bluuebunny/crossref_metadata_2025 huggingface dataset:
Temporal development of ion beam mean charge state in pulsed vacuum arc ion sources
Authors: Oks E. M. , Yushkov G. Yu. , Anders A.
Published: January 2008

Vacuum arc ion sources, commonly also known as “Mevva” ion sources, are used to generate intense pulsed metal ion beams. It is known that the mean charge state of the ion beam lies between 1 and 4, depending on cathode material, arc current, arc pulse duration, presence or absence of magnetic field at the cathode, as well as background gas pressure. A characteristic of the vacuum arc ion beam is a significant decrease in ion charge state throughout the pulse. This decrease can be observed up to a few milliseconds, until a “noisy” steady-state value is established. Since the extraction voltage is constant, a decrease in the ion charge state has a proportional impact on the average ion beam energy. This paper presents results of detailed investigations of the influence of arc parameters on the temporal development of the ion beam mean charge state for a wide range of cathode materials. It is shown that for fixed pulse duration, the charge state decrease can be reduced by lower arc current, higher pulse repetition rate, and reduction of the distance between cathode and extraction region. The latter effect may be associated with charge exchange processes in the discharge plasma.
Inverted end-Hall-type low-energy high-current gaseous ion source
Authors: Oks E. M. , Vizir A. V. , Shandrikov M. V. , Yushkov G. Yu. , Grishin D. M. , Anders A. , Baldwin D. A.
Published: January 2008

A novel approach to low-energy, high-current, gaseous ion beam generation was explored and an ion source based on this technique has been developed. The source utilizes a dc high-current (up to 20A) gaseous discharge with electron injection into the region of ion generation. Compared to the conventional end-Hall ion source, the locations of the discharge anode and cathode are inverted: the cathode is placed inside the source and the anode outside, and correspondingly, the discharge current is in the opposite direction. The discharge operates in a diverging axial magnetic field, similar to the end-Hall source. Electron generation and injection is accomplished by using an additional arc discharge with a “cold” (filamentless) hollow cathode. Low plasma contamination is achieved by using a low discharge voltage (avoidance of sputtering), as well as by a special geometric configuration of the emitter discharge electrodes, thereby filtering (removing) the erosion products stemming from the emitter cathode. The device produces a dc ion flow with energy below 20eV and current up to 2.5A onto a collector of 500cm2 at 25cm from the source edge, at a pressure ⩾0.02Pa and gas flow rate ⩾14SCCM. The ion energy spread is 2to3eV (rms). The source is characterized by high reliability, low maintenance, and long lifetime. The beam contains less than 0.1% of metallic ions. The specific electric energy consumption is 400eV per ion registered at the collector. The source operates with noble gases, nitrogen, oxygen, and hydrocarbons. Utilizing biasing, it can be used for plasma sputtering, etching, and other ion technologies.
Arc plasma simulation of the KAERI large ion source
Authors: In S. R. , Jeong S. H. , Kim T. S.
Published: January 2008

The KAERI large ion source, developed for the KSTAR NBI system, recently produced ion beams of 100keV, 50A levels in the first half campaign of 2007. These results seem to be the best performance of the present ion source at a maximum available input power of 145kW. A slight improvement in the ion source is certainly necessary to attain the final goal of an 8MW ion beam. Firstly, the experimental results were analyzed to differentiate the cause and effect for the insufficient beam currents. Secondly, a zero dimensional simulation was carried out on the ion source plasma to identify which factors control the arc plasma and to find out what improvements can be expected.
A 140mA cw deuteron electron cyclotron resonance source for the IFMIF-EVEDA project
Authors: Gobin R. , Beauvais P.-Y. , Delferrière O. , De Menezes D. , Tuske O. , Adroit G. , Gauthier Y. , Harrault F.
Published: January 2008

In the framework of the IFMIF-EVEDA phase (International Fusion Materials Irradiation Facility—Engineering Validation and Engineering Design Activities), the CEA-Saclay is in charged of the design and realization of the 140mA cw deuteron source. The IFMIF EVEDA demonstrator will be installed in Japan in the next six years and will have to accelerate the deuteron beam up to 9MeV. CEA will build the source and the low energy beam line (LEBT) and will test the cw high intensity deuteron production at Saclay. The SILHI source is an electron cyclotron resonance (ECR) source, operating at 2.45GHz. In 2001, it produced more than 130mA of deuteron beam in pulsed mode to minimize neutron production. Such a result pushes to develop a new ECR source based on the SILHI design and equipped with a specific extraction system. Several options of the accelerator column will be implemented in order to improve the reliability and the efficiency of the source. The IFMIF source and LEBT design will be reported.
Compact E×B mass separator for heavy ion beams
Authors: Wada M. , Hashino T. , Hirata F. , Kasuya T. , Sakamoto Y. , Nishiura M.
Published: January 2008

A compact E×B mass separator that deflects beam by 30° has been designed and built to prove its principle of operation. The main part of the separator is contained in a shielding box of 11cm long, 9cm wide, and 1.5cm high. An electromagnet of 7cm pole diameter produced variable magnetic field in the mass separation region instead of a couple of permanent magnets which is to be used in the final design. The experimental result agreed well with the theoretical prediction, and larger mass ions is bent with less magnetic field with the aid of the deflection electric field. The reduction in resolving power for mass separation due to the deflection electric field has been investigated experimentally.
High current multicharged metal ion source using high power gyrotron heating of vacuum arc plasma
Authors: Vodopyanov A. V. , Golubev S. V. , Khizhnyak V. I. , Mansfeld D. A. , Nikolaev A. G. , Oks E. M. , Savkin K. P. , Vizir A. V. , Yushkov G. Yu.
Published: January 2008

A high current, multi charged, metal ion source using electron heating of vacuum arc plasma by high power gyrotron radiation has been developed. The plasma is confined in a simple mirror trap with peak magnetic field in the plug up to 2.5T, mirror ratio of 3–5, and length variable from 15to20cm. Plasma formed by a cathodic vacuum arc is injected into the trap either (i) axially using a compact vacuum arc plasma gun located on axis outside the mirror trap region or (ii) radially using four plasma guns surrounding the trap at midplane. Microwave heating of the mirror-confined, vacuum arc plasma is accomplished by gyrotron microwave radiation of frequency 75GHz, power up to 200kW, and pulse duration up to 150μs, leading to additional stripping of metal ions by electron impact. Pulsed beams of platinum ions with charge state up to 10+, a mean charge state over 6+, and total (all charge states) beam current of a few hundred milliamperes have been formed.
Efficient negative ion production in rf plasmas using a mesh grid bias method
Authors: Okada Junichi , Nakao Yuichi , Tauchi Yasushi , Fukumasa Osamu
Published: January 2008

Using a grid bias method for plasma parameter control, volume production of hydrogen negative ions H− is studied in pure hydrogen rf plasmas. Relationship between the extracted H− ion currents and plasma parameters is discussed. It is confirmed that both high and low electron temperature Te plasmas are produced in the separated regions when the grid is negatively biased. In addition, with changing grid potential Vg, values of ne increase while Te decrease in their values. The negative ion production depends strongly on the grid potential and related plasma conditions.
Reliable low-power control of ultrafast vortex-core switching with the selectivity in an array of vortex states by in-plane circular-rotational magnetic fields and spin-polarized currents
Authors: Kim Sang-Koog , Lee Ki-Suk , Yu Young-Sang , Choi Youn-Seok
Published: January 2008

The authors investigated the technological utility of counterclockwise (CCW) and clockwise (CW) circular-rotating fields (HCCW and HCW) and spin-polarized currents with an angular frequency ωH close to the vortex eigenfrequency ωD, for the reliable, low-power, and selective switching of the bistate magnetization (M) orientations of a vortex core (VC) in an array of soft magnetic nanoelements. CCW and CW circular gyrotropic motions in response to HCCW and HCW, respectively, show remarkably contrasting resonant behaviors, (i.e., extremely large-amplitude resonance versus small-amplitude nonresonance), depending on the M orientation of a given VC. Owing to this asymmetric resonance characteristics, the HCCW(HCW) with ωH∼ωD can be used to effectively switch only the up (down) core to its downward (upward) M orientation, selectively, by sufficiently low field (∼10Oe) and current density (∼107A∕cm2). This work provides a reliable, low power, effective means of information storage, information recording, and information readout in vortex-based random access memory, simply called VRAM.
Satellite transitions acquired in real time by magic angle spinning (STARTMAS): ‘‘Ultrafast’’ high-resolution MAS NMR spectroscopy of spin I=3∕2 nuclei
Authors: Thrippleton Michael J. , Ball Thomas J. , Wimperis Stephen
Published: January 2008

The satellite transitions acquired in real time by magic angle spinning (STARTMAS) NMR experiment combines a train of pulses with sample rotation at the magic angle to refocus the first- and second-order quadrupolar broadening of spin I=3∕2 nuclei in a series of echoes, while allowing the isotropic chemical and quadrupolar shifts to evolve. The result is real-time isotropic NMR spectra at high spinning rates using conventional MAS equipment. In this paper we describe in detail how STARTMAS data can be acquired and processed with ease on commercial equipment. We also discuss the advantages and limitations of the approach and illustrate the discussion with numerical simulations and experimental data from four different powdered solids.
QM:QM electronic embedding using Mulliken atomic charges: Energies and analytic gradients in an ONIOM framework
Authors: Hratchian Hrant P. , Parandekar Priya V. , Raghavachari Krishnan , Frisch Michael J. , Vreven Thom
Published: January 2008

An accurate first-principles treatment of chemical reactions for large systems remains a significant challenge facing electronic structure theory. Hybrid models, such as quantum mechanics:molecular mechanics (QM:MM) and quantum mechanics:quantum mechanics (QM:QM) schemes, provide a promising avenue for such studies. For many chemistries, including important reactions in materials science, molecular mechanics or semiempirical methods may not be appropriate, or parameters may not be available (e.g., surface chemistry of compound semiconductors such as indium phosphide or catalytic chemistry of transition metal oxides). In such cases, QM:QM schemes are of particular interest. In this work, a QM:QM electronic embedding model within the ONIOM (our own N-layer integrated molecular orbital molecular mechanics) extrapolation framework is presented. To define the embedding potential, we choose the real-system low-level Mulliken atomic charges. This results in a set of well-defined and unique embedding charges. However, the parametric dependence of the charges on molecular geometry complicates the energy gradient that is necessary for the efficient exploration of potential energy surfaces. We derive an efficient form for the forces where a single set of self-consistent field response equations is solved. Initial tests of the method and key algorithmic issues are discussed.
A new parameter-free correlation functional based on an average atomic reduced density gradient analysis
Authors: Tognetti Vincent , Cortona Pietro , Adamo Carlo
Published: January 2008

A new parameter-free correlation functional based on the local Ragot-Cortona approach [J. Chem. Phys. 121, 7671 (2004)] is presented. This functional rests on a single ansatz for the gradient correction enhancement factor: it is assumed to be given by a simple analytic expression satisfying some exact conditions and containing two coefficients. These coefficients are determined without implementing the functional and without using a fitting procedure to experimental data. Their values are determined by requiring that the functional gives a correct average reduced density gradient for atoms, which, to some extent, can be considered an intrinsic atomic property. The correlation functional is then coupled with the Perdew-Burke-Erzernhof (PBE) exchange and compared with the original PBE approach as well as with some other pure density or hybrid approaches. Standard tests for atomic and molecular systems show that our new functional significantly improves on PBE, showing very interesting properties.
Quantum mechanics/molecular mechanics minimum free-energy path for accurate reaction energetics in solution and enzymes: Sequential sampling and optimization on the potential of mean force surface
Authors: Hu Hao , Lu Zhenyu , Parks Jerry M. , Burger Steven K. , Yang Weitao
Published: January 2008

To accurately determine the reaction path and its energetics for enzymatic and solution-phase reactions, we present a sequential sampling and optimization approach that greatly enhances the efficiency of the ab initio quantum mechanics/molecular mechanics minimum free-energy path (QM/MM-MFEP) method. In the QM/MM-MFEP method, the thermodynamics of a complex reaction system is described by the potential of mean force (PMF) surface of the quantum mechanical (QM) subsystem with a small number of degrees of freedom, somewhat like describing a reaction process in the gas phase. The main computational cost of the QM/MM-MFEP method comes from the statistical sampling of conformations of the molecular mechanical (MM) subsystem required for the calculation of the QM PMF and its gradient. In our new sequential sampling and optimization approach, we aim to reduce the amount of MM sampling while still retaining the accuracy of the results by first carrying out MM phase-space sampling and then optimizing the QM subsystem in the fixed-size ensemble of MM conformations. The resulting QM optimized structures are then used to obtain more accurate sampling of the MM subsystem. This process of sequential MM sampling and QM optimization is iterated until convergence. The use of a fixed-size, finite MM conformational ensemble enables the precise evaluation of the QM potential of mean force and its gradient within the ensemble, thus circumventing the challenges associated with statistical averaging and significantly speeding up the convergence of the optimization process. To further improve the accuracy of the QM/MM-MFEP method, the reaction path potential method developed by Lu and Yang [Z. Lu and W. Yang, J. Chem. Phys. 121, 89 (2004)] is employed to describe the QM/MM electrostatic interactions in an approximate yet accurate way with a computational cost that is comparable to classical MM simulations. The new method was successfully applied to two example reaction processes, the classical SN2 reaction of Cl−+CH3Cl in solution and the second proton transfer step of the reaction catalyzed by the enzyme 4-oxalocrotonate tautomerase. The activation free energies calculated with this new sequential sampling and optimization approach to the QM/MM-MFEP method agree well with results from other simulation approaches such as the umbrella sampling technique with direct QM/MM dynamics sampling, demonstrating the accuracy of the iterative QM/MM-MFEP method.
Water clusters (H2O)n, n=6–8, in external electric fields
Authors: Rai Dhurba , Kulkarni Anant D. , Gejji Shridhar P. , Pathak Rajeev K.
Published: January 2008

Structural evolution of water clusters, (H2O)n, n=6–8, induced by a uniform static external electric field is studied within the density functional theory. The electric field is seen to stretch the intermolecular hydrogen bonds in the water clusters, eventually breaking them at some characteristic threshold value, triggering a conformational transformation to a lower energy. The transformed configurations appear as local minima on the cluster’s multidimensional potential energy landscape with the applied field as an extra coordinate. This transformation is accompanied by a rather abrupt increase in the electric dipole moment over and above its steady, albeit nonlinear increase with the applied field. The overall effect of the applied field is the “opening up” of three dimensional morphologies of water clusters to form linear, branched, or netlike structures by making the dipolar water monomers align along the field axis. Consequently, the number of hydrogen bonds in a cluster decreases, in general, with an increase in the field strength. It has been observed that moderately low fields (Field strength ⩽0.008a.u.) markedly alter the ordering of the lowest energy configurations.
The optimal P3M algorithm for computing electrostatic energies in periodic systems
Authors: Ballenegger V. , Cerda J. J. , Lenz O. , Holm Ch.
Published: January 2008

We optimize Hockney and Eastwood’s particle-particle particle-mesh algorithm to achieve maximal accuracy in the electrostatic energies (instead of forces) in three-dimensional periodic charged systems. To this end we construct an optimal influence function that minimizes the root-mean-square (rms) errors of the energies. As a by-product we derive a new real-space cutoff correction term, give a transparent derivation of the systematic errors in terms of Madelung energies, and provide an accurate analytical estimate for the rms error of the energies. This error estimate is a useful indicator of the accuracy of the computed energies and allows an easy and precise determination of the optimal values of the various parameters in the algorithm (Ewald splitting parameter, mesh size, and charge assignment order).
Solving the spin-boson model of strong dissipation with flexible random-deterministic scheme
Authors: Zhou Yun , Shao Jiushu
Published: January 2008

The zero-temperature dynamics of the spin-boson model with strong dissipation has been a challenging problem for more than 20years. To solve this and quantum dynamics of dissipative systems at large, we recently proposed a mixed random-deterministic method. This scheme has been successfully used to simulate the time evolution of the spin-boson model at zero temperature for weak to moderate dissipation. For a better numerical performance, the approach is further modified so that it is flexible to convert a certain part of the random treatment to a deterministic one à la hierarchical equations. Applying the new method to the strong dissipated spin-boson model at zero temperature, we observe that the population in the localized state obeys a simple decay dynamics and the time scale is proportional to the reciprocal of the cutoff frequency.
Predicting the infrared transition intensities in the Ar–HF complex: The key role of the dipole moment surface accuracy
Authors: Jankowski Piotr , Ziółkowski Marcin
Published: January 2008

The method proposed earlier for the generation of the full-dimensional energy surface for van der Waals complexes [P. Jankowski, J. Chem. Phys. 121, 1655 (2004)] is used to obtain a fulldimensional dipole moment surface for the atom-diatom complex in calculations based on the coupled-cluster with single, double, and noniterative triple excitation approach and the aug-cc-pVQZ basis sets. This surface has been employed to calculate transition intensities of the infrared spectra of Ar–HF. Special attention has been paid to study the problem of relative intensities of the different bands which have not been properly predicted within the long-range models of the dipole moment [A. E. Thornley and J. M. Hutson, J. Chem. Phys. 101, 5578 (1994)]. The intensities calculated with the present dipole moment surface agree very well with the experimental data, which indicate that the short-range interactions significantly affect the dipole moment surface and the calculated intensities. To investigate the role of the accuracy of the dipole moment surface on infrared transition intensities in atom-diatom complexes, four models of increasing complexity are studied. Their performance is shown to strongly depend on the region of the interaction energy surface probed by the initial and final states of the individual transitions.
Experimental and theoretical study of the photodissociation of bromo-3-fluorobenzene
Authors: Karlsson Daniel , Anders Borg O. , Lunell Sten , Davidsson Jan , Karlsson Hans O.
Published: January 2008

The UV photodissociation of bromo-3-fluorobenzene under collisionless conditions has been studied as a function of the excitation wavelength between 255 and 265nm. The experiments were performed using ultrafast pump-probe laser spectroscopy. To aid in the interpretation of the results, it was necessary to extend the theoretical framework substantially compared to previous studies, to also include quantum dynamical simulations employing a two-dimensional nuclear Hamiltonian. The nonadiabatic potential energy surfaces (PES) were parameterized against high-level MS-CASTP2 quantum chemical calculations, using both the C–Br distance and the out-of-plane bending of the bromine as nuclear parameters. We show that the wavelength dependence of the photodissociation via the S0→π1π*→π1σ* channel, accessible with a ∼260nm pulse, is captured in this model. We thereby present the first correlation between experiments and theory within the quantitative regime.
An integral equation theory for inhomogeneous molecular fluids: The reference interaction site model approach
Authors: Ishizuka R. , Chong S.-H. , Hirata F.
Published: January 2008

An integral equation theory which is applicable to inhomogeneous molecular liquids is proposed. The “inhomogeneous reference interaction site model (RISM)” equation derived here is a natural extension of the RISM equation to inhomogeneous systems. This theory makes it possible to calculate the pair correlation function between two molecules which are located at different density regions. We also propose approximations concerning the closure relation and the intramolecular susceptibility of inhomogeneous molecular liquids. As a preliminary application of the theory, the hydration structure around an ion is investigated. Lithium, sodium, and potassium cations are chosen as the solute. Using the Percus trick, the local density of solvent around an ion is expressed in terms of the solute-solvent pair correlation function calculated from the RISM theory. We then analyze the hydration structure around an ion through the triplet correlation function which is defined with the inhomogeneous pair correlation function and the local density of the solvent. The results of the triplet correlation functions for cations indicate that the thermal fluctuation of the hydration shell is closely related to the size of the solute ion. The triplet correlation function from the present theory is also compared with that from the Kirkwood superposition approximation, which substitutes the inhomogeneous pair correlation by the homogeneous one. For the lithium ion, the behavior of the triplet correlation functions from the present theory shows marked differences from the one calculated within the Kirkwood approximation.
Systematic study of the thermal diffusion in associated mixtures
Authors: Polyakov Pavel , Wiegand Simone
Published: January 2008

We performed systematic temperature and concentration dependent measurements of the Soret coefficient in different associated binary mixtures of water, deuterated water, dimethyl sulfoxide (DMSO), methanol, ethanol, acetone, methanol, 1-propanol, 2-propanol, and propionaldehyde using the so-called thermal diffusion forced Rayleigh scattering method. For some of the associating binary mixtures such as ethanol/water, acetone/water, and DMSO/water, the concentration xw± at which the Soret coefficient changes its sign does not depend on temperature and is equal to the concentration xw× where the Soret coefficient isotherms intersect. For others such as 1-propanol/water, 2-propanol/water, and ethanol/DMSO, the sign change concentration is temperature dependent, which is the typical behavior observed for nonassociating mixtures. For systems with xw±=xw×, we found that xw± depends linearly on the ratio of the vaporization enthalpies of the pure components. Probably due to the similarity of methanol and DMSO, we do not observe a sign change for this mixture. The obtained results are related to structural changes in the fluid observed by nuclear magnetic resonance, mass spectrometric, and x-ray experiments in the literature. Furthermore, we discuss the influence of hydrophilic and hydrophobic interactions and the solubility on thermal diffusion behavior.
The densities produced by the density functional theory: Comparison to full configuration interaction
Authors: Bochevarov Arteum D. , Friesner Richard A.
Published: January 2008

We investigate one of the fundamental observables, electronic charge density, as produced by a number of popular functionals of the density functional theory (DFT): SVWN5, B3LYP, B3LYP, OLYP, O3LYP, BP86, B3P86, O3P86, and PBE using restricted and unrestricted orbitals. Measuring and comparing the quality of the densities could tell us more about the physical soundness of the functional models. The study is performed on the small molecules He, H2, LiH, H4 in an extensive range of correlation-consistent basis sets. We compare DFT densities to those of full configuration interaction (FCI) under the assumption that the FCI density in the largest employed basis set is sufficiently close to the exact one. For LiH and H4, we also compare the DFT densities to those of CCSD. The SVWN5 functional consistently shows the worst performance. The OPTX exchange functional regularly beats the Becke exchange. Among the best performers are all the hybrid functionals, the novel O3P86 being the most accurate in most cases. The popular functional B3LYP was consistently outmatched by O3LYP, and produced, in fact, some of the poorest densities among the hybrids. CCSD was found to produce much more accurate densities than any DFT functional in the case of LiH in equilibrium geometry, but was sometimes outperformed by DFT in the case of slightly stretched H4, where CCSD theory itself starts to break down. Surprisingly, as one stretches the H2 molecule, BP86 and PBE improve the description of density although such behavior is not observed in other systems. We conclude by reasoning how functionals such as B3LYP, despite being quite average for density, could still be very successful in predicting thermodynamic properties.
Calculations of the exciton coupling elements between the DNA bases using the transition density cube method
Authors: Czader Arkadiusz , Bittner Eric R.
Published: January 2008

Excited states of the double-stranded DNA model (A)12⋅(T)12 were calculated in the framework of the Frenkel exciton theory. The off-diagonal elements of the exciton matrix were calculated using the transition densities and ideal dipole approximation associated with the lowest energy ππ* excitations of the individual nucleobases as obtained from time-dependent density functional theory calculations. The values of the coupling calculated with the transition density cubes (TDC) and ideal dipole approximation (IDA) methods were found to be significantly different for the small interchromophore distances. It was shown that the IDA overestimates the coupling significantly. The effects of structural fluctuations of the DNA chain on the magnitude of dipolar coupling were also found to be very significant. The difference between the maximum and minimum values was as large as 1000 and 300cm−1 for the IDA and TDC methods, respectively. To account for these effects, the properties of the excited states were averaged over a large number of conformations obtained from the molecular dynamics simulations. Our calculations using the TDC method indicate that the absorption of the UV light creates exciton states carrying the majority of the oscillator strength that are delocalized over at least six DNA bases. Upon relaxation, the excitation states localize over at least four contiguous bases.
Free transverse vibration of the fluid-conveying single-walled carbon nanotube using nonlocal elastic theory
Authors: Lee Haw-Long , Chang Win-Jin
Published: January 2008

The effects of flow velocity on the vibration frequency and mode shape of the fluid-conveying single-walled carbon nanotube are analyzed using nonlocal elastic theory. Results show that the frequency and mode shape are significantly influenced by the nonlocal parameter e0a/L. Increasing the nonlocal parameter decreases the real component of frequency and the decrease is more obvious for a lower flow velocity and a higher-order mode. In addition, a higher mode shape is observed with increasing the value of e0a/L. When a critical flow velocity is reached, the combination of first and second modes takes place. The mode shape for the combination is large relative to mode 3 due to the coupled frequency effect, especially including negative imaginary frequency. Furthermore, the mode shape of the combination increases as the nonlocal effect increases.
Electronic structure of the organic semiconductor copper phthalocyanine: Experiment and theory
Authors: Aristov V. Yu. , Molodtsova O. V. , Maslyuk V. V. , Vyalikh D. V. , Zhilin V. M. , Ossipyan Yu. A. , Bredow T. , Mertig I. , Knupfer M.
Published: January 2008

The electronic structure of the organic semiconductor copper-phthalocyanine (CuPc) has been determined by a combination of conventional and resonant photoemission, near-edge x-ray absorption, as well as by the first-principles calculations. The experimentally obtained electronic valence band structure of CuPc is in very good agreement with the calculated density of states results, allowing the derivation of detailed site specific information.
The barrier height of the F+H2 reaction revisited: Coupled-cluster and multireference configuration-interaction benchmark calculations
Authors: Werner Hans-Joachim , Kállay Mihály , Gauss Jürgen
Published: January 2008

Large scale coupled-cluster benchmark calculations have been carried out to determine the barrier height of the F+H2 reaction as accurately as possible. The best estimates for the barrier height of the linear and bent transition states amount to 2.16 and 1.63kcal∕mol, respectively. These values include corrections for core correlation, scalar-relativistic effects, spin-orbit effects, as well as the diagonal Born-Oppenheimer correction. The CCSD(T) basis-set limits are estimated using extrapolation techniques with augmented quintuple and sextuple-zeta basis sets, and remaining N-electron errors are determined using coupled-cluster singles, doubles, triples, quadruples calculations with up to augmented quintuple-zeta basis sets. The remaining uncertainty is estimated to be less than 0.1kcal∕mol. The coupled-cluster results are used to calibrate multireference configuration-interaction calculations with empirical scaling of the correlation energy.
Energy-consistent relativistic pseudopotentials for the 4d elements: Atomic and molecular applications
Authors: Figgen Detlev , Peterson Kirk A. , Stoll Hermann
Published: January 2008

Recently reported energy-consistent relativistic pseudopotentials have been used with series of matching correlation consistent basis sets in benchmark calculations of various atomic and molecular properties. The basis set convergence of the 4d metal electron affinities and 5s2→5s0 excitation energies are reported at the CCSD(T) level of theory, and the effects of valence and 4s4p correlation are investigated. In addition the impact of correlating the low-lying 3d electrons was also studied in all-electron Douglas-Kroll-Hess (DKH) calculations, which also included the ionization potentials and 5s2→5s1 excitation energies. For all four atomic properties, higher order coupled cluster calculations through CCSDTQ are reported. The final calculated values are generally all within 1kcal∕mol of experiment. A notable exception is the ionization potential of Tc, the currently accepted experimental value of which is suggested to be too high by about 3kcal∕mol. Molecular calculations are also reported for the low-lying electronic states of ZrO and RuF, as well as the ground electronic state of Pd2. The effects of spin-orbit coupling are investigated for these cases in pseudopotential calculations. Wherever possible, the pseudopotential results have been calibrated against DKH calculations with correlation consistent basis sets of triple-zeta quality. In all cases the calculated data for these species are in very good agreement with experiment. In particular, the correct electronic ground state for the RuF molecule (Φ9∕24) was obtained, which was made possible by utilizing systematic sequences of correlation consistent basis sets.
Effect of molecular topology on the transport properties of dendrimers in dilute solution at Θ temperature: A Brownian dynamics study
Authors: Bosko Jaroslaw T. , Ravi Prakash J.
Published: January 2008

Structure and transport properties of dendrimers in dilute solution are studied with the aid of Brownian dynamics simulations. To investigate the effect of molecular topology on the properties, linear chain, star, and dendrimer molecules of comparable molecular weights are studied. A bead-spring chain model with finitely extensible springs and fluctuating hydrodynamic interactions is used to represent polymer molecules under Θ conditions. Structural properties as well as the diffusivity and zero-shear-rate intrinsic viscosity of polymers with varied degrees of branching are analyzed. Results for the free-draining case are compared to and found in very good agreement with the Rouse model predictions. Translational diffusivity is evaluated and the difference between the short-time and long-time behavior due to dynamic correlations is observed. Incorporation of hydrodynamic interactions is found to be sufficient to reproduce the maximum in the intrinsic viscosity versus molecular weight observed experimentally for dendrimers. Results of the nonequilibrium Brownian dynamics simulations of dendrimers and linear chain polymers subjected to a planar shear flow in a wide range of strain rates are also reported. The flow-induced molecular deformation of molecules is found to decrease hydrodynamic interactions and lead to the appearance of shear thickening. Further, branching is found to suppress flow-induced molecular alignment and deformation.
Structures and electronic transport of water molecular nanotubes embedded in carbon nanotubes
Authors: Li H. , Zhang X. Q. , Liew K. M.
Published: January 2008

In this paper, ice nanotubes confined in carbon nanotubes are investigated by molecular dynamics. The trigonal, square, pentagonal, and hexagonal water tubes are obtained, respectively. The current-voltage (I-V) curves of water nanotubes are found to be nonlinear, and fluctuations of conductance spectra of these ice nanotubes show that the transport properties of ice nanotubes are quite different from those of bulk materials. Our studies indicate that the conductance gap of ice nanotube is related to the difference value from the Fermi energy EF to the nearest molecular energy level E0. Increasing the diameter of a water molecular nanostructure results in the increase of the conductance.
Bright, low voltage europium doped gallium oxide thin film electroluminescent devices
Authors: Wellenius P. , Suresh A. , Muth J. F.
Published: January 2008

Europium doped gallium oxide thin film electroluminescent devices with bright, red emission (611nm) and relatively low threshold voltages of 60V were produced using pulsed laser deposition. The use of transparent conducting electrodes of amorphous InGaZnO on transparent aluminum titanium oxide/indium tin oxide/7059 Corning glass substrates resulted in a device that is transparent throughout the visible spectrum. At 100V, with 1kHz excitation, the luminance was 221cd∕m2. The Sawyer-Tower circuit analysis and time dependent emission measurements suggest that the charge trapping at the aluminum titanium oxide/Ga2O3:Eu interface plays an important role in producing efficient emission.
Implicit-solvent mesoscale model based on soft-core potentials for self-assembled lipid membranes
Authors: Revalee Joel D. , Laradji Mohamed , Sunil Kumar P. B.
Published: January 2008

An efficient implicit-solvent model for self-assembled lipid bilayers is presented and analyzed using Langevin molecular dynamics simulations. The model is based on soft interactions between particles and short-range attractive interaction between lipid tails, leading for the self-assembly of a lipid bilayer without an explicit solvent. This allows for efficient simulations of large membranes over long times. The model exhibits a fluid phase at high temperatures and a gel phase at low temperatures, identified with the Lβ-phase. The melting transition is investigated via analysis of the diffusivity of the lipid molecules, the chain-orientational order parameter, the sixfold bond-orientational order parameter, and the positional and bond-orientational correlation functions. The analysis suggests the existence of a hexatic phase over a narrow range of temperatures around the melting transition. The elastic properties of the membrane in the fluid phase are also investigated.
Effects of postnitridation annealing on band gap and band offsets of nitrided Hf-silicate films
Authors: Chung K. B. , Cho M.-H. , Hwang U. , Kang H. J. , Suh D. C. , Sohn H. C. , Ko D.-H. , Kim S. H. , Jeon H. T.
Published: January 2008

The effects of film composition and postnitridation annealing on band gap and valence band offset were examined in nitrided Hf-silicate films prepared using direct plasma nitridation. Regardless of the composition of Hf-silicate films, the band gap characteristics were similar after direct plasma nitridation (4.5±0.1eV) and postnitridation annealing (5.6±0.1eV). The decrease in band gap after direct plasma nitridation was caused by the formation of Si–N and Hf–N bonds, while the recovery of band gap by postnitridation annealing was influenced by the dissociation of unstable Hf–N bonds. The difference in valence band offset was strongly related to the chemical states of Si–N bonds.
Characterization of plasma waves in gated two-dimensional electron systems
Authors: Narahara Koichi , Suzuki Yuta
Published: January 2008

The characteristics of plasma waves in gated two-dimensional electron gas systems are discussed. We numerically discuss the effect of the intrinsic nonlinearity of plasma waves on their propagation. Moderate nonlinearity results in the development of solitons, as suggested by [Govorov et al., JETP Lett. 70, 488 (1999)]. In addition, backward-traveling pulses develop when the degree of nonlinearity becomes large.
Magnetocaloric effect in charge ordered Nd0.5Ca0.5MnO3 manganite
Authors: Karmakar Shilpi , Bose E. , Taran S. , Chaudhuri B. K. , Sun C. P. , Yang H. D.
Published: January 2008

The magnetocaloric effect (MCE) has been observed in three well-defined temperature intervals (region I: 2–115 K, region II: around charge order transition TCO, and region III: around room temperature) in polycrystalline Nd0.5Ca0.5MnO3 (NCMO) manganite system showing a first order (CO) transition at a temperature, TCO∼250 K. The magnitude of ΔSM(H) increases monotonically with applied magnetic field but does not reach saturation even at fields as high as 60 kOe. In the three given temperature regions, negligible magnetic and thermal hysteresis are found, which satisfies the requirements of using NCMO as an effective magnetocaloric material for magnetic refrigeration.
Kinetics of mesa overlayer growth: Climbing of adatoms onto the mesa top
Authors: Han Yong , Liu Feng , Li Shao-Chun , Jia Jin-Feng , Xue Qi-Kun , Lee Byeong-Joo
Published: January 2008

We have calculated the energy barriers for an adatom climbing up onto a Pb mesa top either over a facet-facet edge or through a facet-step joint, using a modified embedded atom method. We found that the second process is not only thermodynamically more favorable than the first one but also much faster with a diminishing barrier. Our results provide a plausible explanation for the experimentally observed intriguing growth behavior of a Pb mesa. The underlying mechanisms can be generally applicable to other systems.
Structural analysis of life tested 1.3 μm quantum dot lasers
Authors: Beanland R. , Sánchez A. M. , Childs D. , Groom K. M. , Liu H. Y. , Mowbray D. J. , Hopkinson M.
Published: January 2008

We present the results of an accelerated life test study of quantum dot lasers operating at 1310 nm. The devices were run at 1 and 2 kA/cm2 (∼10 and ∼70 times Ith, depending on facet coatings), at temperatures of 80 and 100 °C for 1350 h. Some devices, particularly those with higher current densities, showed significant drops in output power and increase in threshold current over this time. The devices were examined using electroluminescence, which shows nonradiative recombination centers in the active region of the device as dark spots. A clear correlation between the density of dark spots and degradation is observed. The defect structure responsible for the dark spots has been identified using conventional and high-resolution cross-section transmission electron microscopy of selected structures. The defects consist of an inverted stacking fault pyramid or microtwin enclosing the dot. The more extensive defects observed after the life test are consistent with their growth by climb, i.e., addition and/or removal of point defects. It is proposed that quantum dot devices show enhanced resistance to the growth of these defects in comparison with quantum well lasers.
Impedance of an induction coil at the opening of a borehole in a conductor
Authors: Theodoulidis Theodoros P. , Bowler John R.
Published: January 2008

The electromagnetic field of a cylindrical eddy current probe coil near the open end of a borehole in a conductor has been calculated analytically accounting for edge effects. Calculations of the coil impedance as a function of position and excitation frequency have been made allowing theoretical results to be compared with experimental measurements. Comparisons have been carried out for special cases in which a cylindrical coil has its axis either perpendicular or parallel to the axis of the hole. In the approach used, the field is expressed in terms of transverse electric and transverse magnetic potentials defined with respect to the axis of the hole. The domain of the problem is truncated in the axial direction in order to express the solution in the form of eigenfunction expansions. The truncation modifies the original unbounded domain problem, but the additional boundaries can be made as remote from the coil as desired so that they have a negligible effect on numerical estimates of the coil field. The truncated region approach has proved to be accurate and computationally efficient but more significantly, it allows new solutions to be found for problems that are otherwise analytically intractable. The results model eddy current inspections of boreholes including edge effects at the opening of the hole.
Model-based detector and extraction of weak signal frequencies from chaotic data
Authors: Zhou Cangtao , Cai Tianxing , Heng Lai Choy , Wang Xingang , Lai Ying-Cheng
Published: January 2008

Detecting a weak signal from chaotic time series is of general interest in science and engineering. In this work we introduce and investigate a signal detection algorithm for which chaos theory, nonlinear dynamical reconstruction techniques, neural networks, and time-frequency analysis are put together in a synergistic manner. By applying the scheme to numerical simulation and different experimental measurement data sets (Hénon map, chaotic circuit, and NH3 laser data sets), we demonstrate that weak signals hidden beneath the noise floor can be detected by using a model-based detector. Particularly, the signal frequencies can be extracted accurately in the time-frequency space. By comparing the model-based method with the standard denoising wavelet technique as well as supervised principal components analysis detector, we further show that the nonlinear dynamics and neural network-based approach performs better in extracting frequencies of weak signals hidden in chaotic time series.
A succession of relaxor ferroelectric transitions in Ba0.55Sr0.45TiO3
Authors: Singh Satyendra , Singh Satendra Pal , Pandey Dhananjai
Published: January 2008

We present here the results of frequency dependent dielectric, polarization, and powder x-ray powder diffraction studies in the 300–100K temperature range for Ba0.55Sr0.45TiO3. The dielectric results indicate a succession of three relaxor ferroelectric transitions accompanying the cubic to tetragonal to orthorhombic to rhombohedral phase transitions confirmed by x-ray diffraction studies.
Multipeak emission of the fast electron beams along the target surface in ultrashort laser interaction with solid targets
Authors: Yuan X. H. , Li Y. T. , Xu M. H. , Zheng Z. Y. , Chen M. , Liang W. X. , Yu Q. Z. , Zhang Y. , Liu F. , Bernhardt J. , Wang S. J. , Wang Z. H. , Wei Z. Y. , Zhao W. , Zhang J.
Published: January 2008

The spatial and energy distributions of fast electrons emitted from foil targets irradiated by ultrashort intense laser pulses are measured. Four groups of collimated emissions of fast electrons along the front and rear target surfaces are observed for an incidence angle of <60°. This multipeak characterization is found to be independent of laser polarization states. Numerical simulations reveal that the electron beams are formed due to the deformation of the target surface and then guided by the induced quasistatic electromagnetic fields.
Role of hydrogen in Ge∕HfO2∕Al gate stacks subjected to negative bias temperature instability
Authors: Rahim N. , Misra D.
Published: January 2008

This work investigates the role of hydrogen and nitrogen in a Ge∕HfO2∕Al gate stack by comparing the negative bias temperature instability (NBTI) characteristics with and without the surface nitridation of Ge surface prior to HfO2 deposition. Flatband voltage shift, change in interface state density, and stress induced leakage current were also monitored as a function of stress time. Virtually unchanged interface state density as a function of NBTI indicates no atomic hydrogen release from the dangling bond sites. However, the low n value in power law dependence of flatband voltage shift suggests diffusion of molecular hydrogen absorbed at the interface.
Paraxial energy transport of a focused Gaussian beam in ruby with nondegenerate two-wave couplinglike mechanism
Authors: Gao Feng , Xu Jingjun , Zhang Guoquan , Bo Fang , Liu Haixu
Published: January 2008

Paraxial energy transport of an intensity-modulated and focused Gaussian beam is studied in ruby. Deformations of the temporal profiles are observed under different conditions and explained by the interplay among a nondegenerate two-wave couplinglike mechanism, the population oscillations, and the Fraunhofer diffraction. Experimental evidence is provided for the existence of a nondegenerate two-wave couplinglike mechanism. The self-superluminal can be observed under proper conditions. A group velocity of −83m∕s is achieved with a Gaussian-like-modulated pulse of 30ms with background illumination. Based on these results, a method to control the group velocity of a single beam in such media is proposed.
On-wafer microwave signal-to-noise enhancer using NiFe films
Authors: Khivintsev Y. V. , Zagorodnii V. V. , Hutchison A. J. , Camley R. E. , Celinski Z. J.
Published: January 2008

This paper presents experimental results on a tunable microstrip signal-to-noise enhancer based on NiFe films. Structures involving the first- and second-order Suhl instabilities are compared. The enhancement factor is 10dB for a structure involving only the second order instability and an enhancement factor of 30dB is found for a structure where the first order instability is allowed.