Showing papers on "Saturation (graph theory) published in 1991"

Relative abundances of primary ion radicals in .gamma.-irradiated DNA: cytosine vs. thymine anions and guanine vs. adenine cations

Abstract: An ESR study of the relative distribution of ion radicals formed in DNA equilibrated with D{sub 2}O and {gamma}-irradiated at 77 K is presented. The ESR spectra of irradiated DNA and polynucleotides (poly(dG){center dot}poly(dC) and poly(dAdT){center dot}poly(dAdT)) were obtained and employed in a computer-assisted analysis for the individual ion-radical distribution. Analysis of spectra as a function of power allowed the separation of the spectra of the pyrimidine anions (T{sup {sm bullet}{minus}}, C{sup {sm bullet}{minus}}) from the spectra of the purine cations (G{sup {sm bullet}+}, A{sup {sm bullet}+}). The spectra of the mononucleotide ion radicals, dCMP{sup {sm bullet}{minus}}, dTMP{sup {sm bullet}{minus}}, dGMP{sup {sm bullet}+}, and dAMP{sup {sm bullet}+}, were produced in 8 M LiCl glasses. In addition, the spectra of the ion radicals of all of the mononucleotide ion radicals except dAMP{sup +} were simulated by using hyperfine and g tensors from the literature. Basis spectra derived from (1) power saturation experiments, (2) polynucleotide and mononucleotide spectra, (3) spectra of mononucleotides alone, and (4) anisotropic simulations were used to fit the spectra of DNA by use of a linear least-squares analysis. Each of the four separate analyses confirms that the cytosine anion dominates the spectra of DNA at 100 K. Threemore » analyses included the cationic composition, and they strongly favor the guanine cation over the adenine cation. An average of the authors results gives the DNA ion radicals' relative to abundances as ca. 77% C{sup {sm bullet}{minus}}, 23% T{sup {sm bullet}{minus}} for the anions and >90% G{sup {sm bullet}+} for the cations about equal amounts of anions and cations are present. No difference in results is found for DNA irradiated in frozen D{sub 2}O solutions or simply exchanged at 100% D{sub 2}O humidity.« less

Optical pulse propagation in doped fiber amplifiers.

Abstract: This paper provides a general treatment of the pulse-propagation problem in doped fiber amplifiers within the rate-equation approximation. The dopants are modeled as a two-level system whose dynamic response is governed by the population relaxation time ${\mathit{T}}_{1}$ and the dipole relaxation time ${\mathit{T}}_{2}$. For incident optical pulses with a width ${\mathit{T}}_{0}$ such that ${\mathit{T}}_{1}$\ensuremath{\gg}${\mathit{T}}_{0}$\ensuremath{\gg}${\mathit{T}}_{2}$, pulse amplification is governed by a Ginzburg-Landau-type equation that includes gain saturation, gain dispersion, fiber dispersion, fiber nonlinearity, and the detuning effects occurring when the carrier frequency of the input pulse does not coincide with the gain peak. In the absence of gain saturation, this equation has solitary-wave solutions in the form of chirped solitons. Our numerical results show that the chirped solitons are stable only in the normal-dispersion regime. In the case of anomalous dispersion, as is the case for erbium-doped fiber amplifiers, the amplified pulse develops many subpulses. The effect of gain saturation on pulse amplification is also discussed.

Geometric and electronic properties of Cs structures on III-V (110) surfaces: From 1D and 2D insulators to 3D metals.

Abstract: We report the structural and electronic properties of Cs adsorbed on room-temperature GaAs and InSb (110) surfaces as observed with scanning tunneling microscopy. Cs initially forms long one-dimensional (1D) zigzag chains on both surfaces. Additional Cs adsorption on GaAs(110) results in the formation of a 2D overlayer consisting of five-atom Cs polygons arranged in c(4\ifmmode\times\else\texttimes\fi{}4) superlattice. The tunneling gap measured over these insulating structures narrows with the transition from 1D to 2D, with metallic characteristics observed following saturation with a second Cs overlayer.

Hydrogen-mediated epitaxy of Ag on Si(111) as studied by low-energy ion scattering.

Abstract: We have investigated the effect of the saturation of surface dangling bonds of Si(111) surfaces with atomic hydrogen upon Ag thin-film growth. By using time-of-flight-type low-energy ion scattering-recoil analysis techniques, we find that the growth mode of Ag thin films is drastically changed by the hydrogen termination of Si(111)-7\ifmmode\times\else\texttimes\fi{}7 surfaces and that the epitaxial growth of A-type Ag(111) films is promoted by the hydrogen atoms residing at the film/substrate interface.

Interdiffusion, growth mechanisms, and critical currents in YBa2Cu3O7-x/PrBa2Cu3O7-x superlattices.

Abstract: {ital Z}-contrast electron microscopy demonstrates that interdiffusion is not affecting the resistive transitions observed in {ital M}{times}{ital N} YBa{sub 2}Cu{sub 3}O{sub 7{minus}2}/PrBa{sub 2}Cu{sub 3}O{sub 7{minus}{ital x}} superlattices. At moderate supersaturations, island growth is observed, the islands comprising one Ba-Y-Ba structural unit in height terminated at the Cu-chain plane, having sides 20--30 nm long along (100) and (010). The associated gradual roughening of the growing surface has no apprent effect on critical currents for {ital M}{ge}2, the temperature and field dependence being comparable to that of a film.

Geometric and electronic structure of Sb on Si(111) by scanning tunneling microscopy.

Abstract: Scanning tunneling microscopy and spectroscopy (STS) are used to investigate the Si(111)-Sb surface. At an Sb coverage of a few percent of a monolayer (ML), Sb substitutes Si adatoms in the 7\ifmmode\times\else\texttimes\fi{}7 reconstruction. At about 1/3 ML, the 7\ifmmode\times\else\texttimes\fi{}7, ``disordered 7\ifmmode\times\else\texttimes\fi{}7,'' and \ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3- R30\ifmmode^\circ\else\textdegree\fi{} reconstructions are observed. This sub-ML \ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 reconstruction is a simple adatom phase. The T4 site is identified as the chemical bonding site of Sb adatoms in these low-coverage reconstructions. Spatially resolved STS shows the complex nature of the electronic structure of Si(111)7\ifmmode\times\else\texttimes\fi{}7-Sb as compared to the clean Si surface. At the saturation coverage of 1 ML, domains of 1\ifmmode\times\else\texttimes\fi{}1, 2\ifmmode\times\else\texttimes\fi{}1, and a different \ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 reconstruction are observed, and structure models are proposed.

Many-particle effects and nonlinear optical properties of gaas/(al,ga)as multiple-quantum-well structures under quasistationary excitation conditions

Abstract: Quasi-two-dimensional (2D) carrier systems of GaAs/(Al,Ga)As multiple-quantum-well structures are studied under quasistationary excitation conditions using the pump and probe beam and the luminescence spectroscopy. In the low- to medium-density regime the saturation of the ${\mathit{n}}_{\mathit{z}}$=1 exciton resonances dominates the nonlinear optical properties. The low-temperature saturation density is found to be ${\mathit{N}}_{\mathit{s}}$\ensuremath{\approxeq}4\ifmmode\times\else\texttimes\fi{}${10}^{16}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$, independent of the well width ${\mathit{L}}_{\mathit{z}}$. The carrier-induced energetic shift of the 1hh-exciton resonance as a function of ${\mathit{L}}_{\mathit{z}}$ shows the dimensional dependence of the screening properties of the carriers. The 2D limit is reached at well widths smaller than 50 \AA{}, whereas the 3D behavior occurs already at ${\mathit{L}}_{\mathit{z}}$=190 \AA{}. In the high-excitation regime, the renormalization of the fundamental band gap is investigated as a function of the electron-hole plasma density. The density and the reduced band gap are determined via systematic evaluations of both gain and luminescence spectra. The observed behavior can be described by a strict 2D theory using effective exciton parameters in order to account for the finite well widths of the structures. The same theory describes very well an n-type modulation-doped quantum well if an independent shift of each subband--according only to its specific carrier density---is assumed. The correlation enhancement of the band-to-band transitions was observed in an n-type modulation-doped sample where all excitonic features were quenched by the doping density. The study of the higher subbands reveals that both exciton bleaching and subband renormalization are due mainly to a direct occupation of the specific subband; we find that the intersubband effects via Coulomb screening are negligible.

Formation of sulfur clusters on Re(0001) surfaces observed with the scanning tunneling microscope.

Abstract: We have studied the various structures formed by sulfur on Re(0001) single-crystal surfaces using a scanning tunneling microscope. At coverages of 0.25 monolayer and below, open lattices formed by single adatoms are formed, mostly with a (2\ifmmode\times\else\texttimes\fi{}2) structure. Between 0.25 and 0.5 monolayer, phases consisting of three-, four-, or six-adatom clusters are observed. The sulfur trimers are formed first randomly in the (2\ifmmode\times\else\texttimes\fi{}2) regions and then ordered as their coverage increases to saturation at 0.45 monolayer. In all cases sulfur atoms are adsorbed in threefold hollow sites. The formation of sulfur clusters above 0.25 monolayer is interpreted as evidence of substrate-mediated many-body forces.

Adsorption of Cs on Si(111)7 x 7: Studies of photoemission from surface states and core levels.

Abstract: We have studied the development of the electronic structure at the early stages of interface formation of Cs on the Si(111)7\ifmmode\times\else\texttimes\fi{}7 surface. Low-energy electron diffraction shows that the 7\ifmmode\times\else\texttimes\fi{}7 surface symmetry is conserved throughout the sub-saturation-coverage region. The changes in the surface electronic states characteristic of the clean silicon surface and the development of Cs-induced surface electronic states with increasing Cs coverage have been studied with angle-resolved ultraviolet photoelectron spectroscopy. Changes observed predominantly in the emission from the adatom dangling-bond state suggest that bonding of Cs occurs primarily at the adatom site on the Si(111)7\ifmmode\times\else\texttimes\fi{}7 surface. At two-thirds of the saturation coverage, a strong Cs-induced surface state dominates the valence-band region. This state exhibits a downward dispersion along the high-symmetry lines in going away from normal emission, thus proving the semiconducting state of this well-ordered Si(111)7\ifmmode\times\else\texttimes\fi{}7-Cs surface. Continued adsorption takes place at the rest-atom sites, making the surface metallic as the coverage reaches saturation. The metallicity is evident from the Fermi cutoff in valence-band photoemission and from the asymmetric broadening of the Cs 4d states, characteristic of metallic materials. Studies of the Si 2p level reveal changes in the line shape, which can be modeled with changes in surface core-level shifts and introduction of asymmetric broadening. These observations suggest that metallization occurs through the development of states at the Fermi level of both Cs and Si character. Studies of the Si 2p level further show an upward band bending with a maximum of 0.38 eV at the saturation coverage.

Theoretical support to the double-layer model for potassium adsorption on the Si(001) surface.

Abstract: K adsorption on the Si(001)-2\ifmmode\times\else\texttimes\fi{}1 surface is studied by first-principles molecular dynamics based on a norm-conserving pseudopotential. The theoretically optimized structure for full coverage (\ensuremath{\Theta}=1.0) agrees fairly well with the double-layer model analyzed by x-ray photoelectron diffraction. At \ensuremath{\Theta}=0.5, the configuration with K adsorbed along the trough is more stable than the Levine model. The mean desorption energies for the two peaks in the thermal desorption spectrum are well accounted for by the present calculation. The calculated work-function change also agrees quantitatively with the observed one. Based on these analyses, it is concluded that the double-layer model should be appropriate for the saturation coverage of K on the Si(001) surface.

Chemisorption and thermal decomposition of benzene on Pd(110): High-resolution electron energy loss spectroscopy, low-energy electron diffraction, and thermal desorption studies

Abstract: The adsorbed state of benzene on Pd(110) at 300 K and its thermal decomposition process in the temperature region up to 700 K have been investigated by using high-resolution electron energy loss spectroscopy, low-energy electron diffraction, and multiplexed thermal desorption spectroscopy. Vibrational spectra show the existence of two adsorbed states of benzene on Pd(110) at 300 K. In one state, benzene is adsorbed with its C ring nearly parallel to the surface (flat benzene), and in the other state, at some angle (tilted benzene). The tilted benzene is located in the c(4{times}2) domains. For a small exposure ({approx lt}0.3 langmuir; fractional coverage {theta}{sub C{sub 6}H{sub 6}}{approx lt}0.07), the flat benzene predominates. With increasing exposure, conversion from flat to tilted benzene occurs, and the amount of tilted benzene is increased relative to that of flat benzene. For the saturation exposure (3 langmuirs: {theta}{sub C{sub 6}H{sub 6}} {approximately} 0.27), the tilted benzene predominates; the flat benzene also exists. Thermal decomposition of benzene on Pd(110) has been studied in detail. For a small exposure (0.3 langmuir), heating to 380-600 K forms the C{sub x}H{sub y}(x {ge} 1, y = 0,1) species as the decomposition products. By heating the sample to 600 K, theymore » find only C adatoms exist on Pd(110). For the saturation exposure (3 langmuirs), by heating the sample to 400 K, they find the C(4{times}2) structure is well developed and the Pd(110) surface is mostly covered by tilted benzene. Heating to 400-650 K forms the C{sub x}H{sub y} (x = 1 or {ge} 3, y = 0,1) species. The decomposition temperature is shifted toward higher temperatures by the site-blocking effect of benzene admolecules themselves. The decomposition is accompanied by h{sub 2} desorption.« less

XY chain with random anisotropy: Magnetization law, susceptibility, and correlation functions at T=0.

Abstract: The random-anisotropy model in one dimension, at zero temperature, is studied analytically and in Monte Carlo simulations, focusing on the magnetization curve as a function of disorder strength D, and on the susceptibility and correlation functions. The predicted scaling of the susceptibility (for weak disorder), \ensuremath{\chi}\ensuremath{\propto}${\mathit{D}}^{\mathrm{\ensuremath{-}}4/3}$, is verified, as is the prediction \ensuremath{\Vert}M-1\ensuremath{\Vert}\ensuremath{\propto}${\mathit{H}}^{\mathrm{\ensuremath{-}}3/2}$ on approaching saturation. The correlation length does not follow the expected behavior, ${\mathit{R}}_{\mathit{f}}$\ensuremath{\propto}${\mathit{D}}^{\mathrm{\ensuremath{-}}2/3}$. Nonequilibrium effects are evident: Hysteresis persists even for small disorder, and (steady-state) correlation functions depend strongly upon the initial state of the system. A new simulation method is introduced, which is more effective than the usual Metropolis algorithm for weak disorder.

Core-level binding energies of cesium-doped carbon(C60) and graphite

Abstract: The carbon core-level binding energies of C{sub 60} and Cs-doped C{sub 60} thin films and Cs-graphite intercalation compounds have been characterized by X-ray photoelectron spectroscopy. Thin films of C{sub 60} deposited on silicon and gold substrates exhibit systematic increases in the carbon core-level (C 1s) binding energy and peak width with Cs doping. At saturation coverage, which corresponds to a stoichiometry of Cs{sub 7.2}C{sub 60}, the binding energy shifts by 1.1 eV. The C 1s binding energy was also found to increase systematically in Cs-intercalated graphite where the maximum shift is 0.5 eV for a stoichiometry of Cs{sub 0.12}C. The observed increases in binding energy have been explained in terms of an increase in the Fermi level position as a result of Cs charge transfer to a delocalized C{sub 60} energy band.

Associative ionization in collisions of slowed and trapped sodium

Abstract: Collisions of trapped atoms occur over time intervals that are often longer than radiative lifetimes. This leads to radiative interruption of excited-state collision dynamics and constrains collisional interactions to atom pairs that are in close proximity at the time of excitation. Using classical dynamics I show that this causes Na associative ionization to be dominated by a four-step process in which the longer-range ${\mathrm{Na}}^{\mathrm{*}}$-Na interaction initiates inward motion that is completed as ${\mathrm{Na}}^{\mathrm{*}}$+${\mathrm{Na}}^{\mathrm{*}}$\ensuremath{\rightarrow}${\mathrm{Na}}_{2}^{\mathrm{\ifmmode^\circ\else\textdegree\fi{}}}$+e. Thus low-temperature associative ionization does not result from ${\mathrm{Na}}^{\mathrm{*}}$+${\mathrm{Na}}^{\mathrm{*}}$ collisions. I predict strong single- and two-wavelength dependences, which have a molecular character due to the close proximity at the time of photoabsorption. These are amenable to experimental observation. I also calculate saturation that results from the depletion of interacting pairs. This produces very-low-power saturation and explains the associative ionization measurements of Gould et al. [Phys. Rev. Lett. 60, 788 (1988)].

Analysis of measured total pressures from irradiated solid and liquid (U, Pu)-mixed oxide

Abstract: This paper analyzes measurements of the total pressure from irradiated (U, Pu)-mixed oxide with respect to the fission product release kinetics and availability for pressure generation in Bethe-Tait excursions Two pressure sources acting on a millisecond time scale were identified: release of grain boundary fission products (gases and volatiles such as cesium) triggered by grain boundary separation and release of formerly intragranular fission products due to fuel boiling The former process can provide pressures on a megapascal scale early, and the latter process, late in the accident progression No fission product release was observed from nonboiling liquid fuel Based on the experimental data, a model was formulated for the total pressure over irradiated (u, Pu)-oxide Fuel vapor and gases interact by a suppression mechanism: P{sub IF} = max (P{sub ag} + P{sub FP}, P{sub sat}) The total pressure over irradiated fuel P{sub IF} is equal to the pressure sum from ambient gas P{sub AG} and released fission products in the gaseous state P{sub FP} when this sum is greater than the saturation vapor pressure of fresh (U, Pu)-oxide P{sub sat} In this regime, fuel boiling is suppressed AT sufficiently high temperatures when P{sub sat} {gt} P{sub AG} + P{sub FP},more » the oxide begins to boil and the total pressure P{sub IF} reaches the fresh fuel saturation vapor pressure P{sub sat} The switch-over in the controlling mechanism occurred at {approximately}5200 K« less

Approximation properties of certain interpolation operators of entire exponential type inL p (−∞,+∞) spaces

Abstract: In this paper, we consider the convergence and saturation problems of the following discrete type interpolation operators: $$(S_\sigma ^U f) (x) = :\sum\limits_{k = - \infty }^{ + \infty } {f(x_k ) U_\sigma (x - x_k )} ,x_k = \frac{{2k \pi }}{\sigma },k = 0, \pm 1, \pm 2, \cdots , \sigma > 0,$$ wheref is any bounded function on the real axisR, whileU(x) is an appropriate entire function of exponential type 1 andUσ(x)=:U(σx), we obtain some resuls similar to those in the trigonometric polynomial interpolation operators’ case[4]. As a special case, our results include Dryanov's results in [5].