3}SiH desorbed near 200 K from CH{sub 3}/Si/Cu(100) prepared with a low Si concentration. With increasing Si concentration a (CH{sub 3}){sub 3}SiH desorption state appeared near 420 K, in addition to the 200 K state. The two observed TPD states of (CH{sub 3}){sub 3}SiH at 200 and 420 K were believed to be due to the thermal reaction of CH{sub 3} with the low Si density and high Si density (i.e., Cu{sub 2}Si) regions, respectively. At a saturation coverage of Si, when the well ordered Cu{sub 2}Si phase covered the surface, only the 420 K peak was present during CH{sub 3}/Si/Cu(100) TPD. Results also suggested that (CH{sub 3})Simore » and possibly some (CH{sub 3}){sub 2}Si intermediates predominated on the surface below room temperature, and (CH{sub 3}){sub 3}Si species were formed on the surface only at temperatures between 250 and 390 K. Surface hydrogen needed for the final evolution of (CH{sub 3}){sub 3}SiH was generated from methyl groups at temperatures above 390 K on the Si-saturated Cu(100).« less

Surface passivation in diamond nucleation

Abstract: Surface passivation is introduced to suppress the deleterious effect of Si surface oxides and thus enhance diamond heteroepitaxial nucleation. Surface composition and diamond nucleation and growth on H-, Br-, and I-passivated Si surfaces were studied. X-ray photoelectron spectroscopy showed that the passivated Si surfaces were free of silicon oxides and carbides. Remarkable enhancement in nucleation was achieved on passivated surfaces and the nucleation density obtained on a Br-passivated Si surface reached ${10}^{10}{\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}.$ Programmable temperature desorption revealed that the adsorbate desorption temperature increased in the order of H, I, and Br passivation. The same order of increase was also observed in the saturation value of electron emission current from the passivated surfaces, which was related to the degree of nucleation. Nucleation enhancement was shown to be greater when the adsorbate desorption temperature is closer to the nucleation temperature, so that more adsorbate- and oxide-free Si surface area would be available for nucleation. The study established that surface passivation is potentially an effective approach for diamond heteroepitaxial nucleation.

Subharmonic resonances in plasmas: exponential and superexponential growth of driven relativistic plasma waves

Abstract: Subharmonic resonant beat-wave excitation of nonlinear relativistic plasma waves is studied analytically and in particle-in-cell simulations. We find that if the frequency separation of the lasers, {Delta}{omega} , is 2{omega}{sub p} or 3{omega}{sub p} ({omega}{sub p} is the plasma frequency), then plasma waves are still excited at {omega}{sub p} but they grow exponentially or superexponentially rather than secularly. Both of these subharmonic resonant instabilities saturate due to relativistic detuning. The analytical growth rates and saturation levels agree with the simulation results.

Transient free convection from a horizontal surface in a porous medium subjected to a sudden change in surface heat flux

Abstract: This paper presents an analytical and numerical study of transient free convection from a horizontal surface that is embedded in a fluid-saturated porous medium. It is assumed that for time \(\bar \tau 0\). Firstly, solutions which are valid for small and large \(\bar \tau \) are obtained. The full boundary-layer equations are then integrated step-by-step for the transient regime from the initial unsteady state (\(\bar \tau = 0\)) until such times at which this forward marching approach is no longer well posed. Beyond this time no valid solutions could be obtained which matched the final solution from the forward integration to the steady state profiles at large times \(\bar \tau \to \infty \).

Two-frequency shell model for hypernuclei and meson-exchange hyperon-nucleon potentials

Abstract: A two-frequency shell model is proposed for investigating the structure of hypernuclei starting with a hyperon-nucleon potential in free space. In a calculation using the folded-diagram method for ${}_{\ensuremath{\Lambda}}^{16}\mathrm{O},$ the $\ensuremath{\Lambda}$ single particle energy is found to have a saturation minimum at an oscillator frequency $\ensuremath{\Elzxh}{\ensuremath{\omega}}_{\ensuremath{\Lambda}}\ensuremath{\approx}10\mathrm{MeV},$ for the $\ensuremath{\Lambda}$ orbit, which is considerably smaller than $\ensuremath{\Elzxh}{\ensuremath{\omega}}_{N}=14\mathrm{MeV}$ for the nucleon orbit. The spin-dependence parameters derived from the Nijmegen NSC89 and NSC97f potentials are similar, but both are rather different from those obtained with the J\"ulich-$B$ potential. The $\ensuremath{\Lambda}\mathrm{NN}$ three-body interactions induced by $\ensuremath{\Lambda}N\ensuremath{-}\ensuremath{\Sigma}N$ transitions are important for the spin parameters, but relatively unimportant for the low-lying states of ${}_{\ensuremath{\Lambda}}^{16}\mathrm{O}.$

Optical absorption in LiF: Mg, Ti and its application to the Unified Interaction Model predictions of thermoluminescence dose response

Abstract: The optical absorption (OA) spectrum of LiF: Mg, Ti has been investigated by I{sub m} - T{sub stop} pulsed annealing in steps of 3-4 C over the temperature range from 100 C to 300 C. In agreement with previous investigations, the 5.45 eV OA band is identified as a likely candidate for the recombination stage competitor responsible for composite peak 5 supra linearity. An additional OA band at 5.7 eV is also identified as a possible recombination stage competitor. First order kinetic analysis yields an activation energy of 0.80 {+-} 0.13 eV for the 4.0 eV band usually associated with peak 5, an energy which is similar in value to the binding energy of the Mg-Li{sub vac} trimer believed to form the peak 5 trapping structure. Dose response measurements result in dose filling constants of {beta} {sub TC} = 0.9 {+-} 0.1 x 10{sup -3} Gy{sup -1} and {beta} {sub cc} = 3.8 {+-} 0.8 x 10 {sup -4} Gy{sup -1} for the 4.0 eV and 5.45 eV OA bands respectively. It is demonstrated quantitatively in the framework of the Unified Interaction Model (UNIM) that the experimentally measured peak 5 supra linearity can be theoretically predicted for all possible dose response characteristics of the competitor, i.e., early, late or no entry into saturation up to dose levels of 5000 Gy. Previous objections in the literature to the identification of the 5.45 eV OA band as the recombination stage competitor are therefore judged invalid. (Author)

Bottom-up thermalization in heavy ion collisions

Abstract: We describe how thermalization occurs in heavy ion collisions in the framework of perturbative QCD. When the saturation scale $Q_s$ is large compared to $\Lambda_{QCD}$, thermalization takes place during a time of order $\alpha^{-13/5}Q_s^{-1}$ and the maximal temperature achieved is $\alpha^{2/5}Q_s$.