Showing papers on "Saturation (graph theory) published in 1997"
TL;DR: Antiphase boundaries (APBs) were observed in single crystal films grown on MgO as discussed by the authors, which is an intrinsic consequence of the nucleation and growth mechanism in films.
Abstract: Antiphase boundaries (APBs) were observed in ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ single crystal films grown on MgO. The APBs are an intrinsic consequence of the nucleation and growth mechanism in films. Across an APB, the intrasublattice superexchange coupling is greatly strengthened, while the intersublattice superexchange coupling is weakened, reversing the dominant interaction from that found in the bulk. Thus the APB separates oppositely magnetized regions, consistent with Lorentz microscopy measurements. The APBs induce very large saturation fields and nearly random magnetization distribution in zero field.
456 citations
TL;DR: In this article, the phase coherence time of six quasi-1D Au wires was measured and it was shown that the phase coherent time of these wires is temperature independent at low temperatures.
Abstract: We present measurements of the phase coherence time ${\ensuremath{\tau}}_{\ensuremath{\varphi}}$ in six quasi-1D Au wires and clearly show that ${\ensuremath{\tau}}_{\ensuremath{\varphi}}$ is temperature independent at low temperatures. We suggest that zero-point fluctuations of the phase coherent electrons are responsible for the observed saturation of ${\ensuremath{\tau}}_{\ensuremath{\varphi}}$. We introduce a new functional form for the temperature dependence and present the results of a calculation for the saturation value of ${\ensuremath{\tau}}_{\ensuremath{\varphi}}$. This explains the observed temperature dependence of our samples as well as many 1D and 2D systems reported to date.
249 citations
TL;DR: In this paper, the formation energy of a fully H-terminated GaN surface was found to be 0.02 eV/cell, compared to 1.95 eV for the bare surface.
Abstract: We present first-principles calculations of the formation energy for H-terminated GaN(101\ifmmode\bar\else\textasciimacron\fi{}0) surfaces. The calculations indicate that H adsorption on GaN(101\ifmmode\bar\else\textasciimacron\fi{}0) will proceed by saturation of pairs of Ga and N dangling bonds rather than through exclusive occupation of only one type of bonding site. At $T=0,$ the surface energy of the fully H-terminated surface is found to be 0.02 eV/cell, compared to 1.95 eV/cell for the bare surface. We present results for the N-H and Ga-H stretching and bending eigenfrequencies. Dissociative adsorption of ${\mathrm{NH}}_{3}$ via the formation of N-H and ${\mathrm{G}\mathrm{a}\ensuremath{-}\mathrm{N}\mathrm{H}}_{2}$ bonds is exothermic and reduces the surface formation energy to a value which is less than 0.1 eV at $T=0.$ The implications of these results for the origin of nanopipe defects in GaN are examined.
79 citations
TL;DR: In this paper, a scanning tunneling microscope study of the initial bonding structure and subsequent reaction mechanism of the Si(001) surface was presented, where a local minimum was observed in the center of the reacted dimer pairs, while the unreacted dimers maintained the normal bean-shaped contour of the clean surface.
Abstract: We present here a scanning tunneling microscope study of the initial bonding structure and subsequent reaction mechanism of ${\mathrm{C}}_{2}{\mathrm{H}}_{2}$ with the Si(001) surface Upon exposure of the sample at room temperature to 02 L of ${\mathrm{C}}_{2}{\mathrm{H}}_{2}$ (approximately 20% coverage) adsorption of the molecule on alternate dimer pairs is observed, leading to either a local $2\ifmmode\times\else\texttimes\fi{}2$ or $c(2\ifmmode\times\else\texttimes\fi{}4)$ structure In the filled-state image, a local minimum is observed in the center of the reacted dimer pairs, while the unreacted dimer pairs maintain the normal bean-shaped contour of the clean surface The molecule forms an overlayer with either local $2\ifmmode\times\else\texttimes\fi{}2$ or $c(4\ifmmode\times\else\texttimes\fi{}2)$ order, leading to a saturation coverage of 05 monolayers Upon annealing the substrate at 775 K the surface becomes disordered and the steps are no longer visible After further annealing at 875 K, SiC clusters are formed and the $2\ifmmode\times\else\texttimes\fi{}1$ structure is again seen between the clusters For a starting coverage of 20%, annealing to higher temperatures around 1100 K leads to pinning of the step movement by the SiC clusters For a starting coverage of 05 monolayer, annealing at 1100 K results in faceting of the surface Further annealing at 1275 K creates anisotropic facets that are oriented along the [1\ifmmode\bar\else\textasciimacron\fi{}10] direction with a typical aspect ratio of approximately 4 to 5 These facets act as nucleation sites for subsequent carbonization and SiC growth
78 citations
TL;DR: In this paper, it was shown that at saturation on Pt(111) terraces, O adatoms favor FCC over Hcp sites by roughly 0.5 eV. This preference is a consequence of d-electron depletion between first and second-neighbor Pt atoms in the fcc binding geometry.
Abstract: First-principles calculations show that at saturation on Pt(111) terraces, O adatoms favor {open_quotes}fcc{close_quotes} over {open_quotes}hcp{close_quotes} sites by roughly 0.5 eV. This preference is a consequence of d-electron depletion between first- and second-neighbor Pt atoms in the fcc binding geometry, which does not occur in the hcp case. The source of the different d-electron distributions is the electrostatic requirement that the d hole on each Pt atom adjacent to an O lie along the line connecting the two. {copyright} {ital 1997} {ital The American Physical Society}
47 citations
TL;DR: In this article, the development of a soft-x-ray laser at 21.2 nm, recently carried out at the Laboratoire de Spectroscopie Atomique et Ionique, was reviewed.
Abstract: In this paper we review the development of a soft-x-ray laser at 21.2 nm, recently carried out at the Laboratoire de Spectroscopie Atomique et Ionique. Amplification is generated by electron collisional pumping on a ${2\mathrm{s}}^{2}$${2\mathrm{p}}^{5}$3p-${2\mathrm{s}}^{2}$${2\mathrm{p}}^{5}$3s, J=0--1 transition in neonlike zinc (${\mathrm{Zn}}^{20+}$). The lasing medium is a \ensuremath{\sim}150 \ensuremath{\mu}m\ifmmode\times\else\texttimes\fi{}2 cm line plasma produced by irradiating slab Zn targets at a net intensity of \ensuremath{\sim}1.4\ifmmode\times\else\texttimes\fi{}${10}^{13}$ W ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$, using a 1.06-\ensuremath{\mu}m, 600-ps-long pulsed laser delivering \ensuremath{\sim}350 J of net energy on the target. Accompanying the driving pulse by a 100-mJ-level prepulse train through deliberately imperfect isolation of the mode-locked laser oscillator, the J=0--1 gain coefficient \ensuremath{\sim}5 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ is generated. The emission appears in an \ensuremath{\sim}100-ps burst and precedes the lasing on both the much weaker J=2--1 lines and the x-ray continuum peak. The J=0--1 output source region, the beam spatial profile, and its time history have been measured and their relevance to the kinetics of this lasing system is discussed. A half cavity was implemented and a strong double-pass amplification at 21.2 nm with a 2-cm-long plasma obtained. With the gain-length product achieving \ensuremath{\sim}17.3 according to the particular configuration, saturation was demonstrated, which is confirmed through a schematic model of the level kinetics. To our knowledge, it is the first neonlike soft-x-ray laser where saturation on the J=0--1 line was achieved by using a half-cavity arrangement. Absolute energy measurements indicate \ensuremath{\sim}400 \ensuremath{\mu}J in the half-cavity-generated beam and a peak output power of \ensuremath{\sim}5 MW, which makes this laser one of the most efficient soft-x-ray lasing systems demonstrated to date.
46 citations
TL;DR: In this paper, the laser frequency is tuned close to the fundamental excitonic transition of the quantum well and saturation sets in, and the magnetization buildup is observed in the absence of the seeding magnetic field.
Abstract: We report a giant photoinduced Faraday rotation in a $\mathrm{CdTe}/{\mathrm{Cd}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{Te}$ multiple quantum well structure when the laser frequency is tuned close to the fundamental excitonic transition of the quantum well and saturation sets in. At the same time we observe stimulated emission and buildup of magnetization through photon spin recycling in the presence of a magnetic field which was analyzed by a time-resolved linearly polarized pump and probe technique. The magnetization buildup is also seen in the absence of the seeding magnetic field and by using a circularly polarized pump beam.
33 citations
TL;DR: Differential cross sections and analyzing powers have been measured for the proton knockout reaction from the orbits of three kinds of target nuclei at ${E}_{p}\phantom{0ex}{0ex}}=\phantom{\rule{ 0ex}{ 0ex}}392\mathrm{MeV}.
Abstract: Differential cross sections and analyzing powers ( ${A}_{y}$) have been measured for the proton knockout reaction from ${s}_{1/2}$ orbits of three kinds of target nuclei at ${E}_{p}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}392\mathrm{MeV}$. The ${A}_{y}$ values are reduced from those of free proton-proton scattering and the reduction rate is found to depend on target. For the present setting, where the recoil momentum is almost zero, an averaged density seen through this reaction can be estimated and it reaches almost half of the saturation density in some cases. The reduction of ${A}_{y}$ shows a monotonically decreasing function of the averaged density, which strongly suggests the existence of a nuclear medium effect on the nucleon-nucleon interaction.
31 citations
TL;DR: In this paper, the Schiavilla-Pandharipande-Riska model for the two-body electromagnetic currents, constructed so as to satisfy the continuity equation with realistic ${v}_{14}$ potentials, is adopted.
Abstract: The electromagnetic inclusive transverse response of nuclear matter at saturation density is studied within the correlated basis function perturbation theory for momentum transfers $q$ from $300$ to 550 MeV/$c$. The correlation operator includes a Jastrow component, accounting for the short range repulsion, as well as longer range spin, tensor, and isospin ones. Up to correlated one-particle--one-hole intermediate states are considered. The spreading due to the decay of particle (hole) states into two-particle--one-hole (two-hole--one-particle) states is considered via a realistic optical potential model. The Schiavilla-Pandharipande-Riska model for the two-body electromagnetic currents, constructed so as to satisfy the continuity equation with realistic ${v}_{14}$ potentials, is adopted. Currents due to intermediate $\ensuremath{\Delta}$-isobar excitations are also included. The global contribution of the two-body currents turns out to be positive and provides an enhancement of the one-body transverse response ranging from $\ensuremath{\sim}20%$ for the lower momenta to $\ensuremath{\sim}10%$ for the higher ones. This finding is in agreement with the Green's function Monte Carlo studies of the transverse Euclidean response in $A=3,4$ nuclei and contradicts previous results obtained within the Fermi gas and shell models. The tensor-isospin component of the correlation is found to be the leading factor responsible for such a behavior. The nuclear matter response is compared to recent experimental data on ${}^{40}$Ca and ${}^{56}$Fe.
30 citations
TL;DR: In this paper, the authors re-analyse the situation with the shadowing corrections (SC) in QCD for the proton deep inelastic structure functions and suggest a new nonlinear evolution equation.
Abstract: In this paper we re-analyse the situation with the shadowing corrections (SC) in QCD for the proton deep inelastic structure functions. We reconsider the Glauber - Mueller approach for the SC in deep inelastic scattering (DIS) and suggest a new nonlinear evolution equation. We argue that this equation solves the problem of the SC in the wide kinematic region where $\as \kappa = \as \frac{3 \pi \as}{2 Q^2R^2} x G(x,Q^2) \leq 1$. Using the new equation we estimate the value of the SC which turn out to be essential in the gluon deep inelastic structure function but rather small in $F_2(x,Q^2)$. We claim that the SC in $xG(x,Q^2)$ is so large that the BFKL Pomeron is hidden under the SC and cannot be seen even in such "hard" processes that have been proposed to test it. We found that the gluon density is proportional to $\ln(1/x)$ in the region of very small $x$. This result means that the gluon density does not reach saturation in the region of applicability of the new evolution equation. It should be confronted with the solution of the GLR equation which leads to saturation.
28 citations
TL;DR: In this paper, the authors show that the multiphonon nonradiative decay probabilities of rare-earth ions in a germanate glass are reduced at high excitation state densities for the larger energy gaps.
Abstract: The nonradiative decay process of rare-earth ions in solids has been considered up to now to be independent of excitation intensity. In this paper we show that the multiphonon nonradiative decay probabilities of rare-earth ions in a germanate glass are reduced at high excitation state densities for the larger energy gaps. The classical exponential energy gap law is shown to 'rotate' at higher excitation around the 3.2-phonon point. The observed effect is described in terms of a spatial saturation of the accepting mode term with an effective diffusion length ranging between 45 and 20 \AA{} for excited state density from 2\ifmmode\times\else\texttimes\fi{}${10}^{17}$ to 8\ifmmode\times\else\texttimes\fi{}${10}^{18}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ at an active ion concentration of 2.5\ifmmode\times\else\texttimes\fi{}${10}^{19}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$.
TL;DR: The results of rigorous quantum calculations of the current perpendicular-to-plane giant magnetoresistance (CPP GMR) of finite Co/Cu and Fe/Cr superlattices with perfectly flat interfaces but with growth-induced fluctuations in layer thicknesses are reported in this paper.
Abstract: The results of rigorous quantum calculations of the current-perpendicular-to-plane giant magnetoresistance (CPP GMR) of finite Co/Cu(001) and Fe/Cr(001) superlattices with perfectly flat interfaces but with growth-induced fluctuations in layer thicknesses are reported. They are based on an exact numerical evaluation of the Kubo formula using tight-binding parametrization with s, p, d bands and hopping to first and second neighbors of an ab initio band structure. These calculations show that three distinct regimes of CPP transport occur. When there are no fluctuations, CPP transport is in the ballistic regime. The CPP GMR ratio ${\mathrm{R}}_{\mathrm{CPP}}$ of finite Co/Cu and Fe/Cr superlattices in the ballistic regime reach saturation values equal to ${\mathrm{R}}_{\mathrm{CPP}}$ of an infinite superlattice after only \ensuremath{\approx}3--5 repeats of a superlattice unit cell and the maximum values of ${\mathrm{R}}_{\mathrm{CPP}}$ are of the order of 100%. When small fluctuations in layer thickness corresponding to only one atomic plane at the interface being displaced are introduced, transport changes from ballistic to Ohmic. The calculated GMR ratio ${\mathrm{R}}_{\mathrm{CPP}}$ increases initially linearly with the number N of ferromagnet/spacer bilayers and then saturates for N\ensuremath{\approx}40--50. The theoretical maximum values of ${\mathrm{R}}_{\mathrm{CPP}}$ for Co/Cu and Fe/Cr superlattices in the Ohmic regime are in the region 800--1000 %. The zero-field and saturation-field resistances increase linearly with N (good Ohm's law) and the calculated zero-field resistance of the Co/Cu superlattice is within 10% of the resistance observed in a Co/Cu sample of the same composition and thickness. Small spontaneous (growth-induced) fluctuations in layer thickness can thus account well for the observed CPP GMR. When superlattices with large fluctuations in layer thickness are grown deliberately (pseudorandom spin valves), the Ohmic regime changes into, experimentally as yet unexplored, Anderson localization regime. The results for Co/Cu and Fe/Cr superlattices in which layer thicknesses are made to fluctuate typically between 2 and 10 atomic planes show that strong disorder of the sequence of ferromagnet/spacer interfaces has virtually no effect on the saturation-field resistance ${\mathrm{R}}_{\mathrm{FM}}$, which remains as low as in the Ohmic regime. The zero-field resistance, on the other hand, increases approximately exponentially with the number of bilayers N due to Anderson localization with a localization length \ensuremath{\approx}30--40 nm. The CPP GMR ratio ${\mathrm{R}}_{\mathrm{CPP}}$, therefore, also increases approximately exponentially with N and values as high as ${\mathrm{R}}_{\mathrm{CPP}}$\ensuremath{\approx}3\ifmmode\times\else\texttimes\fi{}${10}^{4}$ are predicted for Fe/Cr valves with N\ensuremath{\approx}50 bilayers. Somewhat smaller (${\mathrm{R}}_{\mathrm{CPP}}$\ensuremath{\approx}${10}^{4}$) enhancement of the CPP GMR is obtained for Co/Cu pseudorandom spin valves. The conditions under which such enhancement should be observable are discussed.
TL;DR: The range of Thomsen's anisotropy parameters {epsilon} and {delta} for vertical transversely isotropic (VTI) media for fine-layered elas-tic materials is considered in this article.
Abstract: The range of Thomsen`s anisotropy parameters {epsilon} and {delta} for vertical transversely isotropic (VTI) media when the anisotropy is due to fine layering of isotropic elas-tic materials is considered. We show that {epsilon} lies in the range -3/8 { -1], for finely lay-ered media having constant density; smaller positive and all negative values of {epsilon} occur for media with large fluctuations in the Lam{acute e} parameter {lambda} We show that sign({delta}) = sign ( - ) for constant density media, so {delta} can be either positive or negative. Among all theoretically possible random media, posi-tive and negative {delta} are equally likely in finely layered media limited to two types of constituent layers. Lay-ered media having large fluctuations in Lam{acute e} {lambda} are the ones most likely to have positive {delta}. Since Gassmann`s results for fluid-saturated porous media show that the effects of fluids influence only the {lambda} Lam{acute e} constant, not the shear modulus {mu}, these results suggest that positive {delta} occurring together with positive but small {epsilon} may be indicative of changing fluid content in layered earth.
TL;DR: In this paper, the time series of chaotic orbits bounded by Kolmogorov-Arnold-Moser barriers is analyzed for subcritical values of the stochasticity parameter.
Abstract: In standard map dynamics, the time series ${\mathrm{x}}_{\mathrm{t}}$ are analyzed for chaotic orbits bounded by Kolmogorov-Arnold-Moser barriers, for subcritical values of the stochasticity parameter. They can be described as a succession of rather regular oscillations of bounded amplitude in basins located near island chains, and of jumps between basins, at ``random'' times. This motion can be adequately modeled by a continuous time random walk, using values of the parameters taken from the numerical data. The resulting theory describes a subdiffusive motion, for which the mean square displacement tends towards a saturation value.
TL;DR: The Barkhausen pattern from the amorphous alloy (Fe, Co, B) showed high sweep-to-sweep reproducibility for most sweeps when driven slowly over a minor hysteresis loop as mentioned in this paper.
Abstract: The Barkhausen pattern from the amorphous alloy ${\mathrm{Fe}}_{21}{\mathrm{Co}}_{64}{\mathrm{B}}_{15}$ showed high sweep-to-sweep reproducibility for most sweeps when driven slowly over a minor hysteresis loop. Small changes in the maximum applied field led to completely different pulse patterns. When driven near saturation the sweep-to-sweep reproducibility was lost. Large features observed on rapid sweeps were also reproducible, but not when the sample was driven to fields about two orders of magnitude larger than the coercive field.
TL;DR: In this paper, the apparent molar heat capacities of aqueous dimethylamine and Dimethylammonium chloride were measured with a Picker flow microcalorimeter over the temperature range 283.15 K ≤ T ≤ 523 K and at different pressures.
Abstract: Data for the apparent molar volumes of aqueous dimethylamine and dimethylammonium chloride have been determined with platinum vibrating tube densimeters at temperatures 283.15 K ≤ T ≤ 523.15 K and at different pressures. Apparent molar heat capacities were measured with a Picker flow microcalorimeter over the temperature range 283.15 K ≤T ≤ 343.15 K at 1 bar. At high temperatures and steam saturation pressures, the standard partial molar volumes
$$V_2^ \circ $$
of dimethylamine and dimethylammonium chloride deviate towards positive and negative discontinuities at the critical temperature and pressure, as is typical for many neutral and ionic species. The revised Helgeson-Kirkham-Flowers (HKF) model and fitting equations based on the appropriate derivatives of solvent density have been used to represent the temperature and pressure dependence of the standard partial molar properties. The standard partial molar heat capacities of dimethylamine ionization
$$\Delta _{{\text{ion}}} C_{{\text{p,2}}}^ \circ $$
, calculated from both models, are consistent with literature data obtained by calorimetric measurements at T ≤ 398 K to within experimental error. At temperatures below 523 K, the standard partial molar volumes of dimethylamine ionization
$$\Delta _{{\text{ion }}} V_2^ \circ $$
agree with those of morpholine to within 12 cm3-mol-1, suggesting that the ionization of secondary amine groups in each molecule is very similar. The extrapolated value for
$$\Delta _{{\text{ion }}} V_2^ \circ $$
of dimethylamine above 523 K is very different from the values measured for morpholine at higher temperature. The difference is undoubtedly due to the lower critical temperature and pressure of (CH3)2NH(aq).
TL;DR: In this paper, a bichromatic field with a resonant ε-Lambda-type coupling to the transition of Rb vapor atoms is used for reflection spectroscopy.
Abstract: We report an experimental investigation of selective reflection spectra using a bichromatic field with a resonant \ensuremath{\Lambda}-type coupling to the ${\mathrm{D}}_{1}$ transition of Rb vapor atoms. The reflection spectra are found to have a more complex structure than equivalent spectra in transmission experiments. This structure is traced back to the effect of wall collisions on velocity selective optical pumping and saturation processes. The spectral line shapes are well described by a theoretical model developed in the text. In addition we report the observation of dark resonances in reflection spectroscopy.
TL;DR: In this paper, a restricted-solid-on-solid model was employed to show that adsorption may result in a decrease of the roughening temperature of a clean surface due to repulsive adsorbate-adsorbate lateral interaction.
Abstract: Employing the restricted-solid-on-solid model, we show that adsorption may result in a decrease of the roughening temperature ${T}_{R}$ due to (i) indirect next-nearest-neighbor adsorbate-substrate repulsion, (ii) attractive nearest-neighbor adsorbate-substrate lateral interaction, or (iii) repulsive adsorbate-adsorbate lateral interaction. In all these cases, the ratio of ${T}_{R}$ near saturation and for a clean surface is given by a simple combination of the corresponding interactions. The coverage dependence of ${T}_{R},$ obtained for case (i) by Monte Carlo simulations, deviates from the mean-field-type prediction. Adsorbate-induced surface roughening observed in catalytic reactions is also briefly discussed.
01 Sep 1997
TL;DR: In this paper, the results of thermal measurements from four thermal-mechanical units, namely Tiva Canyon (TCw), Paintbrush Tuff (PTn), and two Topopah Spring units (TSw1 and TSw2), and from two lithologies, i.e., welded devitrified and nonwelded vitric tuff, are presented.
Abstract: Specimens were tested from four thermal-mechanical units, namely Tiva Canyon (TCw), Paintbrush Tuff (PTn), and two Topopah Spring units (TSw1 and TSw2), and from two lithologies, i.e., welded devitrified (TCw, TSw1, TSw2) and nonwelded vitric tuff (PTn). Thermal conductivities in W(mk){sup {minus}1} averaged over all boreholes, ranged (depending upon temperature and saturation state) from 1.2 to 1.9 for TCw, from 0.4 to 0.9 for PTn, from 1.0 to 1.7 for TSw1, and from 1.5 to 2.3 for TSw2. Mean coefficients of thermal expansion were highly temperature dependent and values, averaged over all boreholes, ranged (depending upon temperature and saturation state) from 6.6 {times} 10{sup {minus}6} to 49 {times} 10{sup {minus}6} C{sup {minus}1} for TCw, from the negative range to 16 {times} 10{sup {minus}6} {center_dot} {degree}C{sup {minus}1} for PTn, from 6.3 {times} 10{sup {minus}6} to 44 {times} 10{sup {minus}6} C{sup {minus}1} for TSw1, and from 6.7 {times} 10{sup {minus}6} to 37 {times} 10{sup {minus}6} {center_dot} {degree}C{sup {minus}1} for TSw2. Mean values of thermal capacitance in J/cm{sup 3}K (averaged overall specimens) ranged from 1.6 J to 2.1 for TSw1 and from 1.8 to 2.5 for TSw2. In general, the lithostratigraphic classifications of rock assigned by the USGS are consistent with the mineralogical data presented in this report.
TL;DR: In this paper, a phenomenological analysis for the pressure-induced change of the hole concentration in the CuO 2 layers was developed, where the external pressure dependence on different oxygen doping and ambient pressure was taken into account.
Abstract: A phenomenological analysis for the pressure-induced change of the hole concentration in the ${\mathrm{CuO}}_{2}$ layers ${(n}_{H})$ is developed. This effect along with the pressure-induced change of the maximum value of critical temperature ${(T}_{c})$ enables us to take account of the external pressure dependence of ${T}_{c}$ on different oxygen doping and different ambient ${T}_{c}$ on the basis of the inverted parabolic relationship between ${T}_{c}$ and ${n}_{H}.$ Detailed calculations for ${\mathrm{HgBa}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{6+\mathrm{\ensuremath{\delta}}}$ $(\ensuremath{\delta}=0.08\ensuremath{\sim}0.35)$ show that the ${T}_{c}(P)$ curve with a saturation at large pressure in the nearly optimally doped but slightly underdoped material is located above the curve with a saturation at relatively low pressure in the slightly overdoped one, and that an experimental curve for the optimal doping lies between the two curves. It is found that the pressure coefficient ${(dT}_{c}/dP)$ decreases with increasing pressure and the initial pressure coefficient declines monotonously with the increase of hole concentration.
TL;DR: The M/G/1 queue with an arrival rate λ that depends weakly upon time, as λ = λ(εt) where ε is a small parameter is considered, and approximations to the probability pn(t) that customers are present at time t are constructed.
Abstract: We consider the M/G/1 queue with an arrival rate \lambda that depends weakly upon time, as \lambda=\lambda (\varepsilon t) where \varepsilon is a small parameter. In the asymptotic limit \varepsilon \rightarrow 0, we construct approximations to the probability p_n (t) that n customers are present at time t. We show that the asymptotics are different for several ranges of the (slow) time scale \tau=\varepsilon t. We employ singular perturbation techniques and relate the various time scales by asymptotic matching.
TL;DR: In this article, the electronic and atomic properties of H chemisorbed on the 2D surface are studied using the extended H\"uckel theory method, and it is concluded that a geometry in which two H atoms are adsorbed per unit cell is the more consistent with the ARP data.
Abstract: The electronic and atomic properties of H chemisorbed on the two-dimensional (2D) ${\mathrm{ErSi}}_{2}$ $(1\ifmmode\times\else\texttimes\fi{}1)$ surface are studied using the extended H\"uckel theory method. Calculated electronic density of states and 2D band structure of various H chemisorption geometries are compared with angle-resolved photoemission (ARP) data. From this comparison it is concluded that a geometry in which two H atoms are adsorbed per unit cell is the more consistent with the ARP data. They are bound respectively to the Si dangling bonds present at the ${\mathrm{ErSi}}_{2}$ surface and to Er in the interstitial voids of the Er hexagonal plane below Si species of the outermost atomic plane of the buckled Si top layer. We have also performed an overlap populations analysis, which allows us to trace back the unique buckling reversal of the Si top layer upon H saturation to an optimization of the Er-Si chemical bonding in H-saturated silicide.
TL;DR: In this paper, different coverages of Cs on the cold Si(100)c(4\ifmmode\times\else\texttimes\fi{}2) surface were studied by photoelectron spectroscopy.
Abstract: Different coverages of Cs on the cold $\mathrm{Si}(100)c(4\ifmmode\times\else\texttimes\fi{}2)$ surface were studied by photoelectron spectroscopy. Valence-band spectra were recorded together with Si $2p$ and Cs $4d$ core-level spectra at a sample temperature of $\ensuremath{\sim}100\mathrm{K}$ for increasing Cs coverage. Emission from Cs atoms at the Si-Cs interface and from the bulk and surface of the Cs overlayer is readily identified in the Cs $4d$ spectra. Three different kinds of plasmon losses contribute to the photoemission background. The plasmons originate from the Si-Cs interface, Cs surface and Cs bulk with energies of around 1.0, 2.2, and 3.2 eV, respectively. The Si $2p$ emission becomes attenuated for increasing Cs coverage and is not detectable at a coverage of $\ensuremath{\sim}4.4$ times the room temperature saturation coverage. From the results of a systematic and quantitative analysis of the spectra combined with low-energy electron diffraction results, we conclude that Cs grows in a layer-by-layer fashion on the cold $\mathrm{Si}(100)c(4\ifmmode\times\else\texttimes\fi{}2)$ surface.
TL;DR: In this paper, a free-electron laser (FEL) configured as a traveling-wave amplifier was used to generate the bunching produced by the amplification of a coherent microwave source to drive appreciable power at the harmonics.
Abstract: Using a free-electron laser (FEL) configured as a traveling-wave amplifier, we have caused the bunching produced by the amplification of a coherent microwave source to drive appreciable power at the harmonics. A 10-kW 24-GHz microwave input signal grows to the $\ensuremath{\sim}200\mathrm{kW}$ level using the lower frequency unstable root of the waveguide FEL dispersion relation. The FEL operates in the TE11 mode, using a helical undulator (1.85-cm period) and a 3-mm-diam 600-kV electron beam contained in an 8.7-mm-i.d. cylindrical waveguide. The harmonic currents set up by the microwave are found to cause growth of harmonic power under two conditions. First, if the design is such that the upper frequency root corresponds to the third harmonic, we see small amounts of third- and second-harmonic power, coherent with the source. Second, we have found kW emission of the seventh harmonic, most likely from the TE72 mode, which travels at the same speed as the 24-GHz wave. In order to excite the seventh-harmonic radiation, the electron beam must be displaced from the axis of the guide by $\ensuremath{\sim}2\mathrm{mm}.$ In both cases, no harmonic power is produced without gain at the fundamental. We present a one-dimensional theoretical model of the experiment, and use the numerical results to interpret our findings. The model predicts that if the microwave signal is strong enough to drive the FEL into saturation, the harmonic radiation should become powerful.
TL;DR: In this article, a-Si:H TFTs with a saturation mobility of 0.7 cm{sup 2}/vsmore ǫ can be fabricated using argon dilution of silane in a 50-MHz glow discharge.
Abstract: The authors discuss various ways to produce hydrogenated amorphous silicon, a-Si:H, at a high deposition rate. They also present results of their recent study on the structural properties of a-Si:H films deposited at high rates using argon (Ar) dilution of silane in a 50-MHz glow discharge. The results of the depositions with Ar dilution are compared to films deposited from pure silane, SiH{sub 4}. The deposition rate r{sub d} is changed by varying the rf power P{sub rf} into the discharge. They focus on the P{sub rf}-dependence of the hydrogen (H) bonding configuration and total H content in the film. It is observed that r{sub d} saturates the H bonding configuration changes from mostly isolated H to mostly clustered H, and back to mostly isolated H. It is argued that Ar* metastable atoms play an important role in the growth mechanism at intermediate P{sub rf}, whereas at high P{sub rf} ion bombardment through Ar{sup +} and SiH{sub x}{sup +} (x {le} 3) ions becomes crucial. Two high-rate a-Si:H films are incorporated in thin-film transistors, TFTs. They present their characteristics before and after illumination with calibrated light. It is shown that a-Si:H TFTs with a saturation mobility of 0.7 cm{sup 2}/vsmore » can be fabricated, with the complete intrinsic layer deposited at 20 {angstrom}/s.« less
TL;DR: In this article, the authors showed that the grain boundaries in poly-Si:H TFTs are indeed very thin and showed that they have a low defect density, low activation energy and low carrier concentration.
Abstract: Poly-silicon films have been prepared by hot-wire chemical vapor deposition (HWCVD) from hydrogen diluted silane gas at a low temperature (430 C). The crystalline volume fraction is 95%. The grains have an average size of 70 nm and coalesce completely. The activation energy (0.54 eV) and the low carrier concentration (6.8 x 10{sup 10} cm{sup {minus}3}) indicate the fully intrinsic nature of the films. The low (<10{sup 17} cm{sup {minus}3}) defect density, the absence of 2,100 cm{sup {minus}1} mode in infrared spectrum, the low activation energy of Hall mobility (0.012 eV) and the ambipolar diffusion length (L{sub D}) of 568 nm all indicate that the grain boundaries in the poly-Si:H films are indeed very thin. Preliminary n-i-p cells incorporating a poly-silicon i-layer yielded 3.15% efficiency and a current density of 18.2 mW/cm{sup 2} for only a 1.5 {micro}m i-layer. In the solar cell, the conducting path is along the columnar grains ((220) preferential orientation from XRD) and the carrier transport bypasses the grain boundary defects. This has been confirmed from the c-Si like optical absorption spectrum (measured by Dual Beam Photoconductivity in the cell configuration) at the low energy transfer and output characteristics similar to those of state of themore » art a-Si:H TFTs: the saturation mobility is 0.4 cm{sup 2}/Vs and the off current is approximately 10{sup {minus}11} A. This leads to the conclusion that the silicon near the SiO{sub 2} interface (the channel region) is still amorphous. This is illustrated by XTEM.« less
TL;DR: In this article, an approach describing the thermal damping width of the giant dipole resonance (GDR) in hot nuclei is presented, where the GDR is generated by the ph excitations within the finite-temperature random-phase approximation (FTRPA), while its damping at finite temperature arises from irreversible coupling of ph configurations to the thermal pp and hh ones beyond the FTRPA.
Abstract: An approach describing the thermal damping width of the giant dipole resonance (GDR) in hot nuclei is presented. The GDR is generated by the ph excitations within the finite-temperature random-phase approximation (FTRPA), while its damping at finite temperature arises from irreversible coupling of ph configurations to the thermal pp and hh ones beyond the FTRPA. A semimicroscopic unification of the quantal spreading and thermal damping widths is undertaken within the framework of motional damping. The numerical calculations are performed, using a schematic model with equally degenerate equidistant shells for a hot nucleus of mass A=112 carrying no angular momentum. The results show that the total width of the GDR increases strongly as a function of the excitation energy up to E{sup {asterisk}}{approximately}120{endash}130 MeV, where it reaches a saturation value. The limiting temperature for the GDR in very hot nuclei is discussed. {copyright} {ital 1997} {ital The American Physical Society}
TL;DR: In this paper, a Monte Carlo simulation of both the single chain structure factor h(q) and the inter-chain structure factor HD(q)-q is presented over a wide range of both wavenumber q and concentration c from the dilute to the concentrated regime, for chain lengths up to N = 256.
Abstract: Off-lattice Monte Carlo simulations of both the single chain structure factor h(q) and the inter-chain structure factor HD(q) of flexible polymers in solutions are presented over a wide range of both wavenumber q and concentration c from the dilute to the concentrated regime, for chain lengths up to N = 256. The single chain properties $\{$gyration radius 〈Rg2〉, $h(q)\}$ are in reasonable agreement with the expected theoretical behavior, showing a crossover from swollen chains $\{\langle R_{\rm g}^2\rangle \propto N^{2
u} ,~ h(q) \propto q^{-1/
u}\}$ to Gaussian chains, and the data comply with a scaling description, with a correlation length ξ∝c-ν/(3ν-1). However, the inter-chain structure factor HD(q) disagrees with the corresponding predictions, we find a behaviour HD(q)∝q-3 only in an intermediate range but this is accidental: rather it is found that HD(q) smoothly bends over from its saturation value at small q to a behavior close to q-4 at q≈1/l, l being the length of effective bonds. This failure is traced back to the condition that the law HD(q)∝q-3 should only be observed for ξ-1≪q ≪l-1, a condition reached neither in the simulation nor in experiments. We also compare our results for HD(q) with the random phase approximation and find strong deviations.
26 Jul 1997
TL;DR: In this paper, the upconversion luminescence efficiencies of single crystals and powders co-doped with Er{sup 3+} and Yb{sup 2++}, and with Tm{sup 4+++] and yb{ sup 3++} were investigated.
Abstract: We discuss the upconversion luminescence efficiencies of phosphors that generate red, green, and blue light. The phosphors studied are single crystals and powders co-doped with Er{sup 3+} and Yb{sup 3+}, and with Tm{sup 3+} and Yb{sup 3+}. The Yb ions are pumped near 980 nm; transfers of two or three quanta to the co-doped rare earth ion generate visible luminescence. The main contribution embodied in this work is the quantitative measurement of this upconversion efficiency, based on the use of a calibrated integrating sphere, determination of the fraction of pump light absorbed, and careful control of the pump laser beam profile. The green phosphors are the most efficient, yielding efficiency values as high as 4 %, with the red and blue materials giving 1 - 2 %. Saturation was observed in all cases, suggesting that populations of upconversion steps of the ions are maximized at higher power. Quasi-CW modeling of the intensity- dependent upconversion efficiency was attempted; input data included level lifetimes, transition cross sections, and cross-relaxation rate coefficients. The saturation of the Yb,Er:fluoride media is explained as the pumping of Er{sup 3+} ions into a bottleneck (long-lived state)- the {sup 4}I{sub 13/2} metastable level, making them unavailable for further excitation transfer. 32 refs., 5 figs., 3 tabs.
TL;DR: In this paper, a comparison of SiH4 and Si2H6 chemical vapor deposition kinetics was performed in a rapid thermal processing (RTP) system at temperatures between 600 and 800°C and reactant gas pressures between 1 and 25 mTorr.
Abstract: A comparison of SiH4 and Si2H6 chemical vapor deposition kinetics was performed in a rapid thermal processing (RTP) system at temperatures between 600 and 800°C and reactant gas pressures between 1 and 25 mTorr. Quantitative assessment of the nucleation parameters and the microstructures of the deposited polycrystalline Si (poly-Si) films on SiO2 have been determined using in situ real time single wavelength and spectroscopie ellipsometry. In addition to ellipsometry, atomic force microscopy and cross-sectional transmission electron microscopy were used ex situ to observe the nucleation stage and the microstructures of the poly-Si films. In the present study we compare the nucleation, poly-Si film microstructure and surface roughness using SiH4 and Si2H6 in the RTP system and show that under the same processing conditions the saturation nuclei density (1010 cm-2) for Si2H6 is about 6 times higher than that for SiH4 and the poly-Si films from Si2H6 are smoother and have better columnar structure than those from SiH4. A particularly important parameter for selective epitaxial depositions is the time for nuclei to form, i.e. the incubation time. An operational incubation time were determined from the real time ellipsometric measurements and confirmed by AFM. The incubation times for using SiH4 and Si2H6 are different, but they show similar activation energies of about Einc = 1 eV in the 600–800°C range. A formula of incubation time tinc was obtained and expressed as follows $${t_{inc}} = \frac{C}{P}{e^{\frac{{{E_{inc}}}}{{kT}}}}$$
The physical meaning and comparison with the previous experimental results from other Labs are also discussed.