bluuebunny/crossref_metadata_2025 huggingface dataset:
Molecular dynamics simulations of gas separations using faujasite-type zeolite membranes
Authors: Jia W. , Murad S.
Published: February 2004

Gas separations with faujasite zeolite membranes have been examined using the method of molecular dynamics. Two binary mixtures are investigated, oxygen/nitrogen and nitrogen/carbon dioxide. These mixtures have been found experimentally to exhibit contrasting behavior. In O2/N2 mixtures the ideal selectivity (pure systems) is higher than the mixture selectivity, while in N2/CO2 the mixture selectivity is higher than the ideal selectivity. One of the key goals of this work was to seek a fundamental molecular level understanding of such divergent behavior. Our simulation results (using previously developed intermolecular models for both the gases and zeolites investigated) were found to replicate this experimental behavior. By examining the loading of the membranes and the diffusion rates inside the zeolites, we have been able to explain such contrasting behavior of O2/N2 and N2/CO2 mixtures. In the case of O2/N2 mixtures, the adsorption and loading of both O2 and N2 in the membrane are quite competitive, and thus the drop in the selectivity in the mixture is primarily the result of oxygen slowing the diffusion of nitrogen and nitrogen somewhat increasing the diffusion of oxygen when they pass through the zeolite pores. In N2/CO2 systems, CO2 is rather selectively adsorbed and loaded in the zeolite, leaving very little room for N2 adsorption. Thus although N2 continues to have a higher diffusion rate than CO2 even in the mixture, there are so few N2 molecules in the zeolite in mixtures that the selectivity of the mixture increases significantly compared to the ideal (pure system) values. We have also compared simulation results with hydrodynamic theories that classify the permeance of membranes to be either due to surface diffusion, viscous flow, or Knudsen diffusion. Our results show surface diffusion to be the dominant mode, except in the case of N2/CO2 binary mixtures where Knudsen diffusion also makes a contribution to N2 transport.
Time-dependent probability of quantum tunneling in terms of the quasisemiclassical method
Authors: Ushiyama Hiroshi , Takatsuka Kazuo
Published: February 2004

In view of the rapid progress in experiments of the tunneling dynamics in the time domain, we develop a quasisemiclassical method that is aimed at a study of the proton-transfer dynamics in a large system such as tropolone and its interesting derivatives, to which not only full quantum mechanics, but even a standard semiclassical theory is never easy to apply. In our very tractable method for multidimensional systems, the tunneling paths are generated in terms of the generalized classical mechanics [K. Takatsuka and H. Ushiyama, Phys. Rev. A 51, 4353 (1995)], but the quantum phases arising from the action integral, the Maslov index, and the semicalssical amplitude factor as well in the semiclassical kernels are entirely neglected. This approach is called the quasisemiclassical method. One of the technical issues involved in the general semiclassical scheme is how to locate points from which a tunneling path emanates. Hence the studies of such tunneling points and the quasisemiclassical method should be examined collectively. We test several ways of determining the tunneling point, including those already proposed in the literature and a newly proposed one. It is shown numerically that the quasisemiclassical method with an appropriate choice of tunneling points reproduces the full quantum mechanical tunneling probability reasonably well. This case study indicates that the present conventional approach is promising to the study of large systems. The role of tunneling points in the initial process of tunneling is also discussed.
Electron spin polarization of the excited quartet state of strongly coupled triplet–doublet spin systems
Authors: Kandrashkin Yuri , van der Est Art
Published: February 2004

The electron spin polarization associated with electronic relaxation in molecules with trip-quartet and trip-doublet excited states is calculated. Such molecules typically relax to the lowest trip-quartet state via intersystem crossing from the trip doublet, and it is shown that when spin–orbit coupling provides the main mechanism for this relaxation pathway it leads to spin polarization of the trip quartet. Analytical expressions for this polarization are derived using first- and second-order perturbation theory and are used to calculate powder spectra for typical sets of magnetic parameters. It is shown that both net and multiplet contributions to the polarization occur and that these can be separated in the spectrum as a result of the different orientation dependences of the ±1/2↔±3/2 and +1/2↔−1/2 transitions. The net polarization is found to be localized primarily in the center of the spectrum, while the multiplet contribution dominates in the outer wings. Despite the fact that the multiplet polarization is much stronger than the net polarization for individual orientations of the spin system, the difference in orientation dependence of the transitions leads to comparable amplitudes for the two contributions in the powder spectrum. The influence of this difference on the line shape is investigated in simulations of partially ordered samples. Because the initial nonpolarized state of the spin system is not conserved for the proposed mechanism, the net polarization can survive in the doublet ground state following electronic relaxation of the triplet part of the system.
Rotation–tunneling analysis of the origin band in the tropolone π*←π absorption system
Authors: Bracamonte Alfredo E. , Vaccaro Patrick H.
Published: February 2004

The tunneling-split origin band of the tropolone Ã 1B2–X̃ 1A1 (π*←π) absorption system was interrogated under ambient, bulk-gas conditions by exploiting high-resolution degenerate four-wave mixing techniques. The inherent complexity of this spectral region was alleviated by performing polarization-resolved measurements, with judicious selection of transverse characteristics for the incident and detected electromagnetic fields enabling rovibronic transitions to be discriminated according to their attendant changes in rotational angular momentum, ΔJ. Quantitative simulation of recorded data sets showed the vibrationless level of the electronically excited state to be bifurcated by Δ0Ã=19.846(25) cm−1, representing a factor of 20 increase in proton-transfer efficiency over the corresponding level of the ground electronic state. Spectroscopic parameters extracted for the 0+ and 0− manifolds of Ã 1B2 tropolone yield unexpectedly large values of the inertial defect, ΔI0+Ã=−0.802(86) amu Å2 and ΔI0−Ã=−0.882(89) amu Å2, strongly suggesting that a loss of molecular planarity accompanies the π*←π electron promotion. These results, as well as complementary information deduced for interloping hot-band resonances, are discussed in terms of the unique structural and dynamical properties exhibited by tropolone and related proton-transfer species.
Elucidating the role of many-body forces in liquid water. I. Simulations of water clusters on the VRT(ASP-W) potential surfaces
Authors: Goldman Nir , Saykally R. J.
Published: February 2004

We test two new potentials for water, fit to vibration-rotation tunneling (VRT) data by employing diffusion quantum Monte Carlo simulations to calculate the vibrational ground-state properties of water clusters. These potentials, VRT(ASP-W)II and VRT(ASP-W)III, are fits of the highly detailed ASP-W (anisotropic site potential with Woermer dispersion) ab initio potential to (D2O)2 microwave and far-infrared data, and along with the SAPT5s (five-site symmetry adapted perturbation theory) potentials, are the most accurate water dimer potential surfaces in the literature. The results from VRT(ASP-W)II and III are compared to those from the original ASP-W potential, the SAPT5s family of potentials, and several bulk water potentials. Only VRT(ASP-W)III and the spectroscopically “tuned” SAPT5st (with N-body induction included) accurately reproduce the vibrational ground-state structures of water clusters up to the hexamer. Finally, the importance of many-body induction and three-body dispersion are examined, and it is shown that the latter can have significant effects on water cluster properties despite its small magnitude.
Femtosecond absorption study of photodissociation of diphenylcyclopropenone in solution: Reaction dynamics and coherent nuclear motion
Authors: Takeuchi Satoshi , Tahara Tahei
Published: February 2004

Reaction dynamics and coherent nuclear motions in the photodissociation of diphenylcyclopropenone (DPCP) were studied in solution by time-resolved absorption spectroscopy. Subpicosecond transient absorption spectra were measured in the visible region with excitation at the second absorption band of DPCP. The obtained spectra showed a new short-lived band around 480 nm immediately after photoexcitation, which is assignable to the initially populated S2 state of DPCP before the dissociation. The dissociation takes place from this excited state (the precursor of the reaction) with a time constant of 0.2 ps, and the excited state of diphenylacetylene (DPA) is generated as the reaction product. The transient absorption after the dissociation decayed with a time constant of 8 ps that is very close to the S2-state lifetime of DPA, but the spectrum of this 8-ps component was different from the S2 absorption observed with direct photoexcitation of DPA. We conclude that the dissociation of DPCP generates the S2 state of DPA that probably has a cis-bent structure. At later delay times (>30 ps), the transient absorption signals are very similar to those obtained by direct photoexcitation of DPA. This confirmed that the electronic relaxation from the S2 state of the product DPA occurs in a similar manner to that of DPA itself, i.e., the internal conversion to the S1 state and subsequent intersystem crossing to the T1 state. In order to examine the coherent nuclear dynamics in this dissociation reaction, we carried out time-resolved absorption measurements for the 480-nm band with 70 fs resolution. It was found that an underdamped oscillatory modulation with a 0.1-ps period is superposed on the decay of the precursor absorption. This indicates that DPCP exhibits a coherent nuclear motion having a ∼330-cm−1 frequency in the dissociative excited state. Based on a comparison with the measured and calculated Raman spectra of ground-state DPCP, we discuss the assignment of the “330-cm−1 vibration” and attribute it to a vibration involving the displacement of the CO group as well as the deformation of the Ph–C=C–Ph skeleton. We consider that this motion is closely related to the reaction coordinate of the photodissociation of DPCP.
On the calculation of single-particle time correlation functions from Bose–Einstein centroid dynamics
Authors: Moffatt Paul , Blinov Nicholas , Roy Pierre-Nicholas
Published: February 2004

The calculation of single-particle time correlation functions using the Bose–Einstein centroid dynamics formalism is discussed. A new definition of the quasidensity operator is used to calculate the centroid force on a given particle for an anharmonic system. The force includes correlation effects due to quantum statistics and is used for the calculation of the classical-like dynamics of phase-space centroid variables within the centroid molecular dynamics approximation. Time correlation functions are then obtained for single-particle quantities. These correspond to the double-Kubo transform of exact quantum-mechanical correlation functions. The centroid dynamics results are compared to those of exact basis-set calculations and a good agreement is found. The level of accuracy is in fact the same as what was observed earlier for the calculation of center-of-mass correlation functions for Fermi–Dirac and Bose–Einstein statistics, and for any correlation function for Boltzmann statistics. These results show that it is now possible to use Bose–Einstein centroid molecular dynamics to calculate single-particle correlation functions for systems where quantum exchange effects are present.
The impact of the multipolar distribution on chiral discrimination in racemates
Authors: Paci I. , Cann N. M.
Published: February 2004

This article explores the impact of the multipolar distribution on chiral discrimination in a series of racemic fluids. Discrimination is measured via the difference between the like–like (LL) and the like–unlike (LU) radial distributions in the liquid. We have found previously [I. Paci and N. M. Cann, J. Chem. Phys. 115, 8489 (2001)] that the magnitude and orientation of the molecular dipole have a decisive impact on the short-ranged enantiomeric imbalance in racemates. Although quadrupolar and octupolar interactions decrease more rapidly with intermolecular separation, they can be significant at small separations, where enantiomeric imbalances occur. We have carefully selected a number of models in which we isolate the effects of the molecular quadrupole and octupole. We find that discrimination can be greatly enhanced by changes in the quadrupole moments. However, for octupole moments, changes in discrimination are small and some octupoles inhibit discrimination. We identify the quadrupole moment closest to the plane perpendicular to the direction of the molecular dipole as the moment that has the greatest favorable effect on chiral discrimination in racemates. In racemates where this moment is large, we have found differences of up to 40% between the LL and the LU radial distributions.
Analytic solutions for Baxter’s model of sticky hard sphere fluids within closures different from the Percus–Yevick approximation
Authors: Gazzillo Domenico , Giacometti Achille
Published: February 2004

We discuss structural and thermodynamical properties of Baxter’s adhesive hard sphere model within a class of closures which includes the Percus–Yevick (PY) one. The common feature of all these closures is to have a direct correlation function vanishing beyond a certain range, each closure being identified by a different approximation within the original square-well region. This allows a common analytical solution of the Ornstein–Zernike integral equation, with the cavity function playing a privileged role. A careful analytical treatment of the equation of state is reported. Numerical comparison with Monte Carlo simulations shows that the PY approximation lies between simpler closures, which may yield less accurate predictions but are easily extensible to multicomponent fluids, and more sophisticate closures which give more precise predictions but can hardly be extended to mixtures. In regimes typical for colloidal and protein solutions, however, it is found that the perturbative closures, even when limited to first order, produce satisfactory results.
Diffusive dynamics of water in tert-butyl alcohol/water mixtures
Authors: Calandrini Vania , Deriu Antonio , Onori Giuseppe , Lechner Ruep E. , Pieper Jörg
Published: February 2004

Quasielastic neutron scattering has been used to investigate the dynamical behavior of H2O in water/tert-butyl alcohol solutions. The measurements were made at fixed temperature (293 K) as a function of tert-butyl alcohol molar fraction, x, in the range 0–0.042. The data have been compared to those of pure water in the temperature range 269–293 K. The effect of tert-butyl alcohol addition on water dynamics is equivalent to that obtained by lowering the temperature of pure water by an amount proportional to the alcohol concentration. The temperature dependence of the diffusivity parameters in pure water and their concentration dependence in tert-butyl alcohol/water solutions can be rescaled to a common curve attributing to each solution a concentration-dependent “structural temperature” lower than the actual thermodynamic one. These results can be understood in terms of Stillinger’s picture of water structuring and of other more recent theoretical pictures that emphasize the influence of the geometrical properties of hydrogen bond networks on water mobility.
Ultrafast collective dynamics in the charge-density-wave conductor K0.3MoO3
Authors: Ren Yuhang , Xu Zhu’an , Lüpke Gunter
Published: February 2004

Low-energy coherent charge-density wave excitations are investigated in blue bronze (K0.3MoO3) and red bronze (K0.33MoO3) by femtosecond pump–probe spectroscopy. A linear gapless, acousticlike dispersion relation is observed for the transverse phasons with a pronounced anisotropy in K0.33MoO3. The amplitude mode exhibits a weak (opticlike) dispersion relation with a frequency of 1.67 THz at 30 K. Our results show for the first time that the time-resolved optical technique provides momentum resolution of collective excitations in strongly correlated electron systems.
Two-electron integrations in the quantum theory of atoms in molecules
Authors: Pendás A. Martı́n , Blanco M. A. , Francisco E.
Published: February 2004

A method to compute two-electron integrals over arbitrary regions of space is introduced and particularized to the basins appearing in the quantum theory of atoms in molecules. The procedure generalizes the conventional multipolar approach to account for overlapping densities. We show that the approach is always convergent and computationally efficient, scaling as N4 in the worst, two-center case. Several numerical results supporting our claims are also presented.
An ab initio study of structural properties and single vacancy defects in Wurtzite AlN
Authors: Hung Andrew , Russo Salvy P. , McCulloch Dougal G. , Prawer Steven
Published: February 2004

The cell parameters, bulk moduli and electronic densities-of-states (DOS) of pure and vacancy defect AlN were computed using generalized-gradient approximation (GGA) and hybrid functional (B3LYP) computational methods within both plane wave-pseudopotential and localized Gaussian basis set approaches. All of the methods studied yielded cell parameters and bulk moduli in reasonable agreement with experiment. The B3LYP functional was also found to predict an optical band gap in excellent agreement with experiment. These methods were subsequently applied to the calculation of the geometry, defect state positions and formation energies of the cation (VAl) and anion (VN) single vacancy defects. For the VAl defect, the plane wave-pseudopotential predicted a significant retraction of the neighboring N away from the vacancy, while for the VN defect, only slight relaxations of the surrounding Al atoms towards the vacancy were predicted. For the computed DOS of both vacancy defects, the GGA methods yielded similar features and defect level positions relative to the valence band maximum, while the B3LYP method predicted higher separations between the defect levels and the valence and conduction bands, leading to higher energy occupied defect levels.
Iterative solutions with energy selected bases for highly excited vibrations of tetra-atomic molecules
Authors: Lee Hee-Seung , Light John C.
Published: February 2004

The use of energy selected bases (ESB) with iterative diagonalization of the Hamiltonian matrix is described for vibrations of tetra-atomic systems. The performance of the method is tested by computing vibrational states of HOOH below 10 000 cm−1 (1296 A+ symmetry states) and H2CO below 13 500 cm−1 (729 A1 symmetry states). For iterative solutions, we tested both the implicitly restarted Lanczos method (IRLM) and the standard (nonreorthogonalizing) Lanczos approach. Comparison with other contracted basis approach as well as direct product grid representation shows superior performance of the ESB/IRLM approach. Of the two systems, H2CO is found to be more challenging than HOOH since it has much stronger couplings among vibrational modes, which leads to a drastically larger primitive basis set. For H2CO we also discuss some interesting behavior of the molecule in the high internal energy regime.
Effects of higher order Jahn-Teller coupling on the nuclear dynamics
Authors: Viel Alexandra , Eisfeld Wolfgang
Published: February 2004

In this paper effects of higher order Jahn-Teller coupling terms on the nonadiabatic dynamics are studied. Of particular interest is the case when the potential energy surfaces of the degenerate state show pronounced anharmonicity. In order to demonstrate the effects a two-dimensional E⊗e Jahn-Teller model system is treated which is based on the e′ stretching vibration of the photoactive E′2 state of NO3 as a realistic example. The sixth order E⊗e Jahn-Teller Hamiltonian is derived in the diabatic representation which is valid for any system with a C3 rotation axis. This diabatization scheme is compared to lower-order Jahn-Teller Hamiltonians and to symmetry adapted as well as ad hoc approximations. Lower-order potentials result in pronounced quantitative and qualitative differences in the dynamics, including differences in the evolution of mean values, the autocorrelation functions (and thus the corresponding spectra), and the electronic population evolution. In the particular example treated, the results of fourth and fifth order potentials are very similar to the sixth order reference system. In contrast, the approximate sixth order Hamiltonians, though the corresponding adiabatic surfaces seem to be nearly identical, results in pronounced differences. The possible consequences for the dynamics of realistic systems with higher dimensionality are briefly discussed.
Relaxation processes of the liquid crystal ME6N in the isotropic phase studied by Raman scattering experiments
Authors: Giorgini Maria Grazia , Arcioni Alberto , Polizzi Ciro , Musso Maurizio , Ottaviani Paolo
Published: February 2004

We have investigated the Raman profiles of the ν(C≡N) and ν(C=O) vibrational modes of the nematic liquid crystal ME6N (4-cyanophenyl-4′-hexylbenzoate) in the isotropic phase at different temperatures and used them as probes of the dynamics and structural organization of this liquid. The vibrational time correlation functions of the ν(C≡N) mode, rather adequately interpreted within the assumption of exponential modulation function (the Kubo–Rothschild theory), indicate that the system experiences an intermediate dynamical regime that gets only slightly faster with increasing temperature. However, this theory fails in predicting the non-exponential behavior that the time correlation functions manifest in the long time range (t>3 ps). For this reason we have additionally approached the interpretation of vibrational correlation functions in terms of the theory formulated by Rothschild and co-workers for locally structured liquids. The application of this theory reveals that the molecular dynamics in this liquid crystal in the isotropic phase is that deriving from a distribution of differently sized clusters, which narrows as the temperature increases. Even at the highest temperature reached in this study (87 °C above the nematic–isotropic transition), the liquid has not yet achieved the structure of the simple liquid and the dynamics has not reached the limit of the single channel process. The vibrational and orientational relaxations occur in very different time scales. The temperature independence of the orientational dynamics in the whole range from 55 °C to 135 °C has been referred to the nonhydrodynamic behavior of the system, arising when local pseudonematic structures persist for times longer than the orientational relaxation. The occurrence of the process of resonant vibrational energy transfer between the C=O groups of adjacent molecules has been revealed in the isotropic phase by a slightly positive Raman noncoincidence effect in the band associated with the ν(C=O) mode. A qualitative interpretation is tentatively given in terms of partial cancellation of contributions deriving from structures having opposite orientations of their C=O groups.
Dynamics of supercooled and glassy dipropyleneglycol dibenzoate as functions of temperature and aging: Interpretation within the coupling model framework
Authors: Prevosto D. , Capaccioli S. , Lucchesi M. , Rolla P. A. , Ngai K. L.
Published: February 2004

Dielectric relaxation measurements of a typical small molecular glassformer, dipropyleneglycol dibenzoate show the presence of two secondary relaxations. Their dynamic properties differ in the equilibrium liquid and glassy states, as well as the changes during structural recovery after rapid quenching the liquid to form a glass. These differences enable us to identify the slower secondary relaxation as the genuine Johari–Goldstein (JG) β-relaxation [G. P. Johari and M. Goldstein, J. Chem. Phys. 53, 2372 (1970)], acting as the precursor of the primary α-relaxation. Agreement between the JG β-relaxation time and the independent relaxation time of the coupling model leads to predicted quantitative relations between the JG β-relaxation and the α-relaxation that are supported by the experimental data.
Optical properties of Cd1−xZnxTe films in a device structure using variable angle spectroscopic ellipsometry
Authors: Paulson P. D. , McCandless B. E. , Birkmire R. W.
Published: February 2004

The optical properties of polycrystalline Cd1−xZnxTe alloy thin films in device structures are reported for energy from 0.8 to 4.6 eV. Cd1−xZnxTe alloy thin films with x from 0 to 1 were deposited on glass/indium-tin-oxide (ITO)/CdS substrates by coevaporation from CdTe and ZnTe sources and were characterized by variable angle spectroscopic ellipsometry, energy dispersive x-ray spectroscopy, and x-ray diffractometry. The Cd1−xZnxTe films are single phase with a zincblende crystal structure over the entire alloy range. The Cd1−xZnxTe optical constants were determined using a multilayer optical model incorporating the optical constants of glass, ITO and CdS, determined independently from glass, glass/ITO, and glass/ITO/CdS specimens. The optical constants of the Cd1−xZnxTe thin films are comparable to literature values reported for single crystals, indicating that the polycrystalline nature of the films does not measurably alter the optical constants. A semiconductor alloy model for determining the composition of CdxZn1−xTe alloy films is developed using the optical data obtained from the analysis. This alloy model can be used to evaluate compositional grading and the effects of Cd1−xZnxTe film processing.
Thermal stability of InGaN multiple-quantum-well light-emitting diodes on an AlN/sapphire template
Authors: Zhang Baijun , Egawa Takashi , Ishikawa Hiroyasu , Liu Yang , Jimbo Takashi
Published: February 2004

InGaN multiple-quantum-well light-emitting diodes (LEDs) were grown on an AlN/sapphire template by metalorganic chemical vapor deposition. The crystalline quality was investigated by x-ray diffraction and electron-beam-induced current. The thermal stability of the LED was demonstrated by measurements of current–voltage, light output power–current, and electroluminescence (EL) spectra at different temperatures. The output power at 200 mA decreased by 7.3% for the LED on the template upon increasing temperature from 25 to 95 °C, while that for the LED on sapphire decreased by 23.9%. The peak external quantum efficiency decreased from 0.23% to 0.22% and from 0.15% to 0.10% for the LEDs on the template and on sapphire, respectively. The EL spectrum peak at 20 mA shifted to lower energy by 17.2 meV for the LED on the template upon increasing temperature, while that for the LED on sapphire shifted by 32.7 meV. The LED on the template exhibited a higher output power and a better thermal stability with respect to the conventional LED on sapphire using a low-temperature GaN buffer layer, which is due to the low threading dislocation density in the active layer and the high thermal conductivity of AlN layer.
Quasi-Langmuir–Blodgett thin film deposition of carbon nanotubes
Authors: Armitage N. P. , Gabriel J.-C. P. , Grüner G.
Published: February 2004

The handling and manipulation of carbon nanotubes continues to be a challenge to those interested in the application potential of these promising materials. To this end, we have developed a method to deposit pure nonoriented nanotube films over large flat areas on substrates of arbitrary composition. The method bears some resemblance to the Langmuir–Blodgett deposition method used to lay down thin organic layers. We show that this redeposition technique causes no major changes in the films’ microstructure and that they retain the electronic properties of as-deposited films laid down on an alumina membrane.
Model of work function of tungsten cathodes with barium oxide coating
Authors: Mishra K. C. , Garner R. , Schmidt P. C.
Published: February 2004

Using a full-potential band structure approach, we have investigated the work function of barium oxide coated tungsten cathodes in low pressure discharge lamps. The main objective of this work is to understand why the work function for such cathodes is lower than that of the uncoated tungsten. The model studied in this work is based on a well known supposition that the source of thermionic electrons is the barium atoms released from the barium oxide coating due to a chemical reaction with the underlying metallic tungsten. For the unrelaxed seven-layer model of (100) surface of barium on barium oxide, the work function is calculated to be 2.22 eV, which is lower than that of BaO, Ba, and W metals separately. For a fully relaxed nine-layer surface, it becomes 1.36 eV. Although this value of the work function is lower than those estimated for the fluorescent cathodes by electrical measurements, which averages contributions from surfaces in all possible random orientations, this model provides a satisfactory explanation of the lowering of the work function of tungsten based cathodes in low pressure fluorescent lamps.
Spectroscopic, topological, and electronic characterization of ultrathin a-CdTe:O tunnel barriers
Authors: Dolog Ivan , Mallik Robert R. , Malz Dan , Mozynski Anthony
Published: February 2004

Ultrathin oxygenated amorphous CdTe (a-CdTe:O) films are prepared by rf sputtering of CdTe in a background of argon or argon/nitrogen/oxygen mixtures. Atomic force microscopy (AFM) is used to characterize the films and shows that they have an island structure typical of most sputtered thin films. However, when sufficiently low powers and deposition rates are employed during sputtering, the resulting films are remarkably smooth and sufficiently thin for use as barrier layers in inelastic electron tunneling (IET) junctions. Four terminal current–voltage data are recorded for Al/a-CdTe:O/Pb tunnel junctions and conductance–voltage curves are derived numerically. WKB fits to the conductance–voltage curves are obtained using a two-component trapezoidal plus square (TRAPSQR) model barrier potential to determine values for the tunnel barrier parameters (height, shape, and width); these parameters are consistent with AFM topological measurements and values from similar devices reported in the literature. IET spectra are presented which confirm that electrons tunnel through ultrathin regions of the a-CdTe:O films, which contain aluminum oxide subregions in a manner consistent with the TRAPSQR barrier model. Because tunneling occurs predominantly through these ultrathin regions, IET spectroscopic data obtained are representative of states at, or within a few tenths of nanometers from, the surface and confirm that the a-CdTe:O surface stoichiometry is very sensitive to changes in the argon/oxygen/nitrogen concentration ratios during film growth. Full IET spectra, current–voltage, and conductance–voltage data are presented together with tunnel barrier parameters derived from (WKB) fits to the data. The results presented here indicate that inelastic electron tunneling spectroscopy is a useful tool for characterizing the surface states of a-CdTe:O and possibly other photovoltaic materials.
Electron field emission from room temperature grown carbon nanofibers
Authors: Smith R. C. , Carey J. D. , Poa C. H. P. , Cox D. C. , Silva S. R. P.
Published: February 2004

The observation of field induced electron emission from room temperature grown carbon nanofibers at low (5 V/μm) macroscopic electric fields is reported. The nanofibers were deposited using methane as a source gas in a conventional rf plasma enhanced chemical vapor deposition reactor using a Ni metal catalyst previously subjected to an Ar plasma treatment. Analysis of the scanning electron microscopy images of the nanofibers show them to possess an average diameter of 300 nm and that the nanofibers are observed to be radially dispersed over an area of 50 μm in diameter. No evidence of hysteresis in the current-voltage characteristic or conditioning of the emitters is observed. The mechanism for emission at low fields is attributed to field enhancement at the tips rather than from the surrounding amorphous carbon film which is shown to have a higher threshold field (20 V/μm) for emission.
Electrical properties along the X and Z axes of LiNbO3 wafers
Authors: Niitsu Gilberto T. , Nagata Hirotoshi , Rodrigues Ana C. M.
Published: February 2004

Electrical conductivity and dielectric relaxation frequency were measured along the X and Z crystallographic axes of a LiNbO3 single-crystal wafer, using the ac impedance spectroscopy technique. The measurements were carried out in the frequency range of 5 Hz–13 MHz and in the temperature range of 460–620 °C. Impedance data were plotted on Nyquist diagrams, which, in addition to the electrical resistance and relaxation frequency of samples, provide information about the charge carriers of the conduction process. Isothermal measurements were also taken as a function of time. Although no difference in the activation energy of conduction was found, a slight but unequivocal difference in electrical conductivity (about 0.16 on a log scale) was established between the two axes. In contrast, a more marked and substantial difference was found between the relaxation frequency of the axes, confirming that differences in the electrical behavior of the X and Z directions of LiNbO3 are more likely due to differences in dielectric constant characteristics than to those which affect the electrical conductivity.
Tunable band gap properties of planar metallic fractals
Authors: Hou Bo , Xu Gu , Wen Weijia
Published: February 2004

We report the effect of finer structures, or more levels, in a particular type of planar metallic fractal pattern on the stop bands or band gaps in the transmission spectrum of electromagnetic wave. It is found that the band gaps shift to low frequencies with the increase of levels, showing tunable band gap properties. In addition, we find from both experiments and theoretical calculations that the bands appearing in long wavelength range are determined by individual patterns which compose a 6×6 fractal array, while those bands located in the range of short wavelength are strongly influenced by their neighboring residents. Our experimental observations are in good agreement with those of finite difference time domain simulations.
Cathodoluminescence of defects in sintered tin oxide
Authors: Maestre D. , Cremades A. , Piqueras J.
Published: February 2004

Cathodoluminescence (CL) in the scanning electron microscope (SEM) has been used to investigate the luminescence mechanisms in tin oxide. Sintered material prepared from high purity powder has been found to show a strong dependence of the CL emission on the thermal treatments applied during sample preparation. SEM images show the presence of nano and microcrystalline grains. The correlation of the grain size and morphology with the optical emission is analyzed by CL microscopy and spectroscopy. The main emission bands appear centered at about 2.58, 2.25, and 1.94 eV depending on the sintering treatment. CL images reveal that the 2.25 and the 2.58 eV bands are associated at specific crystal faces. The evolution of the luminescence bands with mechanical milling shows a complex evolution of the 1.94 and 2.58 eV emissions which is explained by formation and recovery of defects during milling.
Infrared 45° reflectometry of Li doped ZnO films
Authors: Kafadaryan E. A. , Petrosyan S. I. , Hayrapetyan A. G. , Hovsepyan R. K. , Manukyan A. L. , Vardanyan E. S. , Goulanian E. Kh. , Zerrouk A. F.
Published: February 2004

Lithium doped (0–10 at. % Li) ZnO films were grown in the wurtzite structure on sapphire (001) substrates and investigated in the 200–1200 cm−1 frequency range at 300 K by far-infrared reflectivity spectroscopy using polarized oblique (45°) incidence. This technique has enabled us to determine the longitudinal optical phonon frequency E1(LO) at 576 cm−1 of the fundamental lattice vibration at the center of the Brillouin zone, as well as to investigate the LO phonon-plasmon coupling in the low carrier density (N⩽1018 cm−3) ZnO films. The energy shift and halfwidth broadening of the LO phonon band in comparison with the uncoupled mode in high-ohmic ZnO:0.8 at. % Li (ρdc=0.6×106 Ω cm) film have been analyzed to get the concentration and “optical” mobility of charge carriers in the Li doped ZnO films. The results of optical, x-ray diffraction and dc resistivity measurements are discussed.
Comparative study of soft x-ray emission characteristics in a low energy dense plasma focus device
Authors: Bhuyan H. , Mohanty S. R. , Neog N. K. , Bujarbarua S. , Rout R. K.
Published: February 2004

An investigation on the soft x rays emitted in a 2.2 kJ Mather-type dense plasma focus device using a multichannel diode spectrometer and a simple pinhole camera is reported. Emitted x rays associated with different shapes (hollow, solid, and hemispherical) of anode and in hydrogen/nitrogen gas medium are compared. The structure of x-ray emitting sites as well as x-ray yields were found to be strongly influenced by the shape of the anode and the filling gas pressure. The maximum yield of 2.2 J into 4π sr was obtained in the case of hemispherical anode in hydrogen gas medium. The x-ray pinhole images of the collapsed plasma with the hemispherical anode indicated spot-like structure having 500–800 μm in diameter. On the contrary, other anode shapes showed columnar pinched structure of 8–10 mm in length and 1–2 mm in diameter. Results indicated that an appropriate design of the anode could enhance the x-ray yield by more than tenfold in a conventional low energy dense plasma focus device.
Hard sphere perturbation theory for fluids with soft-repulsive-core potentials
Authors: Ben-Amotz Dor , Stell George
Published: February 2004

The thermodynamic properties of fluids with very soft repulsive-core potentials, resembling those of some liquid metals, are predicted with unprecedented accuracy using a new first-order thermodynamic perturbation theory. This theory is an extension of Mansoori–Canfield/Rasaiah–Stell (MCRS) perturbation theory, obtained by including a configuration integral correction recently identified by Mon, who evaluated it by computer simulation. In this work we derive an analytic expression for Mon’s correction in terms of the radial distribution function of the soft-core fluid, g0(r), approximated using Lado’s self-consistent extension of Weeks–Chandler–Andersen (WCA) theory. Comparisons with WCA and MCRS predictions show that our new extended-MCRS theory outperforms other first-order theories when applied to fluids with very soft inverse-power potentials (n⩽6), and predicts free energies that are within 0.3kT of simulation results up to the fluid freezing point.
Interfacing Brownian dynamics simulations
Authors: Geyer T. , Gorba C. , Helms V.
Published: February 2004

Starting from the flux of particles in a Brownian dynamics simulation we derive boundary conditions, which allow us (i) to couple a Brownian dynamics calculation to a reservoir of particles of a given density, i.e., setting up constant density boundary conditions, and (ii) to build an interface between Brownian dynamics and a diffusional treatment of adjacent simulation volumes. With these algorithms it is sometimes possible to dramatically reduce the system size—and therefore the necessary resources—of multiparticle Brownian dynamics calculations. In this paper we give one-dimensional examples which illustrate potential applications and savings.
Oxidation of alloys containing aluminum and diffusion in Al2O3
Authors: Doremus Robert H.
Published: February 2004

The rate of oxidation of iron and nickel alloys containing aluminum is much faster than the rate calculated from measured diffusion coefficients of oxygen and aluminum in alumina. Furthermore the activation energy for oxidation is less than half those for the diffusion coefficients. It is shown that the Wagner theory for oxidation rates involving diffusion of ions and electrons or holes does not apply to alumina. To understand these discrepancies a mechanism of oxidation of these alloys containing aluminum is proposed that involves diffusion of AlO molecules through the alumina. The actual diffusion coefficients of the AlO molecules are larger than the measured or effective ones because they follow a diffusion-exchange mechanism. Reasonable values of the oxidation rates and their temperature dependence can be calculated from these mechanisms.
Doping effect on the dielectric property in bismuth titanate
Authors: Yao Y. Y. , Song C. H. , Bao P. , Su D. , Lu X. M. , Zhu J. S. , Wang Y. N.
Published: February 2004

The dielectric property complemented by the mechanical measurement (internal friction) in the doped Bi4Ti3O12 [Bi4−xLaxTi3O12 (x=0.5,0.75,1) and Bi4−y/3Ti3−yNbyO12 (y=0.015,0.03,0.06)] was systematically investigated from room temperature to 350 °C. In the plot of dielectric loss versus temperature for Bi4Ti3O12 (BiT), a relaxation peak was confirmed to be associated with the motion of the oxygen vacancy. It is found that the La doping at site A is in favor of improvement of the fatigue property, in contrast, the Nb doping at site B can mainly enhance the remanent polarization. Appropriate La doping at site A of perovskite-type unit in BiT enhances the chemical stability of oxygen vacancy by improving the height of the potential barrier for hopping and enhances the mobility of domain by the changing of domain structures. While the Nb doping at site B could induce the distortion of oxygen octahedral and reduce the oxygen vacancy concentration by a compensating effect so that it results in an enhancement of remanent polarization.
The Schrödinger–Poisson self-consistency in layered quantum semiconductor structures
Authors: Ram-Mohan L. R. , Yoo K. H. , Moussa J.
Published: February 2004

We develop a self-consistent solution of the Schrödinger and Poisson equations in semiconductor heterostructures with arbitrary doping profiles and layer geometries. An algorithm for this non-linear problem is presented in a multiband k⋅P framework for the electronic band structure using the finite element method. The discretized functional integrals associated with the Schrödinger and Poisson equations are used in a variational approach. The finite element formulation allows us to evaluate functional derivatives needed to linearize Poisson’s equation in a natural manner. Illustrative examples are presented using a number of heterostructures including single quantum wells, an asymmetric double quantum well, p-i-n-i superlattices, and trilayer superlattices.
Dynamic annealing in III-nitrides under ion bombardment
Authors: Kucheyev S. O. , Williams J. S. , Zou J. , Jagadish C.
Published: February 2004

We study the evolution of structural defects in AlxGa1−xN films (with x=0.0–0.6) bombarded with kilo-electron-volt heavy ions at 77 and 300 K. We use a combination of Rutherford backscattering/channeling spectrometry and cross-sectional transmission electron microscopy. Results show that an increase in Al content not only strongly enhances dynamic annealing processes but can also change the main features of the amorphization behavior. In particular, the damage buildup behavior at 300 K is essentially similar for all the AlGaN films studied. Ion-beam-produced disorder at 300 K accumulates preferentially in the crystal bulk region up to a certain saturation level (∼50%–60% relative disorder). Bombardment at 300 K above a critical fluence results in a rapid increase in damage from the saturation level up to complete disordering, with a buried amorphous layer nucleating in the crystal bulk. However, at 77 K, the saturation effect of lattice disorder in the bulk occurs only for x≳0.1. Based on the analysis of these results for AlGaN and previously reported data for InGaN, we discuss physical mechanisms of the susceptibility of group-III nitrides to ion-beam-induced disordering and to the crystalline-to-amorphous phase transition.
Epitaxial PbTiO3 thin films grown on (100) MgO by pulsed-laser deposition for optical waveguiding properties
Authors: Roemer A. , Millon E. , Vincent B. , Boudrioua A. , Pons-Y-Moll O. , Defourneau R. M. , Seiler W.
Published: February 2004

Perovskite single phased PbTiO3 thin films have been grown by pulsed-laser deposition on (100) MgO single crystals in order to prepare high quality optical waveguides. The substrate temperature and the oxygen pressure during the deposition have been optimized to improve crystalline quality and to grow epitaxial thin films. The films display two main axis orientations according to the MgO (100) basal plane. The waveguiding properties and the optical losses characterized by using m-lines spectroscopy and charge-coupled device camera measurement, respectively, are correlated to the structural properties of the films. Such textured and epitaxied films display very good optical properties, as the optical losses are found to be lower than 1 dB cm−1.
An active tunneling model for the magnetotransport in mixed-phase polycrystalline manganites
Authors: Ju Sheng , Li Zhen-Ya
Published: February 2004

Based on phase separation between metallic and insulating domains, the evolution of magnetoresistance in mixed-phase polycrystalline manganites, as a function of temperature, has been modeled via a bond-disordered random resistor network. The tunneling bond is introduced in the resistor network to account for the contribution from grain boundaries, where the energy barrier is formed. The model is analytically worked out by means of the effective-medium approximation. Compared with the transport in single-phase ferromagnetic-ordered polycrystal, the transport in mixed-phase polycrystal is governed by not only the properties of grain boundaries but also the competing metallic and insulating phases. The fraction of tunneling bonds strongly depends on the temperature and external magnetic field. An active tunneling model is thus established in mixed-phase polycrystalline manganites. Within our model, the effect of crystallinity on the temperature dependence of magnetoresistance in mixed-phase polycrystalline manganites is well explained.
Infrared-laser induced photon avalanche upconversion in Ho3+–Yb3+ codoped fluoroindate glasses
Authors: Lahoz F. , Martı́n I. R. , Briones A.
Published: February 2004

An intense green upconversion emission due to a photon avalanche mechanism has been observed in Ho3+–Yb3+ codoped fluroindate glasses, measured at 200 K, under excitation at around 750 nm. The influence of the Yb3+ concentration on the photon avalanche process has been studied. The pump power threshold of the avalanche mechanism decreases as the Yb3+ concentration is increased. A reduction by about a factor of 3 is observed from the 2.25 mol % Ho3+ single doped glass to the 2.25 mol % Ho3+–2.25 mol % Yb3+ codoped glass. This effect has been related to an extra feeding of the I47 intermediate level of Ho3+ ions via Ho3+→Yb3+ and Yb3+→Ho3+ energy transfer and back transfer processes, respectively.