bluuebunny/crossref_metadata_2025 huggingface dataset:
Hopping transport in doped organic semiconductors: A theoretical approach and its application to p-doped zinc-phthalocyanine
Authors: Schmechel Roland
Published: April 2003

A detailed approach to the complex hopping transport in organic semiconductors is presented and used to describe experimental data from Maennig et al. [Phys. Rev. B 64, 195208 (2001)] on the effect of doping on conductivity, mobility and thermopower. In this approach, the energetic distribution of the charge carriers in a Gaussian shaped density of states (DOS) is calculated under thermal equilibrium conditions and compared to the energetic distribution of the current. The description is based on the Miller–Abraham model for hopping in a disordered material and utilizes the so-called transport energy concept. To include also the case of higher electron concentrations in the tail states of the DOS the Fermi distribution was taken into account. Furthermore, additional trap states in the gap are considered to describe the experimental data at low doping concentration more correctly. In the framework of the model there is no indication of a thermally activated ionization of the dopants. In contrast to other descriptions, the position of the Fermi energy and transport energy are calculated from the model. It is demonstrated that the principal behavior of the transport parameter can be well explained in terms of classical semiconductor physics.
Frequency-tunable high-power terahertz wave generation from GaP
Authors: Tanabe T. , Suto K. , Nishizawa J. , Kimura T. , Saito K.
Published: April 2003

A frequency-tunable terahertz wave was generated from GaP crystals using an optical parametric oscillator as the pump source and a YAG laser (1.064 μm) as the signal source. By tuning the very small angle, θin, between the pump and signal light beam directions, tunable terahertz waves over the frequency range from 0.5 to 3 THz were obtained. The THz frequency changed almost linearly with the angle θin. The precise noncollinear phase matching condition is discussed. The pulsed peak power of the THz wave was as high as 480 mW at 1.3 THz.
Effect of temperature on the optical properties of GaAsSbN/GaAs single quantum wells grown by molecular-beam epitaxy
Authors: Lourenço S. A. , Dias I. F. L. , Poças L. C. , Duarte J. L. , de Oliveira J. B. B. , Harmand J. C.
Published: April 2003

GaAsSbN/GaAs strained-layer single quantum wells grown on a GaAs substrate by molecular-beam epitaxy with different N concentrations were studied using the photoluminescence (PL) technique in the temperature range from 9 to 296 K. A strong redshift in optical transition energies induced by a small increase in N concentration has been observed in the PL spectra. This effect can be explained by the interaction between a narrow resonant band formed by the N-localized states and the conduction band of the host semiconductor. Excitonic transitions in the quantum wells show a successive red/blue/redshift with increasing temperature in the 2–100 K range. The activation energies of nonradiative channels responsible for a strong thermal quenching are deduced from an Arrhenius plot of the integrated PL intensity.
Rho-axis-system Hamiltonian for molecules with one large amplitude internal motion
Authors: Szalay Viktor , Császár Attila G. , Santos Julio , Ortigoso Juan
Published: April 2003

The rho-axis system and the rho-axis-system Hamiltonian were devised and formulated to describe the rotational–internal–rotational motion of molecules possessing certain local geometrical symmetries. Nevertheless, they were employed to molecules without the required symmetries and apparently lying outside the range of their applicability. To justify such applications and to facilitate first-principles calculation of the corresponding spectroscopic parameters, the rho-axis-system and the rho-axis system Hamiltonian must be derived without assuming or imposing symmetry constraints. A derivation satisfying this requirement is described, and the rho-axis method is extended to molecules having a large amplitude internal motion of any kind.
Matching-pursuit for simulations of quantum processes
Authors: Wu Yinghua , Batista Victor S.
Published: April 2003

The matching-pursuit algorithm is implemented to develop an extension of the split-operator Fourier transform method to a nonorthogonal, nonuniform and dynamically adaptive coherent-state representation. The accuracy and efficiency of the computational approach are demonstrated in simulations of deep tunneling and long time dynamics by comparing our simulation results with the corresponding benchmark calculations.
Photoionization threshold shapes of metal clusters
Authors: Wong Kin , Kresin Vitaly V.
Published: April 2003

We demonstrate the efficacy of the Fowler equation, derived for the near-threshold photoelectron yield of bulk metal surfaces, for fitting the ionization spectra of finite metal clusters. The known photoionization curves of KN=30–101 are matched to a Fowler plot. The fitted ionization potentials agree well with the previously determined values and extrapolate even more accurately to the bulk work function; the fitted temperature closely matches the prediction of evaporation ensemble theory.
Effect of heat and film thickness on a photoinduced phase transition in azobenzene liquid crystalline polyesters
Authors: Sánchez C. , Alcalá R. , Hvilsted S. , Ramanujam P. S.
Published: April 2003

The liquid crystal to isotropic phase transition induced with 488 nm light in films of liquid crystalline azobenzene polyesters has been studied as a function of temperature, light intensity, and film thickness. That phase transition is associated with the photoinduced trans–cis–trans isomerizations of azobenzene molecules and it has been found that the 488 nm light power needed to induce the transition to the isotropic state increases when the film thickness decreases. The irradiation with the laser beam heats the film up and this seems to be responsible for the observed thickness dependence. Optical absorption measurements show that azobenzene aggregates present in one of the polymers are broken down in the photoinduced phase transition. The birefringence induced with low power 488 nm light in films before and after undergoing that photoinduced phase transition has also been studied. Birefringence shows a faster growing rate in films which have undergone the transition.
Electron-beam irradiation of porous silicon: Application to micromachining
Authors: Borini Stefano , Amato Giampiero , Rocchia Massimiliano , Boarino Luca , Rossi Andrea Mario
Published: April 2003

Results on electron irradiation of porous silicon are presented and discussed. An electron-beam lithography system is used to irradiate small surface portions of porous silicon, without the use of any sensitive resists. In this way, it has been possible to write pattern with lateral resolution down to 120 nm. It is suggested that direct exposure to electron irradiation provokes the formation of defects, with an enhanced reactivity of exposed areas. This effect is suggested as a possible way to micro- and nanostructuring of porous silicon.
Formation of metallic zinc nanowires
Authors: Yan Yanfa , Liu Ping , Romero M. J. , Al-Jassim M. M.
Published: April 2003

Metallic zinc nanowires are synthesized by evaporating ZnO powder mixed with graphite in an NH3 flowing carrying gas environment. The majority of the as-synthesized zinc nanowires are single crystalline and free from structural defects, whereas a small portion of the nanowires consists of single-crystalline Zn nanorod segments. The zinc nanowires usually have serpentine geometries, with lengths up to a few micrometers and diameters 20 to 200 nanometers. The growth direction of the nanowires is usually ±[001]. Most zinc nanowires are covered with an oxide layer.
Determination of texture by infrared spectroscopy in titanium oxide–anatase thin films
Authors: Pecharromán Carlos , Gracı́a Francisco , Holgado Juan P. , Ocaña Manuel , González-Elipe Agustı́n R. , Bassas J. , Santiso J. , Figueras A.
Published: April 2003

A theoretical model to determine the effective dielectric tensor of heterogeneous materials composed by anisotropic microcrystallites has been introduced to explain the infrared spectral features of textured thin films of uniaxial materials as the function of a textural parameter. This theoretical treatment is able to satisfactorily reproduce the experimental absorbance spectra of TiO2–anatase thin films chosen as a model system. Comparison of texture data obtained from infrared spectroscopy and x-ray diffraction are in good agreement which support the validity of the proposed model.
Interdiffusion and exchange bias in the MnxPt1−x/Co system
Authors: Morales Erie H. , Wang Y. , Lederman D. , Kellock A. J. , Carey M. J.
Published: April 2003

Mn x Pt 1−x (25  nm )/ Co (8.0  nm ) antiferromagnetic/ferromagnetic (AF/F) bilayers were grown via dc magnetron sputtering at room temperature on Si(111) substrates. Samples were annealed in a vacuum in a 1200 Oe magnetic field at various temperatures TA after growth. The Mn concentration x was determined from the MnPt lattice parameters and Rutherford backscattering spectroscopy. The maximum exchange bias (HE) was observed for x∼0.50, in agreement with previous work. Annealing caused the MnPt to form an ordered face-centered-tetragonal CuAu ordered structure which is necessary to observe HE. However, x-ray reflectivity indicates that an interdiffusion region exists at the MnPt/Co interface. The interdiffusion increases with TA, resulting in lower HE. The TA that maximizes HE, 318 °C, is a compromise between the volume fraction of MnPt forming the ordered CuAu structure, which increases HE, and the amount of interdiffusion, which decreases HE.
Ferroelectric field effect in epitaxial LaVO3/(Ba,Sr)/TiO3/(Pb,La)(Zr,Ti)O3/(La,Sr)CoO3 heterostructures
Authors: Choi Woong , Sands Tim
Published: April 2003

A ferroelectric field effect was demonstrated in epitaxial LaVO3/(Ba,Sr)TiO3/(Pb,La)(Zr,Ti)O3/(La,Sr)CoO3 semiconductor–ferroelectric–metal heterostructures grown on (001) SrTiO3 single crystal substrates by pulsed laser deposition. A high degree of c-axis orientation and strong in-plane texture measured by four-circle x-ray diffractometer confirmed the epitaxial crystallographic relationships between the layers. A 30 nm interlayer of (Ba,Sr)TiO3 was required to prevent decomposition of the (Pb,La)(Zr,Ti)O3 during growth of LaVO3 at 500 °C in a vacuum. The capacitance–voltage measurement of the heterostructure exhibited a decrease in capacitance of ∼20% at positive voltages, which was explained by the formation of the depletion region in the semiconducting LaVO3. The observed ferroelectric field effect revealed the modulation of the channel resistance confirming the formation of a depletion region estimated to be ∼7 nm in depth. On- to off-state channel resistance ratios up to 2.5 were measured.
Vector analysis of terahertz transients generated by photoconductive antennas in near- and far-field regimes
Authors: Hughes S. , Tani Masahiko , Sakai Kiyomi
Published: April 2003

We apply a three-dimensional finite-difference time-domain method to simulate the self-consistent creation of near- and far-field THz-frequency vector fields from a femtosecond-pulse-excited photoconductive antenna. The simulation results reveal some key features of the near-field patterns on the photoconductive antenna. The THz wave forms and their Fourier transformed spectra agree well with experimental characteristics. Additionally, we find some significant deviations from a simple Hertzian dipole approximation.
Polymer translocation through a long nanopore
Authors: Slonkina Elena , Kolomeisky Anatoly B.
Published: April 2003

Polymer translocation through a nanopore in a membrane is investigated theoretically. Recent experiments on voltage-driven DNA and RNA translocations through a nanopore indicate that the size and geometry of the pore are important factors in polymer dynamics. A theoretical approach is presented which explicitly takes into account the effect of the nanopore length and diameter for polymer motion across the membrane. It is shown that the length of the pore is crucial for polymer translocation dynamics. The present model predicts that for realistic conditions (long nanopores and large external fields) there are two regimes of translocation depending on polymer size: for polymer chains larger than the pore length, the velocity of translocation is nearly constant, while for polymer chains smaller than the pore length the velocity increases with decreasing polymer size. These results agree with experimental data.
Entropy and thermalization of particles in liquids
Authors: Osorio-González D. , Mayorga M. , Orozco J. , Romero-Salazar L.
Published: April 2003

We describe the entropy of liquids in the context of kinetic theory of dense gases. In the equilibrium regime the statistical entropy has an explicit dependence of the pair correlation function. In order to test the entropy functional, we use the Morse potential to reproduce experimental pair correlation functions of liquid sodium, using the molecular dynamics technique. With this information, we can compare the theoretical entropy with experimental thermodynamic data. On the other hand, from the nonequilibrium point of view, we discuss the entropy-increase-law analyzing the entropy balance equation. The entropy production due to the molecular diffusion processes displays an upper bound which is proportional to the so-called Fisher information. In the regime at which the one-particle distribution function only depends on particles momentum and time, we show that the factor of proportionality which appears in the upper bound is essentially the time integral of the force autocorrelation function between particles. Besides the parameters of the interparticle potential found through molecular dynamics simulations, we find the time scale of particles’ thermalization and therefore the approach to equilibrium for the system.
Mean-field theory of ice phase stability
Authors: Woo Hyung-June , Monson P. A.
Published: April 2003

A simple mean-field theory for the free energy of ice phases applicable to any of the widely used intermolecular potential models of water is proposed. The energetic effect of the hydrogen bond network is calculated via a set of self-consistent equations for the mean multipole moments of each molecule, which serve as the order parameters. An application to the TIP4P model of water yields results in good agreement with simulations, and produces a phase diagram in qualitative agreement with experiment.
Stereographic projections path integral in S1 and (S2)m manifolds
Authors: Russo M. F. , Curotto E.
Published: April 2003

The importance sampling technique for stochastic integration is extended to evaluate imaginary time path integral expressions in two kinds of spacelike curved manifolds that arise frequently in the physics of constrained molecular motion. Using stereographic projection maps, we develop convenient quantum distributions. We explore the issue of energy estimation based on the extension of the virial theorem in curved manifolds and we provide simple numerical criteria to determine if the virial of a system in a curved space approaches the kinetic energy as a stochastic estimator. Simple numerical tests are carried out using both the discretized and the Fourier path integral approaches. The particle in a ring subjected to two different potentials is insightful and is sufficiently simple to simulate by other well established methods.
Glass and polycrystal states in a lattice spin model
Authors: Cavagna Andrea , Giardina Irene , Grigera Tomás S.
Published: April 2003

We numerically study a nondisordered lattice spin system with a first order liquid–crystal transition, as a model for supercooled liquids and glasses. Below the melting temperature the system can be kept in the metastable liquid phase, and it displays a dynamic phenomenology analogous to fragile supercooled liquids, with stretched exponential relaxation, power law increase of the relaxation time, and high fragility index. At an effective spinodal temperature Tsp the relaxation time exceeds the crystal nucleation time, and the supercooled liquid loses stability. Below Tsp liquid properties cannot be extrapolated, in line with Kauzmann’s scenario of a lower metastability limit of supercooled liquids as a solution of Kauzmann’s paradox. The off-equilibrium dynamics below Tsp corresponds to fast nucleation of small, but stable, crystal droplets, followed by extremely slow growth, due to the presence of pinning energy barriers. In the early time region, which is longer the lower the temperature, this crystal-growth phase is indistinguishable from an off-equilibrium glass, both from a structural and a dynamical point of view: crystal growth has not advanced enough to be structurally detectable, and a violation of the fluctuation–dissipation theorem (FDT) typical of structural glasses is observed. On the other hand, for longer times crystallization reaches a threshold beyond which crystal domains are easily identified, and FDT violation becomes compatible with ordinary domain growth.
Picosecond conformational relaxation of singlet excited polyfluorene in solution
Authors: Dias Fernando B. , Maçanita António L. , Seixas de Melo J. , Burrows Hugh D. , Güntner Roland , Scherf Ulli , Monkman Andrew P.
Published: April 2003

Poly[9,9-di(ethylhexyl)fluorene] was studied by steady-state and time-resolved fluorescence techniques in solution in cyclohexane, methylcyclohexane, tetrahydrofuran, and decalin over the temperature range from 343 to 77 K. A decrease in temperature leads to a decrease in the inhomogeneous broadening of the emission band. Fluorescence decays were biexponential, consistent with a two-state model involving two different polymer conformers. Global analysis of the time profiles of luminescence collected at different emission wavelengths shows a long decay-time of 371.5±1.5 ps, which is temperature and solvent independent. The second shorter time (29±3 ps at 313 K and 100±3 ps at 233 K in methylcyclohexane) appears as a decay-time at the onset of the emission spectrum and as a risetime at longer wavelengths. Whilst the slow process was independent of temperature, the fast process showed Arrhenius type behavior, with an activation energy value of 0.10 eV found in both methylcyclohexane and decalin solutions. However, the risetime in the more viscous decalin was longer than that in methylcyclohexane. The observed behavior is interpreted in terms of fast conformational relaxation of the initially excited polymer, leading to a more planar conjugation segment.
Ordering of apolar and polar solutes in nematic solvents
Authors: Dingemans T. , Photinos D. J. , Samulski E. T. , Terzis A. F. , Wutz C.
Published: April 2003

The quadrupolar splittings of deuteriated para- and ortho-dichlorobenzene (1,4-DCB and 1,2-DCB, respectively) are measured by nuclear magnetic resonance (NMR) in the nematic solvents hexyl- and pentyloxy-substituted diphenyl diacetylene (DPDA-C6 and DPDA-OC5, respectively). Measurements are taken for all four combinations of the nominally apolar (1,4-DCB) and polar (1,2-DCB) solutes in the apolar (DPDA-C6) and polar (DPDA-OC5) solvents, and throughout the entire nematic temperature range of the solutions. The temperature dependence of the second-rank orientational order parameters of the solutes are obtained from these measurements and the respective order parameters of the mesogenic cores of solvent molecules are obtained independently from carbon-13 NMR measurements. The order parameter profiles of the two solutes are found to be very different but show little variation from one solvent to the other. The results are analyzed and interpreted in terms of the underlying molecular interactions using atomistic solvent–solute potentials. The influence of electrostatic interactions on solute ordering is directly evaluated by computing the order parameters with and without the electrostatic component of the atomistic potential. It is observed to be small. It is also found that the important interactions in these solvent–solute systems are operative over short intermolecular distances for which the representation of the partial charge distributions in terms of overall molecular dipole and quadrupole moments is not valid.
Selective cross-polarization in solution state nuclear magnetic resonance of scalar coupled spin 12 and quadrupolar nuclei
Authors: Eykyn Thomas R. , Philp David J. , Kuchel Philip W.
Published: April 2003

Semiselective heteronuclear cross-polarization for achieving coherence transfer between a spin I=12 scalar coupled to a spin S⩾12 in isotropic solution is considered. The expansion of the density operator as products of irreducible tensor operators provides a compact formalism for describing cross-polarization involving scalar coupled quadrupolar nuclei. An analytical description of cross-polarization is presented in the limit of strong radio-frequency (rf) fields, with respect to the scalar-coupling constant. Numerical simulations show that reducing the rf field amplitudes does not have a detrimental effect on the efficiency of the transfer provided they are comparable to or greater than the scalar-coupling constant. The use of weak rf fields largely circumvents the reduced efficacy due to Hartmann–Hahn mismatch. Applications of the method for selective observation of scalar-coupled quadrupolar nuclei are considered and experimental results are presented for a mixture of beryllium fluoride complexes.
Accurate quantum mechanical calculation for the N+OH reaction
Authors: Chen Mao-Du , Tang Bi-Yu , Han Ke-Li , Lou Nan-Quan , Zhang John Z. H.
Published: April 2003

Accurate three-dimensional time-dependent quantum wave packet calculations for the N+OH reaction on the A″3 potential energy surface [Guadagnini, Schatz, and Walch, J. Chem. Phys. 102, 774 (1995)] have been carried out. The calculations show for the first time that the initial state-selected reaction probabilities are dominated by resonance structures, and the lifetime of the resonance is generally in the subpicosecond time scale. The calculated reaction cross sections indicate that they are a decreasing function of the translational energy, which is in agreement qualitatively with the quasiclassical trajectory calculations. The rate constants obtained from the quantum mechanical calculations are consistent with the quasiclassical trajectory results and the experimental measurements.
Analytic energy derivatives for regular approximations of relativistic effects applicable to methods with and without correlation corrections
Authors: Filatov Michael , Cremer Dieter
Published: April 2003

Analytic expressions are derived for the evaluation of derivatives of the total molecular energy with respect to external parameters (nuclear coordinates, external electric fields, etc.) within the relativistic regular approximation. The presented formalism employs the spectral resolution of the identity avoiding, however, the explicit use of an auxiliary basis set in the calculation of the matrix elements of the regular relativistic Hamiltonian. The final formulas for the total energy and energy derivatives are presented in matrix form suitable for implementation into standard quantum chemical packages. Results of benchmark calculations for gold containing diatomic molecules and for xenone hexafluoride performed at the Hartree–Fock and various correlation corrected levels of theory are presented and discussed.
NMR shieldings from sum-over-states density-functional-perturbation theory: Further testing of the “Loc.3” approximation
Authors: Fadda Elisa , Casida Mark E. , Salahub Dennis R.
Published: April 2003

The development and implementation of sum-over-states density-functional-perturbation theory (SOS-DFPT) [V.G. Malkin, O.L. Malkina, M.E. Casida, and D.R. Salahub, J. Am. Chem. Soc. 116, 5898 (1994)] has allowed a significant improvement in the accuracy of nuclear magnetic resonance (NMR) chemical shift values over the Hartree–Fock approximation. Furthermore, due to its computational efficiency, SOS-DFPT has opened the way to the study of systems of increased size compared to those that may be approached by more sophisticated but also computationally more intensive methods, such as Møller–Plesset perturbation theory or coupled-cluster theory. The success of SOS-DFPT relies on the introduction of an ad hoc correction to the excitation energy that improves the calculation of the paramagnetic component of the NMR shielding tensor. The lack of a clear physical basis for this approximation has left the SOS-DFPT open to some criticism. We have shown in a previous article [E. Fadda, M.E. Casida, and D.R. Salahub, Int. J. Quantum Chem. 91, 67 (2003)] that the electric field and magnetic field responses are given by equivalent expressions within the Tamm–Dancoff approximation of time-dependent density-functional theory (TD-DFT). This provides an SOS-DFPT expression which, upon restriction to diagonal contributions, yields a new rigorous “Loc.3” approximation. In this article, we more than double our original test set of 10 molecules for C13, N15, and O17 chemical shifts to a set of 25 molecules. In addition, we compare the results of “Loc.3” SOS-DFPT with the results of promising recent functionals for DFT calculations of chemical shifts. The results show not only that the “Loc.3” approximation represents the rigorous physical connection between SOS-DFPT and TD-DFT, but also that it has very good potential for the prediction of NMR shielding constants, opening the way to further developments in DFT-based NMR parameter calculations.
A theoretical model of the static polarizability of carbon buckyonions
Authors: Iglesias-Groth S. , Ruiz A. , Bretón J. , Gomez Llorente J. M.
Published: April 2003

A theoretical approach to determine the static dipole polarizabilities of carbon buckyonions is presented. The microscopic electronic structure of the system is provided by an effective one-electron model and the screening effects are treated within the random phase approximation (RPA). The particular spherical geometry of these multishell fullerenes makes possible the use of electrostatic arguments to derive a simple expression for the RPA which gives the polarizability of the buckyonion and the dipole moment induced on each shell in terms of either the screened or unscreened polarizabilities of the isolated shells. A systematic analysis as a function of the buckyonion size is performed. The relevance of an adequate microscopic description of the electronic structure is demonstrated by contrasting the results provided by two different representations of the electron motion, namely a surface electron gas and a more realistic Hückel model. A comparison between our results and those derived from classical dielectric models is also performed.
Electronic structure of the trimethylenemethane diradical in its ground and electronically excited states: Bonding, equilibrium geometries, and vibrational frequencies
Authors: Slipchenko Lyudmila V. , Krylov Anna I.
Published: April 2003

Accurate equilibrium properties of the ground and the three lowest excited states of the trimethylenemethane (TMM) diradical are calculated by using the novel spin–flip electronic structure method. Changes in structures and vibrational frequencies upon excitation are analyzed. The bonding picture in different states of TMM is derived from wave function analysis and comparison of equilibrium structures with typical values of coupled-cluster (CC) bond lengths, e.g., a double CC bond in ethylene, a single CC bond in twisted ethylene, and a bond in the allyl radical.
First principles prediction of isotopic shifts in H2O
Authors: Schwenke David W.
Published: April 2003

We compute isotope-independent first- and second-order corrections to the Born–Oppenheimer approximation for water and use them to predict isotopic shifts. For the diagonal correction, we use internally contracted multireference configuration interaction wave functions and derivatives with respect to mass-dependent internal coordinates to generate the mass-independent correction functions. For the nonadiabatic correction, we use a self-consistent field wave function for the ground electronic state and single excitation configuration interaction wave functions for the excited states and a generalization of the Handy, Yamaguchi, and Schaefer method to obtain mass-independent correction functions. We find that including the nonadiabatic correction gives significantly improved results compared to just including the diagonal correction when the Born–Oppenheimer potential energy surface is optimized for H2 O16. The agreement with experimental results for deuterium- and tritium-containing isotopes is nearly as good as our best empirical correction, however, the present correction is expected to be more reliable for higher, uncharacterized, levels.
Femtosecond dynamics of solvated oxygen anions. I. Bifurcated electron transfer dynamics probed by photoelectron spectroscopy
Authors: Paik D. Hern , Kim Nam Joon , Zewail Ahmed H.
Published: April 2003

The ultrafast dissociation dynamics of O6−⋅X (X=O2, N2, Xe, or N2O) was investigated by femtosecond photoelectron spectroscopy. The transients, monitoring nascent O2−, exhibit biexponential rises with two distinct time constants—the fast component (τ1∼200 fs) corresponds to the joint rate constant for electron recombination and direct dissociation of the O4− core perturbed by solvent molecules, whereas the slow component (τ2=2.0–7.7 ps, depending on the solvent) corresponds to the process for the liberation of O2−, which is governed by vibrational predissociation and intramolecular vibrational-energy redistribution. These observations are consistent with the mechanism proposed in the earlier communication of this work [Paik et al., J. Chem. Phys. 115, 612 (2001)]. The wave packet bifurcates via two separate dissociation pathways: electron transfer followed by electron recombination, and electron transfer followed by vibrational predissociation. Unlike all other solvents, the anomalous behavior observed for O6−⋅N2O—a threefold increase in τ2 value, compared to the other solvents, and a factor of 10 increase for τ2, compared to that of O6−—reflects the more effective energy dissipation via solute–solvent vibration-to-vibration and rotational couplings. Moreover, for all solvents, the ratio of the slow-rise contribution to the total signal can be correlated with the degree of cooling, supporting the concept of bifurcation in the two channels.
Femtosecond dynamics of solvated oxygen anions. II. Nature of dissociation and caging in finite-sized clusters
Authors: Kim Nam Joon , Paik D. Hern , Zewail Ahmed H.
Published: April 2003

Ultrafast dissociation and recombination dynamics of (O2)n−, n=3–10 was studied using femtosecond, time-resolved photoelectron spectroscopy. The observed transients of nascent fragment anions, following 800 nm fs pulse excitation, exhibit a biexponential rise with two distinct time constants. The time constants, which vary with the number of solvent O2 molecules, clearly show the solvation effect in two different dissociation pathways. Consistent with the bifurcation picture in the preceding paper, the direct subpicosecond dissociation (τ1=110–620 fs, depending on n) is governed by electron recombination and kinematics of the half-collision. The second pathway is indirect (τ2=0.7–8.0 ps, for O6− to O20−) and controlled by intramolecular vibrational-energy redistribution. In the solvent cage, only O16−, O18−, and O20− show the reformation of the bond, with the caging time constant decreasing from 4 ps for the first two to 2 ps for the latter. This caging through ion-induced dipole interaction is then followed by vibrational relaxation on the time scale of 12 to 3 ps, for O16− to O20−. The time scale for the initial direct caging is two to five times slower than that previously observed for diatoms, neutral, or ionic, in van der Waals clusters. We suggest that this initial slower caging is due to the reorientation of O2− and O2 to acquire a proper geometry for O4− bond reformation. In these finite-sized homogeneous clusters, we compare theory with experiment. We also found a correlation between the vertical detachment energy and n−1/3, for n in the range of 2–10, which allow for a connection between the mesoscopic structures and a bulk-type dielectric continuum, with an effective dielectric constant.
Multi-scale scanning tunneling microscopy imaging of self-organized regioregular poly(3-hexylthiophene) films
Authors: Grévin B. , Rannou P. , Payerne R. , Pron A. , Travers J. P.
Published: April 2003

Two-dimensional self-organized poly(3-hexylthiophene) films on highly oriented pyrolytic graphite have been probed at the solid/substrate interface by scanning tunneling microscopy (STM). Structural morphology and typical polymer conformations are visualized and discussed from mesoscopic to nanoscopic scales, including mesoscopic assembly of polycrystals, crystalline monodomain orientations and sizes, grain boundaries, chain folds, and other conformational features. STM estimation of the average chain length is in remarkably good agreement with that derived from size-exclusion chromatography. The multiscale analysis supports a picture where heterogeneities exist at different length scales.
Properties of phase-pure InSe films prepared by metalorganic chemical vapor deposition with a single-source precursor
Authors: Choi In-Hwan , Yu Peter Y.
Published: April 2003

Phase-pure InSe thin films have been prepared by a low-pressure metalorganic chemical vapor deposition technique using a single-source precursor: [(Me)2In(μ−SeMe)]2. These films have been characterized by x-ray diffraction and scanning electron microscopy and found to be single-phased and polycrystalline with a hexagonal lattice. The optical properties of the films have been studied via absorption, photoluminescence, and Raman spectroscopies.
Ferroelectric properties and crystal structure of praseodymium-modified bismuth titanate
Authors: Chon Uong , Shim Jeong Seob , Jang Hyun M.
Published: April 2003

Fatigue-free and highly c-axis oriented Bi4−xPrxTi3O12 (BPrT, x=0.85) thin films were grown on Pt/TiO2/SiO2/Si (100) substrates using the method of metalorganic sol decomposition. The BPrT film capacitor with a top Pt electrode showed significantly improved values of the remanent polarization (Pr) and the nonvolatile charge as compared to those of the Bi4−xLaxTi3O12 (x=0.75) film capacitor, recently known as a promising candidate for nonvolatile memories. The 2Pr value of the BPrT capacitor was 40 μC/cm2 at an applied voltage of 10 V while the net nonvolatile charge was as high as 20 μC/cm2 and remained essentially constant up to 4.5×1010 read/write switching cycles at a frequency of 1 MHz. In addition to these, the capacitor showed excellent charge-retention characteristics with its sensing margin of 16 μC/cm2 and a strong resistance against the imprinting failure.
Interrelations between the local electronic states and the atomic structures in the Si nanoscale island on Si(111)-(7×7) surface
Authors: Negishi R. , Shigeta Y.
Published: April 2003

We have investigated local electronic states and atomic structures of a self-assembled Si nano-island on Si(111)-(7×7) dimer-adatom-stacking fault (DAS) substrate by using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy. The normalized differential conductivity (dI/dV)/(I/V) spectra show several peaks, whose energies depend on each individual dangling bond site on the island, and these states are different from dangling bond states on the (7×7) DAS substrate. STM images at the edge of the island also show some interesting variations as a function of the sample bias voltage. The variations are explained by modifications of dangling bond states on T4 site atoms and on buckled dimer atoms in the vicinity of the edge. From these results, we find a detailed behavior of a redistribution of the electron charge to stabilize the atomic structure of the nano-island.
Magnetic and structural properties of Co, Cr, V ion-implanted GaN
Authors: Lee J. S. , Lim J. D. , Khim Z. G. , Park Y. D. , Pearton S. J. , Chu S. N. G.
Published: April 2003

We report on the magnetic and structural properties of epitaxial metal organic chemical vapor deposition grown p-GaN:Mg/Al2O3 implanted with Co, Cr, and V ions at varying high doses at 350 °C. Magnetic and structural properties were investigated after a short anneal at 700 °C to remove implantation damage. Magnetic properties determined from superconducting quantum interference device magnetometer measurements indicate ferromagnetic-like ordering for Co and Cr doped samples up to 320 K, while V doped samples show paramagnetic behavior for all temperatures considered. For all samples studied, structural characterization techniques such as x-ray diffraction, high-resolution cross-sectional transmission electron microscopy, and selected area diffraction pattern, indicate no second phases that may contribute to the magnetic properties measured. Transport measurements (resistivity as a function of temperature) reveal all samples to show insulating-like behavior.
Synthesis, structure, and magnetic properties of ε-Co nanocrystalline thin films and annealing effects
Authors: Nie X. , Jiang J. C. , Meletis E. I. , Tung L. D. , Spinu L.
Published: April 2003

Cobalt (Co) nanocrystalline thin films consisting of Co nanocrystals (NCs) embedded in an amorphous carbon matrix were directly synthesized at ambient temperature by codeposition of Co and carbon using high vacuum dual electron-beam evaporation. In the as-deposited films, Co was identified to have a cubic ε-Co structure (with a controllable particle size). After annealing at 360 °C for 2 h, the Co NCs with a particle size of 5 nm in the as-deposited film were transformed into hexagonal α-Co (8 nm in size), whereas, the as-deposited Co NCs of 3 nm in size had a tendency to coalesce and interconnect to each other, forming a face-centered-cubic β-Co network. While the ε-Co NCs of 3 nm in size showed superparamagnetism at room temperature, the ε-Co NCs of 5 nm in size exhibited a ferromagnetic state due to coupling interaction or local interparticle connections. The saturation magnetization and coercivity substantially increased after annealing compared to the as-deposited films due to coarsening of Co NCs.
Study of inhomogeneities in the flux density distribution of big monolithic (RE)Ba2Cu3O7−δ melt-textured superconductors
Authors: Gonzalez-Arrabal R. , Eisterer M. , Weber H. W.
Published: April 2003

We investigate the accuracy of Hall probe scanning in the analysis of current limiting effects in big melt-textured monoliths. The flux density distribution is measured on samples with homogeneous or inhomogeneous trapped field profiles prior to and after neutron irradiation. The influence of different current limitations on the trapped field is assessed by comparing the trapped field profiles measured on the big monoliths with the trapped field profiles calculated from the critical current densities measured in small reference samples.
Magnetotransport properties of ferromagnetic La0.8Ba0.2MnO3/antiferromagnetic LaMO3 (M=Cr,Fe) perovskite trilayer films
Authors: Kanki Teruo , Tanaka Hidekazu , Kawai Tomoji
Published: April 2003

We have prepared trilayer structures that contain a ferromagnetic (La,Ba)MnO3 film sandwiched between two antiferromagnetic LaFeO3 (Néel temperature TN=740 K) or LaCrO3 (TN=280 K) layers. It was found that the magnetoresistance (MR) ratio at 0.8 T was enhanced from 43% in the La0.8Ba0.2MnO3 single layer film to 74% in the La0.8Ba0.2MnO3/LaFeO3 trilayer film. The MR ratio in the La0.8Ba0.2MnO3/LaCrO3 trilayer film, however, was almost identical to that of the single layer film. These results could be interpreted as having resulted from the differences in spin-frustration effects due to the different exchange interactions between the respective neighboring layers.
Diffusivity in periodic arrays of spherical cavities
Authors: Berezhkovskii Alexander M. , Zitserman Vladimir Yu. , Shvartsman Stanislav Y.
Published: April 2003

We derive an expression for the effective diffusivity in a model porous medium formed by a periodic array of touching spherical cavities. Our result explicitly links the effective diffusion constant to the microgeometry of the porous material.
Photodissociation of vibrationally excited ammonia: Rotational excitation in the NH2 product
Authors: Bach Andreas , Hutchison J. Matthew , Holiday Robert J. , Crim F. Fleming
Published: April 2003

Vibrationally mediated photodissociation combined with H Rydberg atom photofragment translational spectroscopy reveals the state-to-state photofragmentation dynamics of selected parent rovibronic levels of Ã 1A2″ state ammonia. Analysis of the time-of-flight spectra determines the population of quantum states in the NH2 partner fragment for dissociation from the excited state bending vibration (41). Dissociation from the bending state produces rotationally excited NH2 predominantly (≈75%) in its ground vibrational state, in agreement with theoretical predictions.
Photodissociation spectroscopy of Ca+-formaldehyde
Authors: Lu W.-Y. , Wong T.-H. , Sheng Y. , Kleiber P. D.
Published: April 2003

We have carried out photodissociation spectroscopy studies of Ca+(H2CO). The work is supported by electronic structure calculations of the ground and low-lying excited states of the complex. Our results show that Ca+(H2CO) is bound in a C2v Ca–O–CH2 ground-state equilibrium geometry with a bond dissociation energy of De″(Ca–OCH2)=0.9±0.2 eV. We have assigned five distinct molecular absorption bands of the complex that correlate with Ca+-based 3d and 4p←4s radiative transitions. The absorption bands show clear vibrational resonance structure with partially resolved rotational substructure. Spectroscopic constants are, for the most part, in good agreement with ab initio model predictions. The experimental data give quantitative information about the bonding interactions for electronically excited states of different valence and molecular-orbital character. Results for Ca+(H2CO) are compared with previous results from our lab on the analogous Mg+(H2CO) complex [J. Chem. Phys. 117, 6970 (2002)].
Optical properties of tetragonal Pb(Mg1/3Nb2/3)0.62Ti0.38O3 single crystal
Authors: Wan Xinming , Xu Haiqing , He Tianhou , Lin Di , Luo Haosu
Published: April 2003

A systematic study of the optical properties of tetragonal Pb(Mg1/3Nb2/3)0.62Ti0.38O3 single crystals was calculated out at room temperature. It was found that the crystal is transparent in the visible region and rolls off in near 450 nm. The optical absorption edge is around 400 nm under both unpoled and poled conditions. The refractive indices and their dispersion were accurately measured with the minimum deviation method. The parameters Ai, Bi, Ci, and Di of Sellmeier’s equation for both no and ne are also obtained in terms of least-squares fit. Then, the refractive indices in the wavelength range of 435.8–3100 nm can be calculated. Using the Senarmont compensator method, the effective electro-optic coefficient rc=42.8 pm/V was also obtained. Some discussions about the octahedral BO6 structure are presented on the optical properties of the material.
Different domain structures and their effects on fatigue behavior in Bi3TiTaO9 and SrBi2Ta2O9 ceramics
Authors: Su D. , Zhu J. S. , Wang Y. N. , Xu Q. Y. , Liu J. S.
Published: April 2003

The domain structures of Bi3TiTaO9 (BTT) and SrBi2Ta2O9 (SBT) ceramics, which have the same crystalline structure but different fatigue properties, were studied by transmission electron microscopy. Antiphase boundaries and 90° domain walls in BTT ceramics were confirmed by the dark-field image taken with (031) superlattice reflection and selected area diffraction patterns, respectively. In contrast to SBT, 90° ferroelectric domains in BTT display a high, flat morphology and less density. Additionally, a type of planar defects normal to the c axis, which has not been found in SBT, was observed. The origin of different domain structures is discussed. The mechanism of fatigue behavior related to 90° domains is also proposed.
Theory of magnetization: Twin boundary interaction in ferromagnetic shape memory alloys
Authors: Paul D. I. , Marquiss J. , Quattrochi D.
Published: April 2003

Much interest has been recently given to the development and understanding of ferromagnetic shape memory alloys. In particular, the interaction of crystallographic twin boundaries with an external magnetic field has been put forth as a mechanism for shape deformation and applied to the alloy Ni2MnGa. In this article, we examine this interaction analytically. We consider two crystallographic variants wherein the magnetization directions at their extreme boundaries are along their easy axes. Further, we have included the magnetic exchange energy and have postulated a Gaussian strain distribution localized in one of the variants. Using the basic micromagnetic equations, we determine the magnetization distribution throughout the variants as well as the equilibrium positions of the twin boundary. We show that the resulting magnetic domain wall leads the twin boundary position as long as the magnetic driving force is insufficient to overcome the strain energy impediment. For the reverse situation, both the twin boundary and magnetic domain wall move through the opposing variant. The magnetic coercivity is correspondingly reduced.
Electron-beam interactions in Cu–GeSe2 amorphous thin films
Authors: Romero J. S. , Fitzgerald A. G.
Published: April 2003

Copper migration and surface expansion of irradiated areas are observed in amorphous germanium-based chalcogenide thin films when an electron beam is focused in pulsed or continuous operation on the surface of these thin films. Both phenomena can be explained using a simple model in which the population of D− centers is considered to increase upon electron irradiation. The increase in the D− center population is envisaged as due to the breaking of bonds by the electron radiation and by the constant presence of negative charge in irradiated regions. Changes in copper concentration of 20%–30% and surface expansions of 30%–40% of films thickness have been obtained.
Interfacial and bulk properties of zirconium dioxide as a gate dielectric in metal–insulator–semiconductor structures and current transport mechanisms
Authors: Chim W. K. , Ng T. H. , Koh B. H. , Choi W. K. , Zheng J. X. , Tung C. H. , Du A. Y.
Published: April 2003

In this article, we show the structural and electrical characterization results on aluminum gate/zirconium dioxide/n-type silicon (Al/ZrO2/n-Si) metal–insulator–semiconductor (MIS) devices with equivalent-oxide thickness (EOT) of ∼2.5 nm. About 60% of the devices fabricated with the optimized process conditions showed leakage current density of less than 2 x 10−5 A/cm2 at 1 V accumulation bias, which is lower than devices with silicon dioxide as a gate dielectric of similar EOT. Transmission electron microscopy images showed a ∼1.7-nm-thick interfacial layer (possibly zirconium silicate) and a ∼13-nm-thick bulk ZrO2 layer for the sputter-deposited high-k film. The difference in the dc leakage current of individual devices is due to the varying degrees of crystallization of the bulk ZrO2 layer, and not related to the interface state density. It was found that the interfacial layer between the bulk ZrO2 and the silicon substrate plays an important role in determining the conduction mechanism through the high-k MIS structure. The Frenkel–Poole emission mechanism was found to fit the measured Jg–Vg data between electric fields of 2.0 and 3.2 MV/cm in the interfacial layer (corresponding to 0.7 < Vg<2 V). The electric field in the interfacial layer is generally larger than that in the bulk ZrO2. The injecting field at the cathode, or the n-type silicon substrate in this case, depends on the electric field in the interfacial layer as this is directly in contact with the silicon substrate.