bluuebunny/crossref_metadata_2025 huggingface dataset:
Structure of the exact wave function
Authors: Nakatsuji Hiroshi
Published: July 2002

We investigate the structure of the exact wave function as a solution of the Schrödinger equation, aiming the singles and doubles description of the exact wave function. The basis is that the Hamiltonian involves only one and two body operators. We first present two theorems that indicate a possibility of the singles and doubles description of the exact wave function. We then examine the exponential ansatz, as this theorem implies it to be a possible structure of the exact wave function. Variational CCS (singles) wave function is shown to be certainly exact for one particle Hamiltonian. Thouless transformation plays an important role in the formulation. The conventional CCSD (singles and doubles) function is restrictive, even if it is solved variationally. A wider coupled cluster function with general singles and doubles substitution operators (CCGSD) is also not exact for the existence of noncommuting operators. We then analyze some formal properties of the full CI wave function, and finally, we propose an ansatz of the exact wave function and describe the method of solution. It involves successive solutions of the secular equations of the order of singles and doubles. It is variational and we can calculate both ground and excited states.
Self-epitaxial nucleation origin of fractal aggregation
Authors: Liu Xiang-Yang , Strom C. S.
Published: July 2002

The self-epitaxial nucleation and growth origin of the formation and transformation of crystalline fractal pattern is identified for the first time. Two universal parameters, so-called relative elemental branching period 〈l̃〉 and the anisotropy or structural match parameter m, are introduced to describe the pattern transformation. These two parameters can be predicted according to our model. Fractal patterns arise when 〈l̃〉⩽1. Fractal patterns have a strong anisotropy (m→1) at relatively low surface supersaturations, and a very weak anisotropy (m→−1) at high surface supersaturations. The theoretic results are in excellent agreement with experiments.
The case for a reactive resonance in F+H2
Authors: Chao Sheng Der , Skodje Rex T.
Published: July 2002

We have investigated the F+H2 reaction on the Stark–Werner potential energy surface using quantum scattering theory, adiabatic theory, and spectral quantization. Clear evidence is found for a transition state resonance on this potential surface. Comparison to experimental results suggests that a resonance is also present in the true reaction.
Evidence for La1 fluorescence from jet-cooled 3-methylindole-polar solvent complexes
Authors: Short Kurt W. , Callis Patrik R.
Published: July 2002

The ratio of nonresonant two-photon induced fluorescence excitation spectra using circularly and linearly polarized light for jet-cooled 3-methylindole complexed with a series of increasingly basic hydrogen bond acceptors (water, methanol, ethanol, diethylether, diethylamine and triethylamine) is consistent with an avoided crossing of the two lowest excited singlet states, La1 and Lb1. The dispersed fluorescence of these from this series also reflects the crossing, providing a definitive La1 jet-cooled fluorescence spectrum. The jet-cooled La1 fluorescence spectrum is not broad and redshifted, but has vibronic structure that agrees well with ab initio predictions and is similar to that of La3 phosphorescence.
Correlation of coercivity and microstructure of thin CoFe films
Authors: Platt C. L. , Berkowitz A. E. , Smith David J. , McCartney M. R.
Published: July 2002

The magnetic and structural properties of sputtered Co50Fe50(CoFe) films were examined. CoFe films 300 Å thick deposited on Si substrates at room temperature showed large coercive fields of 140 Oe. When similar thickness films were deposited at 100 °C, the coercivity dropped to 90 Oe, and when they were deposited on CoO, the coercivity was reduced to 12 Oe. Cross-sectional imaging with transmission electron microscopy revealed that the CoO underlayer had induced a columnar grain structure in the CoFe, with grain diameters ranging from 50 to 150 Å. CoFe films grown on Si contained larger grains of 200–350 Å in diameter with fewer distinct vertical grain boundaries. Lorentz microscopy showed that domain walls in the hard CoFe film formed complex, fixed patterns in fields less than the coercivity, whereas walls in the CoFe/CoO sample were more conformal and mobile in response to changing fields. Possible structural origins for the wide variation in coercivity obtained with different substrates, deposition temperature, and thickness of CoFe films are discussed.
Morphology and cathodoluminescence of Li-doped Gd2O3:Eu3+, a red phosphor operating at low voltages
Authors: Park Jung-Chul , Moon Hye-Kyung , Kim Dong-Kuk , Byeon Song-Ho , Kim Bong-Chul , Suh Kyung-Soo
Published: July 2002

A red phosphor, Li-doped Gd2O3:Eu3+ with high cathodoluminescent brightness was developed. In contrast to Gd2O3:Eu3+ which showed an irregular shape of agglomerated fine particles (the mean particle size <0.5 μm), the morphology of Li doped Gd2O3:Eu3+ crystals was quite regular and pseudospherical (the mean particle size ≈2 μm). In particular, the incorporation of Li+ ions into Gd2O3 lattice could induce a remarkable increase of cathodoluminescence efficiency at low voltages (500 V–1 kV). The highest emission intensity was observed with Gd1.84Li0.08Eu0.08O3−δ, whose brightness at 500 V was increased by a factor of 1.8 in comparison with that of commercial Y2O3:Eu3+. This phosphor may be an application to the field emission display operating at low voltages.
Fractional Fokker–Planck equation for nonlinear stochastic differential equations driven by non-Gaussian Lévy stable noises
Authors: Schertzer D. , Larchevêque M. , Duan J. , Yanovsky V. V. , Lovejoy S.
Published: July 2002

The Fokker–Planck equation has been very useful for studying dynamic behavior of stochastic differential equations driven by Gaussian noises. However, there are both theoretical and empirical reasons to consider similar equations driven by strongly non-Gaussian noises. In particular, they yield strongly non-Gaussian anomalous diffusion which seems to be relevant in different domains of Physics. In this paper, we therefore derive a fractional Fokker–Planck equation for the probability distribution of particles whose motion is governed by a nonlinear Langevin-type equation, which is driven by a Lévy stable noise rather than a Gaussian. We obtain in fact a general result for a Markovian forcing. We also discuss the existence and uniqueness of the solution of the fractional Fokker–Planck equation.
B/Si multilayers for soft x-ray and extreme ultraviolet optics
Authors: Ravet M. F. , Bridou F. , Raynal A. , Pardo B. , Chauvineau J. P. , André J.-M.
Published: July 2002

With the development of x-ray sources, high reflectivity and selectivity multilayers for optics are becoming a field of interest for the spectral region of 13–40 nm. In this article, it is shown from theoretical computations that multilayers made of two light materials such as B/Si can be used for these applications. Such multilayers were deposited by electron evaporation. Their physical properties were studied by x-ray reflectometry analyses at two wavelengths: 0.154 and 12.6 nm. The results show that the multilayers are made of dense and pure materials and that the interfacial layer thickness ranges from 0.3 to 0.7 nm.
Generation of hard x rays by ultrafast terawatt lasers
Authors: Guo T. , Spielmann Ch. , Walker B. C. , Barty C. P. J.
Published: July 2002

A compact, tabletop terawatt Ti:sapphire laser drive, ultrafast hard x-ray source for time-resolved x-ray diffraction studies is described. With a copper target the energy conversion efficiency from laser photons (800 nm) to copper K x-ray radiation (1.54 Å) is 0.008%. The optimal laser intensity for generating these x rays is 1018 W cm−2, lower than the highest laser intensity available (5×1018 W cm−2) from the laser system. These results are consistent with a theoretical model proposed on the basis that the x rays are produced as a result of laser driven electron ionization of core level electrons of Cu atoms near room temperature. This source also provides features such as ultrashort pulse duration, extremely small source size, variable wavelengths, high peak spectral brightness, and the potential for multiple beam line experiments. X-ray diffraction patterns from GaAs single crystals and amorphous Ni films recorded with this source are presented.
Dynamic simulation of process control of the reactive sputter process and experimental results
Authors: Malkomes N. , Vergöhl M.
Published: July 2002

The control of reactive sputter processes has been dynamically simulated by integrating the Larsson differential equations. This was done by employing a fast Runge–Kutta step control algorithm, allowing us to simulate sputtering with more than 20-fold real time speed on a pentium 166 Mhz. A simple proportional integral differential (PID) algorithm was implemented to simulate (i) the partial pressure control via reactive gas flow at a fixed current and (ii) the partial pressure control via current at a fixed reactive gas flow. The control cycle time was varied with respect to real life process control. These simulations show that arbitrary setpoints on the stationary s curve resulting from the steady state Larsson equations can be stabilized. However, the cycle time of the PID controller has to be small enough, e.g., less than 600 ms, for a reliable control. The setpoints in the transition mode are highly unstable, so that the process drifts immediately into one of the two corresponding stable steady states (typically within about 3–15 s) after freezing the control. In addition these computations were compared with experimental control results of reactively sputtered TiO2 and Nb2O5 films deposited by the midfrequency technique. In both cases the total s curve was stabilized at a constant oxygen flow. The process stabilization was performed at power densities of up to 5 W/cm2, limited by the generator output. For the oxygen partial pressure measurements a λ-probe with optimized speed was used.
The effects of dispersive Cn/Rn-attraction on M+/Rg bonding (M+=atomic metal ion, Rg=rare gas atom)
Authors: Burns Katherine L. , Bellert D. , Leung Allen W.-K. , Breckenridge W. H.
Published: July 2002

It has been shown, using a “model-potential” analysis, that −Cn/Rn dispersive terms can be an important part of the physical bonding in M+/Rg complexes (M+=atomic metal ion, Rg=rare gas atom) for M+ ions with large, polarizable outer-shell electron clouds. The model potential equation consists of all attractive terms (accurately calculated or estimated) out to 1/R8, as well as an Ae−bR repulsive term. From known De, Re, and ωe values, and the first and second derivatives of the model potential, the repulsive constants A and b as well as the effective charge Z of M+ in a particular M+⋅Rg electronic state, can be determined. For the typical M+⋅Rg states considered here, Z=1.02±0.07, indicating that no extra “chemical” effects are necessary to explain M+/Rg bonding. Furthermore, the trends in the derived Ae−bR repulsive curves make good qualitative sense. A term-by-term analysis for M+⋅Rg states where the M+ ion is small and unpolarizable [such as Na+(2p6)⋅Rg] shows that −Cn/Rn terms contribute only a few percent to the bond strengths, while for M+⋅Rg states where M+ is large and polarizable [for example, Mg+(3s)⋅Rg], the −CnRn terms can contribute on the order of 40%–50% to the bond strengths, thus rationalizing semiquantitatively several heretofore puzzling De, Re, ωe comparative values.
Crystal growth and optical characterization of cerium-doped Lu1.8Y0.2SiO5
Authors: Cooke D. W. , McClellan K. J. , Bennett B. L. , Roper J. M. , Whittaker M. T. , Muenchausen R. E. , Sze R. C.
Published: July 2002

Czochralski growth of cerium-doped Lu1.8Y0.2SiO5 (LYSO) from a 90/10 solution of Lu2SiO5 (LSO) and Y2SiO5 (YSO) is demonstrated. The alloyed scintillator retains the favorable growth properties of YSO and the desirable physical and optical scintillator properties of LSO. Radioluminescence, thermally stimulated luminescence, optical absorption, and lifetime measurements confirm the equivalence of LYSO and LSO optical properties. Advantages of LYSO Czochralski growth relative to LSO include reduced melting point, less propensity for formation of crystalline inclusions, lower cost of starting material, and easier incorporation of cerium into the host lattice. This material offers an attractive alternative to LSO for scintillator applications.
1.4 μm band emission properties of Tm3+ ions in transparent glass ceramics containing PbF2 nanocrystals for S-band amplifier
Authors: Hayashi Hideaki , Tanabe Setsuhisa , Hanada Teiichi
Published: July 2002

Tm 3+ ion doped transparent oxyfluoride glass ceramics containing PbF2 nanocrystals were prepared and infrared fluorescence properties of Tm3+ ions were investigated. From luminescence decay measurement, it is revealed that part of the Tm3+ ions were incorporated into fluoride crystals by annealing as-made glass above glass transition temperature. Temperature dependence of the 1.4 μm band emission properties of Tm3+ ions in transparent glass ceramics was investigated. With lowering temperature, the intensity ratio of the 1.4 μm emission band to the 1.8 μm band increased. At the same time, the center of gravity of the H43-F43 emission shifted to a longer wavelength and it reached about 1485 nm at 25 K. Under 50 K, the peak of the 1.4 μm band split and then the shape of the spectra became flatter. These properties can be utilized for the 1.5 μm (S-band) optical amplifier.
Phase separation in metalorganic vapor-phase epitaxy AlxGa(1−x)N films deposited on 6H–SiC
Authors: Vennéguès P. , Lahrèche H.
Published: July 2002

The microstructure of AlxGa(1−x)N films deposited on 6H–SiC substrates by metalorganic vapor-phase epitaxy is studied by transmission electron microscopy and energy-dispersive x-ray analysis. It is shown that in the first step of the growth (three-dimensional), the deposit is constituted of a thin Al-rich AlGaN wetting layer covering the substrate surmounted by pure GaN islands. In a second step, a homogeneous AlxGa(1−x)N film, with nominal Al concentration, is deposited and smoothing is observed. The results of the high-resolution transmission electron microscopy about the strain state of the different parts of the deposit indicate that the driving force for the phase separation phenomena is the decrease of the interfacial energy due to the low mismatch between Al-rich AlGaN and 6H–SiC (1% for pure AlN).
Alternating fluxes of positive and negative ions from an ion–ion plasma
Authors: Kanakasabapathy Sivananda K. , Overzet Lawrence J. , Midha Vikas , Economou Demetre
Published: July 2002

Relatively electron-free positive- and negative-ion plasmas (ion–ion plasmas) have been achieved in the afterglow of pulsed-power Cl2 discharges. The application of a low-frequency (20 kHz) bias voltage phase locked to the source power modulation and synchronous with the ion–ion plasma, resulted in alternating fluxes of positive (Cl2+) and negative (Cl−) ions on a substrate. These results qualitatively agree with a one-dimensional fluid model. This technique to produce alternate irradiations could be used to reduce differential charging-induced damage in high-aspect-ratio etching processes.
SiGe intermixing in Ge/Si(100) islands
Authors: Capellini G. , De Seta M. , Evangelisti F.
Published: July 2002

We have applied atomic force microscopy and x-ray photoemission spectroscopy to the study of SiGe intermixing in Ge/Si(100) self-assembled islands. We have quantified the Ge/Si alloying as a function of the deposition temperature in the 500–850 °C range. The Si content inside the islands varies from 0% at 550 °C up to 72% at 850 °C. As a consequence of the reduction of the effective mismatch due to the observed SiGe intermixing, the critical base width for island nucleation increases from 25 nm for Tdep<600 °C up to 270 nm for Tdep=850 °C.
A cause for highly improved channel mobility of 4H-SiC metal–oxide–semiconductor field-effect transistors on the (112̄0) face
Authors: Yano Hiroshi , Hirao Taichi , Kimoto Tsunenobu , Matsunami Hiroyuki
Published: July 2002

4H-silicon carbide (SiC) metal–oxide–semiconductor field-effect transistors fabricated on both (112̄0) and (0001) faces were characterized at various temperatures. From the temperature dependence of channel mobility, carrier transport in the inversion layer at the SiO2/4H-SiC(112̄0) interface was found to be affected by phonon scattering (μ0∼T−2.2), while that at the SiO2/4H-SiC(0001) interface was thermally activated (μ0∼T2.6) due to the decrease of Coulomb scattering by emission of electrons from acceptor-like interface states. From the temperature dependence of threshold voltage, the density of acceptor-like interface states near the conduction band edge seems to be low at the SiO2/4H-SiC(112̄0) interface, but quite high (>1013 cm−2 eV−1) at the SiO2/4H-SiC(0001) interface. The low density of acceptor-like interface states near the conduction band edge on the (112̄0) face should be the primary cause for the high inversion-channel mobility.
Two-color fluorescence correlation spectroscopy of one chromophore: Application to the E222Q mutant of the green fluorescent protein
Authors: Jung G. , Bräuchle C. , Zumbusch A.
Published: July 2002

Fluorescence correlation spectroscopy (FCS) is an important method for investigations of diffusion processes as well as of photophysical properties of fluorescing molecules. It has lately been applied in studies of the photodynamics of the green fluorescent protein (GFP). In this case FCS yields valuable information about the population of dark, non-fluorescing states of the molecule. For three-level systems rate constants into and out of the dark state can easily be determined with FCS. This task however becomes significantly more complex for molecules that possess several dark states. Here we present two-color FCS with simultaneous two-color excitation as a method that also yields spectroscopic information about the dark states. This makes the complete analysis of a molecular four-level system possible. The analysis of the GFP mutant E222Q is given as an example of two-color FCS that is readily applicable to other molecules with photoconvertible dark states. For E222Q we determine all the rate constants within the four-level system. With these data we calculate the population of the different molecular states in bulk experiments as encountered, e.g., in microscopic studies.
One-dimensional model of detached plasmas in the scrape-off layer of a divertor tokamak
Authors: Goswami Rajiv , Kaw Predhiman , Warrier Manoj , Singh Raghvendra , Deshpande Shishir
Published: July 2002

Numerical and analytical study of a detached divertor equilibrium is presented. The model uses one-dimensional equations for continuity, momentum and energy balance with radiation, ionization, charge-exchange, and recombination processes. A reasonably simple neutral model is also employed. Analytical calculation, using a simple five-region model for a case with negligible convective heat flux and constant sources/sinks, captures the essence of detailed numerical calculation for the same case. More general cases are handled numerically. The detachment is studied as a function of the ratio Q⊥/S⊥ [the ratio of power and particle volume source, coming from the core to the scrape-off layer (SOL) region]. For low values of Q⊥/S⊥ (detached state), at the midplane and at the target, the ion temperature (Ti) is almost equal to the electron temperature (Te). As this ratio increases (attached state), Ti is larger than Te at the midplane. However at the target, Te is found to be slightly larger than Ti. It is also observed that as Q⊥/S⊥ increases, the region of most intense radiation shifts progressively from closer to the X-point towards the target plate.
Conditions for sustainment of magnetohydrodynamic turbulence driven by Alfvén waves
Authors: Dmitruk P. , Matthaeus W. H. , Milano L. J. , Oughton S.
Published: July 2002

In a number of space and astrophysical plasmas, turbulence is driven by the supply of wave energy. In the context of incompressible magnetohydrodynamics (MHD) there are basic physical reasons, associated with conservation of cross helicity, why this kind of driving may be ineffective in sustaining turbulence. Here an investigation is made into some basic requirements for sustaining steady turbulence and dissipation in the context of incompressible MHD in a weakly inhomogeneous open field line region, driven by the supply of unidirectionally propagating waves at a boundary. While such wave driving cannot alone sustain turbulence, the addition of reflection permits sustainment. Another sustainment issue is the action of the nonpropagating or quasi-two dimensional part of the spectrum; this is particularly important in setting up a steady cascade. Thus, details of the wave boundary conditions also affect the ease of sustaining a cascade. Supply of a broadband spectrum of waves can overcome the latter difficulty but not the former, that is, the need for reflections. Implications for coronal heating and other astrophysical applications, as well as simulations, are suggested.
Deposition factors and band gap of zinc-blende AlN
Authors: Thompson Margarita P. , Auner Gregory W. , Zheleva Tsvetanka S. , Jones Kenneth A. , Simko Steven J. , Hilfiker James N.
Published: July 2002

Successful deposition of zinc-blende AlN films with thickness up to 1000 Å was performed with plasma source molecular beam epitaxy. The films were epitaxial to the Si(001) substrate. The formation of a thin 3C-SiC layer on the Si(001) surface is one of the important factors for the formation of zinc-blende AlN. Evidence for the presence of 3C-SiC is provided by an Auger electron spectroscopy depth profile and a high-resolution transmission electron microscopy plot profile. Spectroscopic ellipsometry was used to determine the optical constants of zinc-blende AlN in the range from 1.85 to 6.5 eV. The extinction coefficient data indicates that zinc-blende AlN is an indirect semiconductor with a band gap of ∼5.34 eV.
Band gap engineering based on MgxZn1−xO and CdyZn1−yO ternary alloy films
Authors: Makino T. , Segawa Y. , Kawasaki M. , Ohtomo A. , Shiroki R. , Tamura K. , Yasuda T. , Koinuma H.
Published: July 2002

We describe the structural and optical properties of II–VI oxide alloys, MgxZn1−xO and CdyZn1−yO, grown by pulsed-laser deposition. Single-phase alloyed films of (Mg,Zn)O and (Cd,Zn)O with c-axis orientations were epitaxially grown on sapphire (0001) substrates. The maximum magnesium and cadmium concentrations (x=0.33 and y=0.07, respectively) were significantly larger than the thermodynamic solubility limits. The band gap energies systematically changed from 3.0 (y=0.07) to 4.0 eV (x=0.33) at room temperature. The photoluminescence peak energy deduced at 4.2 K could be tuned from 3.19 to 3.87 eV by using Cd0.07Zn0.93O and Mg0.33Zn0.67O at both ends, respectively. The lattice constants of the a axis were monotonically increasing functions of the concentrations of both alloys. The exciton–phonon coupling strength was determined in Cd0.01Zn0.99O grown on a lattice-matched ScAlMgO4 substrate.
1.6 μm emission from Pr3+: (3F3,3F4)→3H4 transition in Pr3+- and Pr3+/Er3+-doped selenide glasses
Authors: Choi Yong Gyu , Kim Kyong Hon , Park Bong Je , Heo Jong
Published: July 2002

1.6 μm emission originated from Pr3+: (3F3,3F4)→3H4 transition in Pr3+- and Pr3+/Er3+-doped selenide glasses were investigated under an optical pump of a conventional 1480 nm laser diode. The measured peak wavelength and full width at half maximum of the fluorescent emission were ∼1650 and >100 nm, respectively. A moderate lifetime of the thermally coupled upper manifolds (∼212±5 μs) together with a high stimulated emission cross section of ∼(3±1)×10−20 cm2 promises to be useful for 1.6 μm band fiber-optic amplifier that can be pumped with an existing high-power laser diode. Codoping of Er3+ significantly enhanced the emission intensity by way of a nonradiative Er3+: 4I13/2→Pr3+: (3F3,3F4) energy transfer.
Laser ablation of bicomponent systems: A probe of molecular ejection mechanisms
Authors: Yingling Yaroslava G. , Zhigilei Leonid V. , Garrison Barbara J. , Koubenakis Antonis , Labrakis John , Georgiou Savas
Published: July 2002

A combined experimental and molecular dynamics simulation study of laser ablation of a model bicomponent system with solutes of different volatility provides a consistent picture of the mechanisms of material ejection. The comparison of the ejection yields shows that there are two distinct regimes of molecular ejection, desorption at low laser fluences, and a collective ejection of a volume of material or ablation at higher fluences. Ejection of volatile solutes dominates in the desorption regime, whereas nonvolatile solutes are ejected only in the ablation regime.
Development of a superthermal electron avalanche and tearing mode suppression during electron cyclotron resonant heating
Authors: Lazaros Avrilios
Published: July 2002

The critical velocity, for electron acceleration up to the superthermal region, is expressed in terms of the heating power per particle, instead of the inductively applied electric field. This expression is used to evaluate the superthermal electron production rate in a superthermal electron avalanche during electron cyclotron resonant heating (ECRH). The comparison between the predicted growth time of the superthermal electron avalanche and the experimental decay time of tearing modes provides an additional confirmation of the recent theory for the suppression of magnetohydrodynamic modes by superthermal electrons during ECRH.
Inhomogeneous spatial distribution of reverse bias leakage in GaN Schottky diodes
Authors: Hsu J. W. P. , Manfra M. J. , Lang D. V. , Richter S. , Chu S. N. G. , Sergent A. M. , Kleiman R. N. , Pfeiffer L. N. , Molnar R. J.
Published: July 2002

The reverse bias leakage current in macroscopic GaN Schottky diodes is found to be insensitive to barrier height. Using a scanning current–voltage microscope, we show that the reverse bias current occurs at small isolated regions, while most of the sample is insulating. By comparing the current maps to topographic images and transmission electron microscopy results, we conclude that reverse bias leakage occurs primarily at dislocations with a screw component. Furthermore, for a fixed dislocation density, the V/III ratio during the molecular beam epitaxial growth strongly affects reverse leakage, indicating complex dislocation electrical behavior that is sensitive to the local structural and/or chemical changes.
The ablation-front Rayleigh–Taylor dispersion curve in indirect drive
Authors: Budil K. S. , Lasinski B. , Edwards M. J. , Wan A. S. , Remington B. A. , Weber S. V. , Glendinning S. G. , Suter L. , Stry P. E.
Published: July 2002

The Rayleigh–Taylor (RT) instability, which occurs when a lower-density fluid accelerates a higher-density layer, is common in nature. At an ablation front a sharp reduction in the growth rate of the instability at short wavelengths can occur, in marked contrast to the classical case where growth rates are highest at the shortest wavelengths. Theoretical and numerical investigations of the ablative RT instability are numerous and differ considerably on the level of stabilization expected. Presented here are the results of a series of laser experiments designed to measure the RT dispersion curve for a radiatively driven sample. Aluminum foils with imposed sinusoidal perturbations ranging in wavelength from 10 to 70 μm were ablatively accelerated with a radiation drive generated in a gold cylindrical hohlraum. A strong shock wave compresses the package followed by an ∼2 ns period of roughly constant acceleration and the experiment is diagnosed via face-on radiography. Perturbations with wavelengths ⩾20 μm experienced substantial growth during the acceleration phase while shorter wavelengths showed a sharp drop off in overall growth. These experimental results compared favorably to calculations with a two-dimensional radiation-hydrodynamics code, however, the growth is significantly affected by the rippled shock launched by the drive. Due to the influence of the rippled shock transit phase of the experiment and ambiguities associated with directly extracting the physical amplitude of the perturbations at the ablation front from the simulations, direct comparison to the ablation front RT theory of Betti et al. [Phys. Plasmas 5, 1446 (1998)], was difficult. Instead, a numerical “experiment” was constructed that minimized the influence of the shock and this was compared to the Betti model showing quite good agreement.
The accurate determination of molecular equilibrium structures
Authors: Bak Keld L. , Gauss Jürgen , Jørgensen Poul , Olsen Jeppe , Helgaker Trygve , Stanton John F.
Published: July 2002

Equilibrium structures have been determined for 19 molecules using least-squares fits involving rotational constants from experiment and vibrational corrections from high-level electronic-structure calculations. Equilibrium structures obtained by this procedure have a uniformly high quality. Indeed, the accuracy of the results reported here likely surpasses that reported in most experimental determinations. In addition, the accuracy of equilibrium structures obtained by energy minimization has been calibrated for the following standard models of ab initio theory: Hartree–Fock, MP2, CCSD, and CCSD(T). In accordance with previous observations, CCSD(T) is significantly more accurate than the other models; the mean and maximum absolute errors for bond distances of the 19 molecules are 0.09 and 0.59 pm, respectively, in CCSD(T)/cc-pCVQZ calculations. The maximum error is obtained for ROO in H2O2 and is so large compared with the mean absolute error that an experimental reinvestigation of this molecule is warranted.
Reduction of the ZnSe/GaAs(100) valence band offset by a Te interlayer
Authors: Gleim Th. , Heske C. , Umbach E. , Schumacher C. , Faschinger W. , Ammon Ch. , Probst M. , Steinrück H.-P.
Published: July 2002

For the heterovalent system ZnSe/GaAs(100), we have investigated the influence of a Te pretreatment of the substrate on the electronic structure of the interface by photoelectron spectroscopy. We have paid special attention to correctly determine the valence band maximum in a k-resolved fashion, including the use of photon energies which enable excitation at the Γ point. We find that the Te pretreatment leads to a decrease of the valence band discontinuity as large as 0.3 eV. From photoemission depth profiling we conclude that some Te atoms remain localized at the interface, thus causing the change of the valence band offset while others float on the ZnSe surface, probably acting as surfactants.
Red photoluminescence of gold island films
Authors: Khriachtchev Leonid , Heikkilä Lauri , Kuusela Tom
Published: July 2002

We comparatively study photoluminescence and morphology of gold films prepared with a sputtering method. The ultrathin films (⩽10 nm) efficiently emit light above 600 nm, which dramatically differs from the known emission of thick gold films. The absorption mechanism in ultrathin gold films is presumably contributed by their island structure promoting interacting plasmons. The collective plasmon states of island films decay radiatively, producing the extraordinary red emission observed under excitation at 488.0, 514.5, and 632.8 nm. The red photoluminescence of ultrathin films is substrate dependent, and correlates with the surface morphology analyzed by using atomic force microscopy and resistance measurements.
Surface solvation of halogen anions in water clusters: An ab initio molecular dynamics study of the Cl−(H2O)6 complex
Authors: Tobias Douglas J. , Jungwirth Pavel , Parrinello Michele
Published: July 2002

The structure and dynamics of Cl−(H2O)6 has been studied by ab initio molecular dynamics using the Car–Parrinello approach, and compared to results of ab initio quantum chemical calculations, molecular dynamics based on both polarizable and nonpolarizable empirical potentials, and vibrational spectroscopy. The electronic structure methodology (density functional theory with the gradient-corrected BLYP exchange-correlation functional) used in the Car–Parrinello dynamics has been shown to give good agreement with second-order Møller–Plesset results for the structures and energies of Cl−(H2O)n, n=1–4, clusters. The configurational sampling during the 5 ps ab initio molecular dynamics simulation at 250 K was sufficient to demonstrate that the chloride anion preferred a location on the surface of the cluster which was significantly extended compared to the minimum energy geometry. The structure of the cluster predicted by the polarizable force field simulation is in agreement with the ab initio simulation, while the nonpolarizable force field calculation was in qualitative disagreement, predicting an interior location for the anion. The time evolution of the electronic structure during the ab initio simulation was analyzed in terms of maximally localized orbitals (Wannier functions). Calculation of the dipole moments from the centers of the Wannier orbitals revealed that the chloride anion is significantly polarized, and that the extent of water polarization depends on location in the cluster, thus underscoring the importance of electronic polarization in halogen ion solvation. The infrared absorption spectrum was computed from the dipole–dipole correlation function, including both nuclear and electronic contributions. Aside from a systematic redshift by 3%–5% in the frequencies, the computed spectrum was in quantitative agreement with vibrational predissociation data on Cl−(H2O)5. Our analysis suggests that accounting for anharmonicity and couplings between modes is more important than the fine tuning of the electronic structure method for the quantitative prediction of hydrogen bond dynamics in aqueous clusters at elevated temperatures.
Polyelectrolytes tethered to a similarly charged surface
Authors: Borisov O. V. , Leermakers F. A. M. , Fleer G. J. , Zhulina E. B.
Published: July 2002

Conformations of weakly charged quenched polyelectrolyte chains tethered to a similarly charged planar surface are analyzed on the basis of a combination of scaling, analytical, and numerical self-consistent field (SCF) approaches. Scaling theory predicts universal power law dependences of the large-scale conformational properties (like the end-to-end distance) of grafted chains on the overall surface charge per unit area. The SCF approach allows analysis of the detailed conformational structure of grafted polyions as a function of the distribution of immobilized charges between the surface and grafted chains. The analytical solution is only available in the limiting cases of sparse grafting of polyions to the charged plane and sufficiently dense grafting of polyions to a neutral surface. In the intermediate case when both interchain interactions and interaction of grafted chains with the surface are important, only numerical solutions can be obtained. We consider various ways to distribute the charges between the surface and the brush chains while keeping the sum of the two contributions constant. Upon increasing the charge on the surface, by concomitant reduction of the grafting density of the tethered chains we found: (i) an increase in height of the polymer layer; (ii) the development of a depletion zone of end points near the surface; and (iii) a sharpening of the end-point distribution with the peak shifting away from the surface. In the appropriate limiting cases excellent agreement of analytical SCF predictions is obtained with the numerical results.
Heats of formation and ionization energies of NHx, x=0–3
Authors: Dixon David A. , Feller David , Peterson Kirk A.
Published: July 2002

The heats of formation of NH3, NH2, NH and the ionization energies of NH3, NH2, NH, and N have been calculated at high levels of ab initio molecular orbital theory at 0 K. Geometries and frequencies were calculated with coupled cluster theory, including a perturbative treatment of the connected triple excitations and with correlation consistent basis sets up through augmented sextuple zeta in quality. Subsequent extrapolation of the total energies to the complete one-particle basis set limit was performed to further reduce the basis set truncation error. Additional improvements in the atomization energy were achieved by applying corrections for core/valence correlation, scalar relativistic, spin–orbit, and higher order correlation effects. Zero point energies were taken from anharmonic force fields where available or are based on appropriately scaled values. Using the R/UCCSD(T) method, we find the following heats of formation (kcal/mol) at 0 K: ΔHf(NH3)=−9.10±0.17 (calc.) versus −9.30±0.10 (expt.); ΔHf(NH2)=45.27±0.12 (calc.) versus 45.17±0.09 (expt.); ΔHf(NH)=85.92±0.08 (calc.) versus 90.0±4 (expt.); ΔHf(NH3+)=225.44±0.23 (calc.) versus 225.59±0.08 (expt.); ΔHf(NH2+)=303.00±0.20 (calc.) versus 302.60±0.08 (expt.); and ΔHf(NH+)=396.56±0.12 (calc.).
Solvent effects on heteropolymer adsorption and freezing
Authors: Srebnik Simcha
Published: July 2002

Replica mean field theory is used to study the effect of solvent interactions on the adsorption of random heteropolymers onto heterogeneous surfaces. Random heteropolymers have been shown to undergo a pattern-matching adsorption transition, typified by a sharp transition from weak to strong adsorption on specific heterogeneous surfaces. It is found that polymers with higher excluded volume (good solvent) require higher surface site loadings to undergo the sharp adsorption transition. Furthermore, the final adsorbed polymer fraction is lower for high excluded volume polymers. This information can be used, for example, to design tailor-made membranes that prevent or allow for specific interactions with such protein-like molecules. Studies have shown that solution conditions often play an important role in determining the fouling of membranes by proteins. The results presented in this work may be important in directing applications involving the undesirable adsorption of proteins and other macromolecules that lead to the irreversible fouling of membranes.
Explicit-solvent molecular dynamics simulation at constant pH: Methodology and application to small amines
Authors: Börjesson Ulf , Hünenberger Philippe H.
Published: July 2002

A method is developed for performing classical explicit-solvent molecular dynamics (MD) simulations at constant pH, where the protonation state of each ionizable (titratable) group in a simulated compound is allowed to fluctuate in time, depending on the instantaneous system configuration and the imposed pH. In this method, each ionizable group is treated as a mixed state, i.e., the interaction-function parameters for the group are a linear combination of those of the protonated state and those of the deprotonated state. Free protons are not handled explicitly. Instead, the extent of deprotonation of each group is relaxed towards its equilibrium value by weak coupling to a “proton bath.” The method relies on precalibrated empirical functions, one for each type of ionizable group present in the simulated compound, which are obtained through multiple MD simulations of monofunctional model compounds. In this study, the method is described in detail and its application illustrated by a series of constant-pH MD simulations of small monofunctional amines. In particular, we investigate the influence of the relaxation time used in the weak-coupling scheme, the choice of appropriate model compounds for the calibration of the required empirical functions, and corrections for finite-size effects linked with the small size of the simulation box.
Comparison of the thermal stability of NiSi films in Ni/Pt/(111)Si and Ni/Pt/(100)Si systems
Authors: Liu J. F. , Feng J. Y. , Zhu J.
Published: July 2002

The thermal stability of NiSi films in Ni/Pt/(111)Si and Ni/Pt/(100)Si bilayered systems was investigated. In the Ni/Pt/(111)Si system, the NiSi films formed at 640 °C have an epitaxial relationship of (100)NiSi∥(111)Si and [01̄0]NiSi∥[011̄]Si with the substrates. On the other hand, those formed on (100)Si substrates were polycrystalline although the experimental parameters are the same as those in Ni/Pt/(111)Si samples. In both cases, NiSi and PtSi formed a solid solution Ni1−xPtxSi following Vegard’s law. While the thermal stability of NiSi films is improved in both cases compared with the Ni/Si system, the thermal stability of the textured NiSi films formed on (111)Si substrates is higher than their polycrystalline counterparts formed on (100)Si substrates. This is attributed to the reduced interfacial energy and consequently increased activation energy for the nucleation of NiSi2 due to the formation of textured NiSi films on (111)Si substrates. At 900 °C, the Ni(Pt)Si film formed in Ni/Pt/(100)Si system decomposed into separate phases of NiSi and PtSi with noticeable lattice deformation. Differences between the texture evolution of NiSi films in Ni/Pt/Si and Ni(Pt) alloy/Si systems were also reported and explained from both kinetic and thermodynamic aspects.
Ground state of a spin-1/2 charged particle in a two-dimensional magnetic field
Authors: Hirokawa Masao , Ogurisu Osamu
Published: July 2002

It is investigated that the structure of the kernel of the Dirac–Weyl operator D of a charged particle in the magnetic-field B=B0+B1, given by the sum of a strongly singular magnetic field B0(⋅)=Σνγνδ(⋅−aν) with some singular points aν and a magnetic-field B1 with a bounded support. Here the magnetic field B1 may have some singular points with the order of the singularity less than 2. At a glance, it seems that, following “Aharonov–Casher Theorem” [Phys. Rev. A 19, 2461 (1979)], the dimension of the kernel of D, dim ker D, is a function of one variable of the total magnetic flux (=Σνγν+∫R2B1dxdy) of B. However, since the influence of the strongly singular points works, dim ker D indeed is a function of several variables of the total magnetic flux and each of γν’s.
Two-fluid flowing equilibria of compact plasmas
Authors: Steinhauer Loren C. , Yamada Hideaki , Ishida Akio
Published: July 2002

The properties of two-fluid flowing equilibria are explored. This is facilitated by limiting attention to compact toroids in a “stationary-energy” state with uniform density. Flowing equilibria are found to fall into two classes, force-free and non-force-free, referring to the absence or presence of a j×B force. The force-free class may have significant flows. Spheromaks are in this class. The non-force-free class is diamagnetic and has Alfvénic poloidal flows. Field reversed configurations (FRCs) are in this class. Both classes admit arbitrarily large equilibria. Both classes occupy certain “allowed” regions in “helicity space,” a two-dimensional parameter map with the electron and ion helicities as coordinates. Allowed regions for the two classes overlap; in the overlap region the non-force-free class is energetically favorable. This sheds light on the FRC-spheromak bifurcation observed in experiments. Two-dimensional analytic equilibria are also found that span both classes. These may play a role similar to the familiar Hill’s vortex and Bessel function models in static, magnetohydrodynamic equilibria.
Ionization instabilities and resonant acoustic modes
Authors: Wang Xiaogang , Bhattacharjee A. , Gou S. K. , Goree J.
Published: July 2002

A linear stability analysis of ion-acoustic and dust-acoustic waves is carried out using a multifluid model in the presence of ionization, ion drag, and collisions of ions and dust with the background neutral gas. It is found that an unstable dust-acoustic mode of nonzero real frequency can be generated via a resonance phenomenon. This resonance develops as the frequency of the dust-ion-acoustic mode is reduced sufficiently in the long-wavelength regime that it couples strongly to the dust-acoustic mode. As the charge on dust particles exceeds a threshold, multiple low-frequency modes with large growth rates are excited suddenly. Predictions of the theory are compared with experimental results [D. Samsonov and J. Goree, Phys. Rev. E 59, 1047 (1999)].
Some common misconceptions concerning magnetic refrigerant materials
Authors: Pecharsky V. K. , Gschneidner K. A.
Published: July 2002

The relationships between both extensive and intensive properties quantifying the magnetocaloric effect, i.e., between the isothermal entropy change and the adiabatic temperature change, respectively, have been analyzed. An extensive measure of the magnetocaloric effect alone, without considering another important and also extensive thermodynamic property, i.e., the heat capacity, may lead to biased conclusions about the size of the magnetocaloric effect and, consequently, about the applicability of a magnetic material as a magnetic refrigerant. The near room temperature magnetocaloric properties of the colossal magnetoresistive manganites [(R1−xMx)MnO3, where R=lanthanide metal and M is alkaline earth metal] and the recently discovered Fe-based intermetallic material (LaFe11.47Co0.23Al1.3) have been reaccessed and correctly compared with those of the metallic Gd prototype. Our analysis has shown that these 3d materials are inferior to Gd by a factor of 2 or more because of the high values of the heat capacity per unit mass. Also a comparison of the volumetric isothermal entropy change, which is a critical parameter for the operation of a refrigeration unit, indicates that Gd is superior to these 3d materials for practical applications.
Effects of mechanical compression on the vibrational spectrum of a self-assembled monolayer
Authors: Berg Otto , Klenerman David
Published: July 2002

A molecular monolayer (octadecanethiol on gold) is compressed between a bronze ball and a sapphire prism. Vibrational spectra are obtained by optical sum-frequency generation before, during, and after a pressure cycle to 660 MPa (6.6 kbar). Both reversible and permanent effects are observed on the frequency, intensity, and shape of methyl C–H stretching resonances in the monolayer. Spectral broadening indicates the heterogeneity of chemical environments in the contact, while a pressure-dependent loss of orientational order attenuates the resonant signals. These results from a model system demonstrate that contact-modifying molecular films can be investigated spectroscopically.
Growth and characterization of Y2O3:Eu on Si and yttria-stabilized zirconia
Authors: Joo Rhee Seuk , White Jeffrey O. , Lee Sangwoo , Chen Haydn
Published: July 2002

Y 2 O 3 films doped with Eu are grown by metalorganic chemical vapor deposition on Si(100) and yttria-stabilized zirconia (YSZ) (100). The mismatch is only 2.6% between the lattice constant of YSZ and half the lattice constant of Y2O3. The samples are characterized by x-ray diffraction, scanning electron microscopy, Rutherford backscattering, and photoluminescence. The films deposited on Si are polycrystalline. When films are deposited on YSZ, the Y2O3(100) direction is aligned with YSZ(100). The luminescence and the narrow x-ray diffraction lines indicate that a Y2O3:Eu film with high crystallinity is obtained without annealing.
Hydrodynamic equations for mixed quantum states. I. General formulation
Authors: Burghardt I. , Cederbaum L. S.
Published: July 2002

Quantum-mechanical hydrodynamic equations are considered for mixed quantum states, and the corresponding equations for pure quantum states are derived as a particular case. A generalization of the “quantum potential” of Bohmian mechanics is formulated. In the mixed-state case, an infinite hierarchy of kinetic equations arises that may be truncated by introducing suitable approximations. The influence of dissipation on the kinetic equations is discussed.
Attractors for second-order lattice dynamical systems with damping
Authors: Shengfan Zhou
Published: July 2002

We consider the existence and the approximation of the global attractor for second-order damped lattice dynamical systems.