bluuebunny/crossref_metadata_2025 huggingface dataset:
Two-dimensional circularly polarized pump–probe spectroscopy
Authors: Cho Minhaeng
Published: October 2003

A novel two-dimensional (2D) circularly polarized (CP) pump–probe (PP) spectroscopy is theoretically studied and proposed. Utilizing circularly polarized pump field, one can measure both the left- and right-CP PP spectra in the 2D frequency space spanned by the pump and probe field frequencies. Although the dominant contribution to the measured signal is all electricdipole-allowed four-wave-mixing term, it can be removed by taking the difference between the left- and right-CP PP signals. Taking the first-order terms with respect to either magnetic dipole- or quadrupole-pump field interaction, we find that two distinctively different contributions to the third-order CP-PP polarization are important. However, by controlling the crossing angle between the pump and probe field propagation directions to be the magic angle, θ=tan−1(1/√), it is shown that the magnetic dipole–field interaction contribution to the third-order polarization can be selectively measured. It is observed that the relationship between the linear polarization PP and CP-PP is similar to that between the linear absorption and circular dichroism. Numerically calculated 2D CP-PP spectra for model systems are presented and compared with the absorption, circular dichroism, and linearly polarized PP spectra.
Thermal conductivities of naked and monolayer protected metal nanoparticle based nanofluids: Manifestation of anomalous enhancement and chemical effects
Authors: Patel Hrishikesh E. , Das Sarit K. , Sundararajan T. , Sreekumaran Nair A. , George Beena , Pradeep T.
Published: October 2003

Thermal conductivities of two kinds of Au nanoparticles were measured in water and toluene media. The water soluble particles, 10–20 nm in mean diameter, made with citrate stabilization showed thermal conductivity enhancement of 5%–21% in the temperature range of 30–60 °C at a loading of 0.000 26 (by volume). The effect was 7%–14% for Au particles stabilized with a monolayer of octadecanethiol even for a loading of 0.011%. Comparatively lower thermal conductivity enhancement was observed for larger diameter Ag particles for significantly higher loading. Effective enhancement of 9%, even at vanishing concentrations, points to additional factors in the thermal conductivity mechanism in nanofluids. Results also point to important chemical factors such as the need for direct contact of the metal surface with the solvent medium to improve enhancement.
Connections between ground-state energies from optimized-effective potential exchange-only and Hartree–Fock methods
Authors: Ivanov Stanislav , Levy Mel
Published: October 2003

By using perturbation theory, an expression is derived for the difference between the Hartree–Fock (HF) and optimized effective potential (OEP) exchange-only ground-state energies. With this expression and a model three-electron system, we provide the first formal proof of a specific example where the HF and OEP exchange-only ground-state energies are different. We also give a constrained-search variational argument that the two energies are generally, nevertheless, expected to be very close, consistent with known numerical results.
Effect of field dependent trap occupancy on organic thin film transistor characteristics
Authors: Rashmi  , Balakrishnan V. R. , Kapoor Ashok K. , Kumar Vikram , Jain S. C. , Mertens R. , Annapoorni S.
Published: October 2003

A model to calculate the current voltage characteristics of organic thin film transistors is presented. The model takes into account the influence of high electric field on trap occupancy. The case of the traps at a single energy level is considered in detail. Relevant equations are solved numerically using an iteration method. The field dependent trap occupancy model is found to change free surface charge Qsf and drain saturation current Id significantly. The model is compared with the experimental data published by two different groups. There is good agreement between experimental data and our model.
Experimental findings in support of atomic transport of hydrogen in silica
Authors: Hörnlund E. , Hultquist G.
Published: October 2003

Data supporting a view of both molecular and atomic transport of hydrogen in quartz is presented. By studying isotopic equilibration, it is shown that virtually all of the molecules diffusing through a 1.5 mm thick quartz membrane at 900 °C have undergone dissociation. The catalytic properties of platinum for the dissociation of H2 are used to increase the surface concentration of atomic hydrogen on a quartz membrane. It is found that the high surface concentration increases the total flux of hydrogen through the membrane. It is also found by comparison with literature data that diatomic molecules probably diffuse in two modes in quartz. We suggest that these two modes are one atomic and one molecular.
Enantiomeric purification of nonpolarized racemic mixtures using coherent light
Authors: Frishman Einat , Shapiro Moshe , Gerbasi David , Brumer Paul
Published: October 2003

Angular momentum constraints for coherently controlling chiral purification of racemic mixtures using the dipole-electric field interaction are examined in detail for two different scenarios. First shown is that achieving enantiomeric control in our earlier scheme [M. Shapiro, E. Frishman, and P. Brumer, Phys. Rev. Lett. 84, 1669 (2000)], using parallel laser pulses, requires that the system be M-polarized, where M is the projection of the total angular momentum along the axis of laser polarization. An alternate scenario is then introduced that allows chiral control in an unpolarized racemic mixture by using three mutually perpendicular linearly polarized light fields. Analytic expressions for the enantiomeric excess in both cases are derived and computational results are presented.
The electronic states of 2-furanmethanol (furfuryl alcohol) studied by photon absorption and electron impact spectroscopies
Authors: Giuliani A. , Walker I. C. , Delwiche J. , Hoffmann S. V. , Limão-Vieira P. , Mason N. J. , Heyne B. , Hoebeke M. , Hubin-Franskin M.-J.
Published: October 2003

The photoelectron spectrum of 2-furanmethanol (furfuryl alcohol) has been measured for ionization energies between 8 and 11.2 eV and the first three ionization bands assigned to π3, π2, and no ionizations in order of increasing binding energy. The photoabsorption spectrum has been recorded in the gas phase using both a synchrotron radiation source (5–9.91 eV, 248–125 nm) and electron energy-loss spectroscopy under electric-dipole conditions (5–10.9 eV, 248–90 nm). The (UV) absorption spectrum has also been recorded in solution (4.2–6.36 eV, 292–195 nm). The electronic excitation spectrum appears to be dominated by transitions between π and π* orbitals in the aromatic ring, leading to the conclusion that the frontier molecular orbitals of furan are affected only slightly on replacement of a H atom by the –CH2OH group. Additional experiments investigating electron impact at near-threshold energies have revealed two low-lying triplet states and at least one electron/molecule shape resonance. Dissociative electron attachment also shows to be widespread in furfuryl alcohol.
Analytical fit of the I–V characteristic for cylindrical and spherical Langmuir probes
Authors: Morales Crespo R. , Fernández Palop J. I. , Hernández M. A. , Ballesteros J.
Published: October 2003

A unified radial model of the collisionless sheath for both cylindrical and spherical Langmuir probes is developed. The I–V characteristics are obtained and analytically fitted, taking into account both the probe radius and the probe biasing potential as parameters of the model. This parameterization allowed us to obtain an analytical approach for the potential profile, the floating potential and the sheath edge according to the Nachmann and Tahn criterion [M. Nachmann and P. C. Tahn, IEEE Trans. Plasma Sci. 19, 423 (1991)]. An alternative criterion for the sheath edge is established. This is referred to as the curvature criterion and it matches the Bohm criterion in the limiting case of planar Langmuir probes. The case of a planar sheath is obtained as a limiting case of the model when the probe radius is large enough.
Effective three-body potentials for Li+(aq) and Mg2+(aq)
Authors: Spångberg Daniel , Hermansson Kersti
Published: October 2003

A method for the extraction of effective three-body potential parameters from high-level ab initio cluster calculations is presented and compared to effective pair potentials extracted at the same level. Dilute Li+(aq) and Mg2+(aq) solutions are used as test cases and long molecular-dynamics simulations using these newly developed potentials were performed. Resulting thermodynamical, structural, and dynamical properties are compared to experiment as well as to the empirical effective pair potentials of Åqvist. Moreover, a new time-saving method for the correction of cluster energies computed with a relatively cheap ab initio method, to yield expensive, high-level ab initio energies, is presented. The effective pair approach is shown to give inconsistent results when compared to the effective three-body potentials. The performance of three different charge compensation methods (uniform charge plasm, Bogusz net charge correction, and counter ions) is compared for a large number of different system sizes. For most properties studied here, the system-size dependence is found to be small for system sizes with 256 water molecules or more. However, for the self-diffusion coefficients, a 1/L dependence is found, i.e., a very large system-size dependence. A very simple method for correcting for this deficiency is proposed. The results for most properties are found to compare reasonably well to experiment when using the effective three-body potentials.
Atomic structure of random and c-axis oriented YMnO3 thin films deposited on Si and Y2O3/Si substrates
Authors: Kim Yong Tae , Kim Ik Soo , Kim Seong Il , Yoo Dong Chul , Lee Jeong Yong
Published: October 2003

We have studied the atomic structure of YMnO3 deposited on Si and Y2O3 with high-resolution transmission electron microscopy and fast Fourier transforms-filtered lattice image analysis during furnace and rapid thermal annealing (RTA) processes. For the YMnO3/Si, it is found that the YMnO3 layer is c-axis oriented with an amorphous bottom region after furnace annealing at 850 °C for 1 h. In contrast, after RTA at 850 °C for 3 min the bottom region forms YMnO3 polycrystalline layer with the {1̄21̄2} plane parallel to the surface. When an Y2O3 layer is interposed between YMnO3 and Si, a c-axis oriented YMnO3 layer grows on a [111]-oriented Y2O3 layer. Memory window and leakage current density of the c-axis YMnO3/[111] Y2O3 bilayers are strongly improved due to an aligned [0001] unipolar axis.
Properties and lanthanum distribution of Bi4−xLaxTi3O12−SrBi4−yLayTi4O15 intergrowth ferroelectrics
Authors: Zhu Jun , Hui Rong , Mao Xiang Yu , Lu Wang Ping , Chen Xiao Bing , Zhang Zeng-Ping
Published: October 2003

Using the standard solid-state reaction method, several lanthanum-doped intergrowth ferroelectric ceramics of type Bi4−xLaxTi3O12−SrBi4−yLayTi4O15 [BLT–SBLT (x+y)] were synthesized with the lanthanum doping content, (x+y), ranging from 0.00 to 1.50. The remnant polarization (2Pr) of the samples, which varies with (x+y), has a maximum when x+y=0.50, that is nearly 60% greater than at zero doping. The Curie temperature Tc decreases monotonously with (x+y). By assumpting that the Curie temperature of BLT-SBLT (x+y) is the average of Tcs for its two constituents BLT and SBLT, La distribution in each constituent is estimated. The result demonstrates that when (x+y) is small (⩽1.25), La concentration in BLT constituent is higher than that in SBLT; but when x+y=1.50, more La3+ ions would prefer substituting the Bi3+ ions in SBTi. This finding agrees well with the behavior of d(118) lattice space as well as the relaxation characteristics observed in BLT–SBLT(1.50).
Hybrid silicon–organic nanoparticle memory device
Authors: Kolliopoulou S. , Dimitrakis P. , Normand P. , Zhang Hao-Li , Cant Nicola , Evans Stephen D. , Paul S. , Pearson C. , Molloy A. , Petty M. C. , Tsoukalas D.
Published: October 2003

We demonstrate a nonvolatile electrically erasable programmable read-only memory device using gold nanoparticles as charge storage elements deposited at room temperature by chemical processing. The nanoparticles are deposited over a thermal silicon dioxide layer that insulates them from the device silicon channel. An organic insulator deposited by the Langmuir–Blodget technique at room temperature separates the aluminum gate electrode from the nanoparticles. The device exhibits significant threshold voltage shifts after application of low-voltage pulses (⩽±6 V) to the gate and has nonvolatile retention time characteristics.
Rapid single-flux quantum logic using π-shifters
Authors: Ustinov A. V. , Kaplunenko V. K.
Published: October 2003

We have found that the size of some rapid single-flux quantum (RSFQ) logic cells based on conventional 0-type Josephson junctions can be significantly reduced by using a π-type junction as a phase shifter in passive (nonswitching) mode. In comparison with the recently suggested active (switching) π-junctions mode, the passive mode offers much greater operation margins for their critical current Icπ. This gives π-junctions a chance to be implemented in RSFQ designs in the near future. As an example, we have simulated the operation of a toggle flip flop with zero-geometrical inductance of the fluxon storage loop. Simulations show that the parametric inductance of the π-junction and its normal resistance Rn form a low-pass filter, which sets the low limit for π-junctions IcπRn product, but offers a wide range of variations of the other parameters. The possible reduction of RSFQ cell size by using π-junctions opens the way to scale superconducting logic circuits down to the submicron dimensions.
Spatially heterogeneous dynamics investigated via a time-dependent four-point density correlation function
Authors: Lačević N. , Starr F. W. , Schrøder T. B. , Glotzer S. C.
Published: October 2003

Relaxation in supercooled liquids above their glass transition and below the onset temperature of “slow” dynamics involves the correlated motion of neighboring particles. This correlated motion results in the appearance of spatially heterogeneous dynamics or “dynamical heterogeneity.” Traditional two-point time-dependent density correlation functions, while providing information about the transient “caging” of particles on cooling, are unable to provide sufficiently detailed information about correlated motion and dynamical heterogeneity. Here, we study a four-point, time-dependent density correlation function g4(r,t) and corresponding “structure factor” S4(q,t) which measure the spatial correlations between the local liquid density at two points in space, each at two different times, and so are sensitive to dynamical heterogeneity. We study g4(r,t) and S4(q,t) via molecular dynamics simulations of a binary Lennard-Jones mixture approaching the mode coupling temperature from above. We find that the correlations between particles measured by g4(r,t) and S4(q,t) become increasingly pronounced on cooling. The corresponding dynamical correlation length ξ4(t) extracted from the small-q behavior of S4(q,t) provides an estimate of the range of correlated particle motion. We find that ξ4(t) has a maximum as a function of time t, and that the value of the maximum of ξ4(t) increases steadily from less than one particle diameter to a value exceeding nine particle diameters in the temperature range approaching the mode coupling temperature from above. At the maximum, ξ4(t) and the α relaxation time τα are related by a power law. We also examine the individual contributions to g4(r,t), S4(q,t), and ξ4(t), as well as the corresponding order parameter Q(t) and generalized susceptibility χ4(t), arising from the self and distinct contributions to Q(t). These contributions elucidate key differences between domains of localized and delocalized particles.
Identification of the mechanism-limiting nitrogen diffusion in metallic alloys by in situ photoemission electron spectroscopy
Authors: Figueroa C. A. , Ferlauto A. S. , Alvarez F.
Published: October 2003

In situ photoemission electron spectroscopy is used to identify the mechanism limiting the thermally activated nitrogen diffusion in metallic alloys. The samples were prepared by bombarding stainless steel with a broad ion source in a high-vacuum chamber. The photoemission spectra evolution on increasing controlled oxygen partial pressure is correlated with bulk material properties. The presence of oxygen inhibits the formation of iron nitrides and γN-phase (austenitic face-centered-cubic lattice containing nitrogen), which proved to be fundamental for efficient nitrogen penetration in the alloy.
Role of interface states in electronic properties of (ZnSe)n/(Si2)n (001) superlattices
Authors: Laref A. , Aourag H. , Belgoumene B. , Tadjer A.
Published: October 2003

In a semiempirical tight-binding scheme, the detailed electronic structure and optical properties of (ZnSe)n/(Si2)n (001) superlattices (SLs) are studied with a wide range of n=1–20 giving special attention to the role of the interface states at the Zn–Si and Se–Si polar interfaces. The states at the conduction and valence-band edges are confined two dimensionally in the Si layers. Furthermore, we have found two interface bands in the lower and upper regions of the gap. The states of the lower interfaces band are located at the Zn–Si interface, while those of the upper interface band are located at the Se–Si interface. The energies of the interface states depend on the parameters representing the Zn–Si and Se–Si bond lengths and the valence band discontinuity between ZnSe and Si, but the interface states do not disappear from the gap with reasonable choices of the parameters. It is shown that the heterointerface bond relaxation strongly affects interface band in the band gap. In this system, relaxed Si bonds at the heterointerface induce a vacant interface band and a filled interface band in the band gap. By decreasing the SL period n, the energy gap between the confined band-edge states increases (2.07 eV at the Γ point for n=2) due to the quantum confinement effect. A sudden shrinkage in the band gap (Eg=1.76 eV at the M point) is obtained for n=1. The origin of the band-gap shrinkage is related to the fact that the interface states increasingly overlap and combine as band states. Furthermore, the calculated absorption spectra of the superlattices are found to be quite different from those of bulk ZnSe and Si, but fairly close to their average.
Electrical characterization of InGaN/GaN light emitting diodes grown by molecular beam epitaxy
Authors: Hirsch L. , Barrière A. S.
Published: October 2003

We studied the electrical behavior of multiple InGaN/GaN quantum well based light emitting diodes grown by molecular beam epitaxy and we determined three different domains of current-voltage dependence. We then described the charge carrier transport mechanisms for these three domains. The first domain, corresponding to leakage currents (V<1.5 V), takes place in the mesa side wall, in a semi-insulating material formed by air contamination, after the etching process. Below room temperature, electrical transport occurs by hopping between localized states and above room temperature by the Poole–Frenkel mechanism. The second domain, that of injection currents (1.5 V<V<3.5 V), corresponds to a hole injection from the valence band of p-GaN to the first InGaN quantum well. At low temperature (T<250 K) this hole injection is due to tunnel transfer and above 250 K to thermoionic emission. In the third domain, which corresponds to series resistor (V>3.5 V), the current is limited by the p-GaN zone. In this zone, the density of the free holes is controlled by the combined effects of the temperature and the applied forward bias. The results obtained enables the electronic states resulting from the magnesium doping to be localized at 190 meV above the valence band maximum.
Dose dependence of carrier and heat dynamics at an ion-implanted silicon surface measured using lens-free heterodyne transient grating method
Authors: Katayama Kenji , Yamaguchi Masahiro , Sawada Tsuguo
Published: October 2003

The lens-free heterodyne transient grating method was shown to reveal the dynamics of photoexcited carriers and heat on the surface region of an ion-implanted silicon in the dose range of 1011–1015 cm−2. In addition to the fact that the detection limit of the dose was superior to that for conventional methods, several physical properties of the carrier and heat can be obtained by analyzing transient responses. Theoretical analysis provided the lifetime of carriers and thermal diffusion coefficients in the ion-implanted surface region.
Tuning the emission characteristics of top-emitting organic light-emitting devices by means of a dielectric capping layer: An experimental and theoretical study
Authors: Riel H. , Karg S. , Beierlein T. , Rieß W. , Neyts K.
Published: October 2003

The emission characteristics of top-emitting organic light-emitting devices (OLEDs) have been studied experimentally and theoretically to derive a quantitative understanding of the effect of a dielectric capping layer. We demonstrated that the angular intensity distribution and the spectral characteristics can be tuned and the light outcoupling enhanced simply by varying the optical thickness of a dielectric layer deposited on top of a semitransparent metal electrode. With the capping-layer concept, the outcoupled light intensity in forward direction was increased by a factor of 1.7, and concomitantly a high color purity achieved. An optical model based on a classical approach was used to calculate the emission characteristics. The excellent agreement between measured and simulated data shows that the capping layer controls the interplay between different interference effects such as wide-angle and multiple-beam interference occurring in top-emitting OLEDs. The strength of the capping layer concept is in particular that the optical and the electrical device performance can be optimized separately.
Velocity redistribution of excited atoms by radiative excitation transfer. II. Theory of radiation trapping in collimated beams
Authors: Bezuglov N. N. , Ekers A. , Kaufmann O. , Bergmann K. , Fuso F. , Allegrini M.
Published: October 2003

We have developed a theory of resonance radiation imprisonment in collimated atomic beams. Treating the integral master equation describing imprisonment as a generalized wave (Schrödinger) equation and using the geometrical quantization technique for its solution, we obtained analytical representations for the effective radiative lifetime, mean scattering number, and trapping factors. We apply this theory to explain the recent observation of a dramatic velocity redistribution of excited atoms by radiative excitation transfer after the photofragmentation of Na2. In this process, the fast Na(3p) photofragments transfer their excitation energy efficiently via radiation to the abundant Na(3s) atoms from the primary particle beam. The influence of the hyperfine splitting of the ground state of Na atoms on this process is discussed. The ratio of the number of Na(3p) atoms produced by the radiative excitation transfer to the number of Na(3p) photofragments was found to be 0.13 and 0.19 for photodissociation of Na2 molecules in the vibrational levels v″=17 and v″=23, respectively. This is in good agreement with the corresponding experimental values of 0.16 and 0.22.
Generalized coupling to a heat bath: Extension of the Gaussian isokinetic dynamics and effect of time scaling
Authors: Morishita Tetsuya
Published: October 2003

The Hamiltonian formulation of the Gaussian isokinetic (GIK) thermostat [C. P. Dettmann and G. P. Morriss, Phys. Rev. E 54, 2495 (1996)] is extended to generalize the coupling of a physical system to a heat bath in molecular dynamics (MD) simulations. This extension gives a theoretical basis for the statistical ensemble recently proposed to understand the characteristics of the weak coupling (wc) thermostat [T. Morishita, J. Chem. Phys. 113, 2976 (2000)]. It is shown that the scaling of the momentum and the time in the Hamiltonian formulation gives a clear picture for understanding the relation between the generalized coupling thermostat and the GIK or wc thermostat. We also discuss time scaling in a Lagrangian formulation and show its effect on equations of motion and statistical ensembles in thermostatted MD simulations.
Collapse of homopolymer chains with two fixed terminals
Authors: Xue Bin , Wang Jun , Wang Wei
Published: October 2003

We present molecular dynamics studies on the collapse of homopolymer model chains when two terminals are fixed over a selected distance. At low temperature, short chains show only β-hairpinlike conformations in their equilibrium; while the chain is long enough, helixlike conformations occur. A critical chain length serves as a boundary to distinguish the systems with different equilibrium behaviors and to separate three regimes of scaling behaviors between the mean square radius of gyration and the chain length. For short chains with a small end separation, namely, the end-to-end distance is below a specific value, the radius of gyration and asphericity index of the chain are independent of the end-to-end distance. When the end-to-end distance is larger than a specific distance, the radius of gyration and asphericity index grow linearly. For long chains, both the radius of gyration and asphericity index show multistage processes for different end-to-end separations. The Lindemann index and the asphericity index are combined together with the specific heat to describe the transitions of the chain’s conformation following the variation of temperature.
“Rounding” of the sulfur living polymerization transition under spatial confinement
Authors: Kalampounias A. G. , Andrikopoulos K. S. , Yannopoulos S. N.
Published: October 2003

“Rounding” effects and other specific changes of liquid sulfur’s polymerization transition are studied using Raman scattering in the case of fluid’s spatial confinement in nanonoporous sol-gel glasses. Specifically, in this paper we demonstrate the smearing or “rounding” of the monomer↔polymer transition caused by geometrical confinement of the liquid, a phenomenon that is in marked contrast to ordinary (bulk) liquid sulfur, where the polymerization transition is quite sharp, bearing a close resemblance to a second-order-type transition. In noticeable agreement with recent theories of living polymerizations, the limited growth of polymeric species brings about a diminishing of the strength of the transition as evidenced in the temperature dependence of many physical properties. The careful determination of the extent of polymerization by analyzing Raman spectra made it possible to recalculate the magnitude of certain thermodynamic parameters changes across the polymerization transition. As a result, the temperature dependence of the equilibrium constants for the initiation and propagation step of polymerization and the number average chain length of polymeric species have been reconsidered. A comparison between the temperature dependence of these quantities for the case of bulk (unconfined) and the corresponding ones for the case of confined liquid has been advanced in the framework of recent mean-field models of living equilibrium polymerization.
Influence of electrode configuration and liquid crystalline polymer type on electrorheological effect
Authors: Takesue Naoyuki , Furusho Junji , Inoue Akio
Published: October 2003

In the many studies on electrorheological (ER) fluids, which change viscosity upon application of an electric field, they have generally been placed between homogeneous electrodes. In our previous paper we proposed a one-sided (unilateral) electrode configuration, in which the ER fluid is placed between a nonconductive disk and a disk bearing concentric-circular and radial electrode patterns, and described the resulting ER effects of the liquid crystalline polymer ER fluid. Here we describe the ER effects of the same ER fluid between various configurations with homogeneous electrodes, patterned electrodes, and nonconductive disks, and those of a second type of liquid crystalline polymer ER fluid between the patterned electrodes and nonconductive disks.
Compact boundary-condition-determined wave function for positronium hydride (PsH)
Authors: Bressanini Dario , Morosi Gabriele
Published: October 2003

A simple, compact, and accurate wave function for positronium hydride is written as a product of Pade’ approximants for electron–nucleus interactions and of Jastrow functions for electron–electron interactions. Most of the parameters are fixed taking into account both the correct cusp conditions when two particles collide and the correct asymptotic behavior when one or two particles go to infinity. The remaining parameters were optimized by variational Monte Carlo calculations. The energy of this single term wave function is −0.786073(6) hartree and favorably compares with very long configuration interaction expansions and even with explicitly correlated function expansions. The exam of the wave function and of various two-dimensional distribution functions shows that the PsH structure is similar to the hydrogen anion structure, with the positron slightly perturbing it and its motion strongly correlated to the electrons that are squeezed towards each other and towards the nucleus.
Information uncertainty-type inequalities in atomic systems
Authors: Guevara Nicolais L. , Sagar Robin P. , Esquivel Rodolfo O.
Published: October 2003

The one-electron Shannon information entropy sum is reformulated in terms of a single entropic quantity dependent on a one-electron phase space quasiprobability density. This entropy is shown to form an upper bound for the entropy of the one-electron Wigner distribution. Two-electron entropies in position and momentum space, and their sum, are introduced, discussed, calculated, and compared to their one-electron counterparts for neutral atoms. The effect of electron correlation on the two-electron entropies is examined for the helium isoelectronic series. A lower bound for the two-electron entropy sum is developed for systems with an even number of electrons. Calculations illustrate that this bound may also be used for systems with an odd number of electrons. This two-electron entropy sum is then recast in terms of a two-electron phase space quasiprobability density. We show that the original Bialynicki-Birula and Mycielski information inequality for the N-electron wave function may also be formulated in terms of an N-electron phase space density. Upper bounds for the two-electron entropies in terms of the one-electron entropies are reported and verified with numerical calculations.
Integral coalescence conditions in D⩾2, dimension space
Authors: Pan Xiao-Yin , Sahni Viraht
Published: October 2003

We have derived the integral form of the cusp and node coalescence conditions satisfied by the wave function at the coalescence of two charged particles in D⩾2 dimension space. From it we have obtained the differential form of the coalescence conditions. These expressions reduce to the well-known integral and differential coalescence conditions in D=3 space. It follows from the results derived that the approximate Laughlin wave function for the fractional quantum Hall effect satisfies the node coalescence condition. It is further noted that the integral form makes evident that unlike the electron–nucleus coalescence condition, the differential form of the electron–electron coalescence condition cannot be expressed in terms of the electron density at the point of coalescence. From the integral form, the integral and differential coalescence conditions for the pair-correlation function in D⩾2 dimension space are also derived. The known differential form of the pair function cusp condition for the uniform electron gas in dimensions D=2,3 constitutes a special case of the result derived.
Electronic structures of azafullerene C48N12
Authors: Brena Barbara , Luo Yi
Published: October 2003

Two recently proposed low-energy azafullerene C48N12 isomers have been theoretically characterized using x-ray spectroscopies. The x-ray photoelectron spectroscopy, the near-edge absorption fine structure, the x-ray emission spectroscopy, and the ultraviolet photoelectron spectroscopy for both isomers have been predicted at the gradient-corrected density functional theory level. These spectroscopies together give a comprehensive insight of the electronic structure on the core, valence, and unoccupied orbitals. They have also provided a convincing way for identifying the isomer structures.
Lone pair interactions with coinage metal atoms: Weak van der Waals complexes of the coinage metal atoms with water and ammonia
Authors: Antušek Andrej , Urban Miroslav , Sadlej Andrzej J.
Published: October 2003

Interaction energies between the coinage metal atoms (Cu, Ag, and Au) and lone-pair donating molecules (H2O and NH3) are calculated by the spin adapted restricted open-shell Hartree–Fock coupled cluster method with the scalar relativistic effects accounted for by the Douglas–Kroll approximation. All ammonia complexes CuNH3, AgNH3, and AuNH3 are found to be of C3v symmetry with the counterpoise corrected interaction energies equal to −16.68, −6.87, and −14.64 mH for Cu, Ag, and Au, respectively. In the case of the water molecule the complexes are much weaker with the counterpoise corrected interaction energies equal to −3.78, −1.81, and −1.77 mH, for the three metal atoms, respectively. Moreover, all complexes with the water molecule are nonplanar. For both lone-pair donating molecules the structure and energetics of their complexes with the coinage metal atoms is mostly due to electron correlation effects. The relativistic effects are found to increase the bonding energies in the series of the ammonia complexes, whereas they reduce the bonding energy in the AgOH2 complex and are essentially negligible for CuOH2 and AuOH2. The calculated complex geometries and interaction energies are discussed in terms of different models. The pattern of interaction energies is discussed in terms of the balance between long-range induction and dispersion contributions and short-range forces. Also the possibility of some charge transfer from the lone-pair donor to the metal atom is considered and supported by analysis of the ionization potential and electron affinity data. The relativistic reduction of the size of the coinage metal atoms is found to be of importance as well. The calculated structural data are used to interpret the experimental observation concerning the existence of well resolved resonantly enhanced multiphoton ionization (REMPI) spectra of the ammonia–silver complexes and the absence of the corresponding spectra of the water–silver complex. This experimental difference between the ammonia and water complexes is explained in terms of the very flat interaction energy function for the wagging motion of the water molecule in AgOH2. This large amplitude vibration makes the structure of the complex undefined and is responsible for the nonexistence of the vibrationally resolved REMPI spectrum of the complex.
Interelectronic angles of group 14, 15, and 16 atoms in their low-lying multiplet states
Authors: Koga Toshikatsu
Published: October 2003

The average interelectronic angles 〈θ12〉 in position space and 〈θ̄12〉 in momentum space are examined for the multiplet states arising from the ground electronic configurations of the group 14, 15, and 16 atoms. The angles 〈θ12〉 and 〈θ̄12〉 are greater than 90 degrees with no exceptions and two electron vectors prefer inverse directions to the same direction on average. Common to all 15 atoms, it is found that in position space, lower multiplets have larger interelectronic angles 〈θ12〉 than higher multiplets. However, the opposite is true in momentum space and the interelectronic angles 〈θ̄12〉 are smaller in lower multiplets. The differences in the angles Δ〈θ12〉 and Δ〈θ̄12〉 between multiplets have good linear correlations with the relative stability ΔE of the multiplets.
Theoretical study and rate constant calculation of the CH2O+CH3 reaction
Authors: Liu Jing-yao , Li Ze-sheng , Wu Jia-yan , Wei Zhi-gang , Zhang Gang , Sun Chia-chung
Published: October 2003

The potential energy surface of the CH2O+CH3 reaction is explored at the MP2/6-311++G(d,p), MP4SDQ/6-311G(d,p), and QCISD(T)/6-311+G(3df,2p) (single point) levels of theory. Theoretical calculations suggest that the major product channel (R1) is the hydrogen abstraction leading to the product P1 CHO+CH4 (R1), while the addition process leading to P2H+CH3CHO (R2) appears to be negligibly small. The calculated enthalpies and dissociation activation energies for CH3CH2O and CH3OCH2 radicals involved in the reaction are in line with the experimental values. Dual-level dynamics calculation is carried out for the direct hydrogen abstraction channel. The energy profile of (R1) is refined with the interpolated single-point energies (ISPE) method at the QCISD(T)//MP2 level. The rate constants, which are evaluated by canonical variational transition-state theory (CVT) including small-curvature tunneling (SCT) correction, are in good agreement with the available experimental data. It is shown that tunneling effect plays a significant role in the rate constant calculation; and as a result, the CVT/SCT rate constants exhibit typical non-Arrhenius behavior over a wide temperature range 300–2000 K. The three parameter expression is k=6.35×10−26 T4.4 exp(−2450/T) cm3 molecule−1 s−1.
The hydration of the OH radical: Microsolvation modeling and statistical mechanics simulation
Authors: Cabral do Couto P. , Guedes R. C. , Costa Cabral B. J. , Martinho Simões J. A.
Published: October 2003

The hydration of the hydroxyl OH radical has been investigated by microsolvation modeling and statistical mechanics Monte Carlo simulations. The microsolvation approach was based on density functional theory (DFT) calculations for OH–(H2O)1–6 and (H2O)1–7 clusters. The results from microsolvation indicate that the binding enthalpies of the OH radical and water molecule to small water clusters are similar. Monte Carlo simulations predict that the hydration enthalpy of the OH radical, ΔhydH(OH,g), is −39.1 kJ mol−1. From this value we have estimated that the band gap of liquid water is 6.88 eV, which is in excellent agreement with the result of Coe et al. [J. Chem. Phys. 107, 6023 (1997)]. We have compared the structure of the hydrated OH solution with the structure of pure liquid water. The structural differences between the two systems reflect the strong role played by the OH radical as a proton donor in water. From sequential Monte Carlo/DFT calculations the dipole moment of the OH radical in liquid water is 2.2±0.1 D, which is ∼33% above the experimental gas phase value (1.66 D).
Nanocrystal molecules and chains
Authors: Dı́az J. G. , Planelles J. , Jaskólski W. , Aizpurua J. , Bryant G. W.
Published: October 2003

The electron energy structure of linear artificial molecules and one-dimensional chains formed of spherical semiconductor nanocrystals is investigated with and without an applied magnetic field. Both uniform and multilayer nanocrystals are studied. The calculations are performed within the effective mass model by numerically integrating the effective mass equation on a two-dimensional cylindrical grid. Some calculations are, for comparison, performed also in the tight-binding approach. Density contours are presented to illustrate the transformation of states in systems of strongly interacting coupled quantum dots. Strong interaction between the quantum-dot–quantum-well structures in a chain of nanocrystals can lead to the formation of a very narrow ground-state miniband, well separated from the excited levels with the energies almost independent of the magnetic field.
Ewald summation of electrostatic multipole interactions up to the quadrupolar level
Authors: Aguado Andrés , Madden Paul A.
Published: October 2003

Ewald summation expressions are derived for the energy, interatomic forces, pressure tensor, electric fields, and electric field gradients in a periodic computer simulation system of molecules with charges, induced dipoles, and quadrupoles. The full expressions including quadrupolar multipole interactions are given; those involving dipoles are included for completeness. The expressions apply not only to molecules with permanent multipoles, but also to systems of polarizable ions where the multipoles are induced by the interionic interactions. The expressions which allow for this generalization consider the ions to be spherically polarizable, but include the consequences of the coupling between dipolar and quadrupolar induction. The phonon dispersion curves of MgO and the structural lattice constants of Al2O3 in its corundum phase are discussed as specific examples. In simulations with a dipole- and quadrupole-polarizable interaction model, a full Ewald summation of quadrupolar interactions is shown to have a significant effect.
Second-harmonic generation and Maxwell displacement current spectroscopy of chiral organic monolayers at the air–water interface
Authors: Fujimaki Hiroshi , Manaka Takaaki , Ohtake Hiroshi , Tojima Atsushi , Iwamoto Mitsumasa
Published: October 2003

The molecular orientation and chiral properties of the S-citronelloxy-cyanobiphenyl and 4′-hexyloxy-4-cyanobiphenyl monolayer at the air–water interface were investigated with optical second-harmonic generation (SHG) and Maxwell displacement current (MDC) measurements during monolayer compression. MDC was used to detect the phase transition from planar alignment phase to polar orientational phase in monolayers, and SHG was used to detect the orientation and the chirality of monolayers. The vectorial formulas for the SH process of the monolayer composed of chiral molecules were expressed using tensor components of the nonlinear optical (NLO) susceptibility, where the relationship between the NLO susceptibility and the molecular hyperpolarizabilities was also derived. According to the experiment, a polarized angle dependence measurement could reveal the chirality of this monolayer. The tensor component of the second-order NLO susceptibility, which is related to the chirality of the monolayer, was confirmed to be quite small by a fitting of the polarized angle dependence results, and it was also reasonably supported from the nonlinear molecular hyperpolarizabilities obtained from the quantum chemical calculation.
Recombination kinetics in subdiffusive media
Authors: Seki Kazuhiko , Wojcik Mariusz , Tachiya M.
Published: October 2003

We study the kinetics of the recombination reaction in subdiffusive media, where the displacement of reactants r(t) follows 〈r2(t)〉∝tα with 0<α<1. We derive a rigorous fractional reaction–diffusion equation from a continuous time random walk model and calculate the kinetics of recombination reaction on the basis of this equation. The survival probability of a particle starting at r0 has an asymptotic time dependence of t−α/2 for both the perfectly absorbing and the partially reflecting boundary conditions. The change in the boundary condition alters only the coefficient for the asymptotic time dependence. The asymptotic time dependence of the survival probability is confirmed by the numerical simulations and supported by the results of a lattice model.
Defects in CeO2/SrTiO3 fabricated by automatic feeding epitaxy probed using positron annihilation
Authors: Uedono A. , Shimoyama K. , Kiyohara M. , Yamabe K.
Published: October 2003

Vacancy-type defects in the CeO2/SrTiO3 structure were studied by means of positron annihilation. Thin CeO2 films were grown on SrTiO3 substrates by molecular-beam epitaxy without using an oxidant; oxygen was supplied by diffusion from the substrate (this process is referred to as automatic feeding epitaxy). A preferential epitaxial growth of CeO2(001) on SrTiO3(001) was observed, but an increase in the film thickness or annealing in an O2 atmosphere caused a partial growth of CeO2(110). The introduction of this phase was found to correlate with the reduction in the concentration of vacancy-type defects. The species of defects introduced into the SrTiO3 substrate was found to be not only oxygen vacancies but also Sr vacancies or their complexes. We discuss the mechanism that introduced cation vacancies in terms of oxygen nonstoichiometry. We found the trapping rate of positrons by Sr vacancies to depend on the sample temperature. This was associated with the temperature dependence of the dielectric constant near the defects or the presence of shallow trapping centers.
Full band modeling of the excess current in a delta-doped silicon tunnel diode
Authors: Rivas Cristian , Lake Roger , Frensley William R. , Klimeck Gerhard , Thompson Phillip E. , Hobart Karl D. , Rommel Sean L. , Berger Paul R.
Published: October 2003

The current of a molecular beam epitaxially grown Sb and B delta-doped Si tunnel diode is simulated in all regions of tunneling: peak, valley, and post-valley turn-on. All three regions of the I–V are qualitatively captured by the calculations. The inclusion in the model of bandtail states gives rise to the excess current and the post-valley turn on of the tunnel current. This excess current is dominated by the direct coherent tunneling component of the current tunneling from gap state to gap state. The crossover between phonon-assisted and direct occurs immediately after the valley minimum. The calculated voltages quantitatively match the experimental measurements. The magnitude of the calculated current is approximately a factor of 5.4 too small. Sources of error are analyzed. The current calculations use a second neighbor sp3s* planar orbital basis within the nonequilibrium Green function formalism.
Spin reorientation induced by Ni atoms in Fe/Cu(001)
Authors: Zhao Y. , Ye J. , Gao C. L. , Ma B. , Jin Q. Y.
Published: October 2003

We determined the spin reorientation transition (SRT) in face-centered-tetragonal (fct) Fe/Cu(001) from out of plane to in plane, as induced by Ni-capping atoms, by observing the magneto-optical Kerr effect. The SRT takes place when the Ni-capping layer thickness exceeds a critical value dcNi, at which point the perpendicular anisotropy of the fct-Fe film becomes equal to the in-plane anisotropy of the Ni-capping layer. We found that dcNi decreases linearly with increasing Fe film thickness. By extrapolating to zero Ni thickness, the “true” critical thickness for the fct-Fe films is estimated to be 4.3 monolayers. This result is very close to the value obtained using Co-capping layers as reported in literature.
Density functional study on structure and stability of bimetallic AuNZn (N⩽6) clusters and their cations
Authors: Tanaka Hiromasa , Neukermans Sven , Janssens Ewald , Silverans Roger E. , Lievens Peter
Published: October 2003

A systematic study on the structure and stability of zinc doped gold clusters has been performed by density functional theory calculations. All the lowest-energy isomers found have a planar structure and resemble pure gold clusters in shape. Stable isomers tend to equally delocalize valence s electrons of the constituent atoms over the entire structure and maximize the number of Au–Zn bonds in the structure. This is because the Au–Zn bond is stronger than the Au–Au bond and gives an extra σ-bonding interaction by the overlap between vacant Zn 4p and valence Au 6s(5d) orbitals. No three-dimensional isomers were found for Au5Zn+ and Au4Zn clusters containing six delocalized valence electrons. This result reflects that these clusters have a magic number of delocalized electrons for two-dimensional systems. Calculated vertical ionization energies and dissociation energies as a function of the cluster size show odd–even behavior, in agreement with recent mass spectrometric observations [Tanaka et al., J. Am. Chem. Soc. 125, 2862 (2003)].