bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetization reversal of patterned spin–tunnel junction material: A transmission electron microscopy study
Authors: Ardhuin H. , Chapman J. N. , Aitchison P. R. , Gillies M. F. , Kirk K. J. , Wilkinson C. D. W.
Published: July 2002

Electron beam lithography and reactive ion etching have been used to pattern micron-size magnetic elements in the free layer of spin–tunnel junctions. The magnetization reversal processes of elements with dimensions in the range from 15×1 μm2 to 1×1 μm2 have been studied using Lorentz microscopy in the transmission electron microscope. Under the application of a field parallel to the bias direction, elongated elements reverse by the growth and subsequent annihilation of a quasiperiodic domain structure which evolves from the ends of the elements. Similar processes occur in both halves of a magnetization cycle. By contrast, the reversal of square elements involves the formation of more complex domain structures which differ significantly according to the direction in which the field in applied.
Absolute quantum photoyield of ion damaged diamond surfaces
Authors: Laikhtman A. , Hoffman A. , Kalish R. , Breskin A. , Chechik R.
Published: July 2002

We report on the absolute quantum photoyield (QPY) measurements from defective diamond surfaces in the 140–200 nm spectral range. The effect of defects on the photoemission properties of polycrystalline diamond films is studied by intentionally introducing damage using room temperature 30 keV Xe+ ion bombardment at doses ranging from 2×1013 to 2×1015 ions/cm2. Ion bombardment results in a drastic degradation of the QPY, to less than 1% at 140 nm, even at the lowest implantation dose, compared to ∼11.5% measured for the unimplanted diamond film. The decrease in QPY is associated with a change of the electron affinity from negative to positive as determined by secondary electron emission measurements. Microwave hydrogen plasma treatment of the damaged diamond films results in complete regeneration of the photoemission properties for diamond films implanted to Xe+ doses up to 2×1014 ions/cm2; however, only partial recovery is obtained for films irradiated with higher ion dose.
Effects of doping impurity and growth orientation on dislocation generation in GaAs crystals grown from the melt: A qualitative finite-element study
Authors: Zhu X. A. , Tsai C. T.
Published: July 2002

Dislocations in gallium arsenide (GaAs) crystals are generated by excessive thermal stresses induced during the crystal growth process. The presence of dislocations has adverse effects on the performance and reliability of the GaAs-based devices. It is well known that dislocation density can be significantly reduced by doping impurity atoms into a GaAs crystal during its growth process. A viscoplastic constitutive equation that couples the microscopic dislocation density with the macroscopic plastic deformation is employed in a crystallographic finite element model for calculating the dislocation density generated in the GaAs crystal during its growth process. The dislocation density is considered as an internal state variable and the drag stress caused by doping impurity is included in this constitutive equation. A GaAs crystal grown by the vertical Bridgman process is adopted as an example to study the influences of doping impurity and growth orientation on dislocation generation. The calculated results show that doping impurity can significantly reduce the dislocation density generated in the crystal. The level of reduction is also influenced by the growth orientation during the crystal growth process.
Magnetic properties of Fe+-implanted silica films after post-implantation annealing
Authors: Ding Xing-zhao , Tay B. K. , Shi X. , Chiah M. F. , Cheung W. Y. , Wong S. P. , Xu J. B. , Wilson I. H.
Published: July 2002

Magnetic properties of Fe-implanted silica films after different annealing processes were studied. The magnetization of the samples annealed at Ta⩽600 °C was very weak, and hard to saturate due to the superparamagnetic effect. At Ta=800 °C, the saturation magnetization increased significantly, but still much lower than that of bulk Fe. The coercivity of the films, either as-implanted or annealed, was much higher than that of bulk Fe. No evident magnetic domain structure can be observed for the as-implanted and 600 °C annealed samples, which implied that the magnetic particles at lower temperatures were small and deeply embedded in the film. During the annealing process at 800 °C, the implanted Fe atoms gradually diffused outward with the formation of islands on the film surface. These islands were demonstrated to be ferromagnetic. At the initial stage of annealing (ta⩽1 h), all the islands were of a single-domain magnetic structure. Besides these islands, some embedded magnetic particles could also be detected. When the annealing time was prolonged to 2 h, double-domain or multidomain magnetic structures could be observed in some larger islands, and no detectable magnetic particles remained in the films.
Band-gap engineering in two-dimensional periodic photonic crystals
Authors: Kushwaha M. S. , Djafari-Rouhani B.
Published: July 2002

A theoretical investigation is made of the dispersion characteristics of plasmons in a two-dimensional periodic system of semiconductor (dielectric) cylinders embedded in a dielectric (semiconductor) background. We consider both square and hexagonal arrangements and calculate extensive band structures for plasmons using a plane-wave method within the framework of a local theory. It is found that such a system of semiconductor-dielectric composite can give rise to huge full band gaps (with a gap to midgap ratio ≈2) within which plasmon propagation is forbidden. The most interesting aspect of this investigation is the huge lowest gap occurring below a threshold frequency and extending up to zero. The maximum magnitude of this gap is defined by the plasmon frequency of the inclusions or the background as the case may be. In general we find that the greater the dielectric (and plasmon frequency) mismatch, the larger this lowest band gap. Whether or not some higher energy gaps appear, the lowest gap is always seen to exist over the whole range of filling fraction in both geometries. Just like photonic and phononic band-gap crystals, semiconducting band-gap crystals should have important consequences for designing useful semiconductor devices in solid state plasmas.
Structure and vibrational spectra of H+(H2O)8: Is the excess proton in a symmetrical hydrogen bond?
Authors: Ciobanu Cristian V. , Ojamäe Lars , Shavitt Isaiah , Singer Sherwin J.
Published: July 2002

The energetics, structure, and vibrational spectra of a wide variety of H+(H2O)8 structures are calculated using density functional theory and second-order Møller–Plesset ab initio methods. In these isomers of H+(H2O)8 the local environment of the excess proton sometimes resembles a symmetric H5O2+ structure and sometimes H3O+, but many structures are intermediate between these two limits. We introduce a quantitative measure of the degree to which the excess proton resembles H5O2+ or H3O+. Other bond lengths and, perhaps most useful, the position of certain vibrational bands track this measure of the symmetry in the local structure surrounding the excess proton. The general trend is for the most compact structures to have the lowest energy. However, adding zero-point energy counteracts this trend, making prediction of the most stable isomer impossible at this time. At elevated temperatures corresponding to recent experiments and atmospheric conditions (150–200 K), calculated Gibbs free energies clearly favor the least compact structures, in agreement with recent thermal simulations [Singer, McDonald, and Ojamäe, J. Chem. Phys. 112, 710 (2000)]. © 2000 American Institute of Physics.
Application of nonreflecting boundary condition for numerical simulation of molecular photoionization dynamics
Authors: Rozgonyi Tamás , Glaß Andreas , Feurer Thomas
Published: July 2002

Simulating ionization processes on a molecular time scale requires an appropriate treatment of the ionization continuum. In general this is realized by discretizing the ionization continuum. Since in simulations only a limited number of discrete continuum levels can be considered, the discrete expansion series has to be truncated at some level. Therefore, the optimal number of levels used for the simulations is determined by two constraints. First, the computation time has to be kept low, and second, reflection of population at the last level has to be avoided. It is shown that an approximate nonreflecting boundary condition meets both requirements. The formalism has been applied to the photodissociation process of the CsCl molecule.
Kohn–Sham calculations using hybrid exchange-correlation functionals with asymptotically corrected potentials
Authors: Allen Mark J. , Tozer David J.
Published: July 2002

The theory is presented for asymptotically correcting the potentials of hybrid exchange-correlation functionals, i.e., those that include a fraction of orbital exchange. The Kohn–Sham equations involve a multiplicative potential due to the continuum part of the hybrid functional and a nonmultiplicative term due to the orbital exchange. In asymptotic regions the multiplicative σ-spin potential is corrected to take the form (CX−1)/r+εHOMO,σ+Iσ, where CX is the fraction of orbital exchange; εHOMO,σ is the σ-spin self-consistent highest occupied Kohn–Sham eigenvalue; and Iσ is an approximate ionization energy. For the hydrogen atom, the asymptotic correction leads to a potential that closely resembles the exact potential; the eigenvalue spectrum is intermediate between the Schrödinger and Hartree–Fock eigenvalues, reflecting the presence of orbital exchange. Kohn–Sham orbitals and eigenvalues determined from this procedure have been used to calculate singlet vertical excitation energies for CO, N2, H2CO, C2H4, and C6H6. The correction significantly improves excitation energies to Rydberg states, with mean absolute errors below 0.2 eV. However, despite including orbital exchange, the results do not represent an improvement over the results obtained by asymptotically correcting a recently developed GGA functional. The asymptotic correction is also shown to reduce static isotropic polarizabilities.
Highly efficient GaInAs/(Al)GaAs quantum-dot lasers based on a single active layer versus 980 nm high-power quantum-well lasers
Authors: Klopf F. , Reithmaier J. P. , Forchel A.
Published: July 2002

Highly efficient 980 nm GaInAs/(Al)GaAs quantum-dot (QD) and quantum-well (QW) lasers based on a single active layer have been fabricated and compared in view of high-power applications. QD lasers show a significantly reduced temperature shift of the emission wavelength and achieve external quantum efficiencies of 80% (>1 W/A for 1-mm-long devices). For longer cavity lengths (>2.5 mm), QD lasers show lower threshold current densities than QW lasers. Threshold current densities as low as 54 A/cm2 are reached.
Study on Ge/Si ratio, silicidation, and strain relaxation of high temperature sputtered Co/Si1−xGex structures
Authors: Huang Hsiang-Jen , Chen Kun-Ming , Chang Chun-Yen , Huang Tiao-Yuan , Chen Liang-Po , Huang Guo-Wei
Published: July 2002

As the transistors continue to scale down, the characteristics of high-temperature-sputtered Co/Si1−xGex junction have received lots of attention because of its potential applications to heterojunction bipolar transistors. In this study, we have fabricated Co/Si1−xGex junction using room-temperature and high-temperature (i.e., at 450 °C) sputtered Co on top of strained Si0.86Ge0.14 and Si0.91Ge0.09 layers prepared by ultrahigh vacuum chemical molecular epitaxy. The relative composition of Ge in Ge-rich Si1−zGez precipitate and the solid solution of ternary phase silicide of Co–Si–Ge system were compared between room-temperature and high-temperature sputtered samples. We found that the high-temperature-sputtered samples are more effective in inhibiting lattice relaxation, which would be beneficial for manufacturing metal silicide/Si1−xGex structure devices. Mechanisms were proposed to explain the large difference between the room-temperature and high-temperature sputtered samples. It is believed that the mixed Co–Si–Ge solution on high-temperature-sputtered samples is responsible for the different silicidation behaviors.
Exact solutions of the generalized Lane–Emden equation
Authors: Goenner Hubert , Havas Peter
Published: July 2002

A generalized Lane–Emden equation with indices (α,β,ν,n) is discussed, which reduces to the Lane–Emden equation proper for α=2, β=1, ν=1. General properties of the set of solutions of this equation are derived, and exact solutions are given. These include a singular solution without free integration constant for arbitrary n and for particular relations between n, ν, and α. Among the two-parameter solutions nonequivalent families of solutions of the same equation are obtained.
Dynamic linear Poisson–Boltzmann theory
Authors: Campbell Victoria S. , Grayce Christopher J.
Published: July 2002

We construct an extended-Lagrangian Brownian dynamics algorithm for charged polymers in simple electrolyte solutions which explicitly couples fluctuations in polyion conformation to charge density fluctuations in the ion atmosphere without explicit simulation of solvent and counterions. The algorithm is applicable in the Debye–Hückel limit of low solvent ionic strength. The theoretical description of the system has the appearance of conventional linearized Poisson–Boltzmann theory, except that the screening lengths become space- and time-dependent, hence we consider the algorithm a “dynamic” extension of linearized Poisson–Boltzmann theory. Use of the algorithm is illustrated in a simple model system, and some polyion properties are compared to exact results obtained by conventional Brownian dynamics simulation.
Theoretical study of the N(2D)+O2(X 3Σg−)→O+NO reaction
Authors: Braunstein M. , Duff J. W.
Published: July 2002

Potential energy surfaces are computed for all electronic states relevant for the reaction N(2D)+O2(X 3Σg−)→O+NO at the complete-active-space-self-consistent-field plus second-order perturbation theory (CASSCF-MP2) level using a 6311G(d) basis set. For those states with barriers low enough to contribute significantly at low to moderate temperatures, adiabatic global potential energy surfaces are fit with a functional form using at least 1000 computed ab initio points. Quasi-classical trajectory (QCT) calculations, excluding nonadiabatic effects, are performed and rates and final state vibrational distributions are compared with available experimental data. The peaked vibrational distribution observed in the experimental data is reproduced in these calculations, slightly shifted to higher vibration. These calculations show that from low to moderate temperatures the dynamics are dominated by the 2 2A′ and 1 2A″ states of NO2, which have similar bent, early entrance channel barriers. Although production of O(1D)+NO(X 2Π) is allowed in these calculations, the barrier connecting this adiabatic channel to products is much too high to contribute, and only the O(3P)+NO(X 2Π) branch is formed.
Ring comb copolymer brushes
Authors: Flikkema Edwin , Subbotin Andrei , ten Brinke Gerrit
Published: July 2002

The equilibrium conformations of isolated comb copolymer ring molecules in an athermal solution are investigated by off-lattice Monte Carlo simulations. The molecules considered consist of a closed flexible backbone densely grafted with flexible or rigid side chains. The study focuses on the influence of the length of the side chains on the conformational behavior. As a function of the side chain length the structure gradually stiffens until the size of the side chains approaches the diameter of the ring. Longer side chains do not influence the backbone conformation any further. The results are compared with the large body of knowledge available for linear cylindrical comb copolymer brushes.
Quantum dynamics in condensed phases via extended modes and exact interaction propagator relations
Authors: Schwartz Steven D.
Published: July 2002

This paper presents a new approach to the study of quantum dynamics in condensed phases. The methodology is comprised of two main components. First, a formally exact method is described which allows the description of the liquid as a collection of coupled (through kinetic and potential coupling) harmonic modes. The modes are related to the Fourier modes of the component particle densities. Once the modes have been defined, a canonical transformation from the standard classical interparticle Hamilton function describes a new Hamilton function, which is exactly equivalent and defined on these harmonic coordinates. The final step in this section is the transformation of this Hamilton function into a quantum Hamiltonian operator. The second step in the process is the derivation of a new quantum mechanical evolution operator which is exact and allows the correction from a reference evolution operator, which is formed by adiabatic evolution on an approximate potential. A particular approximate potential which we suggest will be useful, is the collection of harmonic modes given in the Zwanzig Hamiltonian, weighted by the spectral density. Application of the reference interaction propagator methodology can then correct the approximate adiabatic evolution operator based on the approximate potential to the exact Hamiltonian of Fourier modes described above. A test problem of a double well nonlinearly coupled to a harmonic oscillator shows that the methodology obtains rapid numerical convergence. The paper closes with a description of how the methodology would be applied to a many-dimensional (hundreds of degrees of freedom) picture of reaction in a condensed phase.
Exciplex absorptions and emissions in solid rare gases involving atomic hydrogen
Authors: Eloranta Jussi , Kunttu Henrik
Published: July 2002

Excited ionic states of diatomic RgH, where Rg=Kr or Xe, and their electronic transition dipoles have been characterized using the multireference configuration interaction method. The obtained results are in good agreement with the experimental observation of a very intense 6.4 eV absorption/emission in the gas phase as well as in ternary rare gas matrices containing H and Xe. Three different triatomic Xe hydrides (Xe–Xe–H, Xe–H–Xe, and triangular Xe2H) were also studied and used as model fragments for the multibody environment of the solid state. These calculations suggest that the absorptions observed in hydrogen doped solid Xe samples originate from an hydrogen atom trapped in an octahedral site. Upon vertical excitation from the van der Waals bound neutral ground state, a charge transfer configuration Xe6+H− is transiently formed and relaxes into a triatomic Xe2+H− species, which emits near 5 eV. This model is quantitatively consistent with the experimental observations.
Effect of charge transfer interaction and disorderness on transport properties of polyaniline systems
Authors: Chakrabarti Swapan , Bhattacharya S. N. , Banerjee D. , Bhattacharyya R.
Published: July 2002

Films of pure polyaniline and polyaniline-tetracyanoquinodinodimethane have been prepared. The optical absorption spectra of both the samples in the emeraldine base forms are explained in light of the three-dimensional exciton model. A temperature dependence study of the electrical conductivity of the samples shows an interesting crossover phenomenon (Tc=143 K) which may be considered as competition of the two opposingly directed factors, namely, charge transfer interaction and the disorderness parameter (r). While charge transfer interaction is evident from optical absorption spectroscopy, the extent of disorderness is reflected by the results of CHN microanalysis and temperature dependent electrical conductivity measurement.
Surface plasmon quantum cascade lasers at λ∼19 μm
Authors: Tredicucci Alessandro , Gmachl Claire , Wanke Michael C. , Capasso Federico , Hutchinson Albert L. , Sivco Deborah L. , Chu Sung-Nee G. , Cho Alfred Y.
Published: July 2002

The longest wavelength III–V semiconductor laser to date has been realized employing a quantum cascade configuration for the active material. It operates at λ∼19 μm on interminiband transitions in graded superlattices. The waveguide is based on surface plasmon modes confined at a metal–semiconductor interface. The devices emit peak output powers of 14 mW per facet at 10 K and laser action is achieved up to the maximum temperature of 145 K.
Kinetic modeling of a one-dimensional, bounded plasma in the ambipolar regime
Authors: Goswami Monojoy , Ramachandran H.
Published: July 2002

In this paper we present a self-consistent kinetic simulation of a diffusion dominated bulk plasma region. Collisions have been modeled by a velocity-dependent Krook collision operator. The technique is capable of handling large systems—the results presented here are for systems 100λmfp in extent—yet retains the details of the edge physics present. The distribution functions for the trapped and the transiting orbits and their moments are obtained. The density and potential profiles inside the bulk shows overall agreement with ambipolar predictions. The kinetic equivalent of DA is obtained and compared to the fluid prediction. The validity of the code and the observed deviations from fluid treatments are discussed.
A rotating shell and stabilization of the tokamak resistive wall mode
Authors: Gimblett C. G. , Hastie R. J.
Published: July 2002

The finite resistivity of the wall that surrounds any toroidal plasma confinement device can lead to a branch of instabilities known as the resistive wall mode (RWM). Theory indicates that the RWM is potentially activated whenever the plasma equilibrium is unstable with the wall placed at infinity. In particular, advanced tokamak power plant designs require the plasma β to be above the critical value for this condition to be satisfied. Accordingly, it is important to find a method of stabilizing this mode. In this work we describe a method of stabilizing the tokamak RWM that utilizes a secondary rotating conducting shell surrounding the plasma and first wall. This scheme was first thought of for the reversed-field pinch, but must be reexamined for the tokamak as the mode involved has different characteristics. It is shown that provided the second wall is suitably positioned, RWM stabilization of a tokamak is possible even in the absence of plasma rotation.
Differential geometry of GLh(1|1)
Authors: Çelik Salih
Published: July 2002

We construct a differential calculus on the quantum supergroup GLh(1|1) and obtain the h-deformed superalgebra of GLh(1|1).
Structure of the chlorobenzene–argon dimer: Microwave spectrum and ab initio analysis
Authors: Oh Jung Jin , Park Inhee , Wilson Robb J. , Peebles Sean A. , Kuczkowski Robert L. , Kraka Elfi , Cremer Dieter
Published: July 2002

The rotational spectra of the Cl35 and Cl37 isotopes of the chlorobenzene–argon van der Waals dimer have been assigned using Fourier transform microwave spectroscopy techniques. Rotational constants and chlorine nuclear quadrupole coupling constants were determined which confirm that the complex has Cs symmetry. The argon is over the aromatic ring, shifted from a position above the geometrical ring center towards the substituted carbon atom, and at a distance of about 3.68 Å from it. This distance is 0.1–0.2 Å shorter than the similar distance in the benzene–argon and fluorobenzene–argon complexes. Experimental results are confirmed and explained with the help of second-order Mo/ller–Plesset perturbation calculations using a VDZP+diff basis set. The complex binding energy of the chlorobenzene–argon complex is 1.28 kcal/mol (fluorobenzene–argon, 1.17; benzene–argon, 1.12 kcal/mol) reflecting an increase in stability caused by larger dispersion interactions when replacing one benzene H atom by F or by Cl. The structure and stability of Ar⋅C6H5–X complexes are explained in terms of a balance between stabilizing dispersion and destabilizing exchange repulsion interactions between the monomers.
Alkali–helium exciplex formation on the surface of helium nanodroplets. II. A time-resolved study
Authors: Reho J. , Higgins J. , Lehmann K. K. , Scoles G.
Published: July 2002

We have monitored the time evolution of the fluorescence of K*He exciplexes formed on the surface of helium nanodroplets using reversed time-correlated single photon counting. In modeling the present data and that from our previous work on Na*He, we find that partial spin–orbit coupling as well as the extraction energy of helium atoms from the droplet contribute to the observed dynamics of both K*He and Na*He formation, which differ considerably after either D1(n 2P1/2←n 2S1/2) or D2(n 2P3/2←n 2S1/2) excitation for both K(n=4) and Na(n=3). Our quantitative prediction of the Na*He formation dynamics coupled with preliminary data on and modeling of the formation dynamics of K*He allow for extrapolation to the case of Rb*He. Spin–orbit considerations combined with a simple model of helium atom extraction from the matrix reveal the following predicted trend: as the choice of the alkali guest atom is moved down the periodic table, alkali atom–He exciplex formation along the 1 2Π3/2 surface occurs faster while formation along the 1 2Π1/2 surface occurs more slowly, ceasing to occur at all in the case of Rb.
Relativistic magnetohydrodynamics revisited
Authors: Sandoval-Villalbazo Alfredo , Garcı́a-Colı́n Leopoldo S.
Published: July 2002

Magnetohydrodynamics is discussed within the framework of irreversible thermodynamics. The nonrelativistic version is reviewed by introducing the electromagnetic field as an external force. Results are discussed and emphasis is placed on the fact that the transport equations, being of a parabolic type, violate causality. The relativistic version is next considered using Kaluza’s ideas about unifying fields in terms of the corresponding space–time curvature for a given metric. The outcome of this approach is rewarding. The conservation equations follow in a direct way as well as the entropy balance equation with an entropy production whose form suggests the type of constitutive equations that are consistent with its semipositive character. Further, the resulting transport equations are of a hyperbolic type in agreement with causality. Therefore, relativistic magnetohydrodynamics is placed within a thermodynamic framework consistent with the second law.
Electro-acoustic damping effects on the parametric decays of electromagnetic waves in electron–positron plasmas
Authors: Muñoz Vı́ctor , Gomberoff Luis
Published: July 2002

Parametric decays of a linearly polarized electromagnetic wave in an electron–positron plasma, including weakly relativistic effects and damping on the electro-acoustic pseudomodes, are studied. The effect of nondissipative Landau damping is simulated through a phenomenological collisional-like term in the fluid equations. It is shown that in general, there are a number of instabilities due to the coupling between electro-acoustic and electromagnetic pseudomodes, and between electromagnetic modes. On the other hand, damping effects reduce the growth rate of these instabilities and increase the instability range. It is also shown that there are no threshold effects on any of the decay modes.
Analysis of dislocation mechanism for melting of elements: Pressure dependence
Authors: Burakovsky Leonid , Preston Dean L. , Silbar Richard R.
Published: July 2002

In the framework of melting as a dislocation-mediated phase transition we derive an equation for the pressure dependence of the melting temperatures of the elements valid up to pressures of order their ambient bulk moduli. Melting curves are calculated for Al, Mg, Ni, Pb, the iron group (Fe, Ru, Os), the chromium group (Cr, Mo, W), the copper group (Cu, Ag, Au), noble gases (Ne, Ar, Kr, Xe, Rn), and six actinides (Am, Cm, Np, Pa, Th, U). These calculated melting curves are in good agreement with existing data. We also discuss the apparent equivalence of our melting relation and the Lindemann criterion, and the lack of the rigorous proof of their equivalence. We show that the would-be mathematical equivalence of both formulas must manifest itself in a new relation between the Grüneisen constant, bulk and shear moduli, and the pressure derivative of the shear modulus.
Thermodynamic study of the gaseous molecules Al2N, AlN, and Al2N2 by Knudsen cell mass spectrometry
Authors: Meloni G. , Gingerich K. A.
Published: July 2002

The Knudsen effusion mass spectrometric method has been employed to measure the equilibrium partial pressures of the Al2N molecule over the AlN–Au–graphite system. Theoretical computations were carried out to determine the structure, molecular parameters, and thermodynamic properties of Al2N. The partial pressures have been combined with the calculated thermal functions to determine the atomization enthalpy, ΔaH0o, and enthalpy of formation, ΔfH298.15o, in kJ mol−1, of 783.2±15 and 342.7±15 for Al2N, respectively. Upper values for the dissociation energy of AlN, D0o(AlN,g)⩽368±15 kJ mol−1, and for the atomization enthalpy of Al2N2, ΔaH0o(Al2N2,g)⩽1402 kJ mol−1 have been obtained. These results are discussed and compared with recent theoretical literature values.
Ab initio and quasiclassical trajectory study of the N(2D)+NO(X 2Π)→O(1D)+N2(X 1Σg+) reaction on the lowest A′1 potential energy surface
Authors: González Miguel , Valero R. , Sayós R.
Published: July 2002

In this work we have carried out ab initio electronic structure calculations, CASSCF/CASPT2 with the Pople’s 6-311G(2d) basis set on the ground singlet potential energy surface (1 1A′ PES) involved in the title reaction. Transition states, minima and one 1 1A′/2 1A′ surface crossing have been characterized, obtaining three NNO isomers with the energy ordering: NNO (1Σ+)<cyclic−C2v NON(1A1)<NON(1Σg+). Approximately 1250 ab initio points have been used to derive an analytical PES which fits most of the stationary points, with a global root-mean-square deviation of 1.12 kcal/mol. A quasiclassical trajectory study at several temperatures (300–1500 K) was performed to determine thermal rate constants, vibrational and rovibrational distributions and angular distributions. The dynamics of this barrierless reaction presents a predominant reaction pathway (96% at 300 K) with very short-lived collision complexes around the NNO minimum, which originate backward scattering and a similar fraction of vibrational and translational energy distributed into products. At higher temperatures other reaction pathways involving NON structures become increasingly important as well as the N-exchange reaction (3.02% of the branching ratio at 1500 K), this latter in accord with experimental data. It is concluded that the physical electronic quenching of N(2D) by NO should be negligible against all possible N(2D)+NO reaction channels.
CO
2
laser–plume interaction in materials processing
Authors: Kim K. R. , Farson D. F.
Published: July 2002

In laser materials processing, localized evaporation caused by focused laser radiation forms a plume of mixed vapor and ambient gas above the material surface. The beam is refracted and absorbed as it traverses the plume, thus modifying its power density on the surface. In this work, plume–beam interaction is studied using an axisymmetric, high-temperature gas-dynamic model of a plume formed by vapor from an iron surface. The beam propagation in the plume is calculated from the paraxial wave equation including absorption and refraction. The simulation results quantify the effects of plasma plume properties on the beam radius and laser power density variations at the material surface. It is shown that absorption and refraction in the plume have significant impacts on the laser–material interaction. Absorption of the beam in the plume has much less direct effect on the power density at the material surface than refraction does. However, absorption is essential for the formation of the plume, without which there is no refraction. Helium gas is more efficient than argon for reducing the beam refraction and absorption effects. Laser energy reflected from the material surface has significant effects on the plume properties.
Near constant loss in glassy and crystalline LiAlSi2O6 from conductivity relaxation measurements
Authors: Rizos Apostolos K. , Alifragis J. , Ngai K. L. , Heitjans Paul
Published: July 2002

Polycrystalline and glassy LiAlSi2O6 are studied by dielectric relaxation measurements for the purpose of characterizing the nearly frequency independent contribution to the dielectric loss (near constant loss), which is commonly found in glassy ionic conductors independent of the chemical and physical structures. The data show the near constant loss is present in both the polycrystalline and glassy states of LiAlSi2O6. Further, its magnitude and temperature dependence is comparable in both forms of the same substance. The implications of these findings on the mechanism that gives rise to the near constant loss are discussed.
Terahertz vibration–rotation–tunneling spectroscopy of water clusters in the translational band region of liquid water
Authors: Keutsch Frank N. , Brown Mac G. , Petersen Poul B. , Saykally Richard J. , Geleijns Michel , van der Avoird Ad
Published: July 2002

We report the first direct observation of the hydrogen-bond stretching vibration for a water cluster. A perpendicular band of (D2O)3 was measured by terahertz laser vibration–rotation–tunneling spectroscopy at 142.8 cm−1 in the “translational band” region of the liquid corresponding to the hindered translational motions of water molecules. We have tentatively assigned the spectrum to transitions from the vibrational ground state to the degenerate hydrogen-bond stretch or a combination or mixed state of the degenerate stretch and a torsional vibration. Comparison with theoretical results shows that calculated frequencies are much too high, presumably because they do not include coupling between the torsional and stretching vibrations.
Kinetic aspects of the morphology of self-assembled InAs quantum dots on GaAs(001)
Authors: Patella F. , Fanfoni M. , Arciprete F. , Nufris S. , Placidi E. , Balzarotti A.
Published: July 2002

We analyzed by atomic force microscopy self-assembled quantum dots of InAs on GaAs(001) in a series of samples prepared by molecular beam epitaxy (MBE). Two different growth procedures have been applied, namely, the usual continuous growth and the migration-enhanced growth. At equal depositions of InAs, larger than the critical thickness for the two- to three-dimensional transition, marked differences are found in the evolution of the nanoparticle density and volume, despite of the same set of growth parameters were used. Above 2 ML, a small fraction of ripened islands is also present, which is responsible for the nonlinear increase of the total volume of the dots with InAs coverage caused by an anomalous participation of the underlying layers. The different morphologies obtained substantiate the overwhelming role of kinetics on thermodynamics in the nonequilibrium MBE growth.
Electron transport process in quantum cascade intersubband semiconductor lasers
Authors: Kalna K. , Cheung C. Y. L. , Shore K. A.
Published: July 2002

Detailed self-consistent calculations have been performed of the electron transport and capture aspects of the dynamics of electrically pumped quantum cascade intersubband semiconductor lasers. Specific attention is given to the dependence of the characteristic carrier relaxation times on carrier temperature and density at different applied biases. We have found that the capture and intersubband relaxation times oscillate with increasing electric field. Correlative oscillations can be observed when electron temperature and electron density is plotted as a function of the same applied bias at each energy subband of the laser active region. The temperature and density amplitude are rather less pronounced than those of the relaxations time. Analysis of this typical behavior suggests that the most effective laser structure should work under 70 meV bias.
Direct observation of void morphology in step-like electromigration resistance behavior and its correlation with critical current density
Authors: Huang J. S. , Shofner T. L. , Zhao J.
Published: July 2002

Electromigration-induced resistance behavior of via-terminated metal lines was studied. We found a strong correlation between void morphology and step-like resistance behavior. For the lines which exhibit pronounced resistance steps, voids are formed at the top or bottom Al/TiN interface. For the lines which exhibit gradual resistance increases, only complete void formation severing the whole line occurs. The step-like resistance behavior is more pronounced when the current density is close to the critical current density. Possible mechanisms for the resistance steps are discussed.
Magnetization in quasiperiodic magnetic multilayers with biquadratic exchange coupling
Authors: Bezerra C. G. , de Araújo J. M. , Chesman C. , Albuquerque E. L.
Published: July 2002

A theoretical study of the magnetization curves of quasiperiodic magnetic multilayers is presented. We consider structures composed by ferromagnetic films (Fe) with interfilm exchange coupling provided by intervening nonferromagnetic layers (Cr). The theory is based on a realistic phenomenological model, which includes the following contributions to the free magnetic energy: Zeeman, cubic anisotropy, bilinear, and biquadratic exchange energies. The experimental parameters used here are based on experimental data recently reported, which contain sufficiently strong biquadratic exchange coupling.
Misfit strain induced lattice distortions in heteroepitaxially grown LaxCa1−xMnO3 thin films studied by extended x-ray absorption fine structure and high-resolution x-ray diffraction
Authors: Miniotas Andrius , Vailionis Arturas , Svedberg Eric B. , Karlsson Ulf O.
Published: July 2002

Mn–O and Mn–Mn interatomic distances were determined in LaxCa1−xMnO3 films grown heteroepitaxially on single crystal SrTiO3 and LaAlO3 substrates by molecular beam epitaxy. Mn–O bond lengths were found to be fixed at ∼1.975 Å for both types of substrates, while the Mn–Mn distance was detected to be larger for films grown on SrTiO3 substrates than for films grown on LaAlO3. The deviation of Mn–Mn interatomic distances, and subsequently Mn–O–Mn bond angles, in epitaxial LaxCa1−xMnO3 films is attributed to the misfit strain: compressive for LaAlO3 and tensile for SrTiO3 substrates.
Simulation of protein folding by reaction path annealing
Authors: Eastman Peter , Grønbech-Jensen Niels , Doniach Sebastian
Published: July 2002

We present a systematic application of reaction path sampling to computer simulations of the folding of peptides and small proteins at atomic resolution in the presence of solvent. We use a simulated annealing protocol to generate an ensemble of room temperature folding trajectories of fixed length, which connect predetermined initial and final states. The trajectories are distributed according to a discretized version of the Onsager–Machlup action functional. We show that, despite the enormous practical restrictions placed on the number of time slices which can be explored, some of the basic kinetic features found experimentally for the folding of peptides and small proteins are exhibited in the nature of the reaction paths sampled. We test the method on three systems: A 12 residue α-helical peptide, a 16 residue β-hairpin peptide, and the 36 residue avian Pancreatic Polypeptide (aPP). All systems are represented at atomic resolution, and include explicit water molecules. For the 12 residue α-helix, we find that (i,i+3) hydrogen bonds can play a significant role in the folding pathway, with specific (i,i+3) bonds appearing, then transforming to the corresponding (i,i+4) hydrogen bond for some, but not all of the native hydrogen bonds. For the β-hairpin and aPP, hydrophobic interactions play a dominant role, with nonbonded interactions consistently appearing before hydrogen bonds. This is true both at the level of tertiary structure, and at the level of individual hydrogen bonds which tend to form only after stabilizing nonbonded interactions have already formed between the residues involved.
Single molecule waiting time distribution functions in quantum processes
Authors: Cao Jianshu
Published: July 2002

The statistics of single molecule blinking events often reveal underlying quantum mechanisms. The golden rule rate expression for quantum transitions is shown to be the inverse of the mean waiting time. The distribution function for the waiting time is related to the density of states such that simple power-law distribution functions can be predicted based on the functional form for the density of states. Explicit formulas are derived for waiting time distribution functions in three kinetic processes: Quantum tunneling, intersystem conversion, and nonstationary electron transfer.
Electronic excited-state wave functions for quantum Monte Carlo: Application to silane and methane
Authors: Porter A. R. , Al-Mushadani O. K. , Towler M. D. , Needs R. J.
Published: July 2002

We investigate the efficient construction of guiding wave functions for use in diffusion Monte Carlo calculations of electronic excited states. We test guiding wave functions obtained from singles-only configuration interaction, time-dependent density functional theory, and complete active space self-consistent field methods. The techniques are used to study the first ionization potentials and excited states of silane and methane.
States of quantum systems and their liftings
Authors: Kupsch Joachim , Smolyanov Oleg G. , Sidorova Nadejda A.
Published: July 2002

Let H1, H2 be complex Hilbert spaces, H be their Hilbert tensor product and let tr2 be the operator of taking partial trace, with respect to the space H2, of trace class operators in H. The operation tr2 maps states in H (=positive trace class operators in H with trace equal to 1) into states in H1. In this paper we give the full description of mappings that are linear right inverse to tr2. More precisely, we prove that any affine mapping F(W) of the convex set of states in H1 into the states in H that is right inverse to tr2 is given by W⟼W⊗D for some state D in H2. In addition we investigate a representation of the quantum mechanical state space by probability measures on the set of pure states and a representation—used in the theory of stochastic Schrödinger equations—by probability measures on the Hilbert space. We prove that there are no affine mappings from the state space of quantum mechanics into these spaces of probability measures.
Ab initio studies of π-water tetramer complexes: Evolution of optimal structures, binding energies, and vibrational spectra of π-(H2O)n (n=1–4) complexes
Authors: Tarakeshwar P. , Kim Kwang S. , Djafari S. , Buchhold K. , Reimann B. , Barth H.-D. , Brutschy B.
Published: July 2002

The optimal structures, binding energies, and harmonic vibrational frequencies of clusters containing a substituted benzene molecule microsolvated by four water molecules, termed as π-(water tetramer) clusters (π: p-difluorobenzene, fluorobenzene, benzene, toluene) have been evaluated at the second order perturbation level of theory (MP2) using both the 6-31+G* and aug-cc-pVDZ basis sets. In sharp contrast to the complexes of smaller water clusters with these π systems, wherein the water subcluster is most strongly bound to toluene, the water tetramer is most strongly bound to fluorobenzene. This exceptionally high binding energy results from both a π⋅⋅⋅OH H-bond and a competing σ F⋅⋅⋅OH bond between the water tetramer moiety and the aromatic molecule. The magnitudes of the many-body energy terms and their contribution to the binding energies of these π-(water tetramer) systems indicates that the contributions of three- and higher-order terms are much smaller when compared to the neutral water clusters. The two-body terms associated with the π- and σ-type of interaction indicates that in both the fluorobenzene and p-difluorobenzene complexes, the increase in the size of the water cluster enhances the π-H-bonding interaction and weakens the σ F⋅⋅⋅H interaction. This observation is in consonance with the calculated and experimentally observed redshifts of the OH vibrational frequencies. Thus, with an increase in the size of a water cluster bound to the fluorinated π system, there is a lowering of the redshift induced by the σ F⋅⋅⋅H interaction and an increase in the redshift due to the π-H interaction. The calculated redshift of the π H-bonded OH mode is very much dependent on the basis set, with larger basis sets yielding shifts which are in better agreement with the experimentally determined shifts.
Pinholes may mimic tunneling
Authors: Rabson D. A. , Jönsson-Åkerman B. J. , Romero A. H. , Escudero R. , Leighton C. , Kim S. , Schuller Ivan K.
Published: July 2002

Interest in magnetic-tunnel junctions has prompted a re-examination of tunneling measurements through thin insulating films. In any study of metal–insulator–metal trilayers, one tries to eliminate the possibility of pinholes (small areas over which the thickness of the insulator goes to zero so that the upper and lower metals of the trilayer make direct contact). Recently, we have presented experimental evidence that ferromagnet-insulator-normal trilayers that appear from current–voltage plots to be pinhole-free may nonetheless, in some cases, harbor pinholes. Here, we show how pinholes may arise in a simple but realistic model of film deposition and that purely classical conduction through pinholes may mimic one aspect of tunneling, the exponential decay in current with insulating thickness.
Vibrational relaxation of aqueous CS2
Authors: Thomsen C. L. , Tho/gersen J. , Keiding S. R.
Published: July 2002

Vibrational relaxation of aqueous CS2 formed by geminate recombination of CS+S after photolysis of CS2(aq) at 200 nm is studied by subpicosecond transient absorption spectroscopy. Comparison of the transient absorption measured from 200 to 313 nm with calculated absorption transients pertaining to vibrationally relaxing CS2 shows that relaxation to the vibrational ground state of CS2 predominantly occurs in the asymmetric stretch on a 5 ps time scale.
Identifying and removing intruder states in multireference Mo/ller–Plesset perturbation theory
Authors: Choe Yoong-Kee , Witek Henryk A. , Finley James P. , Hirao Kimihiko
Published: July 2002

The lowest-lying excited states of N2 and CO are examined for the presence of intruder states that adversely influence the perturbative convergence in computations using multireference Mo/ller–Plesset perturbation theory. Intruder states are found in the computations of the Πu3 state of N2 and the Π3 state of CO. Since, in second order, these intruders—under normal circumstances—should make a negligible contribution to the target state, their contributions to the second-order energies are simply removed from the perturbation expansion. With the intruder state removed, the vertical excitation for the Πu3 (3Π) state of N2 (CO) is only 0.23 eV (0.35 eV) below the experimental value, compared to differing by 0.72 eV (0.44 eV) with the intruder state present. The intruder states are also removed by performing energy-denominator shifts using a redefined zeroth-order Hamiltonian H0 that differs from previous treatments, since only the energy denominator associated with the intruder state is shifted. This approach removes intruders, but makes no other changes to the perturbation expansions. Two-state models are used to identify the intruder states and suggest remedies.