bluuebunny/crossref_metadata_2025 huggingface dataset:
Density functional theory predictions of anharmonicity and spectroscopic constants for diatomic molecules
Authors: Sinnokrot Mutasem Omar , Sherrill C. David
Published: July 2002

The reliability of density functional theory and other electronic structure methods is examined for anharmonicities and spectroscopic constants of the ground electronic states of several diatomic molecules. The equilibrium bond length re, harmonic vibrational frequency ωe, vibrational anharmonicity ωexe, rotational constant Be, centrifugal distortion constant D̄e, and vibration-rotation interaction constant αe have been obtained theoretically for BF, CO, N2, CH+, and H2. Predictions using Hartree–Fock, coupled-cluster singles and doubles (CCSD), coupled cluster singles and doubles with perturbative triples [CCSD(T)], and various density functional methods (S-VWN, BLYP, and B3LYP) have been made using the 6-31G*, aug-cc-pVDZ, and aug-cc-pVTZ basis sets and compared to experimental values. Density functional theory predictions of the spectroscopic constants are reliable (particularly for B3LYP) and often perform as well as the more expensive CCSD and CCSD(T) estimates.
Exploration of conformational phase space in polymer melts: A comparison of parallel tempering and conventional molecular dynamics simulations
Authors: Bedrov Dmitry , Smith Grant D.
Published: July 2002

Parallel tempering molecular dynamics simulations have been performed for 1,4-polybutadiene polymer melts in the 323 K–473 K temperature domain at atmospheric pressure. The parallel tempering approach provides a vast improvement in the equilibration and sampling of conformational phase space for the atomistic melt chains in comparison with conventional molecular dynamics simulations even for molecular weights and temperatures considered to be routinely accessible via the latter technique.
A new compensating element for a femtosecond photoelectron gun
Authors: Qian Bao-Liang , Elsayed-Ali Hani E.
Published: July 2002

Design and analysis of a new compensating element for improving the electron pulse front and compressing the pulse duration in a femtosecond photoelectron gun are described. The compensating element is a small metallic cylindrical cavity in which an external voltage is applied in such a way that a special electric field forms and interacts with the electron pulse. This electric field reduces the distances between the faster and slower electrons inside the cavity and efficiently compensates for electron pulse broadening caused by the photoelectron energy spread and space charge effects. Poisson’s equation and the equation of motion are solved to obtain the electron trajectories. Results highlight the important design parameters of the new compensating element and show its feasibility in compressing electron pulses in the femtosecond regime.
Theory of rubber friction and contact mechanics
Authors: Persson B. N. J.
Published: July 2002

When rubber slides on a hard, rough substrate, the surface asperities of the substrate exert oscillating forces on the rubber surface leading to energy “dissipation” via the internal friction of the rubber. I present a discussion of how the resulting friction force depends on the nature of the substrate surface roughness and on the sliding velocity. I consider in detail the case when the substrate surface has a self affine fractal structure. I also present a theory for the area of real contact, both for stationary and sliding bodies, with elastic or elastoplastic properties. The theoretical results are in good agreement with experimental observation.
Stability of electrodeless ferroelectric domains near a ferroelectric/dielectric interface
Authors: Li X. , Mamchik A. , Chen I.-W.
Published: July 2002

The stability of an electrodeless near-surface ferroelectric domain, such as the one written by the tip of an atomic-force microscope, is considered. The analysis takes into account the domain-wall energy, depolarization energy, and coercive field. The domain is found unstable unless the surface charge is compensated, which was experimentally verified. A high-dielectric substrate is found to stabilize the domain while a low-dielectric substrate has the opposite effect.
Temporal instability of swirling gas jets injected in liquids
Authors: Parthasarathy R. N. , Subramaniam K.
Published: July 2002

A temporal linear stability analysis of an inviscid incompressible swirling gas jet injected into a co-flowing liquid was conducted. The ratio of the tangential velocity to the axial velocity (swirl number) played a significant role in the destabilization of the gas jet. Even at small gas Weber numbers, the presence of swirl caused the higher order azimuthal modes to become unstable; the growth rates, and the dominant and limiting wave numbers of the higher order modes were greater than those of the varicose and sinuous modes. The differences in growth rates, limiting and dominant wave numbers of the various azimuthal modes became significant at large gas Weber numbers. An increase in liquid viscosity resulted in a reduction in the growth rates and the dominant wave numbers. The liquid co-flow velocity controlled the phase velocity of the unstable modes.
The stability of noncolumnar swirling flows in diverging streamtubes
Authors: Rusak Z. , Judd K. P.
Published: July 2002

A linear stability analysis of a family of steady, noncolumnar and axisymmetric, swirling flows that may develop in a finite-length slightly diverging pipe is presented. These flow states are described by the asymptotic analysis of Rusak et al. (1998). There exists a limit level of the incoming flow swirl ratio ωcσ1 which is the corrected critical swirl as a result of the pipe divergence. When the swirl ratio is in a certain range below ωcσ1, two steady states can exist for the same inlet, outlet, and wall conditions: One which describes a near-columnar vortex state and another which describes a swirling flow with a large-amplitude disturbance. When the swirl level is above ωcσ1, no near-columnar, steady, and axisymmetric state exists. The stability of this family of flows is examined by studying the linearized dynamics of an unsteady and axially symmetric perturbation which also satisfies the boundary conditions. The stability analysis shows that ωcσ1 is a point of exchange of stability for the family of the noncolumnar vortex flows. The near-columnar states have a linearly stable mode of disturbance whereas the states with large disturbances are unstable. Also, the near-columnar states lose their stability characteristics as the swirl level approaches ωcσ1. Therefore, the analysis implies that when the swirl level of the incoming flow is above ωcσ1, the flow in the pipe must develop a transition process that involves large-amplitude perturbations and may lead to vortex breakdown states. The effect of the increase of pipe divergence on the flow dynamics and transition to breakdown states is also discussed.
Thermal effects on liquid film flow during spin coating
Authors: Kitamura Akihiro
Published: July 2002

A theoretical analysis of the thermal effects on the free-surface film flow on a flat rotating disk is presented. Assuming a small aspect ratio of the initial film thickness to the disk radius and neglecting peripheral effects of the liquid film, the evolution equation describing the shape of thin liquid film interface is obtained as a function of space and time and is solved using perturbation analysis. The results reveal the effects of inertial, gravitational, surface-tension and thermocapillary forces and of variable viscosity on the film planarization and thinning. Among these, it was found that the variable viscosity has the most profound effect on the transient film thickness.
Specific rate constants k(E,J) for the dissociation of NO2. II. Linewidths of rotationally selected NO2 near to the dissociation threshold
Authors: Abel B. , Lange N. , Troe J.
Published: July 2002

Optical double-resonance “fluorescence depletion pumping,” has been employed to investigate rovibronic states of jet-cooled NO2 in the region of the dissociation threshold E0. The method allows for the selection of angular momenta of the reacting molecules. In particular, molecules with J=0.5 and 1.5 or with J=2.5 and 3.5 have been selectively investigated. In the first 10 cm−1 above E0, the linewidths for J=0.5 and 1.5 as well as 2.5 and 3.5 have been found to fluctuate around average values which correspond to dissociation rate constants of 2×1010 s−1 and 1.65×1010 s−1, respectively. The experimental results for J=0.5 and 1.5 are in agreement with time-resolved data and quantum mechanical calculations. The difference in average linewidths for the molecules with J=0.5/1.5 and 2.5/3.5 was found to be small. The appearance of the double-resonance spectra does not principally change for higher angular momentum J, i.e., some sharp lines on top of a congested unresolved background are still observed. It is concluded that the dependence of the average lifetimes on J in the present low J-experiments very close to the dissociation threshold is only weak and hardly detectable, which appears to be consistent with recent measurements in the time domain.
Electron transport in metal/multiwall carbon nanotube/metal structures (metal=Ti or Pt/Au)
Authors: Kanda Akinobu , Ootuka Youiti , Tsukagoshi Kazuhito , Aoyagi Yoshinobu
Published: July 2002

We report observation of the Coulomb blockade effect in a metal-on-tube metal/multiwall carbon nanotube (MWNT)/metal structure (metal=Ti or Pt/Au). We find that the tunnel barrier is located in the interface between the MWNT and the metal electrode, and that the tunnel resistance depends crucially on the electrode material. A possible origin of the high contact barrier is discussed.
Complete characterization of the a 6Δ state of the FeH radical
Authors: Wilson Catherine , Cook Helen M. , Brown John M.
Published: July 2002

The electronic spectrum of the FeH radical at near ambient (∼450 K) temperature has been re-investigated at high resolution. More than 150 lines have been assigned by the use of LIF and dispersed LIF techniques. Dispersed fluorescence experiments in the green region have shown surprisingly strong ΔΩ=0, +1, +2… transitions in the e 6Π–a 6Δ system, for which the case (a) selection rule is ΔΩ=−1. The analysis of ΔΩ=0 and −1 sub-bands in this system has given term values in the previously unseen Ω=1/2 and −1/2 components of a 6Δ (v=0), e 6Π−1/2 (v=0) and a 6Δ1/2 (v=1). The a 6Δ state is now experimentally confirmed as a state of sextet nature and is only the second sextet state of FeH to be seen in all spin-orbit components (the other is c 6Σ+). The a 6Δ state is not well-described by a single state effective Hamiltonian; the best fit to this model gives a standard deviation of around 125 times the experimental error. In addition, term values have been determined for a set of levels whose term value origin lies at 3175 cm−1 above the ground state. These have been tentatively assigned to the v=0 level of the C 4Φ9/2 component and are hypothesised to be the cause of the known perturbation to the a 6Δ9/2 (v=1) level.
Magnetization reversal in sub-100 nm pseudo-spin-valve element arrays
Authors: Castaño F. J. , Hao Y. , Hwang M. , Ross C. A. , Vögeli B. , Smith Henry I. , Haratani S.
Published: July 2002

The magnetization reversal exhibited by arrays of 70-nm-wide pseudo-spin-valve (PSV) elements has been investigated by measurements of minor hysteresis loops. Samples were patterned from sputtered NiFe (6 nm)/Cu (3 and 6 nm)/Co (4 nm)/Cu (4 nm) magnetic thin film stacks. The overall room temperature magnetic behavior of the arrays can be understood by considering a distribution of switching fields for both the hard (Co) and soft (NiFe) magnetic layers. Such layers interact through exchange and magnetostatic coupling. Increasing the lengths of the elements leads to narrower switching field distributions and higher mean switching fields (particularly for the hard layer). On the other hand, decreasing the thickness of the Cu spacer leads to an increase of the switching field of the hard layer. Results obtained are well described by a model that treats each PSV as a coupled pair of rectangular single-domain films and uses the values of the interaction field between layers deduced from experimental minor loops.
Electronic structure of halogen-substituted methyl radicals: Excited states of CH2Cl and CH2F
Authors: Levchenko Sergey V. , Krylov Anna I.
Published: July 2002

Electronically excited states in CH2Cl and CH2F radicals are studied at the EOM–CCSD/6-311(3+, 3+)G(3df, 3pd) level of theory. Excited states’ characters and changes in the electronic spectrum in the CH3→CH2F→CH2Cl sequence are interpreted in terms of a simple molecular orbital picture. The key factors determining the electronic structure of these radicals are (i) the presence of lone pairs on the halogen and (ii) how strongly these lone pairs are bound to the halogen. In CH2Cl, the small energy gap between the unpaired electron on carbon and the lone pair on chlorine results in additional π-bonding between C and Cl. Moreover, the relatively weak binding energy of the chlorine’s lone pairs is responsible for the presence of several low-lying valence states in CH2Cl. In CH2F, where the lone pairs have a considerably lower energy, no additional bonding is found. The character of two lowest valence states in CH2F is similar to that of the lowest states in CH2Cl, but the excitation energies are considerably higher. The low-lying Rydberg states appear to be similar in all three radicals.
A computational evaluation of the Ergun and Forchheimer equations for fibrous porous media
Authors: Papathanasiou T. D. , Markicevic B. , Dendy E. D.
Published: July 2002

The results of a comprehensive computational evaluation of the Ergun and Forchheimer equations for the permeability of fibrous porous media are reported in this study. Square and hexagonal arrays of uniform fibers have been considered, as well as arrays in which the fiber size is allowed to change in a regular manner, expressed by a size variation parameter (δ). The range of porosity (φ) examined is from 0.30 to 0.60, the Reynolds number ranges between 0 and 160, and the size variation parameter (δ) between 0 (corresponding to the uniform array) and 0.90 (in which case the diameter of the large fibers in the array is 19 times that of the small ones). We obtain computational results for pressure drop and flow rate in a total of 440 cases mapping the (φ,δ,Re) space; these are presented in terms of a friction factor and are compared to the predictions of the Ergun and Forchheimer equations, both widely used models for the permeability of porous media. In the limit of creeping flow (Re<1), the Forchheimer equation is in excellent agreement with the computational results, while the Ergun equation is unable to capture the behavior of fiber arrays in which the flow has a strong contracting/expanding element. The Forchheimer equation, in its original form, is in closer agreement with the computational results. When the Forchheimer term (F) is expressed as a function of porosity, we obtain a modified form of the Forchheimer equation that is in excellent agreement with computational results for the entire range of (φ,δ,Re) examined.
Three-dimensional acoustic scattering by vortical flows. II. Axisymmetric scattering by Hill’s spherical vortex
Authors: Llewellyn Smith Stefan G. , Ford Rupert
Published: July 2002

The general theory of Part I is applied to the the specific case of scattering of a wave incident along the axis of Hill’s spherical vortex. The full asymptotic solution to the initial-value problem is calculated. Results agree with the general approach, showing that the conditions required for the latter to hold apply in the case of Hill’s spherical vortex.
Effect of field-dependent photogeneration on the rate of grating formation in photorefractive polymers
Authors: Binks D. J. , West D. P.
Published: July 2002

The effect of field-dependent photogeneration on the dynamics of holographic grating formation in photorefractive polymer composites is analyzed. An expression for the space-charge field rise time, τsc, is found by incorporating Onsager’s [J. Phys. Chem. 2, 599 (1934)] theory of field-assisted dissociation into the standard theory of photorefractivity. Experimentally, the variation of τsc with applied field is found by using an analytical form to describe the measured transients in composites containing reorientable chromophores. It is found that the observed variation of space-charge field formation rate, τsc−1, with applied field is entirely attributable to the field dependence of photogeneration. This result is consistent with the presence of charged trap sites and exclusively Langevin trapping.
High field electron paramagnetic resonance of Gd3+-doped glasses: Line shapes and average ion distances in silicates
Authors: Smirnov Alex I. , Sen S.
Published: July 2002

The average distances between Gd3+ ions in multicomponent silicate glasses doped with 0.058 to up to 4.64 wt % Gd2O3 are characterized by electron paramagnetic resonance (EPR) line broadening effects at high-field (95 GHz, W band). Increase in the EPR resonant field/frequency above that of the conventional X band (9.5 GHz) simplifies the Gd3+ EPR spectra and increases the sensitivity of the EPR linewidth to concentration induced broadening effects. Least squares line shape simulations show that the broadening can be described by a Lorentzian function with a width proportional to the average Gd3+ concentration which is in good agreement with the existing theories. Analyses of the concentration dependence of Lorentzian line broadening indicate an onset of clustering of Gd3+ ions at > 1wt % doping levels.
Formation of metal particle nanowires induced by ultrashort laser pulses
Authors: Kaempfe M. , Graener H. , Kiesow A. , Heilmann A.
Published: July 2002

We report a simple method to generate metallic nanowire arrays. By irradiating thin organic films with embedded silver or gold nanoparticles with series of ultrashort laser pulses, wire arrays containing more than 30 nearly parallel wires several microns in length are formed. The individual nanowires are between 100 and 200 nm wide with equal line space ratios, significantly smaller than the laser wavelength used. The wire direction correlates to the linear polarization of the laser pulses. This structural dependence on the laser polarization is mirrored in corresponding anisotropic optical and electrical film properties.
Electrical activation of ultralow energy As implants in Si
Authors: Whelan S. , Privitera V. , Mannino G. , Italia M. , Bongiorno C. , La Magna A. , Napolitani E.
Published: July 2002

Arsenic implants performed in Si at ultralow energy have been extensively studied with structural, chemical, and electrical analysis. The near-surface damage annealing and its influence on the electrical activation of ultrashallow As in Si as a function of the anneal ambient has been investigated. Double alignment medium energy ion scattering, high resolution transmission electron microscopy, and low energy secondary ion mass spectrometry have been used to assess the dopant behavior and crystal recovery in the near-surface regions. The electrical activation of As in Si has been measured with spreading resistance profiling, four point probe, and van der Pauw methods. Major redistribution of the dopant into the SiO2–Si interface region occurred during crystal regrowth of the damaged Si layer. An inactive meta-stable As solid solution was formed in the near-surface region after amorphous layer regrowth. Electrical activation of the dopant occurred upon dissociation of the As solid solution, when the dopant concentration fell to the steady state level. The As diffusion observed has been shown to be enhanced for short (10 s) anneal times at 1100 °C. When annealing at high temperature in an oxidizing ambient the dopant is retained at a high concentration in the solid and a higher level of electrical activation is observed. Significant outdiffusion of the dopant is observed during high temperature annealing in nonoxidizing conditions which reduced the level of activation.
Vertical and lateral heterogeneous integration
Authors: Geske Jon , Okuno Yae L. , Bowers John E. , Jayaraman Vijay
Published: July 2002

A technique for achieving large-scale monolithic integration of lattice-mismatched materials in the vertical direction and the lateral integration of dissimilar lattice-matched structures has been developed. The technique uses a single nonplanar direct-wafer-bond step to transform vertically integrated epitaxial structures into lateral epitaxial variation across the surface of a wafer. Nonplanar wafer bonding is demonstrated by integrating four different unstrained multi-quantum-well active regions lattice matched to InP on a GaAs wafer surface. Microscopy is used to verify the quality of the bonded interface, and photoluminescence is used to verify that the bonding process does not degrade the optical quality of the laterally integrated wells. The authors propose this technique as a means to achieve greater levels of wafer-scale integration in optical, electrical, and micromechanical devices.
Structural evolution of epitaxial SrRuO3 thin films grown on SrTiO3(001)
Authors: Kim Sang Sub , Kang Tae Soo , Je Jung Ho
Published: July 2002

Microstructural changes in epitaxial SrRuO3 thin films grown on SrTiO3(001) during the early stages of growth by pulsed laser deposition were studied using synchrotron x ray scattering. We find that at a very early stage of growth (⩽110 Å), the film is composed of a single (110) domain only, in sharp contrast to double (110) domains reported previously for thick (⩾1000 Å) films. As the film grows further (>110 Å), another, 90° rotated, (110) domain starts to form, coexisting with the original (110) domain. This domain evolution is attributed to strain relaxation, eventually resulting in significant atomic ordering in the in-plane direction.
Theoretical analysis of electronic absorption spectra of vitamin B12 models
Authors: Andruniow Tadeusz , Kozlowski Pawel M. , Zgierski Marek Z.
Published: July 2002

Time-dependent density-functional theory (TD-DFT) is applied to analyze the electronic absorption spectra of vitamin B12. To accomplish this two model systems were considered: CN-[CoIII-corrin]-CN (dicyanocobinamide, DCC) and imidazole-[CoIII-corrin]-CN (cyanocobalamin, ImCC). For both models 30 lowest excited states were calculated together with transition dipole moments. When the results of TD-DFT calculations were directly compared with experiment it was found that the theoretical values systematically overestimate experimental data by approximately 0.5 eV. The uniform adjustment of the calculated transition energies allowed detailed analysis of electronic absorption spectra of vitamin B12 models. All absorption bands in spectral range 2.0–5.0 eV were readily assigned. In particular, TD-DFT calculations were able to explain the origin of the shift of the lowest absorption band caused by replacement of the–CN axial ligand by imidazole.
Response to “Comment on ‘Mixture model description of the T-, P dependence of the refractive index of water’ ” [J. Chem. Phys. 115, 7795 (2001)]
Authors: Cho C. H. , Urquidi J. , Gellene Gregory I.
Published: July 2002

The valid range of the emphasized experimental data and need, in very precise comparisons, to compensate for the very small temperature shifts resulting from revisions in the accepted standard for the International Temperature Scale over the years, are acknowledged. However, it is emphasized that these corrections do not invalidate the approach and that the physical motivation underlying the mixture model analysis leads to physically meaningful fitting parameters; a result not found in any previous treatment.
Bimodal distribution of Indium composition in arrays of low-pressure metalorganic-vapor-phase-epitaxy grown InGaAs/GaAs quantum dots
Authors: Saint-Girons G. , Patriarche G. , Largeau L. , Coelho J. , Mereuta A. , Moison J. M. , Gérard J. M. , Sagnes I.
Published: July 2002

Low-pressure metalorganic-vapor-phase-epitaxy (LP-MOVPE) grown InGaAs/GaAs quantum dots (QDs) emitting around 1.3 μm have been studied by photoluminescence and transmission electron microscopy (TEM). We demonstrate the presence of a bimodal inhomogeneous broadening of the photoluminescence, correlated with a bimodal QDs contrast distribution in the TEM micrographs. Increasing the growth temperature of the dots induces a decrease of the ratio between the number of In-poor and In-rich QDs, illustrating the crucial influence of indium desorption on the LP-MOVPE growth of InGaAs QDs.
Quantum structure and rotational dynamics of HCN in helium clusters
Authors: Viel Alexandra , Whaley K. Birgitta
Published: July 2002

We present diffusion Monte Carlo calculations of ground states and rotationally excited states of HCN Hen4, using our recently developed algorithm for importance sampled rigid body diffusion Monte Carlo [Viel et al., Comput. Phys. Commun. (in press, 2001)] within the mixed frame implementation. Excited states are studied with both fixed node approximations, and the Projection Operator Imaginary Time Spectral Evolution (POITSE) method that allows nodal constraints to be circumvented. Improvements in the POITSE algorithm allow excited states of clusters with up to 80 degrees of freedom to be determined here. The results presented here show that the rotational dynamics of the HCN molecule in He4 clusters are very different from the behavior of heavier molecules such as SF6. Detailed analysis of ground state densities shows that the lighter HCN molecule induces negligible adiabatic following of the helium density as a result of its rotational motion. The excited state calculations show that for small numbers of He4 atoms the nodal structure does not correspond to that of a freely rotating molecule. Nevertheless, the POITSE calculations indicate that there is some admixture of this nodal structure in the low-lying rotational excitations. It is found that a relatively large number of He4 atoms are required to achieve saturation of the effective rotational constant at the experimental value, in contradistinction to the small numbers of atoms required to saturate the rotational constant for heavier molecules such as SF6 and OCS.
The rheology and morphology of phase-separating fluids with viscosity contrast
Authors: Zhang Zhenli , Zhang Hongdong , Yang Yuliang
Published: July 2002

In this paper, the effects of viscosity contrast between the components on the rheology and morphology of phase-separating binary fluids have been studied by numerically solving time-dependent Ginzburg–Landau equation and Navier–Stokes equation. It is found that, the viscosity contrast between the components strongly affects the bulk contribution to the overall rheological behavior. When the viscosity of the minor phase is higher, the trend of bulk contribution is contrary with that of interfacial contribution. Therefore, the non-Newtonian behavior is weakened. When the viscosity of the minor phases is lower, the evolution of bulk contribution is the same as interfacial contribution, and thus the non-Newtonian behavior is strengthened. However, the interfacial contribution still plays the crucial role in the overall rheological behavior. When the system contains two droplets or many domains, due to the merging of domains induced by shear flow, there is a decrease of interfacial volume fraction. Therefore, the interfacial contribution of rheology decreases and forms a peak at small shear strain. Correspondingly, the bulk contribution has also been altered. However, whether the bulk contribution increases or decreases will depend on the viscosity contrast.
Degenerate four-wave-mixing spectroscopy on epitaxially laterally overgrown GaN: Signals from below the fundamental absorption edge
Authors: Omae Kunimichi , Kawakami Yoichi , Fujita Shigeo , Kiyoku Yukihiro , Mukai Takashi
Published: July 2002

We performed photoluminescence (PL) and degenerate four-wave-mixing (FWM) spectroscopy in epitaxially laterally overgrown GaN at 10 K. Optical transitions based on exciton complexes such as biexciton emission, exciton–exciton scattering, electron–hole plasma, and so on, were revealed by PL under a wide range of excitation densities. The FWM signals were observed from states below the fundamental excitonic absorption edge, showing that nonlinear photoswitching can be performed with a transmittance geometry. The origin of such nonlinearlity was discussed by correlating with feasible many-body effects between excitons, biexcitons, and free carriers.
Photoconductive properties of nanometer-sized Si dot multilayers
Authors: Hirano Y. , Sato F. , Aihara S. , Saito N. , Miyazaki S. , Hirose M.
Published: July 2002

Nanometer-sized Si dot multilayers have been prepared by repeating a sequence of low-pressure chemical vapor deposition for dot formation and thermal oxidation for dot isolation. For the multilayer with Si dots in the range of 3–5 nm, the onset of photoconductivity is observable at a photon energy of 2 eV, corresponding to the optical absorption edge of the Si dots. This result indicates that the nanometer-sized Si dot multilayers are promising as photoconductors that work at various wavelengths because the optical band gap can be varied by controlling the dot size.
Two new long-pass cells for infrared and visible spectroscopy
Authors: Steyert David W. , Sirota J. Marcos , Mickelson Michael E. , Reuter Dennis C.
Published: July 2002

We describe two very long path absorption cells for sensitive laser absorption spectroscopy which are based on a design first described by Chernin and Barskaya [Appl. Opt. 30, 51 (1991)]. The cells distribute a nearly arbitrary number of spots across both dimensions of the field mirror in an efficient, compact array: confining up to 500 traversals in a 26 cm diam cooled cylindrical vacuum housing. The first cell, constructed at Denison University, is one of several multipass cells in the Laser Spectroscopy Laboratory used in the visible and near infrared (400–1100 nm). Optical paths of up to 1 km have been attained in this 3 m long cell at pressures from near vacuum to several atmospheres. The cell operates at temperatures down to 125 K. Conventional Pyrex optics are used. The second cell is part of a long path laboratory laser absorption spectrometer used in the thermal infrared: 350–2000 cm−1 at Goddard Space Flight Center. Path lengths up to 476 m have been achieved at frequencies less than 500 cm−1. Two unique features of the Goddard cell are the piezoelectric micrometers, which simplify considerably the controls for the optical alignment, and the diamond-turned spherical mirrors, which significantly reduce the cost and the complexity of cryogenic operation. In addition, we discuss criteria for determining the optimal pathlength of a given multipass cell for the measurement of weak spectral lines.
Enhancement of magnetic coercivity and ferromagnetic transition temperature by proximity effects in the GaMnAs–ZnMnSe multilayer system
Authors: Liu X. , Sasaki Y. , Furdyna J. K.
Published: July 2002

We have investigated the degree to which the ferromagnetic properties of GaMnAs can be modified by the proximity of a Mn-based magnetic layer in which the Mn–Mn interactions are aniferromagnetic. Epitaxial GaMnAs capped with ZnMnSe was used for this purpose. We observed that the presence of the ZnMnSe overlayer leads to a dramatic modification of the coercive field of GaMnAs, which remains high almost up to the Curie temperature. The presence of ZnMnSe overlayers also leads to a significant increase in the Curie temperature of GaMnAs, as compared to “bare” GaMnAs.
p–n
diode with hole- and electron-doped lanthanum manganites
Authors: Mitra C. , Raychaudhuri P. , Köbernik G. , Dörr K. , Müller K.-H. , Schultz L. , Pinto R.
Published: July 2002

The hole-doped (p-) manganite La0.7Ca0.3MnO3 and the electron-doped (n-) manganite La0.7Ce0.3MnO3 undergo an insulator-to-metal transition at around 250 K, above which both behave as a polaronic semiconductor. We have fabricated an epitaxial trilayer (La0.7Ca0.3MnO3/SrTiO3/La0.7Ce0.3MnO3), where SrTiO3 is an insulator. At room temperature, i.e., in the semiconducting regime, it exhibits asymmetric current–voltage (I–V) characteristics akin to a p–n diode. The observed asymmetry in the I–V characteristics disappears at low temperatures where both the manganite layers are metallic. These results indicate that using the polaronic semiconducting regime of doped manganites, a p–n diode can be constructed.
Microwave-power handling capability of HgBa2CaCu2O6+δ superconducting microstrip lines
Authors: Aga Roberto S. , Xie Yi-Yuan , Yan Shao-Lin , Wu Judy Z. , Han Siyuan S.
Published: July 2002

Measurement of microwave-power handling capability, performed on high-temperature superconducting HgBa2CaCu2O6+δ (Hg-1212) microstrip lines, was carried out at a microwave frequency of 1 GHz and temperatures ranging from 77 to 120 K. These samples were prepared using a different route in which direct patterning on Hg-1212 was avoided to minimize their moisture exposure. Microwave input power was swept to high levels to drive the samples from a linear (less dissipative) state into nonlinear (more dissipative) state. Critical input power (Pc) was determined to be the crossover between the two states. The Pc values observed for Hg-1212 microstrip lines were 29.3, 25.8, 23, and 16 dBm at 80, 90, 100, and 110 K, respectively. These Pc values, comparable with the best obtained so far on other superconductors at lower temperatures, are orders of magnitude better at 100 K or higher, suggesting that Hg-1212 is promising for microwave devices and circuits operating above 100 K or higher temperatures.
Wavelength selection of fingering instability inside Hele–Shaw cells
Authors: Fernandez J. , Kurowski P. , Limat L. , Petitjeans P.
Published: July 2002

Fingering instabilities involving fluids confined between two plates sometimes give rise to a typical wavelength λ proportional to the gap h. This unexplained behavior is investigated for the case of the Rayleigh–Taylor instability between two liquids of the same viscosity. Using qualitative scaling arguments and linear stability analysis for a simplified model of hydrodynamics, we show that, in the miscible case, h becomes a natural cut-off when diffusion is negligible, i.e., when the Péclet number Pe=h3Δρg/(ηD) is large (η viscosity, g gravitational acceleration, D diffusivity, Δρ density difference). The same result holds in the immiscible case for large capillary number Ca=h2Δρg/(12γ) (γ surface tension). In this saturation regime, the dominant wavelength is given by λ≈2.3h, while in the opposite limit (low Pe or low Ca) λ scales, respectively, as h/Pe or h/Ca1/2. These results are in agreement with a recent experimental study.
Role of molecular distortions in the spin–orbit coupling between the singlet and triplet states of the 4π electron systems C4H4, C5H5+, and C3H3−
Authors: Shiota Yoshihito , Kondo Masakazu , Yoshizawa Kazunari
Published: July 2002

How molecular distortions enhance the strength of spin–orbit coupling (SOC) between the singlet and triplet states of cyclobutadiene, cyclopentadienyl cation, and cyclopropenyl anion is described. The crossing region of the two potential energy surfaces of cyclobutadiene is characterized by a Jahn–Teller active vibrational mode that can connect the singlet and triplet structures. The spin inversion from triplet to singlet occurs in cyclobutadiene with a structural change from D4h to D2h, but the in-plane distortion along the Jahn–Teller mode cannot directly enhance the strength of SOC. Molecular distortions along some C–H out-of-plane bending modes significantly strengthen the SOC in cyclobutadiene. Also in cyclopentadienyl cation, C–H out-of-plane distortions play an essential role in enhancing the strength of SOC. The out-of-plane motions destroy the planarity of cyclobutadiene and cyclopentadienyl cation, leading to rehybridization of their σ and π orbitals. This is a main reason that the strength of SOC is enhanced by the C–H out-of-plane bending distortions in these planar molecules. On the other hand, in cyclopropenyl anion the carbon-ring distortion that can connect the triplet and singlet structures is a main factor that dominates the transition between the two states, due to its nonplanarity.
Ab initio study of the ground and two low-lying electronic excited states of FeC
Authors: Itono Sachiko S. , Taketsugu Tetsuya , Hirano Tsuneo , Nagashima Umpei
Published: July 2002

Spectroscopic constants and energy levels of the ground Δi3 state (X 3Δi) and two low-lying excited states, Δ1 and ∏,5 of iron carbide FeC have been calculated by the ab initio multireference singles and doubles configuration interaction (MR-SDCI) molecular orbital method taking relativistic and spin–orbit coupling effects into account. Predicted rotational constant B0 (0.6697 cm−1) and spin–orbit coupling constant ASO (−181 cm−1) for the X 3Δ2 state agree well with experimental values. The first Δ1 state which is in isoconfiguration with the X 3Δ state is predicted to lie at 3528 cm−1 above the X 3Δ2 state with B0, ωe, and ωexe values of 0.6861, 923, and 7 cm−1, respectively. The lowest ∏25 state described by one electron excitation from nonbonding 1δ orbital to antibonding 4π orbital should be located at 7248 cm−1 above the X 3Δ2 state with B0, ωe, and ωexe values of 0.6268, 850, and 5 cm−1, respectively. Thus, considering the coincidence in the predicted excitation energy and spectroscopic constants for the Δ1 state, the recently observed state at 3460 cm−1 above the X 3Δ2 state by Aiuchi et al. [Chem. Phys. Lett. 309, 229 (1999)], though tentatively assigned to the ∏25 state, should be reassigned to the Δ1 state.
Coupled surface and channel transport in semiconductor heterostructures
Authors: Ridley B. K.
Published: July 2002

Electron transport in a channel separated from the surface by a thin insulating barrier layer is shown to be profoundly affected by electrical conditions on the surface. Nonuniformities in the coupled surface and channel systems lead to depletion and accumulation regions in the channel and the depletion regions can cause the current to saturate at unusually low voltages. Even when there are no nonuniformities it is shown that the coupled system exhibits a density instability that leads to similar phenomena. These effects are described in terms of an analytical model that takes into account diffusion and hot-electron transport applied to the AlGaN/GaN system. It is noted that the effect of any extension of the channel under the cathode (source) can produce depletion and current saturation that can be mistaken for the result of a nonohmic contact resistance.
Comparative anomalous small-angle x-ray scattering study of hotwire and plasma grown amorphous silicon–germanium alloys
Authors: Goerigk G. , Williamson D. L.
Published: July 2002

The nanostructure of hydrogenated amorphous silicon–germanium alloys, a-Si1−xGex:H, prepared by the hotwire deposition technique (x=0.06–0.79) and by the plasma enhanced chemical vapor deposition technique (x=0 and 0.50) was analyzed by anomalous small-angle x-ray scattering experiments. For all alloys with x>0 the Ge component was found to be inhomogeneously distributed with correlation lengths of about 1 nm. A systematic increase of the separated scattering was found due to the increasing Ge concentration. The different preparation techniques show significant differences in the Ge distribution.
Ballistic magnetoresistance in a nanocontact between a Ni cluster and a magnetic thin film
Authors: Muñoz M. , Qian G. G. , Karar N. , Cheng H. , Saveliev I. G. , Garcı́a N. , Moffat T. P. , Chen P. J. , Gan L. , Egelhoff W. F.
Published: July 2002

We present measurements of ballistic magnetoresistance in nanocontacts grown by electrodeposition of Ni microclusters on magnetic thin films covered by aluminum oxide layers, using a technique proposed by Schad et al. [D. Allen, R. Schad, G. Zangari, I. Zana, D. Yang, M. C. Tondra, and D. Wang, J. Vac. Sci. Technol. A. 18, 1830 (2000); Appl. Phys. Lett. 76, 407 (2000); D. Allen, R. Schad, G. Zangari, I. Zana, D. Yang, M. C. Tondra, D. Wang, and D. Reed, J. Appl. Phys. 89, 6662 (2001)]. The measurements are made on single Ni clusters in contact with a Ni and Co thin film. We measure the magnetoresistance and observe the relaxation of the magnetization and electrical resistance as a function of time. The clusters are electrodeposited under several different experimental conditions. Some are deposited randomly on an unpatterned film and some through various patterned photoresists that control the location at which the cluster is grown. The typical contact size is estimated from the electrical resistance to be 10–30 nm. Ballistic magnetoresistance values up to 14% are obtained in these first experiments.
Photoreactions of water and carbon at 90 K
Authors: Chakarov D. V. , Gleeson M. A. , Kasemo B.
Published: July 2002

Ultraviolet (UV) irradiation (λ<400 nm) of H2O co-adsorbed with alkali metal atoms (AM=K, Cs) on the graphite (0001) surface at 90 K leads to the formation of a range of desorbing products (H2, CH4, CO, and CO2); and to the formation of oxygen-rich AM–O–C complexes on the surface. The reaction proceeds via hydrated AM complexes [AM–(H2O)n], which represent the main photo-reactive center in the co-adsorption systems. The proposed mechanism is initiated by the formation of hot electron–hole pairs in the substrate under UV irradiation. Attachment of a hot electron to an AM–(H2O)n complex can lead to selective dissociation of the associated water molecules. The resultant release of reactive species (e.g., excited H; OH) initiates a range of branched/chain reactions yielding the observed desorbing species.
Mechanism of bias-dependent contrast in scanning-capacitance-microscopy images
Authors: Smoliner J. , Basnar B. , Golka S. , Gornik E. , Löffler B. , Schatzmayr M. , Enichlmair H.
Published: July 2002

In this work, the physical processes leading to contrast in scanning capacitance microscopy (SCM) are investigated both experimentally and theoretically. Using a p-type epitaxial doping staircase on silicon, we show that a monotonic dependence of the SCM signal on the doping level is only obtained, if the tip bias is adjusted in a way that the sample is either in accumulation or depletion. In the transition region, the SCM signal is nonmonotonic as a function of doping and depends on the bias. Therefore, any doping concentration can yield a maximum SCM signal size. We also show that this behavior is in agreement with the conventional model of a metal-oxide-semiconductor junction.
Hole Schottky barrier height enhancement and its application to metal–semiconductor–metal photodetectors
Authors: Lin S. D. , Lee C. P.
Published: July 2002

Hole Schottky barrier heights on GaAs have been studied experimentally by using a conventional metal–semiconductor–metal photodetector (MSMPD) structure. The Schottky barrier height for holes was obtained directly by the hole-current dominated dark current measurement of the MSMPD. With a thin, highly doped surface layer, control of the Schottky barrier heights for holes from 0.48 to 0.79 eV was obtained. By using these engineered Schottky contacts in the MSMPDs, over three orders of magnitude reduction in the dark currents of the MSMPDs was achieved.
Validation of mesoscopic theory and its application to computing concentration dependent diffusivities
Authors: Lam R. , Basak T. , Vlachos D. G. , Katsoulakis M. A.
Published: July 2002

Despite numerous molecular simulation studies on the diffusion of small molecules through nanoporous films and on surfaces, most simulations have been limited to equilibrium conditions and relatively short time and length scales. Currently, there is a lack of a suitable computational framework which links molecular scale information with diffusion over larger scales. Here we employ a recently derived mesoscopic theory, which is exact in the limit of an infinite range potential, to model diffusion by a hopping mechanism through nanoporous films or on surfaces. One- and two-dimensional lattice gradient continuous time Monte Carlo simulations are compared to solutions of mesoscopic models for various finite range potentials and microscopic diffusion dynamics. The latter are solved using finite difference and spectral methods. We have found that the mesoscopic models quantitatively describe diffusion even for relatively short-range potentials, enabling comparison of model predictions to experimental data. It is shown that the accuracy of mesoscopic theories for a finite range potential improves with increasing system dimensionality. We have found that parameterization of a concentration dependent diffusion coefficient depends on various parameters in a complex way and can be multiple-valued. The implications of these results for modeling diffusion through membranes are also discussed.
Coherent anti-Stokes Raman scattering spectroscopy of solid nitrogen to 22 GPa
Authors: Hellwig Holger , Daniels William B. , Hemley Russell J. , Mao Ho-kwang , Gregoryanz Eugene , Yu Zuhe
Published: July 2002

The Raman-active vibron modes of solid nitrogen have been investigated by coherent anti-Stokes Raman scattering (CARS) spectroscopy to 22 GPa at room temperature. Frequencies and linewidths were measured with an accuracy of 0.1 to 0.2 cm−1. From the pressure dependence of the linewidths a dynamical model for the transitions between the δ, δloc, and ε phases has been developed. These phase transitions are characterized by different degrees of ordering of the N2 molecules. The processes can be described by an increase in the orientational order with increasing pressure and a decrease in number in the rotational degrees of freedom at the phase transitions coupled with changes in crystal structure. A structural model for the δloc phase is given, in which the δ–δloc–ε transition sequence arises from a group/subgroup relationship and can therefore be considered ferroelastic in nature. Sample annealing was found to have a significant effect on the results.
Itinerant metamagnetism and possible spin transition in LaCoO3 by temperature/hole doping
Authors: Ravindran P. , Fjellvåg H. , Kjekshus A. , Blaha P. , Schwarz K. , Luitz J.
Published: July 2002

The electronic structure of the perovskite La1−xSrxCoO3 has been obtained as a function of Sr substitution and volume from a series of generalized-gradient-corrected, full-potential, spin-density-functional band-structure calculations. The energetics of different spin configurations are estimated using the fixed-spin-moment (FSM) method. From the total energy versus spin magnetic moment curve for LaCoO3 the ground state is found to be nonmagnetic with the Co ions in a low-spin (LS) state, a result that is consistent with the experimental observations. Somewhat higher in energy, we find an intermediate-spin (IS) state with spin moment ∼1.2μB/%f.u. From the anomalous temperature dependent susceptibility along with the observation of an IS state we predict metamagnetism in LaCoO3 originating from an LS-to-IS transition. The IS state is found to be metallic and the high-spin (HS) state of LaCoO3 is predicted to be a half-metallic ferromagnet. With increasing temperature, which is simulated by a corresponding change of the lattice parameters, we have observed the disappearance of the metamagnetic solution that is associated with the IS state. The FSM calculations on La1−xSrxCoO3 suggest that the hole doping stabilizes the IS state and the calculated magnetic moments are in good agreement with the corresponding experimental values. Our calculations show that the HS state cannot be stabilized by temperature or hole doping since the HS state is significantly higher in energy than the LS or IS state. Hence the spin-state transition in LaCoO3 by temperature/hole doping is from an LS to an IS spin state and the present work rules out the other possibilities reported in the literature.