bluuebunny/crossref_metadata_2025 huggingface dataset:
Planar superconductor-normal-superconductor Josephson junctions in MgB2
Authors: Burnell G. , Kang D.-J. , Lee H. N. , Moon S. H. , Oh B. , Blamire M. G.
Published: July 2002

We report the successful creation of planar MgB2 junctions by localized ion damage in thin (100 nm) films of MgB2 on sapphire by milling a 50 nm trench with a focused-ion beam across tracks of widths between 1 and 5 μm. When the depth of the trench is between 70% and 80% of the film thickness, devices show critical currents (IC) for temperatures below 25 K. The IC of these devices is strongly modulated by applied microwave radiation and magnetic field. The product of the critical current and normal state resistance (ICRN) is remarkably high, implying a potential for very-high-frequency applications.
Metalorganic molecular beam epitaxy of (In)GaAsN with dimethylhydrazine
Authors: Jin C. , Nikishin S. A. , Kuchinskii V. I. , Temkin H. , Holtz M.
Published: July 2002

Single layers and superlattices of GaAsN/GaAs and InGaAsN/GaAs were grown using metalorganic molecular beam epitaxy with dimethylhydrazine, trimethylindium, triethylgallium, and conventional arsenic sources. Nitrogen incorporation into the solid was investigated as a function of the substrate temperature and fluxes. The nitrogen incorporation kinetics and growth mechanism have been modeled by assuming formation of an adduct arising from reactions between triethylgallium and dimethylhydrazine, while neglecting reactions between precursors of trimethylindium and dimethylhydrazine. The model accounts for the experimentally observed relationship between growth rates and nitrogen incorporation in GaAsN and InGaAsN. Our experiments show that the absolute arsenic flux and the As/N flux ratio play a critical role in the growth of single phase GaAsN.
Magnetoresistance of spin-dependent tunnel junctions with composite electrodes
Authors: Ho C. H. , Lin Minn-Tsong , Yao Y. D. , Lee S. F. , Liao C. C. , Chen F. R. , Kai J. J.
Published: July 2002

Spin-dependent tunnel junctions, Co/Al2O3/Co (CoFe)/NiFe, were fabricated to investigate the effect of the additional Co (CoFe) interlayer on tunneling magnetoresistance. The quality of the junction was examined with a cross-sectional image generated by high-resolution transmission electron microscopy, and an electron energy loss spectra map. For junctions with a Co (CoFe) interlayer in the top electrode thinner than 0.8 nm (1.0 nm), the tunneling magnetoresistance ratio increases with interlayer thickness. For junctions with a 0.8–2.0 nm Co (1.0–2.0 nm CoFe) interlayer in the top electrode, the tunneling magnetoresistance ratio reaches the maximum value of 2.16 (4.45) times that without any Co (CoFe) interlayer in the top electrode. The increase in the tunneling magnetoresistance ratio may be attributed to the increased effective ferromagnetic electrode polarization and the various spin-flip scattering factors.
The mechanism of HF/H2O chemical etching of SiO2
Authors: Kang Jeung Ku , Musgrave Charles B.
Published: July 2002

Quantum chemistry is used to investigate the HF/H2O chemical etching mechanism of silicon dioxide. Etching proceeds through four sequential steps to remove silicon as SiF4(g) for silicon dioxide, eventually leading to a fluorine-terminated silicon surface which HF attacks, resulting in a hydrogen-passivated silicon surface. Our predicted activation barriers show that the concerted attack by HF and H2O enhances the etch rate over etching by HF alone by reducing the barrier for each etching step. This is consistent with experimental observations that HF etching is enhanced by the presence of water. The QCISD barrier for the rate-limiting step of this catalytic HF/H2O etching mechanism is 22.1 kcal/mol, while the QCISD etching barrier for HF etching is 35.1 kcal/mol. In addition, we find that the F-terminated silica surface is not readily hydrolyzed by exposure to water since the forward barrier of 32.1 kcal/mol for hydrolysis of F-terminated silicon dioxide is larger than the reverse barrier of 26.2 kcal/mol.
Electrical isolation of n-type GaAs and InP using helium ion irradiation at variable target temperatures
Authors: Ahmed S. , Too P. , Gwilliam R. , Sealy B. J.
Published: July 2002

A single-energy isolation scheme is applied to both n-type GaAs and n-type InP using helium ion irradiation at room temperature (RT), 100 °C and 200 °C to create thermally stable highly resistive regions. It is found that optimum isolation and its persistence to the further heat treatment is very much influenced by the implant temperature in the case of n-type GaAs. Isolation caused by hot implants in GaAs layers is more effective and has an improved thermal stability over RT implants which is due to the formation of thermally stable defects during implantation at elevated temperatures. No such behavior is found in the case of n-type InP though the same isolation-implant conditions were applied to both materials. For helium-isolated n-type InP samples, RT implants are as effective as hot implants and the behavior of the sheet resistivity for all substrate temperatures is identical. The isolation scheme used in this work looks promising since such high isolation values (∼107 Ω/)□ with a broad thermally stable window are obtained for n-type InP material.
Pump-power dependence of THz radiation from InAs surfaces under magnetic fields excited by ultrashort laser pulses
Authors: Migita M. , Hangyo M.
Published: July 2002

THz radiation from InAs surfaces excited by ultrashort laser pulses has been studied under low (∼0.6 μJ/cm2 fluence) and high (∼6 mJ/cm2 fluence) pump power densities. The THz radiation amplitude is affected in both cases. We found that the radiation wave form is broadened by applying magnetic fields for the low-density-excitation case whereas it is not for the high-density-excitation case. This result is explained by the difference of the effect of the magnetic field on the THz radiation due to the transient photocurrent and optical rectification effects.
Specific features of geminate radical pair recombination in high magnetic field
Authors: Shushin A. I.
Published: July 2002

The specific features of diffusion-assisted geminate radical pair (RP) recombination in high magnetic field is analyzed in detail. It is shown that the dependence of the recombination yield R on the intraradical magnetic interaction Ā (hyperfine interaction, etc.) significantly depends on the parameter ξ∼(Ā/Dα2), in which D is the diffusion coefficient and α−1 is the characteristic length of the spin exchange interaction. Simple formula, accurately describing the dependence R(Ā) over a wide region of parameters of the system, is proposed and thoroughly discussed.
Magnetization reversal of Co-rich wires in circular magnetic field
Authors: Chizhik A. , Gonzalez J. , Zhukov A. , Blanco J. M.
Published: July 2002

The process of magnetization reversal of Co-rich amorphous wires in circular magnetic field has been studied by Kerr effect. The transverse Kerr loops had features, related to circular domains nucleation and domain wall propagation. A correlation between the switching field and wire length has been found. A remarkable change of the magnetization reversal under the application of axial dc magnetic field was observed. This could be explained by the presence of an interaction between the axial inner core and circular outer shell of Co-rich wires.
Characterization of gels by Monte Carlo method using a model of radical polymerization with cross linkers
Authors: Nosaka Makoto , Takasu Masako , Katoh Kouichi
Published: July 2002

In this paper, we propose some physical quantities for characterizing gels. Polymer networks (gels) were investigated by Monte Carlo method using a model of free-radical cross-linked polymerization in a continuous system. The distributions of the degree of polymerization for clusters in this simulation are in good qualitative agreement with experimental results. Linkers can be classified into two types according to their role in the network: One forms a simple closed loop within a polymer, and another forms polymer clusters. Their respective amounts and ratios are examined with regard to changes in the distribution of the degree of polymerization.
Influence of nanoscale geometry on the thermodynamics of electron field emission
Authors: Fisher T. S.
Published: July 2002

The effects of nanoscale emitter size on the energy transport in electron field-emission devices are considered, with emitter radii ranging from 5 nm to infinity. Theoretical results for energy distribution and flux indicate that emitter radii less than 50 nm produce significant filtering in favor of high-energy electrons relative to the planar case. An emitter radius of 5 nm is shown to produce a peak energy flux of 250 W/cm2 away from the emitter at a temperature of 300 K. This high rate of energy transport could prove to be useful in the development of direct refrigeration devices.
Oscillations localized near impurity layers in a layered crystal
Authors: Grishaev V. I. , Mamaluı̆ M. A. , Minaev P. A. , Syrkin E. S. , Feodos’ev S. B.
Published: July 2002

It is shown that the interaction between vibrational modes polarized parallel and perpendicular to the layers in a strongly anisotropic layered crystal is very weak for frequencies above the van Hove frequency, which corresponds to a transition from closed isofrequency surfaces to surfaces which are open in the weak-coupling direction. Such mode quasisplitting results in quasi-two-dimensional behavior of phonons polarized along layers and quasi-one-dimensional behavior of phonons polarized in a direction perpendicular to the layers and localized near light or strongly coupled impurity atoms or monolayers.
Coupling one-dimensional time-dependent classical and quantum transport models
Authors: Abdallah N. Ben , Degond P. , Gamba I. M.
Published: July 2002

A transient model for one-dimensional charge transport in an open quantum system is proposed. In the semiclassical limit, it reduces to the inflow boundary value problem for the classical transport equation. On this basis, the coupling of classical and quantum transport models through an interface is investigated. Suitable interface conditions are derived through asymptotic formulas involving the quantum reflection–transmission coefficients and time delays.
Effect of oxygen pressure on the structure and thermal stability of ultrathin Al2O3 films on Si(001)
Authors: Kundu Manisha , Ichikawa Masakazu , Miyata Noriyuki
Published: July 2002

Al 2 O 3 /Si(001) surfaces and interfaces were investigated using scanning reflection electron microscopy, reflection high-energy electron diffraction, x-ray photoelectron spectroscopy, and Auger electron spectroscopy. A uniform, stoichiometric and ultrathin Al2O3 film of about 0.6 nm was grown on an atomically flat Si(001)-2×1 surface, and the resulting Al2O3/Si(001) interface was atomically abrupt. An intentional reoxidation of the Al2O3/Si(001) system under low oxygen pressure (2×10−6, 5×10−6, and 2×10−5 Torr O2) showed that the ultrathin Al2O3 film stoichiometry and the interface abruptness were maintained with progress in reoxidation time. Furthermore, the film and the interface showed no degradation under low-pressure reoxidation at various temperatures (400–750 °C). A high-pressure reoxidation of the Al2O3/Si(001) system at 5×10−5 Torr O2 resulted in the formation of an interfacial SiO2 layer which grew in a layer-by-layer mode with atomic-scale uniformity and had an atomically abrupt interface with Si(001) substrate up to 700 °C. Additionally, a very weak temperature dependence of the growth of interfacial SiO2 was observed. A high-pressure reoxidation at 750 °C led to the formation of crystalline ultrathin Al2O3 film and also caused degradation of the film by formation of SiO2 in the near-surface region, where a slight decrease in the Al2O3 film thickness was observed. This was attributed to the formation of interstitial Si in the interfacial SiO2 layer and the subsequent mobility of Si and Al under this growth condition. Under low-pressure reoxidation, the Si and Al were immobile because of the absence of an interfacial SiO2 layer at the Al2O3/Si(001) interface. These results indicate that the oxygen pressure of the ambience plays an important role in the oxidation of the Al2O3/Si(001) interface, and the mobility, transport, and chemical reactions at various oxidation temperatures (400–750 °C).
Hole mobility enhancements in strained Si/Si1−yGey p-type metal-oxide-semiconductor field-effect transistors grown on relaxed Si1−xGex (x<y) virtual substrates
Authors: Leitz C. W. , Currie M. T. , Lee M. L. , Cheng Z.-Y. , Antoniadis D. A. , Fitzgerald E. A.
Published: July 2002

We have achieved peak hole mobility enhancement factors of 5.15 over bulk Si in metal-oxide-semiconductor field-effect transistors (MOSFETs) by combining tensile strained Si surface channels and compressively strained 80% Ge buried channels grown on relaxed 50% Ge virtual substrates. To further investigate hole transport in these dual channel structures, we study the effects of strain, alloy scattering, and layer thickness on hole mobility enhancements in MOSFETs based upon these layers. We show that significant performance boosts can be obtained despite the effects of alloy scattering and that the best hole mobility enhancements are obtained for structures with thin Si surface layers.
First-principles analysis for the multiplet structures of tetrahedrally and octahedrally oxo-coordinated 3d2 and 3d3 transition metals
Authors: Ishii Takugo , Ogasawara Kazuyoshi , Adachi Hirohiko
Published: July 2002

The multiplet structures of tetrahedrally and octahedrally oxo-coordinated 3d2 and 3d3 transition metals were calculated by the first-principles electronic-structure calculation method for many-electron system. The corresponding notations for the electron configurations, φd2 and φd3 were proposed to distinguish the present method based on a molecular orbital method from traditional methods based on atomic calculation. The multiplet energies depending on metal–oxygen distances were obtained. The calculated energies of the lowest spin-forbidden states decreased in the order Cr4+>Mn5+>Fe6+ in tetrahedrally coordinated φd2 metals, on the other hand, those increased in the order Ti2+>V3+ in octahedrally coordinated φd2 metals, and V2+<Cr3+ in octahedrally coordinated φd3 metals. The decreasing tendency in the tetrahedrally coordinated system originated from the strong covalency. On the other hand, the increasing tendency in the octahedrally coordinated systems originated from the correlation-correction effect considered in the method. A stronger nephelauxetic effect was obtained in the tetrahedrally coordinated system than the octahedrally coordinated systems. This originated from the stronger covalency in the tetrahedrally coordinated system, however, the difference between metals was dominated by correlation-correction effect and differences in free ions rather than covalency.
Lithium metal for x-ray refractive optics
Authors: Dufresne E. M. , Arms D. A. , Clarke R. , Pereira N. R. , Dierker S. B. , Foster D.
Published: July 2002

Lithium metal is the best material for refractive lenses that must focus x-rays with energies below 15 keV, but to date no lens from Li has been reported. This letter demonstrates focusing of 10 keV x-rays with a one-dimensional sawtooth lens made from Li. The lens’ theoretical gain is 4.5, with manufacturing imperfections likely responsible for the threefold gain that is observed. Despite the Li reactivity the lens is stable over months of operation if kept under vacuum.
Electronic correlation in cyclic polyenes. Performance of coupled-cluster methods with higher excitations
Authors: Podeszwa Rafał , Kucharski Stanisław A. , Stolarczyk Leszek Z.
Published: July 2002

In this paper we consider the electrically neutral annulenes CMHM (M=6,10,14,18) and their cations CMHM4+ (M=10,14,18), having the cyclic geometry corresponding to the CM point group, and described by the electronic Hamiltonian of the Pariser–Parr–Pople (PPP) model (in which the strength of electronic correlations is controlled by the magnitude of the resonance integral β). We study the ground-state electronic-correlation effects in these systems by means of the single-reference coupled-cluster (CC) theory employing the restricted Hartree–Fock wave function as the reference. It is known that the basic CC technique—the coupled cluster singles and doubles (CCSD) method (for the annulenes equivalent to the CCD method)—breaks down in the strongly correlated regime of annulenes. In this paper we analyze the performance of the standard extensions of the CCD method, taking into account the connected triple (T) and/or quadruple (Q) excitations in the CC operator T̂: the CCDT, CCDQ, and CCDTQ methods. For comparison we use some modifications of the CCD method that emulate the coupling between the D and Q excitations: the ACP and ACPQ methods. Whenever available, the full configuration-interaction results serve as the reference. We study not only the ground-state correlation energies, but also the quality of the CC amplitudes corresponding to the double excitations, as well as the magnitude of quasidegeneracy effects. Our results provide evidence that not only the CCD, but also the CCDT, CCDQ, and CCDTQ methods break down when the correlation effects become sufficiently strong. This indicates a failure of the standard CC theory (in which the CCD method is gradually augmented by taking into account the T, Q etc. excitations) in the strongly correlated regime of the PPP annulene model.
Simulation of polysilane and polysilyne formation and structure
Authors: Vink R. L. C. , Barkema G. T. , van Walree C. A. , Jenneskens L. W.
Published: July 2002

We present Monte Carlo simulations of the formation and structure of polysilanes and polysilyne networks. The simulation technique allows for the investigation of large systems, containing up to 10 000 monomers. Our results show that ring formation is an important factor for both types of materials. For polysilyne networks, a random structure is found incorporating cyclic substructures, linear chains, and branching points.
Dehydrogenation induced phase transitions of p-aminobenzoic acid on Cu(110)
Authors: Chen Qiao , Frankel Daniel J. , Richardson Neville V.
Published: July 2002

Temperature and coverage dependent studies of p-aminobenzoic acid on a Cu(110) single crystal surface show a novel sequence of periodic structures linked to successive removal of hydrogen from the adsorbed species, as shown by LEED, STM, HREELS, and TPD. At room temperature, flat-lying molecules with a primitive unit cell of (3×4) periodicity with glide plane along the 〈001〉 direction are observed. Annealing this surface to 464 K causes partial desorption of H2 and a variation of the intensity in HREELS of various bands and formation of a (5 −22  4) periodicity. STM images suggest the formation of dimers, as a result of the dehydrogenation. On annealing to higher temperatures (510 K), further dehydrogenation forms (6 −51  2) structure. Finally, at 540 K, a (4 −31  2) periodicity is revealed in which all species correspond to dehydrogenated dimers. Through this sequence, all species retain a flat-lying orientation on the surface.
Production of H− from H2 impinging upon heated saline hydride powder
Authors: Kawano Hiroyuki , Wada Motoi , Sasao Mamiko
Published: July 2002

To develop a type of negative ion source, we did fundamental research; namely, fine powders (∼1 mg) of CaH2, SrH2, or BaH2 deposited on a molybdenum ribbon were heated up to ∼1100 K; those active spots (Ca, Sr, or Ba) having low work function were produced by thermal decomposition (e.g., SrH2→Sr+H2); H2 (or D2) was introduced onto the spots; and the current (J−) of electron and that (IH− or ID−) of H− or D− emitted directly from the spots were measured simultaneously as a function of spot temperature (T) or pressure (PH or PD) of H2 or D2. Analysis of our data yields the results as follows. Below ∼850 K, H− is little produced from H2 because the spots are destroyed by a deactivation reaction (e.g., Sr+H2→SrH2). The spots of Ba are thermally stable even above 1000 K, where little deactivation occurs. IH− from the Ba spots kept at ∼1100 K increases up to ∼1 pA (after mass analysis) per 0.02 cm2 of emitting surface area, proportionally to PH increased up to ∼1 mTorr. As PD increases up to ∼0.1 mTorr, CaH2 (∼1000 K) gets a work function decrease (∼0.1 eV), yielding ID−≃0.3 pA (∼15 pA/cm2).
Electro-rheological effects of liquid crystalline polymer on one-sided pattern electrodes
Authors: Takesue Naoyuki , Furusho Junji , Inoue Akio
Published: July 2002

In most industrial robots, serious problems of vibratory behavior are caused because of elasticity in the drive systems. An electro-rheological (ER) fluid is a substance that can change its apparent viscosity by the application of an electric field, and homogeneous ER fluids are effective in reducing vibration in robots. Conventional research into ER fluids is based on parallel electrodes with ER fluids between them. Since a high voltage is required for use of ER fluids with parallel electrodes, problems of cost, safety and wiring for practical applications are of concern. In this article, we propose one-sided pattern electrodes that solve some of the above problems. A one-sided pattern electrode is an electrode in a pattern arranged on only one of the two surfaces of a pair of electrodes, with an insulator arranged on the other surface. We developed two kinds of one-sided pattern electrodes: a concentric circle form and a radial form, experimentally verifying the manifestation of ER effects in a liquid crystalline polymer (a homogeneous ER fluid). In addition, the effect of disk gap upon viscosity change was investigated.
Structural and optical properties of germanium nanoparticles
Authors: Lin Chung-Wei , Lin Shin-Yen , Lee Si-Chen , Chia Chih-Ta
Published: July 2002

Germanium nanoparticles with spherical shape have been prepared by the thermal evaporation method. The shape and structure of these particles have been studied. Transmission electron microscopy images show that a change of Ge structure from amorphous to crystalline occurs at a growth pressure of 0.4 Torr and is attributed to the change of lattice constant. When the growth pressure becomes larger than 0.4 Torr, a fractallike tree structure made of Ge nanoparticles is observed. As the pressure increases further, the dot shape becomes clearer and many dots overlap. From the Raman spectra, it is found that the diamond structure is dominant for Ge dots.
Enhanced dielectric properties of SrTiO3 epitaxial thin film for tunable microwave devices
Authors: Bouzehouane K. , Woodall P. , Marcilhac B. , Khodan A. N. , Crété D. , Jacquet E. , Mage J. C. , Contour J. P.
Published: July 2002

The performance of tunable microwave devices based on heteroepitaxial YBa2Cu3Ox/SrTiO3 films on (001) LaAlO3 substrates has been evaluated. It was ascertained that “out-of-plane” SrTiO3 lattice parameter is the relevant factor in determining both device agility and dielectric loss. After high temperature annealing (1100 °C, 1 atm O2), only SrTiO3 layers deposited under low oxygen pressure (∼10−5 Torr) show an appreciable reduction of the dielectric losses while maintaining high agility. Annealed samples exhibit voltage independent losses of ∼5×10−3 simultaneously with 55% dielectric agility at 6 GHz and 77 K.
Coaxial ion beam/infrared laser beam spectrometer for investigating infrared spectra of doubly positively charged molecules (molecular dications)
Authors: Abusen Ramadan , Cox Simon G. , Critchley Andrew D. J. , Hughes Alan N. , Kemp Faye , McNab Iain R. , Shiell Ralph C. , Smith Fiona E.
Published: July 2002

We have constructed an apparatus for studying the infrared spectra of molecules with a doubly positive charge (molecular dications). The spectroscopic transitions were recorded indirectly by means of observing a change in the fragmentation rate of the molecular dication when a transition was in resonance. The design and performance of the spectrometer are described, with particular emphasis on the sensitivity achieved for detecting infrared spectra and Zeeman split infrared spectra. The operation and calibration of the spectrometer are discussed and sample results for DCl2+ are presented. It is shown that we achieve the maximum possible signal/noise ratio that could be achieved in this type of experiment.
An efficient algorithm for metric correction forces in simulations of linear polymers with constrained bond lengths
Authors: Pasquali Matteo , Morse David C.
Published: July 2002

The dynamical behavior and equilibrium distribution of linear bead-rod and bead-spring polymers differ even in the limit of infinitely stiff springs. Imposing metric pseudopotential forces on the bead-rod chains yields the behavior of bead-spring chains in Langevin and Brownian Dynamics simulations. Here we present a simple, compact, and efficient algorithm for computing the required metric correction forces at minimal computational cost.
Temperature distribution in polymeric films during ion beam irradiation
Authors: Ide-Ektessabi Ari , Watanabe Yoshinori
Published: July 2002

A simulation code is developed to investigate the cooling efficiency of a prototype ion beam irradiation system for surface modification. A linear type ion source has been used in this system. As a typical case, the temperature increase of poly-ethylene-terephthalate (PET) films during surface modification by the linear type ion source is simulated. In the simulation model, the PET film was placed on a sample holder made of aluminum, the thickness of PET was 50 μm and the thickness of sample holder was 1.5 mm. The incident beam energy was 1500 eV and beam current density was 1.0 A/m2. Using this simulation model, the distribution of the temperature inside of the target and the sample holder was calculated. The relationships between the irradiation time and temperature increase at the target surface was also calculated. The results showed that the behavior of the temperature increase was dominated by the cooling rate, and other parameters have comparatively less influence on temperature distribution. When the cooling rate was 5 W/m2 K, the temperature increase at the target surface was 16.0 K, and when the cooling rate was 50 W/m2 K, the temperature increase at the target surface was 6.4 K, for an irradiation time of 200 s.
Feasibility study of a low pressure barium discharge lamp
Authors: Laski J. , Lister G. G. , Palmer F. , Moskowitz P. E. , Curry J. J.
Published: July 2002

We report the results of an experimental and theoretical study on the feasibility of a barium low pressure discharge as a potential new light source. Barium has a strong neutral resonance line at 553.5 nm, near the center of the eye’s response curve, as well as several red and blue lines from barium ions. Due to the strong reactivity of barium with a wide variety of standard lamp materials, experiments were performed in arc tubes lined with yttria (Y2O3). A wide range of parameter space was explored in buffer gas pressure (1–30 Torr), gas type (argon, neon, and krypton), barium temperature (400–760 °C), and arc current (0.05–1.0 A). The numerical model qualitatively predicted the influence of these parameters on light output. However, the predicted efficacies of up to 100 lm/W were considerably higher than the measured values. The best positive column efficacy achieved was 50 lm/W in a sealed yttria lined quartz tube with 5 Torr argon operating at 400 mA at 760 °C. Possible reasons for this discrepancy are discussed.
Radiometric efficiency of low pressure barium discharges
Authors: Peng X. L. , Curry J. J. , Lister G. G. , Lawler J. E.
Published: July 2002

We have studied, both experimentally and numerically, the radiometric efficiency of low-pressure barium–argon positive column discharges. Using a 14 mm inner diameter alumina discharge tube to reduce chemical reactivity, we have operated discharges with Ba vapor pressures corresponding to a range of oven temperatures from 600 to 800 °C, Ar buffer gas pressures of 0.5 and 5 Torr, and currents of 100–800 mA. The spectral power distribution has been determined by examining the visible radiation emitted perpendicular to the discharge axis from a small aperture in the side of the alumina discharge tube. Absolute emitted powers were determined by comparison to a calibrated tungsten–quartz–halogen lamp. The measured efficiencies for production of visible radiation from the positive column ranged from 2.5% to 52%. These efficiencies correspond to luminous efficacies ranging from 6 to 97 lm/W. Strong emission on Ba ion lines at 455.4, 493.4, 614.2, and 649.7 nm contributed significantly to the best efficiency, 52%, and yielded a white light. A numerical model, incorporating a substantial number of excited levels and atomic processes, has been used to examine the same conditions. The model reproduces most of the major experimental trends.
Small angle neutron scattering studies of a polybutadiene/polystyrene blend with small additions of ortho-dichloro-benzene for varying temperatures and pressures. II. Phase boundaries and Flory–Huggins parameter
Authors: Frielinghaus Henrich , Schwahn Dietmar , Willner Lutz , Freed Karl F.
Published: July 2002

Two polymer blends of polybutadiene and polystyrene are investigated by small angle neutron scattering as a function of pressure and solvent content. We measured the thermal composition fluctuations, which we described with crossover functions. Whereas the crossover behavior of the susceptibility and the correlation length was the subject of Paper I [J. Chem. Phys. 116, 2229 (2002)], here we discuss the phase boundaries and the extracted mean-field effective Flory–Huggins parameter and radius of gyration for the asymptotic behavior at high temperatures. Using a lattice model and the lattice cluster theory of Bawendi and Freed, we developed some general expressions for the dependence of the Flory–Huggins parameter on the excess free volume and on the solvent content. A compensation of the free volume by the solvent molecules was observed. Finally, a discussion of the Clausius–Clapeyron equation allows for a check of the crossover function used to extract mean-field quantities for comparison with theory.
Technological developments in hadron therapy (plenary)
Authors: Bryant P. J.
Published: July 2002

Hadron therapy, which is taken here to exclude neutrons but include ions, has its roots in a paper by Robert R. Wilson in [Radiology 47, 487 (1946)]. His paper spoke mainly of protons and the advantages of their Bragg peak behavior, but it also had the foresight to mention carbon ions. Pioneering work with different ion species was started at LBL in 1975 and continued for about 15 years. Today, there is a small number of working centers and many project proposals that present between them a bewildering range of particles, accelerators, extraction techniques, delivery mechanisms, and diagnostics for the treatment of deep-seated cancers. Recent advances in the understanding of the radio-biological effect of different ions and in accelerator design, reviewed in this article, suggest that there are optimum combinations and that these combinations should be matched to specific cancer types. The deeper the knowledge, the more demanding the requirements have become for the operation and control of the accelerator complex from the ion source down to the patient. The intensity, emittance and purity of the ion species from the source are the first parameters in this chain and the reliability and reproducibility are of paramount importance for the operation. It is the medical community that will compare and ultimately choose between the various accelerator options being developed, but it is the responsibility of the accelerator community to provide the very best tools.
Ion source development at GANIL for radioactive beams and high charge state ions
Authors: Leroy R. , Barué C. , Canet C. , Dupuis M. , Flambard J-L. , Gaubert G. , Gibouin, S. , Huguet Y. , Jardin P. , Lecesne N. , Lehérissier P. , Lemagnen F. , Maunoury L. , Pacquet J.-Y. , Pellemoine-Landré F. , Rataud J-P. , Saint-Laurent M. G. , Villari A. C. C.
Published: July 2002

The GANIL laboratory is in charge of the production of ion beams for nuclear and non-nuclear physics. This article reviews the latest developments that are underway in the fields of radioactive ion beam production, increase of the metallic ion intensities, and production of highly charged ion beams.
Correlation radius of polarization fluctuations in the disordered ferroelectrics
Authors: Glinchuk M. D. , Eliseev E. A. , Stephanovich V. A. , Kirichenko E. V. , Jastrabik L.
Published: July 2002

We calculate correlation radius of polarization fluctuations in the disordered ferroelectrics. The consideration is performed in the model suitable for relaxor ferroelectrics. We consider reference Burns phase in a mean field approximation subjected to random electric field E. The “entire relaxor” is then obtained by averaging of characteristics of the reference phase with distribution function of the random field. For correlation radius (Rc) calculation we take into account the damping of the reference phase soft mode and its dependence on E and perform the averaging over the random field distribution. The obtained temperature dependence of Rc is shown to be strongly dependent on the decay parameter. It appears to be completely different for the disordered system either in the dipole-glass state or in mixed ferroglass phase. The fitting of calculated and measured Rc(T) in PbMg1/3Nb2/3O3 is performed.
Ab initio modeling and experimental study of C–B interactions in Si
Authors: Liu Chun-Li , Windl Wolfgang , Borucki Len , Lu Shifeng , Liu Xiang-Yang
Published: July 2002

We present results of ab initio calculations for the structure and energetics of small boron-carbon-interstitial and carbon-interstitial clusters in silicon and a respective continuum model for the nucleation, growth, and dissolution of these clusters. To test our calculations, we also measure secondary-ion mass spectra of boron and carbon in a SiGeCB system, which our model reproduces well. For the considered concentration regime, we find rapid carbon diffusion, nearly immobile boron, and undersaturation of silicon self-interstitials, resulting mainly from two clusters suggested by our ab initio results, the dicarbon-interstitial cluster and the boron-carbon-interstitial cluster.
Structure of the exact wave function. IV. Excited states from exponential ansatz and comparative calculations by the iterative configuration interaction and extended coupled cluster theories
Authors: Nakatsuji Hiroshi
Published: July 2002

In a previous paper of this series [Paper III: Nakatsuji, J. Chem. Phys. 105, 2465 (2001)], the author showed a high potentiality of the extended coupled cluster (ECC) method to calculate the exact wave function of the ground state. In this paper, we propose ECC-configuration interaction (CI) method, which is an accurate useful method to calculate the excited states from the ECC wave function of the ground state. In contrast to the ECC method, the standard ECC-CI method is approximate, but we can make it exact by generalizing its excitation operator (ECC-CI general). The ECC-CI method is applicable not only to the excited states having the same spin-space symmetry as the ground state, but also to those having different spin-space symmetries and to the ionized and electron-attached states. The theoretical framework of the ECC-CI method is similar to that of the symmetry-adapted-cluster (SAC)-CI method proposed in 1978 by the present author. Next in this paper, we examine the performance of the methods proposed in this series of papers for a simple one-dimensional harmonic oscillator. The iterative configuration interaction (ICI) and ECC methods are examined for the ground state and the ICI-CI and ECC-CI methods for the excited states. The ICI method converges well to the exact ground state and the excited states are calculated nicely by the ICI-CI method in both the standard and general active spaces. In contrast to the simplest (S)ECC examined in Paper III, the ECC2 method shows quite a rapid convergence to the exact ground state, which enables us to calculate the true exact wave function in the ECC form. The ECC-CI methods in both the standard and general active spaces also work well to calculate the excited states. Thus, we conclude that the ICI and ECC approaches have a potentiality to provide useful method to calculate accurate wave functions of the ground and excited states. A merit of ECC is that it provides the exact wave function in a simple explicit form.
Field theory and segmental alignment analysis for a solution of sequence disordered liquid crystalline polymers
Authors: Gutman Lorin , Shakhnovich Eugene
Published: July 2002

A field theory for the many-chain mesogen/flexible sequence model is developed. The theory is solved using a creation–annihilation formalism based on a reference state that recovers the Gupta–Edwards solution for the many-chain homopolymer system. Orientational averages are performed, and a nonlinear equation for the orientational tensor-dependent free energy is derived. The effects of flexibility, stiffness and intersegment alignment on orientational ordering, the nematic/isotropic density threshold, and the segmental orientational ordering at the nematic/isotropic transition is determined.
Band anticrossing effects in MgyZn1−yTe1−xSex alloys
Authors: Wu J. , Walukiewicz W. , Yu K. M. , Ager J. W. , Shan W. , Haller E. E. , Miotkowski I. , Ramdas A. K. , Miotkowska S.
Published: July 2002

The electronic structures of MgyZn1−yTe1−xSex alloys were studied by optical absorption and photoluminescence techniques under applied hydrostatic pressure. In samples with both x and y≠0, the band gap exhibits a strongly nonlinear pressure dependence which is similar to the effects observed previously in ZnTe1−xSex and ZnTe1−xSx ternaries and that is well explained by the anticrossing interaction of the selenium localized electronic states with the conduction band of the matrix. In contrast, the pressure dependence of the band gap in MgyZn1−yTe (i.e., x=0) is not significantly changed in form from that of ZnTe; it is concluded that the effects of alloying MgTe with ZnTe can be well understood within the virtual crystal approximation.
Development of multidimensional Monte Carlo simulation code for H− ion and neutral transport in H− ion sources
Authors: Hatayama A. , Sakurabayashi T. , Ishi Y. , Makino K. , Ogasawara M. , Bacal M.
Published: July 2002

Two multidimensional Monte Carlo simulation codes—(a) neutral (H2,H) transport code and (b) negative ion (H−) transport code—have been developed. This article focuses on the recent simulation results by the neutral transport code for the H− production in a large, hybrid negative ion source, “Camembert III.” Two-dimensional spatial profiles of vibrationally excited molecules H2(v) and H− production are obtained for a given background plasma profile. Both H2(v) and H− ions are mainly produced near the filaments in the driver region. However, the H− source density has double peak in its spatial structure, while the density profile of H2(v) is characterized by the “mushroom” structure with a single peak. These results indicate a large potential of the neutral transport code, not only for the understanding of underlying physics, but also for designing ion sources, including complicating effects of geometry, spatial and velocity distribution of particles, and atomic and wall processes.
Molecular dynamics simulation of the I2(X)⋯Ar isomers population in a free-jet expansion: Thermodynamics versus kinetic control
Authors: Bastida Adolfo , Zúñiga José , Requena Alberto , Miguel Beatriz , Beswick J. Alberto , Vigué Jacques , Halberstadt Nadine
Published: July 2002

A molecular dynamics simulation addressing the problem of thermodynamic versus kinetic control of the isomers population of van der Waals complexes in a supersonic expansion is presented. The populations of the linear and T-shaped isomers of I2(X)⋯Ar in a supersonic beam expansion were determined by molecular dynamics simulation as a function of the distance to the nozzle and compared to the prediction of thermodynamics. The surprising conclusion is that although there is a barrier equal to half the well depth between the two isomers, their populations are consistent with the existence of thermodynamic equilibrium. This result is rationalized by examining the cooling mechanisms in the Ar+I2(X)⋯Ar collisions. In addition to the direct isomerization, a new mechanism (swap cooling), which induces isomerization even for complexes with barriers above the dissociation limit, is evidenced.
Status of the electrostatic plasma lens
Authors: Goncharov A.
Published: July 2002

The electrostatic plasma lens provides an important tool for the manipulation of high current heavy ion beams with moderate energy of 1–100 keV. Applications can include, for example, high dose ion implantation and particle accelerator injection. Here we briefly review the plasma lens (PL) fundamentals and summarize some recent developments and experiments performed at the Institute of Physics NASU (Kiev) and LBNL (Berkeley). We also describe progress in experimental investigations of the nonlinear electron vortex structures that arise due to the drift instability in the high current plasma lens, and lens operation in the low magnetic field regime. We show that there is a very narrow range of low magnetic field for which the focusing properties of the PL improve markedly. Under these conditions a significant decrease (by more than an order of magnitude) in the amplitude of oscillations in the lens volume and focused ion beam are observed.
Chemical reactivity and phase behavior of NH4Cl by molecular dynamics simulations. II. The liquid–vapor coexistence curve
Authors: Guissani Yves , Guillot Bertrand
Published: July 2002

In order to shed some light on the peculiar liquid–vapor phase transition of ammonium chloride (NH4Cl), we make use of a recently developed model potential (see paper I) accounting for the proton transfer reaction (NH4++Cl−⇔NH3+HCl) to investigate by classical molecular dynamics simulation the phase diagram of NH4Cl in the fluid range. A key result of the simulation is that a phase coexistence is found between a liquid mixture composed of ionic (NH4+,Cl−) and covalent (NH3,HCl) species in nearly equal proportion and a vapor composed exclusively of molecules (NH3), findings which explain the conductivity data of the literature. Although the agreement with the experimental coexistence curve is only semiquantitative, the simulation leads to a decisive improvement compared with current theoretical approaches. In addition, the evolution of the reaction kinetics and of the composition of the fluid with the thermodynamic conditions are evaluated, and the sequence of events which accompany the proton transfer is analyzed in detail.
Enhanced nitrogen diffusion in 4H-SiC
Authors: Phelps G. J. , Wright N. G. , Chester E. G. , Johnson C. M. , O’Neill A. G. , Ortolland S. , Horsfall A. , Vassilevski K. , Gwilliam R. M. , Coleman P. G. , Burrows C. P.
Published: July 2002

Experimental evidence is given for boron (B) enhanced diffusion of nitrogen (N) in ion-implanted 4H silicon carbide (4H-SiC), when a nitrogen implant is co-doped within an existing boron p-type well. The co-implanted nitrogen is shown to diffuse continuously with time when samples are annealed at 1600 °C—with little movement of the boron p-well implant profile. An effective nitrogen in boron diffusivity at 1600 °C is determined to be at least 60 times larger than that of a mono-doped nitrogen implant.
Photodissociation spectroscopy of the complexes of Mg+ with di- and tri-ethylamine
Authors: Guo Wenyue , Liu Haichuan , Yang Shihe
Published: July 2002

We present the photodissociation spectroscopy of the complexes of Mg+ with di- and tri-ethylamine in the spectral range of 230–440 nm. Mass spectrometry of the two parent complexes exhibits the persistent product Mg+ from nonreactive quenching throughout the whole wavelength range. As for the reactive channels, C3H7•-elimination products are dominant when the complexes are excited to 3Px,y. Furthermore, photoexcitation of Mg+–NH(C2H5)2 to 3Px,y yields a minor MgH-elimination fragment. On the other hand, photodissociation of Mg+–N(C2H5)3 produces charge-transfer fragments N+(C2H5)3 and (C2H5)2N+CH2. The action spectra of the two complexes consist of two pronounced peaks on the red and blue side of the Mg+ 3 2P←3 2S atomic transition. The calculated absorption spectra of the two complexes using the optimized structures of their ground states agree nicely with the observed action spectra. Photofragment branching fractions of the products are shown to be nearly independent of the photon energy for the 3Px,y excitation, indicating the importance of exit channel effects. A photoreaction mechanism is proposed for C3H7•-elimination, which entails Mg+ insertion into one C–N bond, followed by a nonadiabatic transition to the ground-state surface through a conical intersection. However, for the MgH-elimination channel in the photodissociation of Mg+–NH(C2H5)2, Mg+ rather inserts into the α-C–H bond. Finally, photoexcitation of Mg+–N(C2H5)3 leads to a prominent charge-transfer product [N+(C2H5)3]*, which dissociates further to (C2H5)2N+CH2 by a loss of CH3•.
Orientational preference and influence of rotation on methane mobility in one-dimensional channels
Authors: Bhide Shreyas Y. , Yashonath S.
Published: July 2002

Molecular dynamics investigations of methane in one-dimensional channel systems of AlPO4-5 and the carbon nanotube are reported. Methane shows contrasting orientational preference over 1+3 and 2+2 orientations inside AlPO4-5 and the carbon nanotube. In AlPO4-5, 2+2 orientation is preferred in narrow parts of the channel while 1+3 orientation is predominant in the wider part of the channel. In the carbon nanotube 2+2 is preferred over 1+3 orientation throughout the channel. Thus, the orientation of methane can yield insight into methane–host interactions. Simulations with and without allowing for rotation of methane suggest that rotation aids translational mobility of methane in both the channel systems studied. It is shown that molecules with certain orientations are associated with lower mobility, both in AlPO4-5 and carbon nanotube. Further, translational motion exhibits diffusive behavior for methane in AlPO4-5 and superdiffusive behavior in the carbon nanotube. These results suggest that particular orientation or rotation in general can strongly influence translational mobility of methane in one-dimensional channels. It is seen that levitation effects influence rotational motion in addition to the translational motion.
Bend–stretch Fermi resonance in DCO+
Authors: Robinson Jason D. , Foltynowicz Robert J. , Prentice Krista , Bell Patrice , Grant Edward R.
Published: July 2002

Rydberg series, rovibrationally isolated by double resonant excitation, are extrapolated to determine rotational-state detailed ionization thresholds to form DCO+ in the bending-excited levels (030) and (040). Analysis of the rovibrational structure derived from the positions of these thresholds for (030) agrees with a simple parameterization extended from a fit to lower vibrational levels. For (040), however, the vibrational angular momentum components (0400) and (0420) are observed to be inverted in energy, with the (0400) component displaced approximately 20 cm−1 to a position above that of (0420). This perturbation is interpreted to support a conjecture first made by Hirota and co-workers that the vibrational structure of DCO+ is perturbed by a 4:1 bend–stretch Fermi resonance. Extending the pattern by which vibrational angular momentum components are observed to be split in (020) and (030), we establish the unperturbed position of (0400), from which we estimate the matrix element for bend–stretch coupling. Analysis yields a moderate Fermi matrix element, W(0400)(1000), of 32.43 cm−1, which is about two-thirds the magnitude of coupling found in the systems, CO2 and NO2+, where bend–stretch mixing significantly effects the character of higher vibrationally excited states.