bluuebunny/crossref_metadata_2025 huggingface dataset:
Square water in an electric field
Authors: Girardi M. , Figueiredo W.
Published: October 2002

The physical properties of the square water model, which is a generalization of the square ice to nonzero temperatures, is studied as a function of temperature and electric field. We determined the fraction of hydrogen bonds (HBs), the electric susceptibility, and the entropy of the model. We found that the usual independent-bond approximation gives poor predictions for the HB number when a polarization field is present. We compare the independent-bond results with Monte Carlo simulations, and with more accurate mean-field approximations obtained by the study of clusters of water molecules. At zero temperature, this model presents a first-order phase transition driven by the external electric field. The discontinuity in the HB number gives support to this behavior. We also obtained the exact partition function of the square water model in one dimension employing the transfer matrix technique. The zero field free energy in one dimension displays the same functional form on temperature as the one obtained in the two-dimensional version of the model via mean field approach.
Orientational ordering and disordering of a simple dipolar fluid under shear flow
Authors: McWhirter J. Liam , Patey G. N.
Published: October 2002

Nonequilibrium molecular dynamics simulations are performed for a simple dipolar fluid under planar Couette flow. Under this shear flow, the fluid’s spatial structure becomes distorted, and the pair distribution function is anisotropic. At low shear rates, the dipoles respond to this distortion by orientationally ordering along an axis where the fluid structure is most compressed, giving a shear-induced ferroelectric fluid state. At high shear rates above a critical value, the orientational order decreases. We argue that this disordering can be attributed to the onset of large fluctuations in the director’s orientation about the compression axis at the critical shear rate. We also observe the director can “hop” between “up” and “down” directions along the compression axis. These hops are fast and infrequent events.
Line-narrowed fluorescence spectra of phenanthrene and perylene in polymers: Chromophore, polymer, and vibrational state dependence of coupling to matrix phonons
Authors: Verdal Nina , Kelley Anne Myers
Published: October 2002

Site-selective line-narrowed fluorescence spectra have been obtained for the S1↔S0 transitions of phenanthrene and perylene in polyethylene, polystyrene, and poly(methyl methacrylate) films at 1.5 K. The spectral distributions and intensities of the phonon sidebands on each vibronic transition are analyzed to examine the chromophore, matrix, and vibrational level dependence of the coupling of the chromophore’s electronic transition to matrix and/or intermolecular vibrations. The frequency distribution of the coupled phonons (spectral density) and the strength of the coupling vary considerably among polymers. Different vibronic transitions of the same chromophore in the same matrix exhibit similar spectral densities but slightly different coupling strengths. The coupling strengths appear to correlate with the extent to which the normal mode changes the molecule’s overall dimensions. The more redshifted absorbers within a given chromophore and matrix also exhibit slightly stronger coupling to matrix phonons. Computational simulations of the emission spectra have been performed in order to examine transferability of the spectral densities between different vibronic bands of the same chromophore and between chromophores in the same matrix. Somewhat different spectral densities are required to model vibronic bands of different chromophores in the same polymer.
Relaxation dynamics in photoexcited GaSe nanoparticles
Authors: Chikan V. , Kelley D. F.
Published: October 2002

The photophysics and relaxation dynamics of GaSe nanoparticles have been studied in room temperature solutions, using static and time-resolved emission spectroscopy. Following 400 nm excitation, these particles exhibit a static emission maximum at 480 nm. This emission is polarized and the anisotropy is largest on the blue edge of the emission spectrum. Both the total (unpolarized) emission kinetics and the emission anisotropy kinetics are obtained. Static emission spectra along with wavelength-dependent kinetic results permit the reconstruction of time-dependent spectra. The kinetic results reported here show an 80 ps decay component in the total emission, but not in the anisotropy decay kinetics. There is an ∼270 cm−1 shift in the emission maximum during this decay. This transient is assigned to direct to indirect band edge relaxation. This is followed by a 400 ps decay in both the total emission and the anisotropy kinetics. A very small shift (∼170 cm−1) in the emission maximum occurs during this decay. This transient is assigned to trapping of holes in shallow acceptor levels. Finally, there is a 2.4 ns decay of both the total emission and the emission anisotropy. As this decay occurs, the 480 nm emission is replaced by a much weaker, broad and unpolarized 520 nm emission. This decay is assigned to relaxation of holes into deep traps. The kinetic results are interpreted in terms of a model which is based on the relative energetics of the direct and indirect band edges and the hole trap states in bulk GaSe.
A quantum symmetry preserving semiclassical method
Authors: Babikov Dmitri , Walker Robert B. , T Pack Russell
Published: October 2002

Symmetry constraints are built into a semiclassical propagation scheme. It is then applied to treat H+Ne2 collisions at 30 K, where quantum selection rules restrict the final rotational states of symmetric Ne2 molecules to the even manifold. The cross sections for state-to-state transitions are calculated for symmetric and nonsymmetric isotopic compositions of Ne2. All bound and long-lived quasibound (trapped behind the centrifugal barrier) states of Ne2 are considered. This semiclassical method captures symmetry effects and shows satisfactory agreement with the quantum results.
Two-photon photodissociation of NO through Rydberg levels in the 265–278 nm region: Spectra and photofragment angular distributions
Authors: Cosofret B. R. , Lambert H. M. , Houston P. L.
Published: October 2002

The spectroscopy and dynamics of the NO photodissociation through Rydberg levels near 74 000 cm−1 have been investigated following two-photon excitation. The 6dπ−(v=1) and 5sσ(v=3) levels overlap near 74 070 cm−1. Assignment of the rotational transitions for these levels has been aided by the use of the photoproduct angular distributions measured using product imaging techniques. Product imaging was also used to investigate the 8dπ−(v=1) and 5sσ(v=2) regions assigned by previous investigators. In all cases, the major products were N(2D)+O(3P). The angular distributions vary strongly with rotational transition and with the assumed intermediate in the two-photon excitation scheme and can, for the most part, be predicted by calculation. They demonstrate that, for the Rydberg levels examined, the major contribution to the two-photon line strength is a Π intermediate, likely the C 2Π state, with less than a 30% amplitude contribution from either a Σ or Δ intermediate.
Anharmonic effects on the transition state theory rate constant
Authors: Isaacson Alan D.
Published: October 2002

Accurate rovibrational levels for HCN up to 24 349 cm−1 above the bottom of the vibrational well and vibrational levels at the saddle point of the HCN/HNC isomerization reaction up to 32 809 cm−1 above the saddle point have been computed and used to obtain partition functions over the temperature range 200–2500 K. Under the rigid-rotor approximation, the rovibrational partition function for HCN is found to be exactly separable into vibrational and rotational contributions to first order. Two approximate approaches, second-order perturbation theory and simple perturbation theory, the latter of which obviates the need for a direct summation over energy levels, are shown to yield vibrational partition functions for HCN and at the saddle point that agree with the accurate values within 2%. In contrast, the usual harmonic approximation leads to errors of up to 24% in the individual partition functions, resulting in differences of between 3.5% and 6.5% in conventional transition state theory rate constants calculated with harmonic versus anharmonic vibrations.
The correlation time of mesoscopic chemical clocks
Authors: Gaspard Pierre
Published: October 2002

A formula is proved for the correlation time of nonequilibrium chemical clocks in the presence of molecular noise. The correlation time is defined as the inverse of the damping rate of the autocorrelation functions of the chemical concentrations. Using the Hamilton–Jacobi method for stochastic systems as well as a Legendre transform from the Onsager–Machlup action to a reduced action depending only on the Hamilton–Jacobi pseudoenergy, the correlation time is given in the weak-noise limit in terms of the extensivity parameter, the period of oscillations, as well as the derivative of the period with respect to the pseudoenergy. Using this result, an estimation is obtained for the minimum number of molecules required for the oscillations of the chemical concentrations to remain correlated in time. This estimation puts a fundamental lower limit on the size of chemical clocks. For typical oscillators, the minimum number of molecules is estimated between ten and one hundred, which essentially corresponds to nanometric systems.
Host–guest charge transfer states: CN doped Kr and Xe
Authors: Fiedler S. L. , Vaskonen Kari , Ahokas Jussi , Kunttu Henrik , Eloranta Jussi , Apkarian V. A.
Published: October 2002

The host–guest charge transfer absorption of CN doped krypton and xenon matrices are identified through direct analogy with the previously assigned transitions of Cl/Kr and Cl/Xe. These intense, structured absorption bands appear with the onset at 245 nm in Kr and 360 nm in Xe. Excitation of the CN/Kr charge transfer band at 193 nm leads to emission over CN(A(2Π)→X(2Σ)) transition, indicating that an efficient curve crossing precludes the ionic state from radiating. No emissions were seen in CN/Xe when excited at 193 nm. The charge transfer absorption spectrum of CN/Kr is reproduced through an extended diatomics-in-ionic-systems treatment, using accurate ab initio pair potentials and transition dipoles as input, without further adjustment. The delocalized hole states are then analyzed in real-space, using atomic bases distributed over as many as eleven shells surrounding the CN− center. The ionic states are well described as J=1/2, 3/2 valence bands bound to CN−, with a substructure that cannot be exclusively assigned to a single quantum number. The strong absorptions terminate on states in which 70%–95% of the hole density remains on the first nearest neighbor shell, with hole densities of 1%–5% extended out to R=8 Å. In higher ionic states, with weaker transition dipoles, the hole density maximizes on shells removed by 10 Å from the ionic center. Although these delocalized states provide channels for charge separation via self-trapping of holes, save for a weak signal from the impurity trapped hole at H+ centers, the experiments do not provide evidence for significant charge separation.
Laser-induced dispersed vibration–rotation fluorescence of acetylene: Spectra of ortho and para forms and partial trapping of vibrational energy
Authors: Metsälä Markus , Yang Shengfu , Vaittinen Olavi , Halonen Lauri
Published: October 2002

The laser-induced dispersed vibration–rotation fluorescence method has been developed further when compared with a previous publication [Saarinen et al., J. Chem. Phys. 110, 1424 (1999)]. More than one order of magnitude better signal-to-noise ratio has been achieved in the wave-number region 2900–3500 cm−1 by taking advantage of directionality of the fluorescence signal. The improvement has been applied to overtone spectroscopy of normal acetylene where for high CH stretching excitations separate spectra of ortho and para forms are obtained containing basically just single CH stretching vibrational quantum transitions from the pumped antisymmetric vibrational (ν1+3ν3(Σu+) and ν2+3ν3(Σu+)) and close-lying symmetric vibrational local mode (4ν3(Σg+) and ν1+ν2+2ν3(Σg+)) states. No nuclear spin conversion is observed in these spectra. Two new symmetric vibrational states (ν1+2ν2+4ν40(Σg+)(29%) and (50%)) have been observed and the precision of the spectroscopic parameters of previously published symmetric states has been improved by an order of magnitude. Unexpected fluorescence originating from the antisymmetric CH stretching fundamental state ν3 and some associated states of acetylene have also been observed. These spectra are characterized by both ortho and para forms in normal abundance and by unusual intensity patterns due to strong reabsorption of the fluorescence by ground state acetylene molecules in the sample cell. A simple collisional step-down mechanism is proposed to account for the appearance of the ν3 fluorescence band system. The excess vibrational energy in the sample volume is partly trapped in the form of ν3 mode energy and it decays from the system by radiation.
Symmetry properties of electronically excited states of nitroaromatic compounds
Authors: Abbott James E. , Peng Xianzhao , Kong Wei
Published: October 2002

We report polarization spectroscopy of oriented gas phase nitroaromatic compounds, specifically nitrobenzene and the three isomers of nitrotoluene. Orientation of supersonically cooled molecules was achieved in a uniform electric field. The dependence of the yield of NO on the polarization direction of the excitation laser was used to derive the direction of the transition dipole. The transition to the S2 state centered at 250 nm demonstrated increased yield when the excitation laser was polarized perpendicular to the orientation field. The transition dipole was therefore characterized to be near perpendicular to the permanent dipole. The transition to S3 showed an opposite behavior, indicating that the transition dipole for S3 is primarily parallel to the permanent dipole. Quantitative analysis of the polarization dependence further revealed that in both cases, the transition dipole is not strictly parallel or perpendicular to the permanent dipole, and the tilt is about 20°–30°. This result demonstrates the sophisticated nature of electronically excited states of nitroaromatics. It is therefore not surprising that only two previous theoretical calculations agree with our results, while other calculations and experimental work in condensed phases do not.
State-to-state ArHBr photodissociation quantum dynamics
Authors: Lepetit Bruno , Lemoine Didier
Published: October 2002

We report quantum dynamical calculations on the photodissociation process: ArHBr+hν→H+ArBr or Ar+Br+H. Jacobi coordinates associated with the product arrangement H+ArBr have been used as well as a pseudospectral wave packet propagation method. The radial singularity at the origin is either analytically lifted with a discrete Bessel transform approach, or treated within a sine transform strategy. The relative fraction of a bound ArBr product state is high (of the order of 50%) and decreases with energy. Product vibrational populations decrease with vibrational energies. Rotational distributions show oscillatory patterns characteristic of rotational rainbows, whereas angular distributions have maxima at rainbow angles.
Conformations of dendrimers in dilute solution
Authors: Timoshenko Edward G. , Kuznetsov Yuri A. , Connolly Ronan
Published: October 2002

Conformations of isolated homo-dendrimers of G=1–7 generations with D=1–6 spacers have been studied in the good and poor solvents, as well as across the coil-to-globule transition, by means of a version of the Gaussian self-consistent method and Monte Carlo simulation in continuous space based on the same coarse-grained model. The latter includes harmonic springs between connected monomers and the pair-wise Lennard-Jones potential with a hard core repulsion. The scaling law for the dendrimer size, the degrees of bond stretching and steric congestion, as well as the radial density, static structure factor, and asphericity have been analyzed. It is also confirmed that while smaller dendrimers have a dense core, larger ones develop a hollow domain at some separation from the center.
Structure of nonuniform fluid mixtures: A self-consistent density-functional approach
Authors: Patra Chandra N. , Ghosh Swapan K.
Published: October 2002

A self-consistent density-functional approach is developed for the structure of nonuniform fluid mixtures, where the second-order direct correlation function and the bridge function of the corresponding uniform fluid mixture are used as the input. These two quantities are obtained through integral equation theory using an accurate closure relation. The present theory uses the concept of universality of the free energy density functional to obtain the first-order direct correlation function of the nonuniform system from that of the corresponding uniform system. This new approach is employed to predict the structure of binary hard-sphere fluid mixtures near a hard wall. The calculated density and concentration profiles are shown to compare well with available computer simulation results.
Molecular weight distributions of polydisperse polymers in coexisting liquid phases
Authors: Shresth R. Saurabh , McDonald R. Christopher , Greer Sandra C.
Published: October 2002

We present new experimental measurements of the molecular weight (MW) distributions in coexisting liquid phases for three polymer/solvent systems. We studied samples at the critical compositions and at four temperatures in the two-phase regions. For polystyrene in methylcyclohexane (with an upper critical solution point), we observed the expected fractionation between phases, with a higher average MW in the lower, polymer-rich phase. For poly(ethylene oxide) in 2, 6-lutidine+water (with a lower critical solution point), the total polymer mass is nearly equal in the two phases, but the polymers of higher average MW equilibrate into the upper, 2,6-lutidine-rich phase. For poly(ethylene oxide) in isobutyric acid+water (with an upper critical solution point), most of the total polymer mass is in the upper, isobutyric acid-rich phase, but the polymers of higher average MW equilibrate into the lower, water-rich phase; moreover, the fractionation in this case is quite dramatic, with almost a factor of 2 difference in average MW between the phases and a significant decrease in polydispersity for the lower phase. Our results support prior experimental results on this phenomenon that find that a basic Flory–Huggins (FH) theory is inadequate. More recent theoretical considerations suggest that the difference between FH theory and experiments is due to non-mean field effects, but this theory awaits direct comparison to experiments.
Selectivity of the α and β bond fissions for bromoacetyl chloride upon n→π* excitation: A combined complete-active-space self-consistent field and multireference configuration interaction study
Authors: Ding Wan-Jian , Fang Wei-Hai , Liu Ruo-Zhuang , Fang De-Cai
Published: October 2002

The potential energy surfaces for the BrCH2COCl dissociations into Br+CH2COCl, BrCH2CO+Cl, and BrCH2+COCl in the S0, S1, and T1 states have been investigated at the complete-active-space self-consistent field, density functional theory, and multireference configuration interaction levels with the 6-31G* and cc-pVDZ basis sets, which provide some new insights into the mechanism of the BrCH2COCl photodissociation at 248 nm. It is found that the most probable pathway is the S1 C–Cl α and C–Br β bond fissions, which are a pair of competitive dissociation channels with some preference of the α C–Cl bond cleavage. The C–C α bond fission can take place along the S1 pathway upon photoexcitation at 248 nm, but it is not in competition with the C–Cl α bond cleavage. These results are consistent with the experimental findings. The relative strength of the C–C and C–Cl α bonds is one of the factors that influences the selectivity of the α bond fissions. However, the selectivity is mainly determined by the mechanism of dissociation upon n→π* excitation. The preference of the C–Cl α bond fission over the C–Br β bond cleavage was attributed to the nonadiabatic recrossing in previous studies. The present calculations predict that the distance dependence of intramolecular energy relaxation is another important factor that influences the selectivity of the α and β bond cleavages.
Will the level of seismic noise at Livingston Observatory interfere with the detection of gravity waves from binary inspirals?
Authors: Rizzi Anthony
Published: October 2002

A major impediment to gravity wave detection in interferometer detectors such as Laser Interferometer Gravitational Wave Observatory (LIGO) is nongravitational wave-induced motion of the test masses. All types of noise sources including environmental sources contribute to this problem. Seismic motion is a significant source of such motion. I introduce a method to quantify the contribution of a given noise source to motion that is most deleterious to gravity wave detection, e.g., motion that mimics gravity wave signatures. I define such a benchmark in two senses: A relative benchmark that quantifies the degree of intrinsic interference with the gravity wave detection and an absolute benchmark which incorporates scaling factors appropriate to a given experiment. To give statistical meaning to the method and to illustrate it, I benchmark Gaussian noise and seismic colored Gaussian noise; both benchmark at 0 false events/day, which, applying a simple statistical model, implies ≪1 event for even year-long data runs. Finally, the relative benchmark for seismic noise at the Livingston observatory is (for the band from ∼100–400 Hz), using two 24 h data sets, 40–290 false events/day. Given the LIGO I noise curves, it is shown that the seismic noise should not interfere with the detection of binary inspiral generated gravity waves using optimal filtering. Its absolute benchmark is 0 false events/day, which, applying as above a simple statistical model, implies ≪1 event/year. In rough terms, if only Livingston seismic noise and gravity waves were impinging on the detector, one would expect to see neutron star binary inspiral’s that occur anywhere in the universe.
Radiation-induced damage in x-ray spectroscopy of CdS nanoclusters
Authors: Döllefeld Herwig , McGinley Colm , Almousalami Samir , Möller Thomas , Weller Horst , Eychmüller Alexander
Published: October 2002

Results of x-ray spectroscopy, namely x-ray absorption spectroscopy and x-ray photoelectron spectroscopy, on CdS nanoparticles of different sizes are presented. The signals could be associated with specific atoms inside the cluster structure which is known from single crystal x-ray diffraction. X-ray absorption measurements performed with different beam intensities lead to significant changes in the spectra. Radiation damage in CdS nanoparticles resulting from the high beam intensities is discussed for the first time. Also in x-ray photoelectron spectroscopy performed with very high energy resolution (0.25 eV), evidence is gained for radiation damage, particularly for the smallest nanoparticles investigated.
Ferromagnetism in Cr-doped Ge
Authors: Choi Sungyoul , Hong Soon Cheol , Cho Sunglae , Kim Yunki , Ketterson John B. , Jung Chi-Un , Rhie K. , Kim Bong-Jun , Kim Y. C.
Published: October 2002

We have fabricated Cr-doped bulk Ge single crystal using the vertical gradient solidification method. The material shows ferromagnetic ordering at 126 K, as determined from temperature-dependent magnetization and resistance measurements. A sample with x=0.01 was p type with nh=3×1017 cm−3 at 350 K. The measured magnetic moment per Cr was 0.83μB at 5 K.
Small molecule organic light-emitting diodes can exhibit high performance without conventional hole transport layers
Authors: Huang Qinglan , Cui Ji , Yan He , Veinot Jonathan G. C. , Marks Tobin J.
Published: October 2002

It is generally accepted that hole transport layers (HTLs) with thicknesses on the order of tens of nm are indispensable to the function of small molecule organic light-emitting diodes (OLEDs) if high electroluminescence and quantum efficiencies are to be achieved. In the present letter, small molecule OLEDs with high luminance and external quantum efficiencies are fabricated in which the HTL is replaced solely by an ultrathin (1–2 nm) self–assembled, saturated hydrocarbon organosiloxane monolayer. These results require some reconsideration of conventional design criteria regarding the necessity of HTLs and argue that the role of the self-assembled monolayer here is to enhance hole injection and charge recombination efficiency, while blocking electron transport to the anode. These results therefore offer significantly simplified device fabrication.
La
1−x
Ba
x
MnO
3
epitaxial thin films by pulsed-laser deposition: A consequence of strain stabilization
Authors: Pradhan A. K. , Sahu D. R. , Roul B. K. , Feng Y.
Published: October 2002

We report transport, magnetization, and transmission-electron-microscopic studies of La1−xBaxMnO3/SrTiO3 thin films grown by pulsed-laser deposition. The metal–insulator transition is remarkably sharp, and the high value of the temperature coefficient of resistivity is significantly relevant for the required value for the infrared imaging applications. We have demonstrated that the strain-induced property can be tuned and stabilized in La1−xBaxMnO3 films simply by changing the doping level (La/Ba ratio) and addition of metal ions (Ag) for applications at desirable temperature regime close to room temperature.
Field emission from well-aligned zinc oxide nanowires grown at low temperature
Authors: Lee C. J. , Lee T. J. , Lyu S. C. , Zhang Y. , Ruh H. , Lee H. J.
Published: October 2002

Field electron emission from vertically well-aligned zinc oxide (ZnO) nanowires, which were grown by the vapor deposition method at a low temperature of 550 °C, was investigated. The high-purity ZnO nanowires showed a single crystalline wurtzite structure. The turn-on voltage for the ZnO nanowires was found to be about 6.0 V/μm at current density of 0.1 μA/cm2. The emission current density from the ZnO nanowires reached 1 mA/cm2 at a bias field of 11.0 V/μm, which could give sufficient brightness as a field emitter in a flat panel display. Therefore, the well-aligned ZnO nanowires grown at such low temperature can promise the application of a glass-sealed flat panel display in a near future.
Inhomogeneous nanoscale polymer-dispersed liquid crystals with gradient refractive index
Authors: Ren Hongwen , Wu Shin-Tson
Published: October 2002

Inhomogeneous polymer-dispersed liquid crystal devices having gradient droplet distribution were fabricated and their phase retardation characterized. The gradient refractive index profile can be used as switchable prism gratings, and positive and negative lenses with tunable focal lengths. Such a tunable electronic lens is a broadband device and can be used for unpolarized light.
High-efficiency organic light-emitting diodes with tunable light emission by using aromatic diamine/5,6,11,12-tetraphenylnaphthacene multiple quantum wells
Authors: Qiu Yong , Gao Yudi , Wang Liduo , Wei Peng , Duan Lian , Zhang Deqiang , Dong Guifang
Published: October 2002

Organic light-emitting diodes (OLEDs) with multiple-quantum-well (MQW) structures, which consist of N, N′-bis-(1-naphthyl)-N, N′-diphenyl-1, 1′ biphenyl 4, 4′-diamine and 5,6,11,12-tetraphenylnaphthacene (rubrene), and tris(8-hydroxyquinoline) aluminum (Alq3) as the electron transporting material, have been fabricated. The results demonstrate that the MQW structure can sharply increase the performance of OLEDs. The diode with a MQW number of 4 exhibits efficiency up to 8.1 cd/A, which is four times that of the conventional diodes without the MQW structure. It is also interesting to find that the light emission from Alq3 and rubrene can be obtained together and the electroluminescent spectrum, which is independent of the driving voltage, varies with the well number. For the device with a well number of 6, pure rubrene light emission is obtained. Our work demonstrates that the organic MQW structure not only can efficiently control the carrier transporting, thus conducive to achieve an electron–hole balance, but also help to adjust the emitting zone in the devices, then providing an option to obtain different emission colors.
Modeling of ballistic nanoscale metal-oxide-semiconductor field effect transistors
Authors: Fiori G. , Iannaccone G.
Published: October 2002

We present a code for the quantum simulation of ballistic metal-oxide-semiconductor field effect transistors (MOSFETs) in two dimensions, which has been applied to the simulation of a so-called “well-tempered” MOSFET with channel length of 25 nm. Electron confinement at the Si/SiO2 interface and effective mass anisotropy are properly taken into account. In the assumption of negligible phonon scattering in nanoscale devices, transport is assumed to be purely ballistic. We show that our code can provide the relevant direct-current characteristics of the device by running on a simple high-end personal computer, and can be a useful tool for the extraction of physics-based compact models of nanoscale MOSFETs.
Fabrication of nanostructures with long-range order using block copolymer lithography
Authors: Cheng J. Y. , Ross C. A. , Thomas E. L. , Smith Henry I. , Vancso G. J.
Published: October 2002

Block copolymer lithography makes use of the self-assembling properties of block copolymers to pattern nanoscale features over large areas. Although the resulting patterns have good short-range order, the lack of long-range order limits their utility in some applications. This work presents a lithographically assisted self-assembly method that allows ordered arrays of nanostructures to be formed by spin casting a block copolymer over surfaces patterned with shallow grooves. The ordered block copolymer domain patterns are then transferred into an underlying silica film using a single etching step to create a well-ordered hierarchical structure consisting of arrays of silica pillars with 20 nm feature sizes and aspect ratios greater than 3.
Influence of the quantum-well shape on the light emission characteristics of InGaN/GaN quantum-well structures and light-emitting diodes
Authors: Shim H. W. , Choi R. J. , Jeong S. M. , Van Vinh Le , Hong C.-H. , Suh E.-K. , Lee H. J. , Kim Y.-W. , Hwang Y. G.
Published: October 2002

Structural and optical properties of various shapes of quantum wells (QWs), including rectangular, triangular, trapezoidal, and polygonal ones are investigated. Photoluminescence (PL) measurements show that the highest light emission efficiency and the best reproducibility in the intensity and wavelength are obtained from trapezoidal QWs. The temperature dependence of PL spectra indicates the more localized nature of excitons in the trapezoidal QWs. A plan-view transmission electron microscopy shows that quantum dots (QDs) are formed inside the dislocation loop in trapezoidal QWs. The distribution of QDs in size and composition becomes more uniform with trapezoidal QWs than with rectangular QWs, leading to superior light-emission characteristics. It is suggested that QD engineering and dislocation control are possible, to some extent, by the modulation of the QW shape in InGaN/GaN-based light-emitting devices.
Atomic relocation processes in impurity-free disordered p-GaAs epilayers studied by deep level transient spectroscopy
Authors: Deenapanray P. N. K. , Martin A. , Doshi S. , Tan H. H. , Jagadish C.
Published: October 2002

We have used capacitance–voltage and deep level transient spectroscopy techniques to study the relocation of impurities, such as Zn and Cu, in impurity-free disordered (IFD) p-type GaAs. A four-fold increase in the doping concentration is observed after annealing at 925 °C. Two electrically active defects HA (EV+0.39 eV) and HB2 (EV+0.54 eV), which we have attributed to Cu- and Asi/AsGa-related levels, respectively, are observed in the disordered p-GaAs layers. The injection of gallium vacancies causes segregation of Zn dopant atoms and Cu towards the surface of IFD samples. The atomic relocation process is critically assessed in terms of the application of IFD to the band gap engineering of doped GaAs-based heterostructures.
Thermal stability of (HfO2)x(Al2O3)1−x on Si
Authors: Yu H. Y. , Wu N. , Li M. F. , Zhu Chunxiang , Cho B. J. , Kwong D.-L. , Tung C. H. , Pan J. S. , Chai J. W. , Wang W. D. , Chi D. Z. , Ang C. H. , Zheng J. Z. , Ramanathan S.
Published: October 2002

The kinetics of the interfacial layer (IL) growth between Hf aluminates and the Si substrate during high-temperature rapid thermal annealing (RTA) in either N2 (∼10 Torr) or high vacuum (∼2×10−5 Torr) is studied by high-resolution x-ray photoelectron spectroscopy and cross-sectional transmission electron microscopy. The significant difference of the IL growth observed between high vacuum and relatively oxygen-rich N2 annealing (both at 1000 °C) is shown to be caused by the oxygen species from the annealing ambient. Our results also show that Hf aluminates exhibit much stronger resistance to oxygen diffusion than pure HfO2 during RTA in N2 ambient, and the resistance becomes stronger with more Al incorporated into HfO2. This observation is explained by the combined effects of (i) smaller oxygen diffusion coefficient of Al2O3 than HfO2, and (ii) higher crystallization temperature of the Hf aluminates.
Dielectric and ferroelectric properties of highly oriented (Pb,Nb)(Zr,Sn,Ti)O3 thin films grown by a sol-gel process
Authors: Jiwei Zhai , Cheung M. H. , Xu Zheng Kui , Li Xin , Chen Haydn , Colla Eugene V. , Wu T. B.
Published: October 2002

Antiferroelectric (Pb,Nb)(Zr,Sn,Ti)O3 thin films were deposited via a sol-gel process on LaNiO3-coated silicon substrates. Films showed a strong (001) preferred orientation upon annealing at 500–700 °C for 30 min. The dependence of electrical properties on film thickness has been studied, with the emphasis placed on field-induced phase switching from the antiferroelectric to the ferroelectric state. The decrease of film thickness led to an increase of the phase-switching field along with the appearance of remanent polarization. However, the dielectric constant and maximum polarization decreased with the reduction of film thickness. Saturation polarization was 35 μC/cm2, which is equal to that observed in bulk samples.
Realization and properties of MgB2 metal-masked ion damage junctions
Authors: Kang D.-J. , Peng N. H. , Webb R. , Jeynes C. , Yun J. H. , Moon S. H. , Oh B. , Burnell G. , Tarte E. J. , Moore D. F. , Blamire M. G.
Published: October 2002

Ion beam damage combined with nanoscale focused-ion-beam direct milling was used to create manufacturable superconductor–normal–superconductor type (SNS) Josephson junctions in 100-nm-thick MgB2 with TC of 38 K. The junctions show nonhysteretic current–voltage characteristics between 36 and 4.2 K. Experimental evidence for the dc and ac Josephson effects in MgB2 metal-masked ion damage junctions are presented. This technique is particularly useful for prototyping devices due to its simplicity and flexibility of fabrication and has a great potential for high-density integration.
Surface dihydrides on Ge(100): A scanning tunneling microscopy study
Authors: Maeng Jae Yeol , Lee Jun Young , Cho Young Eun , Kim Sehun , Jo Sam K.
Published: October 2002

We studied the atomic scale surface evolution of Ge(100) exposed at 300 K to gas-phase hydrogen atoms, H(g). Surface H(g) uptake created a 2×1:H phase, quickly reaching ∼1 monolayer H coverage. However, in contrast to the Si(100) surface, dangling bonds of the Ge(100) surface could never be completely removed by H(g) due to their regeneration by highly efficient surface H abstraction. This, together with the instability of surface dihydrides, GeH2(a), inhibited the large-scale formation of 3×1:H and 1×1:H phases. Short GeH2(a) rows, present in small metastable 3×1:H domains formed near defect sites, were etched selectively by H(g), producing line defects.
An investigation into the overseas market servicing strategies of Queen’s Award for Export winners: revisiting the concentration versus spreading debate
Authors: Crick Dave , Bradshaw Robert , Chaudhry Shiv
Published: October 2002

Although a body of knowledge exists on the areas of both export strategy and competitiveness, empirical data have tended to relate to general industrial surveys rather than concentrating on higher‐performing firms. Existing studies have provided mixed results concerning the merits of managers adopting particular international growth strategies. This paper reports on findings from a postal survey of winners of the Queen’s Award for Export, arguably the premier award for export achievement in the UK. Multivariate quantitative analysis of survey data and subsequent findings from interviews indicate that limited statistical differences exist between the performance and competitiveness of two groups drawn from the sample of “successful” firms. The two groups are those that employ as a growth strategy an approach which concentrates on key overseas markets compared with those that spread their efforts over a number of markets.
Necessary conditions for a maximum likelihood estimate to become asymptotically unbiased and attain the Cramer–Rao lower bound. II. Range and depth localization of a sound source in an ocean waveguide
Authors: Thode Aaron , Zanolin Michele , Naftali Eran , Ingram Ian , Ratilal Purnima , Makris Nicholas C.
Published: October 2002

Analytic expressions for the first order bias and second order covariance of a maximum-likelihood estimate (MLE) are applied to the problem of localizing an acoustic source in range and depth in a shallow water waveguide with a vertical hydrophone array. These expressions are then used to determine necessary conditions on sample size, or equivalently signal-to-noise ratio (SNR), for the localization MLE to become asymptotically unbiased and attain minimum variance as expressed by the Cramer–Rao lower bound (CRLB). These analytic expressions can be applied in a similar fashion to any ocean-acoustic inverse problem involving random data. Both deterministic and completely randomized signals embedded in independent and additive waveguide noise are investigated. As the energy ratio of received signal to additive noise (SANR) descends to the lower operational range of a typical passive localization system, source range and depth estimates exhibit significant biases and have variances that can exceed the CRLB by orders of magnitude. The spatial structure of the bias suggests that acoustic range and depth estimates tend to converge around particular range and depth cells for moderate SANR values.
Sweep-spread carrier for underwater communication over acoustic channels with strong multipath propagation
Authors: Kebkal Konstantin G. , Bannasch Rudolf
Published: October 2002

Multipath propagation causes the transmitted signal to take numerous, time-varying different-length paths to the receiver. Exploitation of conventional frequency-constant carrier signals for communication over underwater acoustic channels typically shows that intricate mutual time variations of multipath arrivals create an extremely hard problem for equalizers to compensate for nonstable intersymbol interactions. Communication over such channels can be, however, substantially improved by using a new method based on the implementation of a sweep-spread carrier (S2C). Such a carrier consists of a succession of sweeps, which cause permanent rapid fluctuation of signal frequency. Apart from some additional useful effects such as providing multiuser access and reducing influence of narrow-band noise, the method provides significant processing improvement enabling clear separation of multipath arrivals by converting their time delays into their frequency reallocations—the steeper the sweeps, the better the multipath resolution that can be achieved. When converting the signal into constant intermediate frequencies, the best suitable multipath arrival can be separated not as traditionally in time domains, applying complex equalizers, but in frequency domains by means of usual band-pass filters. High transmission stability of this approach was confirmed in both computer simulations as well as in validation experiments carried out since summer 1999.
Experimental studies of a drumlike silencer
Authors: Choy Y. S. , Huang Lixi
Published: October 2002

The theoretical finding of the broadband performance of a reactive silencer is validated experimentally. The silencer consists of two highly stretched membranes lining part of the duct and backed by two long and shallow cavities. The test rig was built with a small square duct of 5 cm in dimension, and each cavity is 5 cm deep and 25 cm long. Two types of metal foils, stainless steel and copper, were used, and the lowest membrane-to-air mass ratio was 1.3. A transmission loss in excess of 10 dB was achieved over more than one octave band. For one configuration close to the optimal parameters, the predicted ratio of the frequency band limits is 2.47, while the experiment gave 2.35. Three spectral peaks were found in the stopband, as predicted, but the peaks were broader than prediction, indicating the presence of significant sound energy dissipation mechanisms. Comparison with theoretical simulation shows that the cavity damping dominates over membrane friction. Tests using heavier membranes and membrane with different levels of tension also agree with predictions. Issues of practical implementation of the concept as a flow-through silencer are also addressed.
Approximating the head-related transfer function using simple geometric models of the head and torso
Authors: Algazi V. Ralph , Duda Richard O. , Duraiswami Ramani , Gumerov Nail A. , Tang Zhihui
Published: October 2002

The head-related transfer function (HRTF) for distant sources is a complicated function of azimuth, elevation and frequency. This paper presents simple geometric models of the head and torso that provide insight into its low-frequency behavior, especially at low elevations. The head-and-torso models are obtained by adding both spherical and ellipsoidal models of the torso to a classical spherical-head model. Two different numerical techniques—multipole reexpansion and boundary element methods—are used to compute the HRTF of the models in both the frequency domain and the time domain. These computed HRTFs quantify the characteristics of elevation-dependent torso reflections for sources above the torso-shadow cone, and reveal the qualitatively different effects of torso shadow for sources within the torso-shadow cone. These effects include a torso bright spot that is prominent for the spherical torso, and significant attenuation of frequencies above 1 kHz in a range of elevations. Both torso reflections and torso shadow provide potentially significant elevation cues. Comparisons of the model HRTF with acoustic measurements in the horizontal, median, and frontal planes confirm the basic validity of the computational methods and establish that the geometric models provide good approximations of the HRTF for the KEMAR mannequin with its pinnae removed.
Primitive stream segregation of tone sequences without differences in fundamental frequency or passband
Authors: Roberts Brian , Glasberg Brian R. , Moore Brian C. J.
Published: October 2002

Peripheral-channeling theorists argue that differences in excitation pattern between successive sounds are necessary for stream segregation to occur. The component phases of complex tones comprising unresolved harmonics (F0=100 Hz) were manipulated to change pitch and timbre without changing the power spectrum. In experiment 1, listeners compared two alternating sequences of tones, A and B. One sequence was isochronous (tone duration=60 ms, intertone interval=40 ms). The other began isochronously, but the progressive delay of tone B made the rhythm irregular. Subjects had to identify the sequence with irregular rhythm. Stream segregation makes this task more difficult. A and B could differ in passband (1250–2500 Hz, 1768–3536 Hz, 2500–5000 Hz), component phase (cosine, alternating, random), or both. Stimuli were presented at 70 dB SPL in pink noise. Dissimilarity in either passband or phase increased discrimination thresholds. Moreover, phase differences raised threshold even when there was no passband difference. In experiment 2, listeners judged moment-by-moment the grouping of long ABA-ABA-… sequences. The measure was the proportion of time a sequence was heard as segregated. The factors that increased segregation were very similar to those that increased threshold in experiment 1. Overall, the findings indicate that substantial stream segregation can occur without differences in power spectrum. It is concluded that differences in peripheral channeling are not a requirement for stream segregation.
The time-dependent magneto-visco-elastic behavior of a magnetostrictive fiber actuated viscoelastic polymer matrix composite
Authors: Hogea Cosmina S. , Armstrong William D.
Published: October 2002

The paper develops a one-dimensional magneto-elastic model of a magnetostrictive fiber actuated polymer matrix composite material which accounts for a strong viscoelastic response in the polymer matrix. The viscoelastic behavior of the composite polymer matrix is modeled with a three parallel Maxwell element viscoelastic model, the magnetoelastic behavior of the composite fibers is modeled with an anhysteric directional potential based domain occupation theory. Example calculations are performed to identify and explain the dynamical behavior of the composite. These calculations assume that a constant stress and the oscillating magnetic field are applied in the fiber longitudinal direction. The inclusion of matrix viscosity results in an apparent hysteresis loop in the magnetization and magnetostriction curves even though the model does not include magnetoelastic hysteresis in the fibers. The apparent hysteresis is a consequence of the interaction of the time varying fiber stress caused by matrix viscosity with a multidomain state in the fiber. The small increase in fiber longitudinal compressive stress due to matrix viscosity under increasing field inhibits the occupation of domains with magnetization orientations near the fiber longitudinal [112] direction. As a consequence, the summed longitudinal magnetization and magnetostriction is reduced as compared to the decreasing field limb.
Object localization from acoustic emissions produced by other sonars (L)
Authors: Kuc Roman
Published: October 2002

When multiple active sonars, such as dolphins, bats, or mobile robots, interrogate the environment simultaneously, one view is that each sonar considers the signals produced by the others to be interference, while another view is that these signals contain useful information. A listening sonar can typically detect the time delay between another sonar’s interrogation emission and its corresponding echo from an object and possibly also the emission and echo directions. This letter first shows that, even if the directions of these two signals and relative delays are known, determining the object location is an ill-posed problem, supporting the interference view. It then shows the time delay alone provides useful information, indicating that no echo-producing object can be closer than a perceived minimum range. Echoes from specular surfaces provide similar information but also require self echolocation to refine the range estimate. Experimental acoustic waveforms illustrate the approach. The results indicate that traveling in fixed formation and emitting identifiable echolocation sounds faciliate cooperative echolocation by biological sonars.