bluuebunny/crossref_metadata_2025 huggingface dataset:
Resonant tunneling and photoluminescence spectroscopy in quantum wells containing self-assembled quantum dots
Authors: Patanè A. , Polimeni A. , Eaves L. , Main P. C. , Henini M. , Dubrovskii Yu. V. , Belyaev A. E. , Brounkov P. N. , Vdovin E. E. , Khanin Yu. N. , Hill G.
Published: July 2002

We investigate the optical and electrical properties of n-i-n GaAs/(AlGa)As double barrier resonant tunneling diodes (RTDs) in which a layer of InAs self-assembled quantum dots (QDs) is embedded in the center of the GaAs quantum well. A combination of photoluminescence (PL) and electrical measurements indicates that the electronic states and charge distribution in this type of RTD are strongly affected by the presence of the dots. Also, the dot PL properties depend strongly on bias, being affected by tunneling of majority (electrons) and minority (photocreated holes) carriers through the well. The measurements demonstrate nonlinear effects in the QD PL by means of resonant tunneling and the possibility of using the dot PL as a probe of carrier dynamics in RTDs.
Conditions of single guided mode in two-dimensional triangular photonic crystal slab waveguides
Authors: Ryu Han-Youl , Hwang Jeong-Ki , Lee Yong-Hee
Published: July 2002

Photonic band structure analyses are applied to study the effect of slab thickness in two-dimensional photonic crystal slab waveguides. For transverse electric-like modes of a triangular lattice of air holes, the band gap of the asymmetric photonic crystal slab with a drilled low-index cladding do not differ significantly from that of the photonic crystal suspended in air over a wide range of slab thickness. The condition of single guided mode operation is also studied and it is found that the single mode cutoff thickness changes only by a small amount as an air-hole filling ratio varies once the center of the band gap is fixed.
Loss kinetics of carbon atoms in low-pressure high density plasmas
Authors: Ito Haruhiko , Teii Kungen , Funakoshi Hikaru , Hori Masaru , Goto Toshio , Ito Masafumi , Takeo Takashi
Published: July 2002

Vacuum ultraviolet absorption spectroscopy (VUVAS) with a carbon hollow cathode lamp was applied to the measurement of decay rate of C atom density in the afterglow of CO and CO/H2 inductively coupled plasmas. The transition line used for the measurement was 2p3s 3P2–2p2 3P2 at 165.7 nm. The influence of background absorption by the species in plasma other than C atoms on the transition line of C atoms was found to be negligible. This was clarified by measuring the absorption intensities around the center wavelength of C atoms in plasmas with VUVAS employing a xenon microhollow cathode lamp. Moreover, the dependence of the decay rate of C atom density on pressure revealed that C atoms were dominantly lost at the surface rather than in the gas phase in both CO and CO/H2 plasmas. However, in the case of CO/H2 plasma at higher pressures over about 5.0 Pa, C atoms were lost in the gas phase as well as at the surface. The diffusion constants of C atoms in both CO and CO/H2 plasmas were also determined to be 3.1×104 and 3.7×104 cm2 Pa s−1, respectively.
The local structure in liquid methylamine and methylamine–water mixtures
Authors: Kusalik Peter G. , Bergman Dan , Laaksonen Aatto
Published: July 2002

Molecular dynamics (MD) computer simulations are carried out on liquid methylamine and on 10 and 30 wt % aqueous solutions of methylamine. The local three-dimensional structure in these liquid systems is investigated using standard one-dimensional radial distribution functions and the fully three-dimensional measure of the local structure around the molecule, spatial distribution functions. Time correlation functions for the linear and angular motion of methylamine in the molecular coordinate system are also explored. From this analysis, a detailed structural picture emerges, revealing local molecular environments in these liquids that are both complex and varied. Hydrogen bond balance is found to play a key role in determining preferred arrangements. Strong hydrogen bonds are formed around the amino group in the pure liquid and in aqueous solution. At the same time, there is a strong hydrophobic association of methyl groups. The hydration structure in aqueous solution is found to be particularly rich, where in addition to the usual H-bonding nearest neighbors in the first hydration shell of the amino group, there are bridging water molecules and a novel type of distinctly non-H-bonding neighbors. The hydration structure in aqueous methylamine solution differs substantially from that found in methanol–water liquid mixtures.
Role of explicitly cooperative interactions in protein folding funnels: A simulation study
Authors: Eastwood Michael P. , Wolynes Peter G.
Published: July 2002

We discuss an off-lattice simulation of a model protein containing nonpairwise-additive interactions. The effect of varying the strength of nonadditivity within a physically reasonable range on the folding funnel topography, i.e., free energy profiles as a function of global and local order parameters, is determined. A critical comparison is made with existing theories of free energy profiles based on energy landscape ideas. While the global mean-field theory gives the correct trends for the essential simulation results, its barriers are not quantitatively accurate for short range interactions. Variational approximations that allow spatial variation of the order parameter provide quite accurate barriers and accurate pictures of the localization of order.
Photoelectron spectroscopy of SnO−
Authors: Davico Gustavo E. , Ramond Tanya M. , Lineberger W. Carl
Published: July 2002

The 364 nm laser photoelectron spectrum of SnO− is reported. Transitions from both Π1/22 and Π3/22 spin–orbit states of the anion to the X 1Σ+ state of SnO are observed. The electron affinity of SnO is found to be 0.598±0.006 eV.
Integrating the quantum Hamilton–Jacobi equations by wavefront expansion and phase space analysis
Authors: Bittner Eric R. , Wyatt Robert E.
Published: July 2002

In this paper we report upon our computational methodology for numerically integrating the quantum Hamilton–Jacobi equations using hydrodynamic trajectories. Our method builds upon the moving least squares method developed by Lopreore and Wyatt [Phys. Rev. Lett. 82, 5190 (1999)] in which Lagrangian fluid elements representing probability volume elements of the wave function evolve under Newtonian equations of motion which include a nonlocal quantum force. This quantum force, which depends upon the third derivative of the quantum density, ρ, can vary rapidly in x and become singular in the presence of nodal points. Here, we present a new approach for performing quantum trajectory calculations which does not involve calculating the quantum force directly, but uses the wavefront to calculate the velocity field using mv=∇S, where S/ℏ is the argument of the wave function ψ. Additional numerical stability is gained by performing local gauge transformations to remove oscillatory components of the wave function. Finally, we use a dynamical Rayleigh–Ritz approach to derive ancillary equations-of-motion for the spatial derivatives of ρ, S, and v. The methodologies described herein dramatically improve the long time stability and accuracy of the quantum trajectory approach even in the presence of nodes. The method is applied to both barrier crossing and tunneling systems. We also compare our results to semiclassical based descriptions of barrier tunneling.
A molecular dynamics study of the transport coefficients of liquid transition and noble metals using effective pair potentials obtained from the embedded atom model
Authors: Alemany M. M. G. , Gallego L. J. , González L. E. , González D. J.
Published: July 2002

Using effective pair potentials obtained from the Voter and Chen version of the embedded atom model, we carried out molecular dynamics simulations to compute the diffusion coefficients and shear viscosities of liquid Ni, Pd, Pt, Cu, Ag, and Au. The results obtained agree satisfactorily with those derived in previous studies using the full Voter and Chen potentials.
Optimal charge-shaping functions for the particle–particle—particle–mesh (P3M) method for computing electrostatic interactions in molecular simulations
Authors: Hünenberger Philippe H.
Published: July 2002

The application of the particle–particle—particle–mesh (P3M) method for computing electrostatic interactions in molecular simulations relies on the use of a charge-shaping function to split the potential into two contributions, evaluated in real and reciprocal space, respectively. Although the charge-shaping function is traditionally taken to be a Gaussian, many other choices are possible. In the present study, we investigate the accuracy of the P3M method employing, as charge-shaping functions, polynomials truncated to a finite spacial range (TP functions). We first discuss and test analytical estimates of the P3M root-mean-square force error for both types of shaping functions. These estimates are then used to find the optimal values of the free parameters defining the two types of charge-shaping function (width of the Gaussian or coefficients of the TP function). Finally, we compare the accuracy properties of these optimized functions, using both analytical estimates and numerical results for a model ionic system. It is concluded that the use of specific TP functions instead of the traditional Gaussian function leads to improvements in terms of computational speed, simplicity of use, and accuracy of results.
Orbital energy analysis with respect to LDA and self-interaction corrected exchange-only potentials
Authors: Garza Jorge , Vargas Rubicelia , Nichols Jeffrey A. , Dixon David A.
Published: July 2002

The self-interaction correction of Perdew and Zunger with the optimized effective potential using the Krieger–Li–Iafrate approximation is analyzed for atomic and molecular systems in the exchange-only context. Including the self-interaction correction (SIC) orbital by orbital shows that the appropriate asymptotic behavior of the exchange potential can be achieved if just the contribution of the highest occupied molecular orbital (HOMO) is considered. However, if a good description of the exchange potential in the valence region is required, and consequently a good description of the HOMO energy, then all electrons of the valence shell must be taken into account. In contrast, the lowest unoccupied molecular orbital (LUMO) is described adequately if just the HOMO SIC contribution is employed. In addition, if the lowest occupied orbital is also considered in the SIC approximation, there is an improvement in the description of the exchange potential in inner regions of an atom. When all electrons in an atom or in a molecule are considered in the SIC approximation, there exists a linear relationship between their occupied orbital energies and those obtained with the local density approximation (LDA). This indicates that the SIC and LDA occupied orbital energies are related by a shift. Furthermore, for a set of atoms or molecules there is a linear relationship between HOMO energies obtained with LDA and those obtained with the SIC approximation. Using both observations, the shift between the occupied orbital energies LDA and SIC is reported. We found that this shift (obtained for the occupied orbitals) cannot be applied to the virtual orbitals, in particular to the LUMO. However, we do find an additional linear relationship between LUMO energies obtained with LDA and those obtained with the SIC approximation. The difference between the LUMO and HOMO energy (GAP) obtained with the LDA and SIC approximations is compared with that obtained with an exact local-multiplicative exchange potential and all are compared with experimental vertical excitation energies. Whereas the LDA GAP underestimates the excitation energies, the GAP obtained with the SIC approximation and with an exact local-multiplicative exchange potential overestimates this quantity. From an analysis of the exchange energy for simple molecules, and with a similar approach to the modified Xα method, we found a linear relationship between the SIC and Hartree–Fock (HF) methods. We show numerically that the nondiagonal terms of the exact orbital representation of the exchange energy can be approximated by the SIC approach.
Comparative study of GaN and AlN nucleation layers and their role in growth of GaN on sapphire by metalorganic chemical vapor deposition
Authors: Lorenz K. , Gonsalves M. , Kim Wook , Narayanan V. , Mahajan S.
Published: July 2002

Microstructures of low-temperature GaN and AlN nucleation layers (NLs) grown on sapphire by metalorganic chemical vapor deposition and changes in these microstructures and surface morphologies upon annealing at high temperature (HT) were investigated by transmission electron microscopy and atomic force microscopy. Upon annealing, both NLs undergo changes using different pathways, resulting in islands submerged in continuous GaN and AlN NLs. Possible explanations for the rearrangement of the microstructure during annealing of the NLs are suggested for each case. The step flow growth of HT-GaN on GaN NLs initiates directly from the step ledges of the existing rounded nucleation islands, while nucleation of GaN islands on the undulated surface precedes the step flow growth on AlN NLs. The earlier appearance of flat-top islands on AlN NLs was accounted for by localized growth enhancement due to the surface energy difference between AlN and GaN.
Quantum, multibody effects and nuclear reaction rates in plasmas
Authors: Savchenko V. I.
Published: July 2002

Detailed calculations of the contribution from off-shell effects to the quasiclassical tunneling of fusing particles are provided. It is shown that these effects accelerate the Gamow rates of nuclear reactions in dense plasma by several orders of magnitude.
Influence of inertia, gravity, and substrate topography on the two-dimensional transient coating flow of a thin Newtonian fluid film
Authors: Khayat Roger E. , Welke SaraLynn R.
Published: July 2002

The interplay between inertia, gravity, and substrate topography is examined in this study for the transient two-dimensional flow of a thin Newtonian film. Surface tension effect is assumed to be negligible. The fluid emerges from a channel and is driven by a pressure gradient maintained inside the channel. The substrate is assumed to be stationary and of arbitrary shape. The lubrication equations are solved by expanding the flow field in terms of orthonormal modes in the vertical direction and using the Galerkin projection, combined with a time-stepping implicit scheme, and integration along the flow direction. The leading-order mode is found to be clearly dominant. Gravity and substrate topography can have a significant effect on transient behavior, but this effect varies significantly, depending on the level of fluid inertia. The wave and flow structures are examined for high- and low-inertia fluids. It is found that low-inertia fluids tend to accumulate near the channel exit, exhibiting a standing wave that grows with time. This behavior clearly illustrates the difficulty faced with coating high-viscosity fluids. The topography of the substrate has a drastic effect on the flow. A secondary wave emerges in the presence of a bump or a depression in the substrate. The wave structure is again highly dependent on the level of inertia.
Ground-state lasing at room temperature in long-wavelength InAs quantum-dot lasers on InP(311)B substrates
Authors: Saito Hideaki , Nishi Kenichi , Sugou Sigeo
Published: July 2002

InAs quantum dots (QDs) with a high density of 9×1010 cm−2 are formed on InAlGaAs layer/InP (311)B substrates. Lasers having five-period stacked InAs QD layers are operated in the ground state (λ≈1.6 μm) at room temperature, and the maximal modal gain of the ground state is measured to be 20 cm−1. We obtained a threshold current density of 380 A/cm2 at room temperature, and observed the temperature-insensitive threshold current at temperatures from 77 to 220 K.
Electromagnetic effects on plasma microturbulence and transport
Authors: Snyder P. B. , Hammett G. W.
Published: July 2002

Results are presented from three-dimensional kinetic-fluid simulations of pressure gradient driven microturbulence in toroidal long mean-free-path plasmas. A numerically efficient model which includes self-consistent magnetic fluctuations and nonadiabatic electron dynamics is employed. A transition from electrostatic ion-drift turbulence to Alfvénic turbulence is seen at modest values of the plasma pressure. Significant electromagnetic effects on heat conductivity are observed, including an increase as the ideal ballooning threshold is approached, particularly when electron Landau damping is included. Turbulent spectra show a number of similarities to experimental fluctuation measurements.
Excited-state reversible association–dissociation reaction: Renormalized kinetic theory in configuration space
Authors: Kwac Kijeong , Yang Mino , Shin Kook Joe
Published: July 2002

Kinetic theory of bimolecular reactions in liquid [M. Yang, S. Lee, and K. J. Shin, J. Chem. Phys. 108, 117, 8557, 9069 (1998)] is reformulated in the configuration space for the reversible pseudo-first-order association–dissociation reaction. By use of this formulation, we analyze the effect of finite excited-state lifetimes on the time dependence of the survival probabilities of reactant molecules in the presence of a competing bimolecular contact quenching process. In contrast with the excited-state reversible geminate recombination [I. V. Gopich and N. Agmon, J. Chem. Phys. 110, 10433 (1999)], the long time asymptotic behavior of the pseudo-first-order system shows different behavior. With the present results, we also reanalyze the experimental result [D. Huppert et al., Phys. Rev. Lett. 68, 3932 (1992)] qualitatively and make a suggestion for a possible reinterpretation of their experimental data.
First principles molecular dynamics simulations of pressure-induced structural transformations in silicon clusters
Authors: Molteni C. , Martoňák R. , Parrinello M.
Published: July 2002

Using a recently developed constant pressure ab initio molecular dynamics method for studying nonperiodic systems, we have investigated pressure-induced structural transformations in silicon clusters. Pressure is applied and tuned through a liquid described by a classical potential, while the clusters are treated within a density functional theory scheme. Pressure-induced transformations in the Si35H36 and Si71H60 clusters show common characteristics, with a tendency toward metallicity at high pressure and a hysteretic behavior when the pressure is released.
Optical spectroscopy of CrCl63− doped Cs2NaScCl6: Broadband near-infrared luminescence and Jahn-Teller effect
Authors: Wenger Oliver S. , Güdel Hans U.
Published: July 2002

Single crystals of 4.1% Cr3+ doped Cs2NaScCl6 were studied by variable temperature optical absorption and luminescence spectroscopy. The A2g4↔4T2g transition exhibits a great deal of fine structure both in absorption and emission spectra. The dynamic T⊗e Jahn-Teller effect in the Cr3+ T2g4 excited state is analyzed on the basis of the Ham quenching in the T2g4 spinor splitting and the vibrational fine structure in the 7 K T2g4→4A2g luminescence spectrum. The combined effect of the a1g and eg distortions leads to a net axial compression of the CrCl63− equilibrium geometry by 0.029 Å, and an equatorial expansion by 0.088 Å in the T2g4 excited state relative to the A2g4 ground state. These results are compared to those from other Cr3+ doped chloride and bromide elpasolites, and their different room temperature luminescence quantum yields are discussed on the basis of their different T2g4 excited state distortions. Temperature dependent luminescence and lifetime measurements show that in Cr3+:Cs2NaScCl6 at room temperature the luminescence quantum yield is about 0.7.
Influence of intramolecular vibrations in third-order, time-domain resonant spectroscopies. I. Experiments
Authors: Larsen Delmar S. , Ohta Kaoru , Xu Qing-Hua , Cyrier Michelle , Fleming Graham R.
Published: July 2002

This is the first in a two-paper series that investigates the influence of intramolecular vibrational modes on nonlinear, time-domain, electronically resonant signals. Both Transient Grating (TG) and Three Pulse Photon Echo Peak Shift (3PEPS) signals were collected from several probe molecules: Nile Blue, N,N-bis-dimethylphenyl-2,4,6,8-perylenetetracarbonyl diamide, and Rhodamine 6G dissolved in different solvents: benzene, dimethylsulfoxide, and acetonitrile. The effects of excitation of different vibronic transitions on the electronically resonant signals were identified by comparing signals collected with laser pulses at different excitation wavelengths. In the 3PEPS profiles, we find that excitation on the blue edge of the absorption spectrum causes a decreased initial peak shift values and more rapid initial decays, whilst in the TG signals, the magnitude of the “coherent spike” is strongly wavelength dependent. Additional thermally activated vibronic effects were studied via temperature dependent 3PEPS profiles. Our results reveal the sensitivity of the nonlinear signals to the excitation wavelengths and to the distinct vibronic structure of the different chromophores studied. Pronounced modulations in both the 3PEPS and TG signals originating from coherently excited vibrational modes were directly observed. Additional oscillations were observed that are attributed to difference frequencies and higher harmonics of the fundamental modes. In paper II we demonstrate that detailed account of the vibronic nature of the chromophore is required to describe the wavelength dependent signals.
Growth and magnetic properties of ultrathin Co films on Pd(111) investigated by ultrahigh vacuum in situ surface magneto-optical Kerr effect and scanning tunneling microscope
Authors: Kim Jonggeol , Lee Jeong-Won , Jeong Jong-Ryul , Kim Sang-Koog , Shin Sung-Chul
Published: July 2002

Growth and magnetic properties of Co ultrathin films on Pd(111) surface are studied with a ultrahigh vacuum scanning tunneling microscope (STM) and in situ surface magneto-optical Kerr effect measurement as a function of Co thickness. STM images of Co-covered surface reveal that the growth proceeds as a good layer-by-layer epitaxy up to a thickness of 2 ML. Image at a coverage of 1 ML shows almost complete monolayer coverage instead of disconnected islands. At higher coverages, the surface shows uniformly distributed three-dimensional (3D) clusters of ∼5 nm size manifesting 3D-like growth mode. Co films begin to show ferromagnetism at a coverage of 1.5 ML with a strong perpendicular magnetic anisotropy, which persists up to 5 ML. For the Co films thicker than 5 ML, reorientation transition from perpendicular to in-plane magnetic anisotropy is observed to occur over a thickness range of ∼1.5 ML. Interestingly, in this thickness range, Pd overlayer is found to drastically change the magnetic behavior.
Development of thin-film metal hydrides for integration into field emission displays
Authors: Chalamala Babu R. , Reuss Robert H.
Published: July 2002

We report on the development of solid-state hydrogen sources utilizing thin-film metal hydrides. We demonstrate that integration of these metal hydride thin films facilitate a practical method to introduce controlled amounts of hydrogen into sealed field emission display assemblies. To prove the concept, we operated Mo field emitter arrays without emission current loss for 400 h of continuous operation with titanium-hydride-coated stainless steel anode plates. Comparable arrays operated in the absence of hydride films, but in ultrahigh vacuum, had emission current degradation of over 50% in less than 100 h of operation.
Substituent effects on the mechanical quality factor of Pb(Mg1/3Nb2/3)O3–PbTiO3 and Pb(Sc1/2Nb1/2)O3–PbTiO3 ceramics
Authors: Chen Yun-Han , Uchino Kenji , Shen Mingrong , Viehland Dwight
Published: July 2002

The mechanical quality factor of Pb(Mg1/3Nb2/3)O3–PbTiO3(PMN–PT) and Pb(Sc1/2Nb1/2)O3–PbTiO3(PSN–PT) ceramics have been investigated for compositions close to the morphotropic phase boundary. Investigations were performed for various types of lower valent substituents on the B-site cation. These investigations have shown significant differences between the modified PMN–PT and PSN–PT crystalline solutions. In PMN–PT, lower valent substitutents had little effect upon the mechanical quality factor; however in PSN–PT it was found to be significantly increased. A crystal chemical approach is then used to explain these differences.
Intrinsic stress in chemical vapor deposited diamond films: An analytical model for the plastic deformation of the Si substrate
Authors: Jeong J.-h. , Kwon D. , Lee W.-S. , Baik Y.-J.
Published: July 2002

The intrinsic stress in diamond film deposited on a Si substrate is difficult to measure because high-temperature deposition induces plastic deformation in the Si and so renders useless an elastic solution. In this study, an analytical model is proposed to estimate intrinsic stress using a substrate-curvature technique and considering the plastic deformation of substrate. The stress distribution of the as-deposited film is affected not only by the intrinsic stress of the film but also by the bending and plastic deformation of the substrate. In this model, the distribution is formulated, based on elastic/plastic plate-bending theory, in terms of substrate curvatures, intrinsic stress in the film, and yield stress of the substrate. The intrinsic stress of the film together with the yield stress of the substrate can be obtained from experimentally measured substrate curvatures by solving two equilibrium equations and a moment-relaxation equation describing the film removal. Diamond films were deposited by microwave plasma chemical vapor deposition at varying film thicknesses and deposition temperatures. For the application of the model, the curvature of the film-removed substrate was measured as well as that of as-deposited substrate. The results show that overestimated intrinsic stress can be corrected successfully through this new model. The validity of the results was confirmed by stress measurement using a Raman-peak-shift method. In addition, the generation mechanism of intrinsic stress is analyzed as reflecting a competition between a grain-size effect and nondiamond carbon effect.
Extended benchmark studies of coupled cluster theory through triple excitations
Authors: Feller David , Dixon David A.
Published: July 2002

Coupled cluster theory through quasiperturbative triple excitations [CCSD(T)] was used with large correlation consistent basis sets to obtain optimized structures, harmonic vibrational frequencies and atomization energies for 37 molecules from the G2/97 test set. In some cases, it proved possible to include the triple excitations iteratively via CCSDT. Use of various correlation consistent basis set sequences facilitated estimation of frozen core energies in the complete basis set limit. Tight d functions were added for all second row atoms in order to improve the basis set convergence properties. Core/valence correlation corrections were obtained from all electron CCSD(T)/cc-pCVQZ calculations. Scalar relativistic contributions to the atomization energy were obtained from configuration interaction mass-velocity/one-electron Darwin calculations and CCSD(T) Douglas–Kroll–Hess calculations. By combining results from the present work with previously reported findings, a total of 114 comparisons with reliable experimental data for molecular atomization energies were possible. A statistical analysis of the level of agreement with experiment was performed, leading to a mean absolute deviation of 0.8 kcal/mol and maximum absolute error of −4.4 kcal/mol. This represents the most thorough study to date of the reliability of a composite approach to computational thermochemistry based on coupled cluster theory. The approach avoids the use of additivity approximations to estimate the complete basis set limit and does not include empirical corrections to the electronic energy. Results from three parameterized methods (G2, G3, and CBS-Q) for the same set of molecules are compared to the coupled cluster results.
Application of off-specular polarized neutron reflectometry to measurements on an array of mesoscopic ferromagnetic disks
Authors: Temst K. , Van Bael M. J. , Fritzsche H.
Published: July 2002

Using off-specular polarized neutron reflectometry with neutron spin analysis, we determined the magnetic properties of a large array of in-plane magnetized ferromagnetic Co disks. Resonant peaks are clearly observed in the off-specular reflectivity, due to the lateral periodicity of the disk array. Using polarized neutrons, the intensity of the resonant peak in the off-specular reflectivity is studied as a function of the magnetic field applied in the sample plane. Spin analysis of the reflected neutrons reveals the magnetization reversal and saturation within the disks.
Thomson scattering in a low-pressure neon mercury positive column
Authors: Bakker L. P. , Kroesen G. M. W.
Published: July 2002

The electron density and the electron temperature in a low-pressure neon mercury positive column are determined using Thomson scattering. Special attention has been given to the stray light reduction in the Thomson scattering setup. The results are obtained in a discharge tube with a 26 mm diam, 10 mbar of neon, a mercury pressure inbetween 0.14 and 0.85 Pa, and an electric current ranging from 100 to 400 mA. The systematic error in the electron density is 15%–45%, the statistical error is 25%–35%. The total error in the electron temperature is 15%–35%.
Magnetic domain formation in perforated permalloy films
Authors: Welp U. , Vlasko-Vlasov V. K. , Crabtree G. W. , Thompson Carol , Metlushko V. , Ilic B.
Published: July 2002

The magnetization behavior of permalloy films containing a square array of holes has been studied using magnetometry, magneto-optical imaging, and magnetic force microscopy. Two types of domain boundaries are observed. The hole array induces a strongly pinned, complex domain structure on the microscopic level (i.e., within one unit cell of the hole array). We show that magnetization reversal occurs through the nucleation and motion of a domain wall that separates areas of oppositely polarized microscopic domain patterns. These walls display highly mobile kinks that induce the reversal of the microscopic patterns. The interplay between intrinsic anisotropy and pinning by the hole array induces a pronounced anisotropy in the domain patterns and switching mechanism.
Generalized Filinov transformation of the semiclassical initial value representation
Authors: Wang Haobin , Manolopoulos David E. , Miller William H.
Published: July 2002

An efficient method is proposed for the practical solution of the “sign” problem for integrals involved in the semiclassical initial value representation. It is based on a generalization of the conventional Filinov filtering procedure which has the (approximate) effect of incorporating complex initial conditions into the phase space average; it does this by including an explicit oscillatory term in the filtering function that partially cancels the oscillatory part of the original integrand. A systematic procedure is also described for making an optimal choice of the “smoothing parameters,” thus removing this arbitrariness in the overall approach. Tests on systems with chaotic dynamics demonstrate the accuracy and efficiency of the method.
Theoretical study on the photoisomerization of azobenzene
Authors: Ishikawa Takeshi , Noro Takeshi , Shoda Takayuki
Published: July 2002

Ab initio calculations are performed to elucidate the mechanism of the photoisomerization of azobenzene. We obtain the excitation energies of the S1(n→π*), S2(π→π*), and S3(n2→π*2) states by complete active space self-consistent field (CASSCF) and multireference single double configuration interaction (MRSDCI) calculations. Two-dimensional potential surfaces of the ground- and excited states are obtained at the CASSCF level in order to investigate the isomerization pathways. A conical intersection between the ground state and the S1 state is found near the midpoint of the rotation pathway, and causes a radiationless transition. On the other hand, the S2 state has local minima at the cis and trans structures, so that the isomerization proceeds at the S2 surface following the deexcitation.
Axisymmetric flow due to a porous sphere sedimenting towards a solid sphere or a solid wall: Application to scavenging of small particles
Authors: Davis Anthony M. J.
Published: July 2002

A classic difficulty in applying a boundary condition is handled by a new mathematical approximation, correct to second order, which is used to reduce to a double set of difference equations the creeping flow problem posed by the sedimentation of a solid sphere and a porous sphere within which the Brinkman equation is assumed to be valid. It is shown that the permeability allows heavier small solid particles to be captured and markedly reduces the force experienced by a porous sphere approaching a solid plane.
Nonequilibrium partition function in the presence of heat flow
Authors: Al-Ghoul Mazen , Eu Byung Chan
Published: July 2002

In the literature on extended thermodynamics the nonequilibrium partition function in the presence of a heat flux appears in a divergent form, which has been usually evaluated by expanding the divergence causing exponential factor involving the heat flux and by arbitrarily truncating the resulting divergent series of the integrals. In this paper we show how to cast the nonequilibrium partition function in the presence of a heat flux into a convergent form and then calculate such a convergent nonequilibrium partition function in the case of a nonequilibrium dilute monatomic gas. We have used three different methods of evaluation. Thus one of the nagging problems is resolved that arises in the 13 moment approximation to irreversible processes in the nonequilibrium statistical mechanics of extended thermodynamics.
Sensing an edge with a lateral photodetector
Authors: Leung A. F. , Soderberg S.
Published: July 2002

The electrical response of a linear lateral photodetector to three different light projections is examined. First, one section of the photodetector is illuminated and the rest of the detector is covered with a piece of black tape forming a bright/dark edge. Second, the whole photodetector is illuminated and then a wire is placed in front casting a shadow onto the detector. Third, one section of the photodetector is illuminated and the rest of the detector is covered with an optical filter forming a bright/dim edge. The response to each projection is expressed by a set of equations and verified by experiments. The tracking of the position of the bright/dark edge, wire, and bright/dim edge is described.
Spatial mode control of a CO2 laser by use of controllable back mirror focal length
Authors: Amjadi A.
Published: July 2002

A cw CO2 laser, with variable back mirror focal length, has been designed and constructed for studying the stable laser transverse modes. The back mirror is a thin, flat, gold-coated plate of silicon wafer. The curvature of this mirror can be changed easily by varying its back pressure. A simple diaphragm is used inside the laser cavity for considering the effect of different curvatures on different active medium cross sections.
Separator current sheets: Generic features in minimum-energy magnetic fields subject to flux constraints
Authors: Longcope D. W.
Published: July 2002

Equations are found for force-free magnetic equilibria in the “coronal half-space” z>0, subject to boundary conditions on the normal magnetic field at z=0. The distribution of normal field is assumed to be composed of NS isolated unipolar source regions of arbitrary shape, arranged arbitrarily on the plane. The equilibria are found by minimizing the magnetic energy subject to constraints on the total flux interconnecting pairs of source regions. For NS source regions interconnected in ND ways, there are Nc=ND−NS+1 distinct constraints on the field. Minimization subject to these constraints leads to an Nc-dimensional space of equilibria, for given boundary data. All field-lines connecting source regions are current-free, but the equilibrium contains Nc current-sheets lying at certain interfaces. In a two-dimensional magnetic field current sheets occur at points topologically equivalent to X-type neutral points in the potential field. In a three-dimensional field current sheets occur at points topologically equivalent to separators in the potential field. The free magnetic energy is a function of the Nc fluxes used to constrain the variation.
Mechanical quality factor of large mirror substrates for gravitational waves detectors
Authors: Amico P. , Bosi L. , Carbone L. , Gammaitoni L. , Marchesoni F. , Punturo M. , Travasso F. , Vocca H.
Published: July 2002

Thermal noise in the mirror substrates sets a most severe limit to the low-frequency sensitivity of the interferometric gravitational wave detectors presently under construction. The mechanical quality of the mirror substrates and the geometry of their suspension are shown to affect markedly the noise level of the detector output. High mechanical Q have been obtained for different large fused silica substrates under Virgo suspension conditions. Moreover, calcium fluoride substrates are shown to provide a more promising option for the design of future cryogenic, low thermal noise interferometers.
Polaronic and magnetic field effects on the binding energy of an exciton in a quantum well wire
Authors: Bouhassoune M. , Charrour R. , Fliyou M. , Bria D. , Nougaoui A.
Published: July 2002

We present a theoretical study of the binding energy of an exciton in a cylindrical quantum well wire subject to an external magnetic field. Calculations are performed using a variational approach within the effective mass approximation. We describe the effect of the quantum confinement by an infinitely deep potential well taking into consideration the interaction between the charge carriers (electron and hole) and the optical phonons. It is shown that the polaronic correction to the exciton binding energy is important and should not be neglected. The confinement introduced by a magnetic field, in addition to the geometrical one, is also discussed.
Real-time, high-gain, computer controlled amplifier for optical detection systems
Authors: Garcı́a R. Edwin , Hernández-Cordero Juan , Geffroy Enrique , Porta Andrés V.
Published: July 2002

A computer controlled high-gain amplifier has been designed for sensitive detection of optical signals. Based on the use of digital-to-analog converters DACs, the gain and offset of a transimpedance amplifier are adjusted through a computer trying to match ideal amplifier parameters for optical detection with different experimental conditions. The amplifying modules have been developed for optical-rheometry techniques that provide information about the microstructural properties of fluids by measuring optical anisotropies induced by transient flows. Characterization of these programmable gain amplifiers shows that they provide gains and bandwidths more than adequate for experiments involving signals that evolve rapidly and with a large dynamic range. Hence, the use of DACs allows for the possibility of computer controlling both gain and offset in real time, with a significant reduction of the spurious contributions from the amplifying stages during data acquisition. In spite of being designed for optical rheometry, the amplifiers could be useful for any other laboratory arrangements requiring an amplifying stage with real-time gain and offset adjustment capabilities.
Remote-plasma chemical vapor deposition of conformal ZrB2 films at low temperature: A promising diffusion barrier for ultralarge scale integrated electronics
Authors: Sung Junghwan , Goedde Dean M. , Girolami Gregory S. , Abelson John R.
Published: July 2002

High-quality ZrB2 thin films have been deposited at substrate temperatures as low as 300 °C by a new method: remote hydrogen-plasma chemical vapor deposition from the single-source precursor Zr(BH4)4. Carrying out the deposition in the presence of atomic hydrogen generates films with properties that are far superior to those deposited by purely thermal methods; the latter are boron-rich, oxidize readily in air, and adhere poorly to the substrates. In contrast, the films generated at a substrate temperature of 300 °C in the presence of atomic H have a B/Zr ratio of 2, a resistivity of 40 μΩ cm, an oxygen content of ⩽4 at. %, and are fully conformal in deep vias. A 20 nm thick amorphous film of ZrB2 on c-Si(001) prevents Cu indiffusion after 30 min at 750 °C. We propose that the beneficial effects of atomic hydrogen can be attributed to promoting the desorption of diborane from the growth surface.
Chiroptical properties from time-dependent density functional theory. I. Circular dichroism spectra of organic molecules
Authors: Autschbach Jochen , Ziegler Tom , van Gisbergen Stan J. A. , Baerends Evert Jan
Published: July 2002

We report the implementation of the computation of rotatory strengths, based on time-dependent density functional theory, within the Amsterdam Density Functional program. The code is applied to the simulation of circular dichroism spectra of small and moderately sized organic molecules, such as oxiranes, aziridines, cyclohexanone derivatives, and helicenes. Results agree favorably with experimental data, and with theoretical results for molecules that have been previously investigated by other authors. The efficient algorithms allow for the simulation of CD spectra of rather large molecules at a reasonable accuracy based on first-principles theory. The choice of the Kohn–Sham potential is a critical issue. It is found that standard gradient corrected functionals often yield the correct shape of the spectrum, but the computed excitation energies are systematically underestimated for the samples being studied. The recently developed exchange-correlation potentials “GRAC” and “SAOP” often yield much better agreement here with experiments for the excitation energies. The rotatory strengths of individual transitions are usually improved by these potentials as well.
Optical transitions in hydrogenated amorphous silicon
Authors: Malik Saad M. , O’Leary Stephen K.
Published: July 2002

We study how the various kinds of optical transitions which occur within hydrogenated amorphous silicon contribute to the spectral dependence of the complex component of the dielectric function. While the optical transitions from the valence band band states to the conduction band band states are found to play a dominant role in determining the magnitude of this function for photon energies in excess of the optical gap, other types of optical transitions are also found to be important. We suggest that these other types of optical transitions should not be neglected when interpreting the optical properties of hydrogenated amorphous silicon.
X-ray photoemission electron microscopy, a tool for the investigation of complex magnetic structures (invited)
Authors: Scholl Andreas , Ohldag Hendrik , Nolting Frithjof , Stöhr Joachim , Padmore Howard A.
Published: July 2002

X-ray photoemission electron microscopy unites the chemical specificity and magnetic sensitivity of soft x-ray absorption techniques with the high spatial resolution of electron microscopy. The discussed instrument possesses a spatial resolution of better than 50 nm and is located at a bending magnet beamline at the Advanced Light Source, providing linearly and circularly polarized radiation between 250 and 1300 eV. We will present examples that demonstrate the power of this technique applied to problems in the field of thin film magnetism. The chemical and elemental specificity is of particular importance for the study of magnetic exchange coupling because it allows separating the signal of the different layers and interfaces in complex multilayered structures.
Linear analysis of sheared flow stabilization of global magnetohydrodynamic instabilities based on the Hall fluid model
Authors: Sotnikov V. I. , Paraschiv I. , Makhin V. , Bauer B. S. , Leboeuf J. N. , Dawson J. M.
Published: July 2002

A systematic study of the linear stage of sheared flow stabilization of Z-pinch plasmas based on the Hall fluid model with equilibrium that contains sheared flow and an axial magnetic field is presented. In the study we begin with the derivation of a general set of equations that permits the evaluation of the combined effect of sheared flow and axial magnetic field on the development of the azimuthal mode number m=0 sausage and m=1 kink magnetohydrodynamic (MHD) instabilities, with the Hall term included in the model. The incorporation of sheared flow, axial magnetic field, and the Hall term allows the Z-pinch system to be taken away from the region in parameter space where ideal MHD is applicable to a regime where nonideal effects tend to govern stability. The problem is then treated numerically by following the linear development in time of an initial perturbation. The numerical results for linear growth rates as a function of axial sheared flow, an axial magnetic field, and the Hall term are reported.
First-order properties for triplet excited states in the approximated coupled cluster model CC2 using an explicitly spin coupled basis
Authors: Hättig Christof , Köhn Andreas , Hald Kasper
Published: July 2002

An implementation is reported for first-order properties of excited triplet states within the approximate coupled cluster model CC2 using an explicitly spin coupled basis for the triplet excitation manifold and the resolution of the identity (RI) approximation for the electron repulsion integrals. Results are presented for the change of the second moment of charge upon excitation in the ππ* valence and n=3 Rydberg states of benzene. Employing large basis sets with up to 828 functions, we obtain results close to the CC2 basis set limit and are able to resolve an uncertainty in the assignment of the lowest E1u1 states. It is found that the often used %T1 measure for the single excitation contribution to excited states is not reliable for a comparison across different excitation operator manifolds. An alternative diagnostic is proposed which provides a unique measure for the single excitation contribution that is independent of the chosen representation of the excitation operator manifold.
Particle size and magnetic field dependent resistivity and thermoelectric power of La0.5Pb0.5MnO3 above and below metal–insulator transition
Authors: Banerjee Aritra , Pal Sudipta , Bhattacharya S. , Chaudhuri B. K. , Yang H. D.
Published: July 2002

The effect of particle size on the transport properties (resistivity and thermopower) of La0.5Pb0.5MnO3 has been investigated both in the presence and in the absence of magnetic field B=0.0–1.5 T (maximum). Grain size, dc conductivity; and the metal–insulator transition temperature Tp of the sample increase with increasing annealing time. Grain size has, however, comparatively little effect on the Seebeck coefficient S. Magnetoresistance is higher for the samples with smaller grain sizes. dc magnetic susceptibility also increases with increasing grain size. High temperature (T>θD/2) resistivity data well fit the small polaron hopping model. Polaron hopping energy WH decreases but polaron radius rp increases with the increase of grain size. In the metallic regime (for T<Tp), resistivity data fit well with ρ=ρ0+ρ2.5 T2.5 and the transport mechanism is attributed mainly to the magnon-carrier scattering (∼T2.5). In all the samples with different grain sizes, S changes sign below Tp. In contrast to magnetoresistance, application of magnetic field increases S at low temperature (T<Tp) for these samples. Thermopower data in the metallic phase (both for B=0.0 and 1.5 T) can be analyzed by considering a spin-wave fluctuation term (∼T4) in addition to the magnon-scattering term similar to the case of resistivity data. Although the variable range hopping mechanism is supported from the resistivity data (for Tp>T>θD/2), it is hard to justify this model from the temperature dependent thermopower data.