bluuebunny/crossref_metadata_2025 huggingface dataset:
Indium nanowires in thick (InGaN) layers as determined by x-ray analysis
Authors: Krost A. , Bläsing J. , Protzmann H. , Lünenbürger M. , Heuken M.
Published: July 2002

300-nm-thick InGaN layers with In concentrations up to 21% were grown by low-pressure metalorganic chemical-vapor deposition. Besides the InGaN and GaN Bragg peaks, the symmetric (0002) x-ray spectra of strain-relaxed samples show an additional signal which could be identified stemming from metallic tetragonal indium. The indium is incorporated in InGaN with its pseudohexagonal (101) plane parallel to the InGaN(0001) plane in a sixfold configuration. From the widths and intensities of the asymmetric In(211) and the symmetric In(h0h) diffraction peaks, the lateral and perpendicular crystallite sizes of the In inclusions are estimated to be ∼30 and ∼300 nm, respectively, i.e., the In is incorporated in a wire-like manner in the growth direction. In InGaN-based devices such indium wires could act as highly conducting channels detrimental for electronic and optoelectronic applications.
Nanosecond transients in the electroluminescence from multilayer blue organic light-emitting devices based on 4,4′-bis(2,2′diphenyl vinyl)-1,1′-biphenyl
Authors: Savvate’ev V. , Friedl J. H. , Zou L. , Shinar J. , Christensen K. , Oldham W. , Rothberg L. J. , Chen-Esterlit Z. , Kopelman R.
Published: July 2002

Nanosecond electroluminescence (EL) overshoots observed when multilayer blue 4,4′-bis(2,2′-diphenyl vinyl)-1,1′-biphenyl (DPVBi)-based organic light-emitting devices (OLEDs) are excited by rectangular voltage pulses are described. The overshoots occur at the voltage turn-off and exceed the cw brightness by up to an order of magnitude. Time-resolved images of the OLEDs demonstrate that the emission from most of the sample surface decays with a single time constant τ1=13±3 ns. This decay is attributed to recombination of charges which accumulate at the interface of the electron and hole transporting layers, possibly at intrinsic trapping sites. In areas of increased electron injection and EL, such as cathode edges and morphological defects, a second slower decay time 20 ns<τ2<1 μs is observed, apparently due to release of carriers from localized trap states in the organic/cathode interface. Only marginal variations in τ1 are found between bright and dim areas of the devices. At a bias of 10 V, the amplitude of the overshoot is found to peak at a pulse duration of ∼20 μs. Its behavior is believed to result from increased quenching of singlet excitons by the accumulated charges.
Room-temperature electroluminescence from electron-hole plasmas in the metal–oxide–silicon tunneling diodes
Authors: Liu C. W. , Lee M. H. , Chen Miin-Jang , Lin I. C. , Lin Ching-Fuh
Published: July 2002

An electron-hole plasma recombination model is used to fit the room-temperature electroluminescence from metal–oxide–silicon tunneling diodes. The relatively narrow line shape in the emission spectra can be understood by the quasi-Fermi level positions of electrons and holes, which both lie in the band gap. This model also gives a narrower band gap than that of bulk silicon. The surface band bending in the Si/oxide interface is responsible for this energy gap reduction.
Magnetic-field-dependent plasma composition of a pulsed arc in a high-vacuum ambient
Authors: Schneider Jochen M. , Anders André , Hjörvarsson Björgvin , Hultman Lars
Published: July 2002

The effect of a magnetic field on the plasma composition of a pulsed Au plasma stream in a high-vacuum ambient is described. The plasma was formed with a pulsed vacuum-arc-plasma source, and the time-resolved plasma composition was measured with time-of-flight charge-to-mass spectrometry. Plasma impurities due to ionization of nonmetallic species (H+, O+, and N+) were found to be below the detection limit in the absence of a magnetic field. However, in the presence of a magnetic field (0.4 T), the contribution of ionized nonmetal species to the plasma composition was up to 0.22 atomic ratio. These results are characteristic of plasma-based techniques where magnetic fields are employed in a high-vacuum ambient. In effect, the impurity incorporation during thin-film growth pertains to the present findings.
Recombination dynamics of carriers in an InGaN/AlGaN single-quantum-well light-emitting diode under reverse-bias voltages
Authors: Kudo Hiromitsu , Ishibashi Hiroki , Zheng Ruisheng , Yamada Yoichi , Taguchi Tsunemasa
Published: July 2002

The radiative recombination process of the blue emission band in an InGaN single-quantum-well light-emitting diode has extensively been investigated by means of the dependence of an external electric field on photoluminescence and time-resolved photoluminescence spectra. Two emission (higher and lower) components separated by about 40 meV are found in the emission band on the condition of reverse bias at 77 K. It is also found that the luminescence intensity decreases dramatically with increasing reverse-bias voltage at room temperature. The model based on field ionization of excitons cannot explain the present experimental phenomena. It is, therefore, suggested that the free-carrier recombination process is dominant at room temperature.
Nitrogen vacancy scattering in n-GaN grown by metal-organic vapor phase epitaxy
Authors: Zhu Qin-Sheng , Sawaki Nobuhiko
Published: July 2002

Electron mobility limited by nitrogen vacancy scattering was taken into account to evaluate the quality of n-type GaN grown by metal-organic vapor phase epitaxy. The nitrogen vacancy scattering potential used for our mobility calculation has to satisfy two requirements: such potential is (1) spatially short range, and (2) finite and not divergent at the vacancy core. A square-well potential was adopted to calculate the mobility, because it satisfies not only these two requirements, but also simplifies the calculation. As a result, the estimated mobility shows a T−1/2 temperature dependence, and is very sensitive to the potential well width. After introducing the nitrogen vacancy scattering, we obtained the best fitting between the calculated and experimental results for our high quality sample, and it was found that the measured mobility is dominated by ion impurity and dislocation scatterings at the low temperatures, but dominated by optical phonon and nitrogen vacancy scatterings at the high temperatures.
Improvement of output coupling efficiency of organic light-emitting diodes by backside substrate modification
Authors: Madigan C. F. , Lu M.-H. , Sturm J. C.
Published: July 2002

The emission intensity of an organic light-emitting diode at normal viewing angle and the total external emission efficiency have been increased by factors of 9.6 and 3.0, respectively, by applying spherically shaped patterns to the back of the device substrate. The technique captures light previously lost to waveguiding in the substrate and, with proper choice of substrate, light previously lost to waveguiding in the organic/anode layers. A method of applying the technique using laminated films and an optical model for evaluating coupling efficiency are also presented.
Influence of buffer layer and growth temperature on the properties of an undoped GaN layer grown on sapphire substrate by metalorganic chemical vapor deposition
Authors: Wang T. , Shirahama T. , Sun H. B. , Wang H. X. , Bai J. , Sakai S. , Misawa H.
Published: July 2002

The influence of low-temperature buffer layer thickness on the electrical properties of GaN film is investigated, and the surface morphology is also examined by atomic force microscopy. A best surface morphology does not show best electrical properties, which could be attributed to the usual growth mechanism for GaN film on sapphire substrate. The influence of the growth temperature for the final GaN layer is also investigated. When the growth temperature increases to 1100 °C, the mobility is greatly enhanced to 600 cm2/V s with a background carrier density of 3.3×1016/cm3 at room temperature. The emission energy of the near band gap exciton at a low temperature shows a blueshift with increasing growth temperature due to an enhanced thermal stress. The calculation based on a thermal stress model agrees very well with the photoluminescence measurement. This result could partly explain the reason that the previously published values for the near band gap exciton emission energy are scattered.
Connecting disorder and magnetic properties in CoFe thin films
Authors: Freeland J. W. , Bussmann K. , Idzerda Y. U.
Published: July 2002

Through a direct study of the chemical and magnetic interfacial disorder in conjunction with magnetic property measurements, evidence is presented that the grain size is the ultimate factor in determining the magnetic coercivity in CoFe thin film structures.
Photocarrier injection effect on double exchange ferromagnetism in (La, Sr)MnO3/SrTiO3 heterostructure
Authors: Katsu Hayato , Tanaka Hidekazu , Kawai Tomoji
Published: July 2002

We have fabricated a (La1−xSrx) MnO3/SrTiO3 heterostructure and measured the temperature dependence of magnetization and resistivity under white light irradiation. We found that increased resistivity and a decrease of the metal-to-insulator transition temperature by 10 K occur under illumination of light at a wavelength below 400 nm. In particular, an additional photoinduced metal–semiconductor transition is observed below 100 K with decreased magnetization. This behavior can be explained by considering that the hole concentration in the (La1−xSrx) MnO3 layer decreases owing to electron injection following the photoexcitation of SrTiO3. We also confirmed electron injection along a perpendicular direction to the heterostructure using current–voltage measurement.
Picosecond dynamics of terahertz-sideband generation in far-infrared illuminated quantum wells
Authors: Citrin D. S.
Published: July 2002

Optical excitation of a quantum well illuminated by a strong far-infrared (FIR) field is known to give rise to sidebands on the optical spectra separated from the fundamental by multiples of twice the FIR frequency Ω. In this study, we consider the picosecond dynamics of terahertz (THz) sideband formation using picosecond optical excitation pulses. We find that the emission at the THz sidebands largely tracks the temporal evolution of the excitation pulse. Applications to wavelength-division multiplexing are discussed.
Initial oxynitridation of a Si(001)-2×1 surface by NO
Authors: Miyata Noriyuki , Watanabe Heiji , Ichikawa Masakazu
Published: July 2002

We investigated the initial oxynitridation of an atomically flat Si(001)-2×1 surfaces by NO. The first oxynitridation, in which NO reacts with the first Si layer on the Si(001)-2×1 surface, takes place in a layer-by-layer manner caused by two-dimensional nucleation. However, further oxynitridation for the second Si layer proceeds in a three-dimensional manner in which the atomic-scale roughness at the oxynitride/Si interface increases. In addition, it was found that N is incorporated as N≡Si3, even though the oxynitride is ultrathin.
Quantitative evaluation of growth-induced residual stress in InP epitaxial micromechanical structures
Authors: Chitica N. , Strassner M. , Daleiden J.
Published: July 2002

We have investigated the residual stress distribution and its relation to the presence of impurity atoms in InP layers grown by metalorganic vapor phase epitaxy (MOVPE) on top of lattice-matched GaInAs layers. The homogeneous and gradient components of the stress have been directly measured using highly sensitive micromechanical test structures. Secondary ion mass spectrometry depth profiling of the composition evidenced that residual As and Ga atoms are present throughout the relatively thick (>0.5 μm) InP layers. Our investigations show that the distribution of these impurities accounts for the measured residual stress in the layers and, at the same time, it is specific to the MOVPE growth technique.
Morphological instability of growth fronts due to stress-induced mobility variations
Authors: Sage Jennifer F. , Barvosa-Carter William , Aziz Michael J.
Published: July 2002

We report a comparison between theory and experiment for a general stress-induced morphological growth instability that is kinetically rather than energetically driven. Stress variations along a perturbed planar growth front result in variations in interfacial mobility in a manner that is destabilizing under one sign of the stress state and stabilizing under the opposite sign, even for a pure material. Investigation of solid-phase epitaxial growth at a corrugated Si(001) interface under both compression and tension results in good agreement between experiment and theory with no adjustable parameters, demonstrating that this mobility-based mechanism is dominant in determining morphological evolution in this system.
Optical microcavities based on amorphous silicon–nitride Fabry–Pérot structures
Authors: Giorgis Fabrizio
Published: July 2002

All amorphous silicon–nitride (a-Si1−xNx:H) optical microcavities have been grown by plasma-enhanced chemical vapor deposition. The basic structure is a Fabry–Pérot resonator with a N-rich a-Si1−xNx:H layer as optical active medium. By controlling layer thickness and composition, homogeneous structures have been obtained on a large area. Tunable narrow emission bands (few nm) and a strong resonant enhancement of the luminescence yield (more than 1 order of magnitude) have been demonstrated. These results, which can be accounted for by a light intensity spatial redistribution owing to the anisotropic phonon density of states in the cavities, open up new possibilities for applications in thin film flat panel displays.
Steady solutions for bubbles in dipole-driven Stokes flows
Authors: Cummings L. J.
Published: July 2002

In this paper we present a new family of steady bubble solutions in two-dimensional slow viscous flow, driven by a dipole singularity within the flow and surface tension effects at the bubble boundary. These solutions generalize the semi-infinite domain solution constructed by Jeong and Moffatt [J. Fluid Mech. 241, 1 (1992)] to describe their experimental observations for free surface flow in a large tank driven by counter-rotating rollers (the dipole singularity models the action of the rollers).
Scale distributions of fluid interfaces in turbulence
Authors: Catrakis Haris J. , Bond Christopher L.
Published: July 2002

Scale distributions of high- and low-dimensional transects of fluid interfaces in turbulence are analyzed and compared. For mixed-fluid interfaces derived from concentration measurements in turbulent jets at Re∼104 and Sc∼103, the relative coverage dimensions of spatial two-dimensional and one-dimensional transects agree at the smallest and largest scale, as expected, but are found to be different at intermediate scales: the higher-dimensionality transects have a larger relative coverage dimension and, in this sense, exhibit more structure. This behavior has implications for quantifying turbulent mixing in terms of the geometry of fluid interfaces.
A detailed test of mode-coupling theory on all time scales: Time domain studies of structural relaxation in a supercooled liquid
Authors: Hinze G. , Brace David D. , Gottke S. D. , Fayer M. D.
Published: July 2002

The dynamics of supercooled salol (phenyl salicylate) was measured in the time domain using optical Kerr effect techniques. By combining several experimental setups, data spanning more than six decades in amplitude and time (∼100 fs to ∼1 μs) were observed. The data have a complex shape, ranging from high-frequency intramolecular oscillations at short times, to nearly exponential relaxation at long times. As predicted by mode-coupling theory (MCT), the data for some ranges of time appear as power laws. The slowest power law, the von Schweidler power law, has an almost constant exponent of ∼0.59 over the entire temperature range studied (247–340 K). Above the MCT Tc (T>∼1.17 Tg, where Tg is the laboratory glass transition temperature) for t>∼1 ps, the decays are shown to be in excellent agreement with the master curve predicted by ideal MCT when higher order terms are included. However, the data do not display the plateau predicted by ideal MCT. To discuss the data at all temperatures, the intermediate time scale portion of the data, 2<t<10 to 500 ps (depending on the temperature), is modeled as a power law that falls between the critical decay and the von Schweidler power law. This intermediate power law shows significant temperature dependence with an exponent that decreases to a value of ∼−1 below Tc. Calculations using extended MCT, for a full range of hopping times, demonstrate that the temperature dependence of the intermediate time scale data near and below Tc cannot be explained by extended MCT.
New electric quadrupole transitions of K2 observed in a pulsed molecular beam: The 1 1Δg–X 1Σg+ bands near 500 nm
Authors: Joo Duck-Lae , Yoon Youngjee , Lee Yonghoon , Baek Sun Jong , Kim Bongsoo
Published: July 2002

A new electronic transition of K2 is identified near 500 nm by resonance enhanced two photon ionization study. The rotational structure shows O, P, Q, R, and S branches. We obtain molecular constants for the upper electronic state of 39K2 as ωe=79.22±0.02 cm−1, ωexe=0.189±0.002 cm−1, Te=19 855.74±0.02 cm−1, B0′=(5.181 05±0.000 06)×10−2 cm−1, and D0′=(8.65±0.03)×10−8 cm−1. By comparison with the ab initio calculated values and from other experimental evidence we assign the observed bands to the 1 1Δg–X 1Σg+ transitions of K2, which are allowed by electric quadrupole radiation.
Spatial pattern formation of optically excited carriers in photoconductive switches
Authors: Bieler M. , Hein G. , Pierz K. , Siegner U. , Koch M.
Published: July 2002

We have spatially resolved the photoluminescence patterns emitted from a GaAs/Al0.3Ga0.7As quantum well after femtosecond laser excitation for different electric bias fields applied in the plane of the well. These patterns demonstrate substantially different electron–hole dynamics across the excitation laser spot. The density dependence of screening combines with the lateral variation of the carrier density to produce the observed effects. These results prove that the spatial variation of the carrier dynamics is to be taken into account for the optimization of photoconductive switches as ultrafast voltage pulse sources or THz emitters.
Absolute densities of long lived species in an ionized physical vapor deposition copper–argon plasma
Authors: Andrew Y. , Abraham I. , Booske J. H. , Lu Z. C. , Wendt A. E.
Published: July 2002

Optical absorption spectroscopy has been used to measure absolute, average gas phase densities of neutral copper, ground and metastable states, and neutral argon, metastable and resonance states, in an ionized physical vapor deposition plasma. Spectroscopic measurements were carried with a xenon arc lamp as a high intensity, continuum light source, and an optical multichannel detector. Copper radiative transitions in the wavelength range of 324.8–510.6 nm and argon radiative transitions in the 706.7–811.5 nm range were employed. The curve of growth method has been used to calculate the absolute line average densities from fractional absorption data. For a copper–argon plasma of neutral pressure 30 and 10 mTorr copper metastable state densities were found to lie in the range of 1010–1012 cm−3. Comparison of these densities with neutral copper densities derived from independent measurements of neutral copper flux at the substrate indicate gas phase temperatures greater than 1500 K under certain experimental conditions. These values of inferred temperatures indicate the copper metastable state density to be significant in comparison with neutral copper ground state densities at 10 and 30 mTorr with radio frequency heating power of 1 kW. The concentrations of argon 4s sublevels of the first excited state were found to be in the range of 4.5×108–1.5×1011 cm−3 for the experimental conditions studied. The ordering of the relative densities of the argon 4s sublevels and the variation of the lumped first excited state with experimental parameters are discussed.
Domain switching and electromechanical properties of pulse poled Pb(Zn1/3Nb2/3)O3–PbTiO3 crystals
Authors: Yu Hanxing , Gopalan Venkat , Sindel Jürgen , Randall Clive A.
Published: July 2002

Domain switching behavior is studied under pulse field conditions in (1−x)Pb(Zn1/3Nb2/3)O3–xPbTiO3 single crystals oriented at various crystallographic directions. The activation energy, critical field, and domain wall mobilities are determined from simple methods earlier employed to ferroelectric single crystals by Merz [Phys. Rev. 95, 690 (1954)]. Domain structures were observed by atomic force microscopy of etched surfaces and shown to have a dendritic morphology under pulse conditions. The size and density of these dendritic domains are compared for various pulse conditions and discussed in terms of nucleation and domain growth behavior. The pulsed electric fields were also utilized to pole the crystals in the [001]C directions, and the corresponding piezoelectric strains were determined.
Small mesoscopic particles in dilute and semidilute solutions of nonadsorbing polymers
Authors: Eisenriegler E.
Published: July 2002

Polymer-induced interactions between two small mesoscopic particles of radius R and between a particle and a wall are investigated for dilute or semidilute embedding solutions of long flexible nonadsorbing polymer chains with radius of gyration Rg. Asymptotically exact predictions are obtained using a “small radius expansion,” to express the interactions in terms of properties of the polymer solution without particles. The nonmonotonic dependence of the second virial coefficient B2CC of a dilute suspension of particles on the interchain overlap is discussed in detail. The magnitude of the minimum of B2CC/R3 increases as a power law in Rg/R. The exponent and amplitude are quite different from the earlier prediction of an integral-equation approach. For dilute polymers in two dimensions outside two circular disks in contact, exact results are given for the monomer-density depletion profile, the pressure variation along the perimeter of, and the polymer-induced force between the two disks.
Time and temperature invariances in the evolution of properties through the glass transition
Authors: Mascarell Juan Bisquert , Garcia-Belmonte Germà
Published: July 2002

In this paper we analyze relaxation phenomena of amorphous materials near or below the glass transition. A phenomenological theory is suggested that maintains the main ingredients of the widely accepted models, i.e., those quasi-universal properties of structural relaxation which are well established, while a new approach is adopted for constructing the overall relaxation under a given temperature history. The evolution of the relaxational part of a property p under time–temperature changes is described by a first order relaxational equation that states that the instantaneous advance of the relaxation is proportional to the amount of deviation from equilibrium. The model consistently combines three different principles: (a) Linearity of response, (b) time–temperature re-scaling, and (c) power law relaxation at short times. This is achieved by imposing the following requirements on the relaxational equation: that the equation be expressed as a unique function of the reduced time; and that it provides the Kohlrausch–Williams–Watts relaxation law in the particular case of a temperature jump experiment. In addition, the relaxation time is not a function of fictive temperature. This approach provides as an outcome a new type of superposition over past perturbations. The analysis of rate heating/cooling experiments shows that the model reproduces the hysteresis of the fictive temperature and the peaks in heat capacity curves frequently observed in experiment. The physical meaning of the shift relationship between cooling rate and fictive temperature is critically examined on the basis of scaling properties and relaxational properties and some limitations of the standard result are identified. A more general and physically reasonable relationship is obtained by rigorous derivation in the framework of the new model. It is therefore demonstrated that that relationship is not related to nonlinearity, contrary to what is widely believed. In addition, it is shown that the more general relationship involves the parameter β describing the slowing down of the relaxation. This provides the basis for new relations to be inferred between apparently different phenomenological properties. An explanation is advanced for the observed correlations between measured parameters in the Tool–Narayanaswamy–Moynihan phenomenology. © 2000 American Institute of Physics.
A hierarchy of generalized AKNS equations, N-Hamiltonian structures and finite-dimensional involutive systems and integrable systems
Authors: Yan Zhen-ya , Zhang Hong-qing
Published: July 2002

An eigenvalue problem and the associated hierarchy of nonlinear soliton equations are proposed in this paper. In particular, a representative system of generalized AKNS soliton equations in the hierarchy is given, namely, qt=−12 qxx+q2r−2μq2rx−2μrqqx+2μ2q3r2, rt=12 rxx−qr2−2μr2qx−2μqrrx+2μ2q2r3. N-Hamiltonian structures are also established for all the hirarchy of generalized AKNS soliton equations based on N+1 pairs of Hamiltonian pairs. And then the eigenvalue problem is nonlinear as a finite-dimensional completely integrable Hamiltonian system under the Bargmann constraint between the potentials and the eigenvalue functions, and its involutive system is also given. Finally, the involutive solutions of the hierarchy of generalized AKNS soliton equations are found, in particular, the involutive solutions of the system of generalized Schrödinger equations are given.
Electrical domain fixing of photorefractive index gratings in (K0.5Na0.5)0.2(Sr0.75Ba0.25)0.9Nb2O6 crystals
Authors: Shen Xiaonong , Zhao Jianhua , Wang Ruibo , Cheng Zhenxiang , Zhang Shujun , Chen Huanchu
Published: July 2002

Electrical domain fixing in Cu- and Mn-doped (K0.5Na0.5)0.2(Sr0.75Ba0.25)0.9Nb2O6 (KNSBN) crystals is presented. The relation between the applied field and the revealed grating in single and multidomain crystals is investigated. In multidomain Mn-doped KNSBN, a revealing efficiency of over 233% is obtained. The mechanism of domain fixing in single and multidomain crystal is discussed.
Thickness dependence of stress in lead titanate thin films deposited on Pt-coated Si
Authors: Fu Desheng , Ogawa Takeshi , Suzuki Hisao , Ishikawa Kenji
Published: July 2002

Thickness dependence of the soft mode E(1TO) of tetraganol lead titanate thin film, deposited on Pt-coated Si by a chemical solution deposition, was determined with Raman scattering measurements. A downshift of the soft mode was attributed to the residual stress in the thin film, which was estimated in the range of 1.3–2.6 GPa, corresponding to film thickness of 400–50 nm. The variation of the clamped dielectric constants determined by observed mode frequencies was found to agree with the prediction of stress dependence of dielectric constants by Devonshire theory.
Dipole scattering in polarization induced III–V nitride two-dimensional electron gases
Authors: Jena Debdeep , Gossard Arthur C. , Mishra Umesh K.
Published: July 2002

Unusually large spontaneous and piezoelectric fields in the III–V nitrides have led to the making of an entirely new class of two-dimensional electron gas. Fluctuation from a perfectly periodic binary structure in highly polar semiconductor alloys present the same physical situation as a random distribution of microscopic dipoles. The excess dipole distribution in the barrier layers is evaluated by a method similar to the virtual crystal approximation. It is shown that the mobility of electrons in the two-dimensional electron gas formed in highly polar heterostructures is intrinsically limited by scattering from such dipoles.
Prism coupler with variable coupling gap
Authors: Dawson P. , Cairns G. F. , O’Prey S. M.
Published: July 2002

The construction and operation of a prism/variable-gap/sample system (or variable-gap Otto coupler) for the excitation of surface electromagnetic modes is reported. This system has been used for the observation and characterization of surface plasmon polaritons on thin film structures. The initial alignment of prism and sample is performed under gravity and the subsequent gap variation is performed by means of a single actuator operating a flexure stage on which the prism is mounted. The flexure stage ensures the maintenance of good parallelism between sample and prism as the gap dimension is varied. The coupler has also served as a prototype, in terms of design principle, for the construction of a more sophisticated, variable-gap Otto coupler that can operate in vacuum at temperatures from ambient to 85 K.
Evaluation of lattice distortion near c-ZrO2/α-Al2O3 interface by plasmon loss
Authors: Yamada Susumu , Ohta Joji , Kusanagi Hideo
Published: July 2002

Plasmon energies near a cubic-ZrO2 (c-ZrO2)/α-Al2O3 interface were studied by electron energy loss spectroscopy using a field-emission transmission electron microscope. The plasmon energies measured by scanning an electron probe of 1 nm in diameter across the interface showed that, approaching the interface, the plasmon energy of c-ZrO2 increases from 23.9 to 24.3 eV and that of α-Al2O3 decreases from 24.3 to 23.4 eV. Energy dispersive x-ray spectroscopy revealed that fluctuation of the chemical composition across the interface is small (⩽5%) and causes only a slight change in the plasmon energies (⩽0.16 eV). We conclude that the large energy changes (up to 0.9 eV) are mainly caused by lattice distortion. Using the Drude model, it was found that, 3 nm from the interface, the unit cell volume of c-ZrO2 decreases by 3% and that of α-Al2O3 increases by 6% compared with the volumes at 50 nm. The presence of lattice distortion near the interface was also confirmed by the convergent-beam electron diffraction method. The results show that plasmon loss is useful to evaluate lattice distortion.
Edge-type misfit dislocations produced by thermal processing of pre-relaxed InxGa1−xAs/GaAs heterostructures
Authors: Liu X. W. , Hopgood A. A. , Usher B. F. , Wang H. , Braithwaite N. S.
Published: July 2002

Dislocation structures are presented for GaAs/InxGa1−xAs/GaAs heterostructures before and after thermal processing. Cathodoluminescence has allowed nondestructive examination of bulk specimens, while transmission electron microscopy has been used to establish the details of the dislocation configurations. In each as-grown specimen the thickness of the InxGa1−xAs layer was above its critical value, so 60° misfit dislocations were already present. It is shown that new pure edge, i.e., 90°, dislocations are formed at the interfaces by thermal processing at 1040 K. Their Burgers vectors are a/2〈101〉 perpendicular to their 〈010〉 directions. Although individual 90° misfit dislocations are more effective relievers of strain than 60° ones, the self-energy for an array of such dislocations is higher and hence 60° misfit dislocations form first. A model of the formation of 90° edge misfit dislocations is proposed in which the climb of vacancy-producing jogs on pre-existing 60° dislocations leaves a trailing dislocation dipole.
A study of the condensed phases and solid–solid phase transition in toluene: A Monte Carlo investigation
Authors: Kumar A. V. Anil , Yashonath S. , Chaplot S. L.
Published: July 2002

A Monte Carlo study of the orthorhombic(β), monoclinic(α), and liquid phases of toluene in the isobaric isothermal ensemble employing variable shape simulation cell is reported here. The intermolecular potential of Williams and Starr is seen to reproduce the lattice parameters and other known properties reasonably well for the α-phase. The β-phase is not reproduced as well. The structure has been characterized in terms of the radial distribution functions and orientational correlation functions. The transition from the orthorhombic low temperature β-phase to the high temperature monoclinic α-phase has been successfully simulated. The transition is first order and lies between 140 and 145 K in agreement with experiment. The reverse transition from the α- to the β-phase does not take place in agreement with experiment. The liquid phase density and the heat of vaporization are reproduced well. The potential employed predicts an interaction energy which is about 5% in excess of the experimental value. The orientational correlation function and the radial distribution functions are sensitive to the potential and suggest where improvements are possible.
Ab initio calculations of molecular resonant photoemission spectra
Authors: Carravetta V. , Ågren H. , Vahtras O. , Jensen H. J. Aa.
Published: July 2002

An ab initio method for calculations of molecular resonant photoemission (RPE) spectra is described. The method includes a multicenter expansion of both the dipole matrix element—direct emission—and the Hamiltonian matrix element between the resonant state and the autoionizing states—resonant emission. These quantities are relevant for the description of the process both in the two-step model, where the spectrum is computed at the resonance energy only, and in the one-step model where, by a K-matrix approach, the direct-to-resonant interference is taken into account and the electronic line profile is fully described. The resonant two-electron matrix elements are evaluated over the core-excited relaxed orbitals with the outgoing Auger electron orbital expanded on an augmented multicentered Gaussian basis set. Stieltjes imaging is shown to work excellently for such Gaussian basis sets giving correct continuum normalization matrix elements even for RPE electron energies as high as 100–1000 eV. A numerical investigation is carried out for the participator decay of the C 1s→π* and O 1s→π* states of CO.
Spectral and resonance properties of δ- and δ′-type interactions in relativistic quantum mechanics
Authors: Hounkonnou M. N. , Avossevou G. Y. H.
Published: July 2002

We discuss spectral and resonance properties of exactly solvable Dirac Hamiltonians corresponding to a surface δ and δ′ interactions. First, we study spectral properties for δ-sphere and δ′-sphere models. Next, we analyze the resonance phenomena for the same models with specific boundary conditions.
The weakly bound complex CO–orthoD2: Detection of millimeter-wave transitions
Authors: Surin Leonid A. , Dumesh Boris S. , Winnewisser Gisbert , Pak Igor
Published: July 2002

For the first time two rotational transitions of the CO–orthoD2 van der Waals molecular complex have been recorded in a supersonic jet and assigned to the two R-branch transitions, R(1) (J,j,l)=2,1,1←1,0,1 112 806.515(50) MHz, R(2)=3,1,2,←2,0,2 119 746.484(50) MHz. These transitions were detected with the highly sensitive intracavity spectrometer based on the millimeter wave generator, called OROTRON. Their measured frequencies are in close agreement with the frequency positions predicted from the infrared spectrum, which accompanies the fundamental band of CO in the 4.7 μm region, and which has been published recently by McKellar [J. Chem. Phys. 112, 9282 (2000)]. The millimeter wave transitions will allow a precise determination of the rotational levels with different parity in the vibrational vCO=0 ground state.
Rovibrational Hamiltonians for general polyatomic molecules in spherical polar parametrization. III. Global vs local axis system and angular coordinates
Authors: Mladenović Mirjana
Published: July 2002

We study different parametrizations of the angular space of polyatomic molecules for an orthogonal description of the molecular geometry. Kinetic energy operators for pentatomic molecules, given by compact and computationally useful forms in a global and a local formulation of the axis system, are compared and discussed. A new decomposition of T̂ for sequentially bonded pentatomic molecules in conjunction with a basis of Wigner and associated Legendre functions provides kinetic energy matrix elements which are free of singularities. Practical problems caused by an unusual volume element for a description involving only intervector (bending) angles are addressed. The corresponding rovibrational kinetic energy operators are derived for the two-vector body-fixed formulation.
Inhomogeneous viscosity fluid flow in a wide-gap Couette apparatus: Shear-induced migration in suspensions
Authors: Haber Shimon , Brenner Howard
Published: July 2002

The first portion of this two-part paper investigates the short time evolution of a low Reynolds number flow characterized by a spatially inhomogeneous viscosity within the annular domain between two widely separated concentric circular cylinders undergoing relative rotation. The viscosity is regarded as a material property and as such is convected with the fluid. Any possible “diffusion” of this viscosity is supposed negligible, at least in the short times of interest in our calculation. The initial viscosity field, assumed to be only slightly inhomogeneous, is expanded into a Fourier series with respect to the polar angle, and the contributions of the zeroth and first harmonics are subsequently addressed. Approximate short-time analytic solutions for the velocity and viscosity fields are obtained. In the second part of this paper the results of the preceding analysis are employed in an attempt to gain insight into the experimentally observed shear-induced migration of particles in suspensions being sheared in a wide-gap Couette apparatus. The connection of the inhomogeneous viscosity problem studied in the first part to such shear-induced migration phenomena lies in the assumption that the local viscosity of a suspension of (non-Brownian) particles is functionally dependent only upon the local suspended particle volumetric fraction. In such circumstances, the local transport of suspended particles corresponds to a concomitant transport of the local suspension viscosity and vice-versa. Subject to the foregoing interpretation and limited by algebraic tractability to short times, a global radial migration is predicted. It increases with an increase in the annular gap size between the cylinders and depends upon the phase angle between the rotating outer and inner cylinders, but not upon their relative circumferential velocity—a conclusion consistent with experimental observations. Further, to leading order, particle migration is found to be independent of purely radial viscosity disturbances (the zeroth harmonic) and to arise entirely from coupling between circumferential disturbances in the velocity and viscosity (i.e., particle concentration) fields. The solution also indicates that the high shear-rate region, proximate to the inner wall, may either become less viscous on average (thereby predicting net radial migration away from the high shear rate region) or, conversely, more viscous (corresponding to migration toward the high shear rate region migration). The latter case arises in circumstances that involve a large positive radial gradient in the viscosity’s first harmonic.
Mixing in two-dimensional vortex interactions
Authors: Rogberg Peter , Dritschel David G.
Published: July 2002

We examine the mixing of a passive tracer initially contained within several widely separated vortex patches in a two-dimensional, nearly inviscid, incompressible flow. The initial vortex positions and sizes are chosen so that they collapse toward a common center, resulting in a strong interaction. The area of tracer ejected from the vortices is found to be well correlated with the departure of each tracer contour from an ideal elliptical shape. This result appears to be more widely applicable, and, in particular, it may be useful for quantifying small-scale mixing in realistic atmospheric and oceanic flows.
On the deviatoric normal stress on a slip surface
Authors: Chen Kang Ping , Saric William , Stone Howard A.
Published: July 2002

A general formula for the deviatoric normal stress on a slip surface for incompressible flows is derived and its application is discussed.
Regular and singular late-time asymptotes of potential motion of fluid with a free boundary
Authors: Abarzhi S. I.
Published: July 2002

We study theoretically a potential motion of fluid with a free boundary and with no external forces. We derive and integrate the equations describing the flow evolution from the initial to a highly nonlinear stage in 2D and 3D cases and study the influence of the initial conditions in a wide range of parameters. It is shown that at late time a nonlinear structure of bubbles and spikes is generated and the structure is determined by the initial velocity field. The local dynamics of highly symmetric 3D flows has a universal form when expressed in dimensionless units. Bubbles and spikes conserve a near-circular contour, and at a fixed length scale, the 3D solutions depend significantly on the flow symmetry. We show that the influence of the initial conditions could result in nonuniqueness of solutions describing the regular bubble.
Evidence for activated conduction in a single electron transistor
Authors: Morgan Nicole Y. , Abusch-Magder David , Kastner Marc A. , Takahashi Yasuo , Tamura Hiroyuki , Murase Katsumi
Published: July 2002

Conductance measurements are reported for quantum dots fabricated by pattern-dependent oxidation in silicon; the small size of these devices allows observation of single electron behavior to temperatures as high as 200 K. At temperatures above 6 K the zero-bias conductance peaks, periodic in gate voltage, are asymmetric, with heights that increase rapidly with increasing temperature. These features suggest that the dominant conductance mechanism in this temperature range is thermal activation over one of the potential barriers that localize electrons in the quantum dot. Quantitative comparisons are made between the data and the theory of Matveev and Glazman, including fits of the asymmetric line shapes. However, from these fits we find a potential barrier height that is much smaller than the charging energy, indicating that our data cannot be fully explained by this theory. At lower temperatures, the conductance peaks become much less periodic, and the data in this regime suggest the presence of several quantum dots in series.
Critical assessment of the performance of the semiempirical divide and conquer method for single point calculations and geometry optimizations of large chemical systems
Authors: van der Vaart Arjan , Suárez Dimas , Merz Kenneth M.
Published: July 2002

We present a detailed analysis of the performance of the semiempirical divide and conquer method as compared with standard semiempirical MO calculations. The influence of different subsetting schemes involving dual buffer regions on the magnitude of the errors in energies and computational cost of the calculations are discussed. In addition, the results of geometry optimizations on several protein systems (453 to 4088 atoms) driven by a quasi-Newton algorithm are also presented. These results indicate that the divide and conquer approach gives reliable energies and gradients and suggest that protein geometry optimization using semiempirical methods can be routinely feasible using current computational resources.
May quasicrystals be good thermoelectric materials?
Authors: Maciá Enrique
Published: July 2002

We present a theoretical analysis of quasicrystals (QCs) as potential thermoelectric materials. We consider a self-similar density of states model and extend the framework introduced in [G. D. Mahan and J. O. Sofo, Proc. Natl. Acad. Sci. U.S.A. 93, 7436 (1996)] to systems exhibiting correlated features in their electronic structure. We show that relatively high values of the thermoelectric figure of merit, ranging from 0.01 up to 1.6 at room temperature, may be expected for these systems. We compare our results with available experimental data on transport properties of QCs and suggest some potential candidates for thermoelectric applications.
Discontinuous trajectories of Lagrangian systems with singular hypersurface
Authors: Pugliese F. , Vinogradov A. M.
Published: July 2002

It is shown that a Lagrangian system whose Legendre transformation degenerates along a hypersurface behaves in a strange manner by jumping from time to time without any “visible cause.” In such a jump the system changes instantaneously its coordinates as well as its momenta. Necessary elements of the general theory of such systems are reported and a detailed description of a postrelativistic oscillator showing such a behavior is given.
Ultrafast high-contrast all-optical switching using spin polarization in low-temperature-grown multiple quantum wells
Authors: Takahashi Ryo , Itoh Hiroki , Iwamura Hidetoshi
Published: July 2002

We have demonstrated an ultrafast high-contrast surface-reflection all-optical switch using a spin-polarization scheme. The device is made with low-temperature-grown Be-doped strained InGaAs/InAlAs multiple quantum wells. It has a photoresponse of 300 fs due to ultrafast electron trapping. The on/off ratios have been drastically improved by more than 20 dB, and reached over 30 and 40 dB at pump pulse energies of 4 and 14 pJ, respectively. Furthermore, we have obtained polarization insensitive operation by handling the two orthogonal components of the signal pulse at two different points.
A spinor-like representation of the contact superconformal algebra K′(4)
Authors: Poletaeva Elena
Published: July 2002

In this work we construct an embedding of a nontrivial central extension of the contact superconformal algebra K′(4) into the Lie superalgebra of pseudodifferential symbols on the supercircle S1/2. Associated with this embedding is a one-parameter family of spinor-like tiny irreducible representations of K′(4) realized just on 4 fields instead of the usual 16.
Apparatus for thin-film stress measurement with integrated four-point bending equipment: Performance and results on Cu films
Authors: Weihnacht V. , Brückner W. , Schneider C. M.
Published: July 2002

New insight into the mechanical behavior of thin metallic films on substrates can be obtained by a novel experimental technique. It comprises stress measurements by the wafer curvature technique in combination with a four-point bending of beam-shaped samples. A dedicated apparatus was constructed which allows such experiments in high vacuum between room temperature and 500 °C to be carried out. It has a stress measurement sensitivity better than 0.1 MPa and a long-term stability better than ±0.2 MPa over 24 h in the case of a 1 μm thick film. Strains up to 0.8% could be imposed by a four-point bending on films grown on 380 μm thick Si substrates before cracking of the substrates. Both the thermal cycling and the four-point bending technique were used to investigate the plastic behavior of 1 μm thick Cu films on oxidized Si substrates.
Ferromagnetism in germanium clathrate: Ba8Mn2Ge44
Authors: Kawaguchi Tetsuji , Tanigaki Katsumi , Yasukawa Masahiro
Published: July 2002

A unique magnetic nanosystem, constructed from Ge20 dodecahedrons and Mn, is presented, which shows a ferromagnetic transition around 10 K. In this system with the formula Ba8MnxGe46−x (x=1–2), the Mn atoms can be incorporated with accurate control in position of the crystal lattice. The spontaneous magnetization is approximately linearly proportional to the amount of Mn introduced and is maximized at x=2. Magnetic measurements reveal that the d electrons are almost localized on Mn atoms but also affected by conduction electrons spreading over the clathrate network.