bluuebunny/crossref_metadata_2025 huggingface dataset:
SU
(N)
coherent states
Authors: Mathur Manu , Mani H. S.
Published: October 2002

We generalize Schwinger boson representation of SU(2) algebra to SU(N) and define coherent states of SU(N) using 2(2N−1−1) bosonic harmonic oscillator creation and annihilation operators. We give an explicit construction of all (N-1) Casimirs of SU(N) in terms of these creation and annihilation operators. The SU(N) coherent states belonging to any irreducible representations of SU(N) are labeled by the eigenvalues of the Casimir operators and are characterized by (N-1) complex orthonormal vectors describing the SU(N) manifold. The coherent states provide a resolution of identity, satisfy the continuity property, and possess a variety of group theoretic properties.
W
∞
-algebras in n complex dimensions and Kodaira–Spencer deformations: A symplectic approach
Authors: Bandelloni G. , Lazzarini S.
Published: October 2002

It is shown that the notion of W∞-algebra originally carried out over a (compact) Riemann surface can be extended to n complex dimensional (compact) manifolds within a symplectic geometrical setup. The relationships with the Kodaira–Spencer deformation theory of complex structures are discussed. Subsequently, some field theoretical aspects at the classical level are briefly underlined.
High thermal stability of AlGaAs/GaAs V-grooved quantum wire
Authors: Liu Xing-Quan , Wang Xue-Lun , Ogura Mutsuo
Published: October 2002

In this article, we make a systematic comparison of the photoluminescence (PL) properties of AlGaAs/GaAs V-grooved quantum wire (QWR) structures with their counterpart quantum well (QWL) structures grown in the same system. The QWR structures exhibit constant photoluminescence (PL) intensity up to a temperature Tc of 90, 140, and 170 K for a 2, 5, and 9 nm QWR, respectively. The 9 nm QWR sample showed the highest relative thermal stability with respect to its counterpart 9 nm QWL sample, its PL intensity being stronger than that of the 9 nm QWL up to 270 K. This temperature is very close to room temperature, at which most opto-electronic devices operate. The time-resolved PL measurements provide further confirmation of the PL results. These data show that the PL thermal stability of the V-grooved QWR structures, despite the increased interface to volume ratio, is now comparable to that of the counterpart QWL structures, which is a basic condition for the superior properties of QWR structures to be brought into practical play.
Microwave emission due to hypervelocity impacts and its correlation with mechanical destruction
Authors: Takano T. , Murotani Y. , Maki K. , Toda T. , Fujiwara A. , Hasegawa S. , Yamori A. , Yano H.
Published: October 2002

Microwave emission due to hypervelocity impacts on metallic plates has been found. The targets used in the experiment are aluminum plates with various thicknesses. The projectile, a nylon cylinder with metal screw of 0.21 gm, was accelerated up to the velocity of 4 km/s; a heterodyne receiver detected the microwave at 22 GHz. The emission is a random sequence of pulses with several nanosecond width, which lasts more than 10 μs. The phenomenon seems to be dependent on the extent of target destruction through the formation of impact craters or penetration. If so, we could use the characteristics of the phenomena to better understand the mechanical destruction process. We propose several models for the cause of this microwave generation and study them on the basis of timing relation of observed events.
Inelastic and reactive collisions of Cs(9 2P3/2) with hydrocarbons
Authors: Su Hongmei , Bersohn Richard
Published: October 2002

Cs atoms when excited to their 7 2P, 8 2P, and 9 2P states have sufficient internal energy to be able to react with H2 molecules forming CsH and H and they do so. Cs 9 2P has sufficient energy to react with alkanes; in fact, this does not occur. Instead an efficient n changing but l conserving collision occurs producing Cs atoms in the lower P2 states. This phenomenon is shown to result from the fact that in the Rydberg state the valence electron is remote from the Cs+ ion core. Cs 9 2P does react with propene removing an allylic H atom to form CsH.
The effects of vibrational mode and collision energy on the reaction of formaldehyde cation with carbonyl sulfide
Authors: Liu Jianbo , Van Devener Brian , Anderson Scott L.
Published: October 2002

The effects of collision energy (Ecol) and five different H2CO+ vibrational modes on the title reaction have been studied, including measurements of product ion recoil velocity distributions. A series of ab initio and Rice–Ramsperger–Kassel–Marcus calculations were used to examine properties of various complexes and transition states that might be important. Four product channels are observed. Proton transfer (PT) dominates at low Ecol, and is suppressed by Ecol but mildly enhanced by H2CO+ vibrational excitation. PT occurs by a direct mechanism at high energies, but appears to be mediated by reactantlike complexes at low energies. The other major low energy channel corresponds to H2+ transfer, and the majority of these product ions go on to eliminate CO, producing H2S+. Both H2+ transfer and H2S+ channels are strongly inhibited by Ecol and vibrational excitation, which is interpreted in terms of competition with other channels. Charge transfer occurs in short time scale collisions at all energies, and is strongly enhanced by Ecol and by vibrational excitation. The vibrational effects for all channels are mode specific.
Crystal nucleation rates for particles experiencing anisotropic interactions
Authors: Dixit Narendra M. , Zukoski Charles F.
Published: October 2002

We explore crystal nucleation kinetics in suspensions containing particles experiencing short-range anisotropic interactions. A kinetic model is developed where cluster growth is written in terms of the rates of single particle aggregation onto and dissociation from cluster surfaces. Aggregation rates are determined for particles interacting with centrosymmetric interactions and corrected to account for the low probability of bond formation due to orientational constraints. The rates of dissociation are determined as the sum of the independent rates of bond breakage via translational and rotational diffusion of particles on cluster surfaces. The resulting nucleation rates display remarkable sensitivity to the degree of anisotropy. Under identical supersaturations and average strengths of interaction, slight changes in the degree of anisotropy result in several tens of orders of magnitude changes in nucleation rates. Surprisingly, crystal nucleation rates can either increase or decrease depending on how the degree of anisotropy is altered. These studies are discussed in terms of what is known about the equilibrium thermodynamics and kinetics of crystal nucleation in protein solutions.
Dynamics of zonal flow saturation in strong collisionless drift wave turbulence
Authors: Kim Eun-jin , Diamond P. H.
Published: October 2002

Generalized Kelvin–Helmholtz (GKH) instability is examined as a mechanism for the saturation of zonal flows in the collisionless regime. By focusing on strong turbulence regimes, GKH instability is analyzed in the presence of a background of finite-amplitude drift waves. A detailed study of a simple model with cold ions shows that nonlinear excitation of GKH modes via modulational instability can be comparable to their linear generation. Furthermore, it is demonstrated that zonal flows are likely to grow faster than GKH mode near marginality, with insignificant turbulent viscous damping by linear GKH. The effect of finite ion temperature fluctuations is incorporated in a simple toroidal ion temperature gradient model, within which both zonal flow and temperature are generated by modulational instability. The phase between the two is calculated self-consistently and shown to be positive. Furthermore, the correction to nonlinear generation of GKH modes appears to be small, being of order O(ρi2k2). Thus, the role of linear GKH instability in the saturation of collisionless zonal flows, in general, seems dubious.
Stabilization of ion temperature gradient driven turbulence and formation of an internal transport barrier in a tokamak
Authors: Voitsekhovitch I. , Garbet X. , Benkadda S. , Beyer P. , Figarella C. F.
Published: October 2002

The formation of an internal transport barrier (ITB) in a tokamak plasma due to the stabilization of ion temperature gradient (ITG) driven turbulence is analyzed using a three-dimensional turbulence code [Garbet et al., Phys. Plasmas 8, 2793 (2001)]. Two mechanisms leading to a reduction of turbulent transport are studied. First, a bifurcation to an improved confinement state obtained by increasing the heating power above a threshold value with a monotonic q profile is presented. This bifurcation occurs due to a spontaneous increase of self-generated E×B rotation shear. Then, the effect of low and negative magnetic shear on the ITG driven turbulence and transport is studied. The beneficial effects of low order minimum safety factor and low curvature of the safety factor profile are shown in these simulations by varying the magnetic configuration in a large range. The evolution of the statistical properties of turbulence during the ITB formation is presented.
Hybrid resonant modes of two-sided multipactor and transition to the polyphase regime
Authors: Kryazhev A. , Buyanova M. , Semenov V. , Anderson D. , Lisak M. , Puech J. , Lapierre L. , Sombrin J.
Published: October 2002

A plane parallel model of multipactor is studied in detail using both an analytical approach and numerical simulations. The analytical analysis is carried out within a widely exploited approximation, which assumes a fixed value for the initial velocity of secondary electrons. It is shown that the commonly accepted picture of the multipactor zones is not quite complete and should be modified by taking into account the existence of hybrid resonance modes and the important consequences of a secondary emission yield that significantly exceeds unity. Numerical simulations demonstrate that the chart of the multipactor zones is also very sensitive to a spread of the initial velocity of the electrons. In particular, the full effect of initial electron velocities cannot accurately be modeled by using a single fixed value only.
Generation and propagation of monochromatic acoustic phonons in gallium arsenide
Authors: Kent A. J. , Stanton N. M. , Challis L. J. , Henini M.
Published: October 2002

We have generated coherent zone-folded longitudinal acoustic (LA) phonons by femtosecond pulse-laser excitation of a gallium arsenide/aluminum arsenide superlattice. Using the filter effect due to frequency-dependent scattering in the gallium arsenide substrate, we have determined the frequency spectrum of the LA phonons that leak out of the superlattice and propagate to a bolometer on the opposite face of the gallium arsenide substrate. We show that the phonon spectrum has a strong monochromatic component centered on ν=sLA/dSL, where dSL is the superlattice period and sLA is the phonon speed. We propose that such phonons may be used for phonon spectroscopy and in a range of “phonon optics” applications.
Influence of the choice of internal temperatures on the composition of CxHyOzNt plasmas out of thermodynamic equilibrium: Application to CH2 plasma
Authors: Koalaga Zacharie
Published: October 2002

The purpose of this paper is to study the influence of the choice of internal temperatures on the composition of CxHyOzNt plasmas out of thermodynamic equilibrium. The numerical calculation is specially performed for CH2 plasma in the pressure range 0.1–1 MPa and for the electron temperature range 5000–30 000 K. Precisely, the investigation of this plasma allows one to show that the choice of internal temperatures can have more influence on plasma composition than the choice of the form of the two-temperature Saha and Guldberg–Waage laws. Indeed, for one of the supposed hypotheses, it is observed that the two forms of the two-temperature system used here can give the same equilibrium composition by uncoupling the excitation temperature of the diatomic and the monatomic species. Great attention must then be given to the adopted hypothesis for internal temperature and not only to the form of the two temperature system used. An accurate comparison between the two models requires the measurement of plasma parameters such as the various internal temperatures and the species concentration. Therefore, we have also carried out an analysis of the potential experimental diagnostics of these plasma parameters. Such diagnostics can help to test and validate theoretical models.
Analytical study of the dynamics of capillary discharge plasmas for recombination x-ray lasers using H-like ions
Authors: Lee K. , Kim J. H. , Kim D.
Published: October 2002

A simple but appropriate analytical study has been made to investigate the dynamics of a capillary discharge-produced plasma, thus obtaining Z-scaling formulas for the experimental parameters for a collisional recombination pumped x-ray laser using H-like ions. The analysis has been found to be in an excellent agreement with the results from a series of magnetohydrodynamic (MHD) simulations both in the dynamics and in the prediction of the appearance of gain of the Hα line in the H-like ion for the elements with the atomic number z=4–13. The formulas are valuable because (1) they identify key physical processes and (2) serve as a guideline for the design of an experimental setup as well as the full MHD simulation and (3) provide an overview on the application of the capillary discharge to the recombination laser. The comparison of the experimental parameters for collisional recombination x-ray lasers with those for collisional excitation x-ray laser [Shlyaptsev et al. SPIE 2012, 99(1994)] has been made also.
Electron temperature gradient instability in toroidal plasmas
Authors: Dong J. Q. , Sanuki H. , Itoh K. , Chen Liu
Published: October 2002

Electron temperature gradient (ETG) driven instability in toroidal plasmas is studied with gyrokinetic theory. The full electron kinetics including finite Larmor radius effects, toroidal (curvature and magnetic gradient) drift motion ωD, and transit k∥v∥, is considered. The upgraded numerical scheme for solving the integral eigenvalue equations allows the study of both growing and damping modes, and thus direct calculation of critical gradient. A systematic parameter study is carried out for low β(=plasma pressure/magnetic pressure) circular flux surface equilibria. The basic characteristics of the modes are discussed. The scaling of the critical gradient with respect to toroidicity and to the ratio of electron temperature over ion temperature is obtained. Estimation for the transport induced by the modes is given.
Observation of bistable density transitions in magnetized plasma by voltage-biased electrode
Authors: Shinohara Shunjiro , Matsuyama Shoichiro
Published: October 2002

Repeated transition phenomena with abrupt reductions and jumps of the electron density were observed (flip–flop pattern in bistable systems), by voltage biasing to an inserted electrode in the inner region of a rf- (radio frequency-) produced, cylindrical magnetized plasma. These global, self-excited, density transitions and back ones between two states were accompanied by changes of the floating potential profile and the bias current under various parameters. Control of the staying time probability in one of two states with hysteresis loops was attempted, changing the bias voltage, which showed the importance of a sheath region as well as fine structural patterns.
Geometric manipulation of DNA molecules with a laser
Authors: Matsuzawa Y. , Hirano K. , Mizuno A. , Ichikawa M. , Yoshikawa K.
Published: October 2002

The present study reports on the geometric manipulation of DNA molecules condensed by treatment with PEG and salt with laser on a microscope. To date, the majority of previously reported laser studies have dealt with the trapping or transportation of target objects. Here, we demonstrate the successful attachment and transformation of condensed DNA molecules as well as the construction of various configurations. Laser manipulation of DNA has fruitful applicability in chemistry, life science, and biotechnology.
Ion drag and thermophoretic forces acting on free falling charged particles in an rf-driven complex plasma
Authors: Zafiu C. , Melzer A. , Piel A.
Published: October 2002

Monodisperse plastic microspheres have been dropped through a long radio-frequency discharge column. The trajectories of the falling particles have been measured. It was observed that the particles are driven out of the plasma. From the trajectory analysis and plasma measurements the forces on the particles have been derived. Special attention has been paid to the thermophoresis and ion drag forces which are also considered to be responsible for the void formation in microgravity experiments. Two experimental situations have been considered here: first a plasma characterized by its natural symmetric electric potential and temperature distribution and second, a plasma with an asymmetric temperature and electric potential profile. For both cases a good agreement has been found between the measured “trajectory force” obtained from the particle trajectory analysis and the sum of the ion drag, thermophoretic and electric field force.
Nondoped-type white organic electroluminescent devices utilizing complementary color and exciton diffusion
Authors: Tsuji Toshie , Naka Shigeki , Okada Hiroyuki , Onnagawa Hiroyoshi
Published: October 2002

We fabricated nondoped white organic electroluminescent devices using vacuum-deposited thin films of blue-emitting 4,4′-bis[N-1-napthyl-N-phenyl-amino]biphenyl (α-NPD) and orange-emitting 4-(dicyanomethylene)-2-metyl-6-(p-dimethyl aminostyryl)-4H-pyran (DCM), a hole-blocking layer of 2-(4-biphenyl)-5-(p-tert-butylphenyl)-1,3,4-oxadiazole (tBu-PBD) and electron-transporting tris(8-quinolinolato) aluminum (III). Excitons formed at the α-NPD/tBu-PBD interface sequentially transfer their energy to α-NPD via the Förster mechanism. The exciton is captured by an ultrathin DCM layer located within the pure α-NPD layer. The position of the DCM determines the device spectrum, and enables a white emission to be achieved. The spectrum is not sensitive to the voltage applied, and the devices show maximum luminance of about 1000 cd/m2.
Radiation transport coupled particle-in-cell simulation of low-pressure inductive discharges
Authors: Lee Hae June , Verboncoeur J. P.
Published: October 2002

Low pressure (1–5 Torr) argon discharges driven by an inductive radio frequency wave are simulated with a one-dimensional radiation transport coupled particle-in-cell model. The discharge is maintained by an induced azimuthal electric field which is self-consistently coupled with plasma dynamics. The radiation efficiency is investigated for the variations of input power, gas pressure, and cylinder radius, and compared with that of positive column discharges. The radiation efficiency is improved up to 8% compared with that of conventional positive column discharges by virtue of reduced radiation trapping resulting from enhancement of excitation collisions near the wall for inductive discharges.
Laser hohlraum coupling efficiency on the Shenguang II facility
Authors: Chang Tieqiang , Ding Yongkun , Lai Dongxian , Huan Tianxuan , Zhu Shaoping , Zheng Zhijian , Wang Guangyu , Zheng Yongmin , He Xiantu , Pei Wenbing , Duan Qingsheng , Zhang Weiyan , Feng Tinggui , Chen Guangnan , Gu Peijun
Published: October 2002

Recently, hohlraum experiments were performed at the Shenguang-II (SG-II) laser facility [Lin et al., Chin. J. Lasers B10, Suppl. IV6 (2001)]. The measured maximum radiation temperature was 170 eV for the standard hohlraum and 150 eV for a 1.5-scaled one. This paper discusses the radiation temperature and laser hohlraum coupling efficiency in terms of a theoretical model [Phys. Plasmas 8, 1659 (2001)] and numerical simulation. A 2D laser–hohlraum coupling code, LARED-H [Chin. J. Comput. Phys. 19, 57 (2002)], gives a satisfactory coincidence with the measured time-resolved radiation temperature. Upon fitting the time-resolved curve, the theoretical model obtains the hohlraum coupling efficiency and, furthermore, the parameter n+s for the hohlraum wall material (Au) can be determined simultaneously, where n, s are the power exponents of temperature for the radiation Rosseland mean-free path and specific internal energy, respectively.
Streaming instability in plasmas with opposite polarity grains
Authors: Torney M. P. , Shukla P. K. , Bingham R.
Published: October 2002

The dust streaming instability in plasmas with opposite polarity grains is analyzed analytically. For this purpose, a dispersion relation is derived by employing a Boltzmann response for the electrons and ions and a hydrodynamic response to positively and negatively charged dust grains that co-exist in an unmagnetized plasma. The dispersion relation admits new instabilities in both collisionless and collisional regimes. The results may have relevance to understanding the origin of electrostatic turbulence in the Earth’s mesosphere, meteor trails, and cometary tails.
Kinetics of laser-induced low-temperature crystallization of amorphous silicon
Authors: Khait Yu. L. , Beserman R. , Chack A. , Weil R. , Beyer W.
Published: October 2002

A brief report on experimental and theoretical studies of the kinetics of the laser-induced crystallization (LIC) in undoped amorphous hydrogenated silicon is presented. It is shown that the LIC occurs at a substantially lower temperature and occurs at this temperature much faster compared to the thermal crystallization in a furnace. A nanoscopic kinetic electron-related model of the LIC is presented. The model explains the experimental observations as the integral effect of a huge amount of nanoscale picosecond atomic and electronic reconstructions leading to more stable material states which are generated by electron-assisted short-lived (picosecond) large energy fluctuations in nanometer material regions.
Ambipolar diffusion in a dusty plasma
Authors: Amiranashvili Sh. , Yu M. Y.
Published: October 2002

Ambipolar diffusion of electrons and ions in a plasma containing charged dust grains is investigated. As the diffusion is on the ion time scale, the dust distribution remains stationary but the dust charge can vary. An analytical solution showing that the presence of dusts can significantly increase the diffusion length is given. Other solutions of practical interest are obtained numerically.
Ab initio calculations of vacancies in SixGe1−x
Authors: Dalpian G. M. , Venezuela P. , da Silva Antônio J. R. , Fazzio A.
Published: October 2002

Ab initio calculations are used to investigate the structural and electronic properties of vacancies in SixGe1−x. The (++), (+), (0), and (−) charge states are studied and the substitutional disorder of the alloy is considered explicitly. We found a linear relationship between the effective-U for the system formed by the (++), (+), and (0) charge states and the number of Si atoms in the first neighborhood of a vacancy (NSi). The effective-U is positive when NSi is zero, and it is negative when NSi is 2 and 4. In all cases, the absolute value of the effective-U in the alloy is significantly smaller than its value for pure Si and pure Ge.
Dependence of the detective quantum efficiency of photoconductive x-ray image detectors on charge transport parameters and exposure: Application to a-Se
Authors: Kasap Safa O. , Zahangir Kabir M. , Rowlands John A. , Tousignant Olivier , Leboeuf Jonathan , Laperrière Luc , Demers Yves
Published: October 2002

The effects of charge carrier trapping (i.e., incomplete charge collection) on the detective quantum efficiency (DQE) of a photoconductive detector are studied by using a cascaded linear system model. The model includes signal and noise propagations in the following stages: (1) x-ray attenuation, (2) conversion gain, (3) charge collection, and (4) the addition of electronic noise. We examine and discuss the DQE of an a-Se fluoroscopic detector as a function of charge transport parameters and x-ray exposure. The theoretical model shows a very good agreement with experimental DQE versus exposure characteristics.
Evidence for forward domain growth being rate-limiting step in polarization switching in 〈111〉-oriented-Pb(Zr0.45Ti0.55)O3 thin-film capacitors
Authors: Hong Seungbum , Setter Nava
Published: October 2002

We studied polarization switching of 295 nm thick 〈111〉-oriented-Pb(Zr0.45Ti0.55)O3 (PZT) thin-film capacitors through a polarization hysteresis loop and piezoelectric force microscopy (PFM) on top of the Pt electrode. Positive voltage pulses of 450 kV/cm and 2.5 ms were applied to the top as a poling pulse. Negative pulses ranging from −45 to −60 kV/cm at an interval of 3 kV/cm were applied to the top electrode. After each pulse, we performed PFM imaging by applying 17 kHz ac voltage of 1 V peak to peak to the bottom electrode while grounding the top one and the tip. The first-harmonic amplitude and phase of the tip vibration signal were mapped over the scanned area of 10×10 μm2. Subsequently, we measured the polarization hysteresis curve at 100 Hz after each domain image acquisition to estimate the volume fraction of switched polarization induced by the positive voltage pulses. We analyzed the areal fraction of switched domains as a function of pulse voltage from phase images and compared it with the volume fraction obtained from the hysteresis loops. Based on the evolution of domain images and the comparison plots of the areal and volume fractions of switched domains, we conclude that forward domain growth is a rate-limiting step during polarization reversal in 〈111〉-oriented-PZT thin-film capacitors.
Enhancement of the critical current density and flux pinning of MgB2 superconductor by nanoparticle SiC doping
Authors: Dou S. X. , Soltanian S. , Horvat J. , Wang X. L. , Zhou S. H. , Ionescu M. , Liu H. K. , Munroe P. , Tomsic M.
Published: October 2002

Doping of MgB2 by nano-SiC and its potential for the improvement of flux pinning were studied for MgB2−x(SiC)x/2 with x=0, 0.2, and 0.3 and for 10 wt % nano-SiC-doped MgB2 samples. Cosubstitution of B by Si and C counterbalanced the effects of single-element doping, decreasing Tc by only 1.5 K, introducing intragrain pinning centers effective at high fields and temperatures, and significantly enhancing Jc and Hirr. Compared to the undoped sample, Jc for the 10 wt % doped sample increased by a factor of 32 at 5 K and 8 T, 42 at 20 K and 5 T, and 14 at 30 K and 2 T. At 20 K and 2 T, the Jc for the doped sample was 2.4×105 A/cm2, which is comparable to Jc values for the best Ag/Bi-2223 tapes. At 20 K and 4 T, Jc was twice as high as for the best MgB2 thin films and an order of magnitude higher than for the best Fe/MgB2 tapes. The magnetic Jc is consistent with the transport Jc which remains at 20 000 A/cm2 even at 10 T and 5 K for the doped sample, an order of magnitude higher than the undoped one. Because of such high performance, it is anticipated that the future MgB2 conductors will be made using a formula of MgBxSiyCz instead of pure MgB2.
Influence of near-surface nonstoichiometry on the surface magnetization of mixed-valent manganite: A computer simulation study
Authors: Wad U. P. , Ogale Abhijit S. , Ogale S. B. , Venkatesan T.
Published: October 2002

A Monte Carlo simulation study is performed to examine the effects of near-surface nonstoichiometry in a mixed-valent manganite (e.g., La0.7A0.3MnO3, where A=Sr, Ca) on the magnetization of its surface, and that of a few layers underneath. The nonstoichiometry is introduced either in the form of oxygen vacancy gradient or a gradient in the La:A ratio. The corresponding gradient in the exchange constant J is incorporated phenomenologically from the known phase diagram [P. Schiffer et al., Phys. Rev. Lett. 75, 3356 (1995)]. We show that the near-surface nonstoichiometry can account for the temperature dependence of magnetization of the surface layers as revealed by photoemission and x-ray magnetic circular dichroism (X-MCD) experiments [J. Park et al., Nature (London) 392, 794 (1998); J. Park et al., Phys. Rev. Lett. 81, 1953 (1998)]. The demagnetization shows viscous fingeringlike protrusions which may have important consequences for spin-polarized transport across interfaces.
Interfacial layer formation during high-temperature annealing of ZrO2 thin films on Si
Authors: Howard J. M. , Craciun V. , Essary C. , Singh R. K.
Published: October 2002

High-k materials deposited directly on silicon exhibit an interfacial layer between the grown layer and the underlying substrate. This is of particular concern in metal–oxide–semiconductor technologies where these layers have a deleterious effect on the overall capacitance of the resulting devices. In this letter, the growth and properties of this silicatelike interfacial layer are examined after postdeposition anneals in a vacuum, inert, and oxidizing atmospheres. X-ray reflectivity, Fourier transform infrared spectroscopy, and x-ray photoelectron spectroscopy have been used to characterize the growth and properties of this interfacial layer.
Operation of photonic crystal membrane lasers above room temperature
Authors: Lee Po-Tsung , Cao J. R. , Choi Sang-Jun , Wei Zhi-Jian , O’Brien John D. , Dapkus P. Daniel
Published: October 2002

Operation of photonic crystal lasers for substrate temperatures as high as 50 °C is reported. The temperature dependence of the lasing wavelength and the threshold pump power is also investigated. The characteristic temperature To is 37.7 K.
Selective manipulation of InAs quantum dot electronic states using a lateral potential confinement layer
Authors: Kim Eui-Tae , Chen Zhonghui , Madhukar Anupam
Published: October 2002

To further the objective of controlled manipulation of the electronic states in epitaxial island quantum dots (QDs), we introduce the notion of a lateral potential confinement layer (LPCL) whose judicious placement during island capping allows selective impact on ground and excited electron and hole states. The energy states of InAs/In0.15Ga0.85As QDs are manipulated using 10-monolayer-thick In0.15Al0.25Ga0.60As LPCLs positioned at the bottom, upper, and top region of the QDs. The changes in the photoluminescence (PL) and PL excitation spectra reveal the nature of the electronic transitions impacted selectively through the spatial charge distributions of the states involved.
Lateral composition modulation in mixed anion multilayers
Authors: Dorin C. , Mirecki Millunchick J. , Wauchope C.
Published: October 2002

Lateral composition modulation on the group V sublattice has been observed in GaAs/GaSb short period superlattices. Cross sectional transmission electron microscopy and x-ray diffraction reciprocal space maps reveal that all structures are phase-separated with Sb compositions for the strongest modulated structure of x=0.73 in the Sb-rich regions, x=0.55 in the As-rich regions, and wavelengths 15⩽Λ⩽20 nm. The composition modulation observed in these films is not due to spinodal decomposition, because an alloy grown at the same conditions results in a homogeneous layer, but may be related to vertical stacking of quantum dots that nucleate during the growth of the structure.
Effects of hydrostatic and uniaxial stress on the conductivity of p-type GaN epitaxial layer
Authors: Liu Y. , Kauser M. Z. , Nathan M. I. , Ruden P. P. , Dabiran A. M. , Hertog B. , Chow P. P.
Published: October 2002

We present experimental results on the dependence of the electrical conductivity of Mg-doped, p-type GaN on hydrostatic pressure and uniaxial stress. The samples were grown by molecular beam epitaxy on sapphire substrates. Hydrostatic pressure over the range studied (0–7 kbar) leads to a relatively small decrease in the conductivity. Uniaxial stress in the basal plane results in strongly increasing conductivity parallel to the stress direction and in weakly decreasing conductivity perpendicular to the stress direction. We relate the observed symmetry of the piezoconductivity to deformation potential induced changes in the band structure near the top of the valence band.
Oxidation and reduction behavior of Ge/Si islands
Authors: Sass T. , Zela V. , Gustafsson A. , Pietzonka I. , Seifert W.
Published: October 2002

We have investigated the oxidation/reduction behavior of dome-shaped three-dimensional islands of Ge on Si(001) grown by ultrahigh vacuum chemical vapor deposition. The oxidation was done by exposing the surfaces to H2O steam in N2. The reduction was done by H2, which selectively reduces only the GeO2. The results of the oxidation/reduction processes under varying conditions were analyzed by high-resolution transmission electron microscopy. We found that the selective reduction of such structures does not result in a perfect recovery of the former Ge dots, but results in phase-segregated Ge enrichments. In many cases these enrichments show epitaxial relationship with the underlying Si substrate. These structures are of potential interest for Ge dots embedded in an insulating material as well as for lateral epitaxial overgrowth of SiO2/Si(001) by Ge, using the reduced Ge dots as the seeds for epitaxy.
Cathodoluminescence of cubic boron nitride films deposited by chemical vapor deposition
Authors: Zhang W. J. , Kanda H. , Matsumoto S.
Published: October 2002

Cathodoluminescence (CL) characteristics of cubic boron nitride (cBN) films deposited by chemical vapor deposition were investigated. Combined with the results from Fourier-transformed infrared spectroscopy and Raman spectroscopy, the dependence of the emission energy and intensity on the phase composition and crystallinity of the films was revealed. A broad emission band centered at about 3.12 eV for UCL (or US-1) as observed previously in undoped cBN single crystals synthesized by high-temperature, high-pressure method, and an emission band at about 2.48 eV for the A and B bands, due to multivacancy complexes of boron and nitrogen vacancies, were observed in the cBN films with high phase purity and high crystallinity. For the hexagonal boron nitride (hBN) phase, the main emission band up-shifted to high energy of 3.48–3.68 eV. A peak centered at 5.28 eV was observed in the spectra, which is believed the near-band-gap UV emission of hBN.
Growth of ordered, single-domain, alumina nanopore arrays with holographically patterned aluminum films
Authors: Sun Zhijun , Kim Hong Koo
Published: October 2002

Highly-ordered, single-domain (∼cm2), alumina nanopore arrays with controlled symmetry were obtained by anodizing prepatterned aluminum films that were deposited on silica substrates. Holographic lithography, in conjunction with a conformal deposition process, was utilized to form nanoscale corrugations on aluminum film surface prior to anodization. Both the order and symmetry of pore arrays were found to be well controlled and guided by the periodic surface corrugations from the initial stage of pore growth. Ordered nanopore arrays grown on foreign substrates are promising as a host or template for forming various nanodevices and their on-chip integration.
Real-time assessment of In surface segregation during the growth of AlSb/InAs(Sb) heterostructures
Authors: Prévot I. , Vinter B. , Marcadet X. , Massies J.
Published: October 2002

Indium surface segregation is evidenced in real time by reflection high-energy electron diffraction (RHEED) during the molecular beam epitaxial growth of AlSb on InAs(Sb). The resulting interface width is determined from the RHEED specular beam intensity variation during the growth. It extends over several nanometers and increases with the growth temperature. Band structure simulations show that the indium segregation leads to a strong localization of the wave function associated to the first bound hole level at the AlSb on InAs(Sb) interface.
Wideband sub-Poissonian light generation in light-emitting diodes incorporating a heavily-doped active region
Authors: Tanaka K. , Higashi A. , Yuji H. , Masuyama R. , Kadoya Y. , Yamanishi M.
Published: October 2002

We investigated the generation of sub-Poissonian light in light-emitting diodes (LEDs) in which the active region is heavily doped with Be. The squeezing of the intensity-fluctuation below the full-shot-noise level was observed in a wide frequency range, near-dc to 1.5 GHz. From the noise-suppression spectra, we evaluated the radiative recombination lifetime of electrons in the active region and confirmed that the lifetime is reduced with increasing doping density. The results demonstrate clearly the excellent noise properties as well as the capability of high-speed modulation of the LEDs.
Dielectric relaxation studies of Bi-doping effects on the oxygen-ion diffusion in La2−xBixMo2O9 oxide-ion conductors
Authors: Wang X. P. , Fang Q. F. , Li Z. S. , Zhang G. G. , Yi Z. G.
Published: October 2002

Two dielectric relaxation peaks associated with oxygen-ion diffusion in the oxide-ion conductors La2−xBixMo2O9 (x=0.05, 0.1, and 0.15) have been studied. It is found that the activation energies of the two peaks increase with increasing Bi-doping concentration, which results from the blocking effects of the lone-pair electrons of Bi3+ ions. From the different effects of Bi doping on the two peaks, the diffusion paths of oxygen ions corresponding to each peak are confirmed. Significantly, it is revealed that Bi doping could enhance the ionic conductivity of La2Mo2O9 at lower temperatures.
Self-heating effects at high pump currents in deep ultraviolet light-emitting diodes at 324 nm
Authors: Chitnis A. , Sun J. , Mandavilli V. , Pachipulusu R. , Wu S. , Gaevski M. , Adivarahan V. , Zhang J. P. , Khan M. Asif , Sarua A. , Kuball M.
Published: October 2002

We present a detailed high-pump-current study of self-heating effects in ultraviolet light-emitting diodes (LEDs) grown on sapphire. For deep ultraviolet LEDs on sapphire, our results establish self-heating to be a primary cause of premature power saturation under dc pumping. Even the flip-chip packaged devices undergo a steady-state temperature rise to about 70 °C at a dc pump current of only 50 mA (at 8 V) resulting in a significant decrease in LED output. Temperature rise values estimated from peak emission wavelength shifts and from micro-Raman mapping of the active devices were in good agreement.
Distributions of noble metal Pd and Pt in mesoporous silica
Authors: Arbiol J. , Cabot A. , Morante J. R. , Chen Fanglin , Liu Meilin
Published: October 2002

Mesoporous silica nanostructures have been synthesized and loaded with Pd and Pt catalytic noble metals. It is found that Pd forms small nanoclusters (3–5 nm) on the surface of the mesoporous structure whereas Pt impregnation results in the inclusion of Pt nanostructures within the silica hexagonal pores (from nanoclusters to nanowires). It is observed that these materials have high catalytic properties for CO–CH4 combustion, even in a thick film form. In particular, results indicate that the Pt and Pd dispersed in mesoporous silica are catalytically active as a selective filter for gas sensors.
Fabrication and subpicosecond optical response of low-temperature-grown GaAs freestanding photoconductive devices
Authors: Adam R. , Mikulics M. , Förster A. , Schelten J. , Siegel M. , Kordoš P. , Zheng X. , Wu S. , Sobolewski R.
Published: October 2002

We have fabricated and tested freestanding low-temperature-grown GaAs photoconducting devices exhibiting subpicosecond photoresponse. The epitaxially grown thin-film microswitches were transferred on top of sapphire and Si/SiO2 substrates and integrated with Ti/Au coplanar strip transmission lines. Our devices exhibited above 200 V breakdown voltages and dark currents at 100 V below 3×10−7 A. The photoresponse of our microswitches was measured using electro-optic sampling technique with 100-fs-wide laser pulses at wavelengths of 810 nm and 405 nm as the excitation sources. For 810-nm excitation, we measured 0.55-ps-wide electrical transients with voltage amplitudes of up to 1.3 V. The ultraviolet excitation resulted in 1.35-ps-wide transients due to the hot carriers generation and intraband relaxation.
Micromachined Fabry–Perot interferometer for motion detection
Authors: Waters Richard L. , Aklufi Monti E.
Published: October 2002

The monolithic integration of a Fabry–Perot interferometer and a (100) silicon photodiode is reported for use as a highly sensitive transduction method in the detection of minute displacements of a proof mass attached to a spring. The combination results in a compact device with active transistor-like amplification and minimal parasitic elements. The transducer is fabricated using standard surface micromachining techniques. The finesse of the optical cavity, incident optical power, and geometry of the mirror and support structure control the sensitivity of the transducer. A transduction of more than 2285 A/m, percent change in transmission with displacement of 3%/nm, small-signal voltage amplification of 460 V/V, output resistance of 100 MΩ and transconductance of 1 mA/V have been obtained thus far for a single device without amplification.
Band-edge photoluminescence of AlN epilayers
Authors: Li J. , Nam K. B. , Nakarmi M. L. , Lin J. Y. , Jiang H. X.
Published: October 2002

AlN epilayers with high optical qualities have been grown on sapphire substrates by metalorganic chemical vapor deposition. Deep ultraviolet photoluminescence (PL) spectroscopy has been employed to probe the optical quality as well as optical transitions in the grown epilayers. Band-edge emission lines have been observed both at low and room temperatures and are 6.017 and 6.033 eV at 10 K. It was found that the peak (integrated) emission intensity of the deep impurity related transition is only about 1% (3%) of that of the band-edge transition at room temperature. The PL emission properties of AlN have been compared with those of GaN. It was shown that the optical quality as well as quantum efficiency of AlN epilayers is as good as that of GaN.
Electrical isolation of ZnO by ion bombardment
Authors: Kucheyev S. O. , Deenapanray P. N. K. , Jagadish C. , Williams J. S. , Yano Mitsuaki , Koike Kazuto , Sasa Shigehiko , Inoue Masataka , Ogata Ken-ichi
Published: October 2002

The evolution of sheet resistance of n-type single-crystal wurtzite ZnO epilayers exposed to bombardment with MeV H1, Li7, O16, and Si28 ions at room temperature is studied in situ. We demonstrate that sheet resistance of ZnO can be increased by about 7 orders of magnitude as a result of ion irradiation. Due to extremely efficient dynamic annealing in ZnO, the ion doses needed for isolation of this material are about 2 orders of magnitude larger than corresponding doses in the case of another wide-bandgap semiconductor, GaN. Results also show that the ion doses necessary for electrical isolation close-to-inversely depend on the number of ion-beam-generated atomic displacements. However, in all the cases studied, defect-induced electrical isolation of ZnO is unstable to rapid thermal annealing at temperatures above ∼300 °C.
Lattice instabilities in (Pb,Cd)TiO3 alloys
Authors: Halilov S. V. , Fornari M. , Singh D. J.
Published: October 2002

We report density functional calculations of the lattice instabilities of CdTiO3 and Pb0.5Cd0.5TiO3 supercells. The dominant instabilities in CdTiO3 are against TiO6 octahedral rotation, as may be expected from its Pnma structure. However, Pb substitution for Cd rapidly supresses this instability in favor of A-site driven ferroelectricity. At the 50% Pb composition, the rotational, rhombohedral ferroelectric and tetragonal (TET) ferroelectric instabilities of the cubic perovskite structure are comparable in energy, and moreover a rather large c/a ratio is found for the TET state. This strongly suggests investigation of the Pb rich part of the (Cd,Pb)TiO3 pseudobinary and minor modifications to it as potential high performance piezoelectric materials.
Multiple-wavelength second-harmonic generation in aperiodic optical superlattices
Authors: Liu H. , Zhu S. N. , Zhu Y. Y. , Ming N. B. , Lin X. C. , Ling W. J. , Yao A. Y. , Xu Z. Y.
Published: October 2002

We present a kind of aperiodic structure for the generation of multiple-wavelength second harmonic in a quasi-phase-matching scheme. In order to confirm its efficiency, a LiTaO3 superlattice with such an aperiodic domain-inverted structure was designed and fabricated. The second-harmonic generation at four present wavelengths was experimentally demonstrated from the superlattice with high and nearly equal conversion efficiencies. The tested result is in good agreement with theoretical consideration. The method may be used for the design of optical superlattices to construct multiple-wavelength lasers and wavelength converters for all-optical network.
Improved stability of hydrogenated amorphous-silicon photosensitivity by ultraviolet illumination
Authors: Branz Howard M. , Xu Yueqin , Heck Stephan , Gao Wei
Published: October 2002

Postdeposition ultraviolet (UV) illumination, followed by etching, improves the stability of hydrogenated amorphous-silicon thin films against subsequent light-induced degradation of photosensitivity. The etch removes a heavily damaged layer extending about 100 nm below the surface, but beneath the damage, the UV has improved the stability of 200 to 300 nm of bulk film. The open-circuit voltage of Schottky solar cells is also stabilized by UV-etch treatment. Possible mechanisms are discussed.
Electric-field-aligned growth of single-walled carbon nanotubes on surfaces
Authors: Ural Ant , Li Yiming , Dai Hongjie
Published: October 2002

Aligned single-walled carbon nanotubes are grown onto the surfaces of SiO2/Si substrates in electric fields established across patterned metal electrodes. Calculations of the electric field distribution under the designed electrode structures, the directing ability of electric fields, and the prevention of surface van der Waals interactions are used to rationalize the aligned growth. The capability of synthesizing oriented single-walled nanotubes on surfaces shall open up many opportunities in organized architectures of nanotubes for molecular electronics.
Angle-dependent x-ray absorption spectroscopy study of Zn-doped GaN
Authors: Chiou J. W. , Mookerjee S. , Rao K. V. R. , Jan J. C. , Tsai H. M. , Asokan K. , Pong W. F. , Chien F. Z. , Tsai M.-H. , Chang Y. K. , Chen Y. Y. , Lee J. F. , Lee C. C. , Chi G. C.
Published: October 2002

As-grown and Zn-implanted wurtzite GaN samples have been studied by angle-dependent x-ray absorption near edge structure (XANES) measurements at the N and Ga K-edges and the Ga L3-edge. The angle dependence of the XANES spectra shows that the Ga–N bonds lying in the bilayer have lower energies than bonds along the c-axis, which can be attributed to the polar nature of the GaN film. The comparison of the Ga L3-edge XANES spectra of as-grown and Zn-doped GaN reveals significant dopant induced enhancement of near-edge Ga d-derived states.
Upside-down tuning of light- and heavy-hole states in GaNAs/GaAs single quantum wells by thermal expansion and quantum confinement
Authors: Ya M. H. , Cheng W. Z. , Chen Y. F. , Lin T. Y.
Published: October 2002

Polarization resolved photoluminescence from a cleaved sample edge (edge photoluminescence) was used to investigate the valence-band splitting in GaNAs/GaAs single quantum wells. The spectra resulting from the heavy- and light-hole transitions shows an interesting phenomenon, in which the light-hole state is above the heavy-hole state at low temperatures, they degenerate at about 195 K, and finally the light-hole state is below the heavy-hole state at higher temperatures. We point out that the underlying origin of our observation can be attributed to the competitive effect between the tensile strain induced by lattice mismatch and quantum confinement. It is known that the large density of heavy-hole states is beneficial for laser diodes. Our result is therefore very useful for the application of GaNAs/GaAs quantum wells in optical devices.