bluuebunny/crossref_metadata_2025 huggingface dataset:
Tunneling magnetoresistance at up to 270 K in La0.8Sr0.2MnO3/SrTiO3/La0.8Sr0.2MnO3 junctions with 1.6-nm-thick barriers
Authors: Obata Takeshi , Manako Takashi , Shimakawa Yuichi , Kubo Yoshimi
Published: July 2002

Magnetic tunneling junctions are fabricated from epitaxially grown La0.8Sr0.2MnO3/SrTiO3/La0.8Sr0.2MnO3 trilayers. A large tunneling magnetoresistance of 150% is observed for a junction with a thin barrier layer (1.6 nm) under a low switching field (<10 Oe) at 5 K. A small tunneling magnetoresistance is observed even at 270 K, which is close to the ferromagnetic Curie temperature (290 K) of the La0.8Sr0.2MnO3 film. The large magnetoresistance and high operating temperature are attributed to the sufficiently thin and uniform barrier layer of SrTiO3.
Optical characterization of the “E2” deep level in GaN
Authors: Hacke P. , Ramvall P. , Tanaka S. , Aoyagi Y. , Kuramata A. , Horino K. , Munekata H.
Published: July 2002

The correspondence between the E2 level (∼Ec−0.55 eV) in n-type GaN undergoing thermoionization and photoionization was established. The optical cross section in the vicinity of the threshold for photoionization of this level was measured by means of capacitance transient spectroscopy. Analysis using the formulation of Chantre yielded the optical activation energy, Eo=0.85 eV, and the Franck–Condon parameter, dFC=0.30 eV at 90 K.
A magnetostatic forward volume wave oscillator-based magneto-optic Bragg cell modulator
Authors: Su Jun , Tsai Chen S.
Published: July 2002

A guided-wave magneto-optic (MO) Bragg cell modulator that utilizes a magnetostatic forward volume wave (MSFVW) oscillator through a feedback loop is reported. The carrier frequency of the MSFVW signals involved in the MO interaction is tuned linearly from 2.112 to 3.274 GHz by increasing an external bias magnetic field from 2350 to 2900 Oe. Compared with a conventional delay line-based MO Bragg cell modulator, the oscillator-based MO modulator can provide higher diffraction efficiency by twofold to fourfold owing to reduction of insertion losses in the MSFVW oscillator. Furthermore, such oscillator-type MO modulator can be more easily integrated with other monolithic microwave integrated circuit components.
Photonic band-gap effects upon the light emission from a dye–polymer–opal composite
Authors: Romanov S. G. , Maka T. , Sotomayor Torres C. M. , Müller M. , Zentel R.
Published: July 2002

A polymeric photonic crystal formed by impregnating an opal with a Coumarin 6 dye containing polymer has been prepared and its emission properties studied by angular-resolved photoluminescence. A strong spectral redistribution of the luminescence intensity was observed in the dye-polymer-opal compared to that of the dye-polymer reference sample. The emission spectrum shows a marked distortion compared with the reference sample, which changes with the rotation of the crystal due to the angular dispersion of the stop band. Under increasing optical pumping power, a band of amplified spontaneous emission was observed at the stop-band edge. The angular dispersion of the stop band in the luminescence observed from the back surface exhibits in a strong directionality at the maximum of the dye emission band.
Optical and structural studies of Ge nanocrystals embedded in AlN matrix fabricated by pulsed laser deposition
Authors: Hassan K. M. , Sharma A. K. , Narayan J. , Muth J. F. , Teng C. W. , Kolbas R. M.
Published: July 2002

We have fabricated Ge nanostructures buried in a matrix of AlN grown on Si(111) by pulsed laser deposition at the substrate temperature of 500 °C. The characterization of these structures was performed using high-resolution transmission electron microscopy (HRTEM), photoluminescence, and Raman spectroscopy. The HRTEM results show that the Ge islands are single crystal with a pyramidal shape. The average size of Ge islands was determined to be ∼15 nm, which could be varied by controlling laser deposition and substrate parameters. The Raman spectrum showed a peak of the Ge–Ge vibrational mode downward shifted up to 295 cm−1 which is caused by quantum confinement of phonons in the Ge dots. The photoluminescence of the Ge dots (size ∼15 nm) was blueshifted by ∼0.266 eV from the bulk Ge value of 0.73 eV at 77 K, resulting in a distinct peak at ∼1.0 eV. The transmission measurements carried out on different samples having Ge dot sizes of 7, 8, and 13 nm deposited on sapphire substrate showed the above band edge transitions of Ge, which were also blueshifted in accordance with the quantum confinement effect. The importance of pulsed laser deposition in fabricating novel nanostructures is emphasized.
The role of potential fluctuations in continuous-wave donor–acceptor pair luminescence of heavily doped materials
Authors: Kuskovsky I. , Li D. , Neumark G. F. , Bondarev V. N. , Pikhitsa P. V.
Published: July 2002

It has recently become apparent that “standard” (low-concentration and low-compensation) theory for donor–acceptor pair (DAP) photoluminescence (PL) is totally incapable of explaining results in highly doped and compensated material. It can be noted that such material is often of high technological interest. It has been argued, mainly qualitatively, that the discrepancies result from potential fluctuations due to random ionic charges. We here present a quantitative theory for cw DAP PL, using an approximate model. We also present data for the concentration and intensity dependence of DAP PL in heavily doped ZnSe:N, and show that the results are explained very satisfactorily by our fluctuation model.
Lasing properties of disk microcavity based on a circular Bragg reflector
Authors: Shaw A. , Roycroft B. , Hegarty J. , Labilloy D. , Benisty H. , Weisbuch C. , Krauss T. F. , Smith C. J. M. , Stanley R. , Houdré R. , Oesterle U.
Published: July 2002

The lasing properties of quantum well structures, where the cavity is defined in the plane of the wells by circular Bragg reflectors are investigated. Diffraction of the in-plane lasing modes into the vertical direction by the circular distributed Bragg reflector (DBR) allows the simultaneous measurement of near-field emission patterns and emission spectra, allowing unambiguous assignment of azimuthal quantum numbers to the lasing modes. The radial quantum number is determined by fitting the lasing spectrum to theory. Lasing is shown to occur in modes whose wave vector is mainly radial, confined by the circular DBR structure, rather than in whispering gallery type modes which are mainly azimuthal.
Pulse autocorrelation measurements based on two- and three-photon conductivity in a GaN photodiode
Authors: Streltsov Alexander M. , Moll K. D. , Gaeta Alexander L. , Kung P. , Walker D. , Razeghi M.
Published: July 2002

We characterize the performance of a GaN p-i-n photodiode as a nonlinear sensor for second- and third-order femtosecond pulse autocorrelation measurements in the visible and near-infrared regimes, respectively. The two- and three-photon absorption coefficients for GaN are also determined.
Cross-sectional nanophotoluminescence studies of Stark effects in self-assembled quantum dots
Authors: Htoon H. , Keto J. W. , Baklenov O. , Holmes A. L. , Shih C. K.
Published: July 2002

By using a cross-sectional geometry, we show the capability to perform single-dot spectroscopy in self-assembled quantum dots using far-field optics. By using this method, we study the quantum-confined Stark effect in self-assembled quantum dots. For single-stack quantum dots (QDs), we find that the spectra are redshifted with an increase in electric field. For vertically coupled double-stack quantum dots, while most of the QDs are redshifted, some QDs show blueshifted spectra, which can be interpreted as an evidence of coupled QD molecules.
Mg doping of GaN layers grown by plasma-assisted molecular-beam epitaxy
Authors: Smorchkova I. P. , Haus E. , Heying B. , Kozodoy P. , Fini P. , Ibbetson J. P. , Keller S. , DenBaars S. P. , Speck J. S. , Mishra U. K.
Published: July 2002

GaN:Mg layers grown by plasma-assisted molecular-beam epitaxy at 650 °C are investigated. Secondary-ion-mass-spectroscopy measurements reveal uniform Mg doping profiles with very sharp boundaries. The amount of incorporated Mg atoms changes approximately linearly with incident Mg flux. Hall measurements on p-type GaN:Mg layers show that about 1%–2% of all Mg atoms are ionized at room temperature. The hole mobility depends strongly on the hole concentration, varying from μp=24 cm2/V s for p=1.8×1017 cm−3 to μp=7.5 cm2/V s for p=1.4×1018 cm−3. GaN p–n diodes with molecular-beam-epitaxy-grown p regions are analyzed using current–voltage measurements.
Saturation effects in Y2SiO5:Tb under low-voltage excitation
Authors: Stoffers C. , Lee R. Y. , Penczek J. , Wagner B. K. , Summers C. J.
Published: July 2002

Low-voltage field emission device phosphors that are excited at high current densities often exhibit brightness saturation with increasing current. The physical processes responsible for saturation can be complex, with several mechanisms contributing, including ground state depletion and excited energy transfer. A two-level model, in conjunction with cathodoluminescence brightness and transient measurements, is used to show the influence of ground state depletion and thermal quenching on the saturation behavior of Y2SiO5:Tb under low-voltage excitation.
An analysis of temperature dependent piezoelectric Franz–Keldysh effect in AlGaN
Authors: Hou Y. T. , Teo K. L. , Li M. F. , Uchida Kazuo , Tokunaga Hiroki , Akutsu Nakao , Matsumoto Koh
Published: July 2002

Strong Franz–Keldysh oscillations near the band gap of AlGaN are observed in the contactless electroreflectance (CER) studies of a GaN/InGaN/AlGaN multilayer structure. The line shape analysis of the CER spectra at different temperatures provides an accurate determination of the AlGaN band gap energies and the built-in electric fields. Using the existing data of the thermal expansion coefficients of GaN and sapphire, and the piezoelectric constants of AlGaN, the temperature dependence of the electric field is estimated and is in good agreement with the experimental results between 15 and 300 K. We attribute such electric field to the piezoelectric strain effect.
High-performance blue light-emitting diode based on a binaphthyl-containing polyfluorene
Authors: Jiang Xuezhong , Liu Sen , Ma Hong , Jen Alex K.-Y.
Published: July 2002

We report efficient blue electroluminescence (EL) from multilayer polymer light-emitting diodes using a binaphthyl-containing polyfluorene as the emitting layer and a series of thermally polymerized triphenylamine/tetraphenyldiaminobiphenyl-containing polymers as the hole transport layer. The polymer possesses a high photoluminescence quantum efficiency of 44%. The polymer light-emitting diodes exhibit EL emission peak at 420 or 446 nm, depending on the hole transporting materials. The EL reaches maximum brightness and external quantum efficiency of 3070 cd/m2 and 0.82%, respectively.
Formation of icosahedral quasicrystalline phase in Zr70Ni10M20(M=Pd, Au, Pt) ternary metallic glasses
Authors: Saida J. , Matsushita M. , Li C. , Inoue A.
Published: July 2002

An icosahedral quasicrystalline phase was confirmed as a primary precipitation phase in melt-spun Zr70Ni10M20 (M=Pd, Au and Pt) ternary metallic glasses with two-stage crystallization process. The onset temperature of the transformation from amorphous to icosahedral phase are 687 K for Pd, 754 K for Au and 783 K for Pt at a heating rate of 0.67 K s−1. The size of the icosahedral particles is in the range of 5–20 nm. The second crystallization reaction results in the formation of Zr2Ni+Zr2Pd and Zr2Ni+ZrPt phases in the Zr–Ni–Pd and Zr–Ni–Pt alloys, respectively, and Zr3Au phase in the Zr–Ni–Au alloy through a single exothermic reaction. The formation of nanoscale icosahedral phase indicates the possibility that icosahedral short-range order exists in the glassy state.
Magnetization reversal of ferromagnetic/antiferromagnetic bilayers
Authors: Li Zhanjie , Zhang Shufeng
Published: July 2002

By incorporating random interfacial exchange interaction into the Landau–Lifshitz–Gilbert equation, we show that magnetization reversal of exchange-biased films possesses unique hysteresis features. A detail magnetization reversal process for different ferromagnetic layer thickness is analyzed.
Mechanical resonant immunospecific biological detector
Authors: Ilic B. , Czaplewski D. , Craighead H. G. , Neuzil P. , Campagnolo C. , Batt C.
Published: July 2002

We have demonstrated high-sensitivity detection of bacteria using an array of bulk micromachined resonant cantilevers. The biological sensor is a micromechanical oscillator that consists of an array of silicon-nitride cantilevers with an immobilized antibody layer on the surface of the resonator. Measured resonant frequency shift as a function of the additional cell loading was observed and correlated to the mass of the specifically bound Escherichia coli O157:H7 cells. Deposition and subsequent detection of E. coli cells was achieved under ambient conditions.
Optical properties, spectral narrowing of photoluminescence and blue electroluminescence of poly(phenylene pyridine) derivatives
Authors: Fujii A. , Ootake R. , Fujisawa T. , Ozaki M. , Ohmori Y. , Laga Tong , Yoshino K. , Lu H.-F. , Chan H. S. O. , Ng S.-C.
Published: July 2002

Optical properties of poly(phenylene pyridine) derivatives, poly(2,5-dialkoxy-1,4phenylenealt-2,5-pyridine) (PHOnPY25), such as optical absorption, photoluminescence (PL), and electroluminescence (EL), have been studied. The electronic energy structures of PHOnPY25 have been determined by optical and electrochemical measurements. Strong PL with high quantum efficiency has been clarified in the films, and spectral narrowing has been observed by pulse excitation of a nitrogen laser. Intense blue EL has also been demonstrated.
Feedback stabilization of nonaxisymmetric resistive wall modes in tokamaks. I. Electromagnetic model
Authors: Liu Y. Q. , Bondeson A. , Fransson C. M. , Lennartson B. , Breitholtz C.
Published: July 2002

Active feedback stabilization of pressure-driven modes in tokamaks is studied computationally in toroidal geometry. The stability problem is formulated in terms of open-loop transfer functions for fluxes in sensor coils resulting from currents in feedback coils. The transfer functions are computed by an extended version of the MARS stability code [A. Bondeson et al., Phys. Fluids B 4, 1889 (1992)] and can be accurately modeled by low order rational functions. In the present paper stability is analyzed for a system with an ideal amplifier (current control). It is shown that feedback with modest gain, and a single coil array poloidally, gives substantial stabilization for a range of coil shapes. Optimum design uses sensors for the poloidal field, located inside the resistive wall, in combination with rather wide feedback coils outside the wall. Typically, the feedback does not strongly modify the plasma-generated magnetic field perturbation. A future companion paper [C. M. Fransson et al., Phys. Plasmas (accepted for publication)] will apply control theory to study the limitations arising for finite time-constant of the amplifier-feedback coil circuit.
Magnetocaloric effect in La1−xSrxMnO3 for x=0.13 and 0.16
Authors: Szewczyk A. , Szymczak H. , Wiśniewski A. , Piotrowski K. , Kartaszyński R. , Da̧browski B. , Koleśnik S. , Bukowski Z.
Published: July 2002

Magnetization of polycrystalline La1−xSrxMnO3 samples with x=0.13 and x=0.16 has been measured as a function of temperature and external magnetic field. Using the gathered data and the thermodynamic Maxwell’s relation, changes of the magnetic contribution to entropy under influence of the magnetic field, allowing to estimate the value of magnetocaloric effect, have been determined. The changes reach ∼7 J/(kg K) in the field of 7 T, which makes the studied samples good candidates for application for magnetic refrigeration. Expressions describing entropy and its change under influence of magnetic field have been derived in the molecular field approximation. Satisfactory agreement between the theoretical curves and the experimental dependencies has been found.
Time-resolved photoluminescence of GaN/Al0.5Ga0.5N quantum wells
Authors: Harris J. C. , Someya T. , Kako S. , Hoshino K. , Arakawa Y.
Published: July 2002

We report photoluminescence (PL) and time-resolved PL measurements of GaN/AlxGa1−xN multiple quantum wells with barriers of high aluminum content, x=0.5. In wells of width 1–2 nm, low temperature recombination appears to be dominated by radiative processes with lifetimes ∼0.5 ns. Dependence of lifetime on emission energy is very small compared to InGaN quantum wells, indicating that carrier localization is very slight and interface quality is high. In 4 nm wells, PL emission at an energy below the bulk GaN band gap and long recombination lifetimes result from the polarization field across the wells.
Non-Hermitian formulation of interference effect in scattering experiments
Authors: Narevicius Edvardas , Moiseyev Nimrod
Published: July 2002

Non-Hermitian quantum mechanics allows one to calculate a physical observable, e.g., scattering cross section, as a sum over a finite number of discrete resonance states. The coefficients in the sum can get complex and negative values even in cases where conventional scattering theory predicts real positive coefficients only. Consequently, structure (or absence of structure) in scattering cross section can be obtained as a result of interference between a small number of discrete resonance states; whereas, conventional scattering theory would require integration over the continuum of scattering states and therefore it is a heavy numerical task. We show here that in electron scattering experiments the interference between overlapping broad resonances leads to oscillations in the phase of transition probability amplitude and to enhancement of the transition state lifetime due to nuclear motion.
Polarization modulation technique for magneto-optical quantitative vector magnetometry
Authors: Vavassori P.
Published: July 2002

This letter presents a method for determining the orientation of the magnetization vector during the reversal process in ferromagnetic films using a conventional setup for magneto-optical ellipsometry based on the photoelastic modulation technique for the modulation of polarization. The orientation is obtained through the determination of three orthogonal reduced (i.e., normalized to saturation value) magnetization components as a function of the external field. Test measurements on a reference sample are also presented, which confirm the reliability of the measurements and show what kind of information can be retrieved.
Predictive study of a plasma structure and function in reactive ion etcher driven by very high frequency: Validity of an extended two-dimensional relaxation continuum model
Authors: Maeshige Kazunobu , Hasebe Masao , Yamaguchi Yukio , Makabe Toshiaki
Published: July 2002

The plasma structure and physical function of a narrow gap reactive ion etcher (RIE), consisting of capacitively coupled parallel plates driven at 100 MHz, have been predicted in a proper manner by an extended relaxation continuum model including gas flow and sputtered particle transport from the substrate. Monitoring the spatiotemporal excitation rate gives validity to the use of the continuum model even at 50 mTorr under higher power condition mainly maintained by an ionization multiplication of the secondary electrons ejected from the powered electrode by ion impacts. The plasma structures are testified by comparing the two-dimensional net excitation rate of Ar(3p5) with the experimental computerized tomography image. A nonvolatile particle transport successive to the physical etching on the substrate has been predicted in the RIE under a feed gas flow.
The role of dc current limitations in Fowler–Nordheim electron emission from carbon films
Authors: Cui J. B. , Teo K. B. K. , Tsai J. T. H. , Robertson J. , Milne W. I.
Published: July 2002

We investigated the electron field emission characteristics of carbon films deposited by filtered cathodic vacuum arc. A special sandwich structure was created using alternating layers of carbon and aluminum. This structure allows both field emission and dc current–voltage characteristics to be measured on the same film. The emission current was found to follow the Fowler–Nordheim law below a critical field, Ecri. Above Ecri, the current deviates from the Fowler–Nordheim law but follows the dc current–voltage characteristics of the film. It is concluded that the frequently observed deviation of electron emission from the Fowler–Nordheim law at high field is due to the current limitation of the resistive carbon film. The important implication of this finding for device fabrication is discussed.
High-energy ion-implantation-induced gettering of copper in silicon beyond the projected ion range: The trans-projected-range effect
Authors: Gueorguiev Y. M. , Kögler R. , Peeva A. , Mücklich A. , Panknin D. , Yankov R. A. , Skorupa W.
Published: July 2002

Five different species, namely B, Si, P, Ge, and As, were implanted at MeV energies into (100)-oriented n-type Czohralski Si, in order to form deep gettering layers during the subsequent annealing. Then the samples were contaminated with Cu by implanting the impurity on the backface and performing additional annealing. The resulting Cu depth distributions were measured by secondary ion mass spectrometry. Strong gettering of Cu atoms beyond the projected ion range RP and formation of a well-defined separate Cu gettering band therein is found for P and As implants. We call this phenomenon the “trans-RP effect.” It arises from the presence of a significant amount of defects in the regions much deeper than RP. Their gettering ability is higher than that of the extended defects around RP, as the amount of Cu atoms gettered beyond RP is, especially for the P implants, much greater than that in the implanted gettering layer at RP. These deep defects have not been detected by transmission electron microscopy, and we suggest that they are small interstitial clusters. A mechanism responsible for the migration of self-interstitials from RP into the trans-RP region and their clustering therein is proposed. An explanation is given of the possible reasons for the differences in the results for the P+ and As+ implants.
Chirality in the context of spin 32 particles
Authors: Marques G. C. , Spehler D.
Published: July 2002

In this paper we apply the chiral spinorial approach to the description of spin 32 particles. Chiral components of rank 3 spinor fields are considered to be the dynamical variables of the theory. The free Lagrangian is built from the same principles as in the case of spin 1 and spin 2 particles: chiral symmetry at the free field level and chiral symmetry breakdown at the interaction level. We show how the chiral spinorial approach provides an unambiguous Lagrangian approach for massless spin 32 particles. This approach provides a fairly rich set of effective Lagrangians for the interaction of spin 32 particles.
Facilitated spin models, mode coupling theory, and ergodic–nonergodic transitions
Authors: Pitts Steven J. , Young Thomas , Andersen Hans C.
Published: July 2002

Facilitated kinetic Ising spin models are models for cooperative relaxation in liquids. Some of them have ergodic–nonergodic transitions of the type predicted by the mode coupling theory of supercooled liquids. We discuss two mode coupling theories, that of Kawasaki and one developed by us, and compare their predictions with the properties of several facilitated kinetic spin models, including the hierarchically constrained kinetic Ising model in one dimension (the East model), the North-east model, and the class of (a,a−1)-Cayley tree models. We present new simulation data for the East model. For models with low dimensionality and low coordination number, there is little or no relationship between the transitions predicted by the mode coupling theories and the actual behavior of the spin systems, with the mode coupling theories generally predicting transitions for models that don’t have them and attributing qualitatively incorrect properties to those transitions that do occur. The mode coupling theories describe the relaxation of the East model well for short times but fail at long times in the vicinity of the incorrectly predicted transition and for the states that are incorrectly predicted to be nonergodic. Simulation evidence is presented for scaling behavior of the relaxation for low temperatures and long times in the East model, but no extant mode coupling theory predicts this behavior correctly. An analogy between liquids and facilitated spin models is proposed, whereby the slightly supercooled liquid regime is analogous to the spin system states near the spurious mode coupling transition, and the low temperature supercooled liquid near its glass transition is analogous to the spin model states just above the actual ergodic–nonergodic transition (in the case of models that have such a transition) or in the low temperature scaling regime (in the case of models with this type of low temperature behavior). According to this analogy, the actual transition or the low temperature scaling behavior of the spin models is analogous to the behavior at or near the thermodynamic transition that is sometimes proposed as the basis for the glass transition in liquids.
Optimized leaky mode spectroscopy with a single planar film
Authors: Leitz M. , Podgorsek R. P. , Franke H. , Woods J.
Published: July 2002

A transparent dielectric layer coated on a high refractive index prism forms a planar leaky waveguide. The leaky mode spectrum of this layer strongly depends on the imaginary part of the layer refractive index κ. As an example the leaky mode spectrum of a doped polymer layer is studied theoretically and experimentally. For κ=0 no modes are detectable. In the range of κ≈10−4, so-called “optimized leaky modes,” can be observed while, in the range κ⩾10−3, broad resonances with reduced reflectivity can be observed. The imaginary part of the polymer refractive index was adjusted by doping with the dye malachit green with an absorption maximum at λ=620 nm.
Surface production of CF, CF2, and C2 radicals in high-density CF4/H2 plasmas
Authors: Sasaki K. , Furukawa H. , Kadota K. , Suzuki C.
Published: July 2002

Surface production of CF, CF2, and C2 radicals in high-density CF4/H2 plasmas was examined using laser-induced fluorescence spectroscopy. No significant amount of surface production was observed in pure CF4 plasmas. The addition of H2 into CF4 plasmas enhanced the surface production of CFx and C2 from fluorocarbon film deposited on the chamber wall. The characteristics of the surface production in cw discharges are reported, in comparison with surface production in pulsed discharges. In addition, it has been found that the surface production rates are determined not by the partial pressure but by the flow rate of H2, suggesting the significant consumption of feedstock H2 in discharges. The surface production of CFx and C2 indicates that these radicals are not the precursors for the deposition of fluorocarbon film in the CF4/H2 plasma. The deposition mechanism of fluorocarbon film in the CF4/H2 plasma is discussed, taking into account the surface production of CFx and C2.
Identification of electron and hole traps in KH2PO4 crystals
Authors: Garces N. Y. , Stevens K. T. , Halliburton L. E. , Demos S. G. , Radousky H. B. , Zaitseva N. P.
Published: July 2002

Electron paramagnetic resonance (EPR) has been used to characterize a hole trap and several electron traps in single crystals of potassium dihydrogen phosphate (KH2PO4 or KDP). The paramagnetic charge states of these centers are produced by ionizing radiation (e.g., x rays or a 266 nm beam from a pulsed Nd:YAG laser) and are stable for days and even weeks at room temperature. One center consists of a hole trapped on an oxygen ion adjacent to a silicon impurity located on a phosphorus site. This defect has a small, but easily observed, hyperfine interaction with the adjacent substitutional proton. The other centers are formed when an electron is trapped at an oxygen vacancy. These latter defects are best described as (PO3)2− molecular ions, where the primary phosphorus nucleus is responsible for a large hyperfine splitting (500–800 G in magnitude). Five EPR spectra representing variations of these oxygen vacancy centers are observed, with the differences being attributed to the relative position of a nearby cation vacancy, either a missing proton or potassium. An angular study of the EPR spectra, conducted at room temperature, provided principal values and principal directions for the g matrices and hyperfine matrices for the hole center and two of the electron centers.
Active wall motions for skin-friction drag reduction
Authors: Kang Sangmo , Choi Haecheon
Published: July 2002

In the present study we investigate a possibility of reducing skin-friction drag in a turbulent channel flow with active wall motions. The wall is locally deformed according to two successful control strategies [J. Fluid Mech. 262, 75 (1994); J. Fluid Mech. 358, 245 (1998)]. Results show that overall 13–17% drag reductions are obtained with the active wall motions, and turbulence intensities and near-wall streamwise vortices are significantly weakened. It is remarkable that instantaneous wall shapes are elongated in the streamwise direction and resemble riblets in appearance. However, the mechanism of the present drag reduction is essentially different from that of riblets.
Direct observation of stacking fault nucleation in the early stage of ZnSe/GaAs pseudomorphic epitaxial layer growth
Authors: Wang N. , Fung K. K. , Sou I. K.
Published: July 2002

The nucleation of stacking faults in the initial stage of growth of ZnSe/GaAs(001) epilayers has been studied by high-resolution transmission electron microscopy. Stacking faults have been observed to nucleate on the {111} planes on the slopes of ZnSe islands and hang over the adjacent (001) surface. The image details of a V-shaped fault originating from a sharp tip at a later stage of growth is in good agreement with the simulated image of stacking faults emerging from a dimer array of three chains. This gives support to residual dimer array on the 2×1 reconstructed (001) surface as the origin of stacking faults in ZnSe epilayers.
Effect of repetition rate of a pulsed microwave diamond forming plasma on the density of C2
Authors: Khachan J. , James B. W. , Marfoure A.
Published: July 2002

The relative density of C2 in a pulsed microwave discharge, used for diamond deposition, was measured by optical emission spectroscopy. It was found that the average density of C2 increases with increasing plasma repetition rate (200 Hz–8 kHz) and approaches a limit at high frequencies. These results are explained with a simple plasma chemical kinetics simulation of the pulsed discharge.
Molecular order in nematic liquid crystals from shape-dependent repulsive and attractive interactions
Authors: Ferrarini Alberta , Moro Giorgio J.
Published: July 2002

A molecular model for orientational ordering in liquid crystal phases is presented. A van der Waals picture is adopted, with repulsive and attractive interactions described, respectively, in terms of excluded volume and surface contact interactions for pairs of molecules. The model, based on a realistic representation of the molecular structure, is applied to the nematic phase of para-azoxyanisole (PAA). Temperature and pressure dependence of order parameters is analyzed, by discussing the role played by repulsive and attractive interactions between molecules.
High-pressure process to produce GaN crystals
Authors: Gilbert Donald R. , Novikov Alexander , Patrin Nikolay , Budai John S. , Kelly Frank , Chodelka Robert , Abbaschian Reza , Pearton Stephen J. , Singh Rajiv
Published: July 2002

High melt temperature and thermal decomposition prevent the use of standard bulk semiconductor crystal growth processes for the production of GaN. We have employed a hydrostatic pressure system to grow GaN crystals. An ultrahigh pressure, high temperature process was developed using a solid-phase nitrogen source to form GaN crystals in a Ga metal melt. Using a thermal gradient diffusion process, in which nitrogen dissolves in the high temperature region of the metal melt and diffuses to the lower temperature, lower solubility region, high quality crystals up to ∼1 mm in size were formed, as determined by scanning electron microscopy, x-ray diffraction, and micro-Raman analysis.
Model for the charge trapping in high permittivity gate dielectric stacks
Authors: Houssa M. , Naili M. , Heyns M. M. , Stesmans A.
Published: July 2002

The generation of traps in SiOx/ZrO2 and SiOx/TiO2 gate dielectric stacks during gate voltage stress of metal-oxide-semiconductor capacitors is investigated. The voltage and temperature dependence of the trap generation rate is extracted from the analysis of the gate current increase observed during the electrical stress. These data can be explained by a model based on a two-stage degradation process, i.e., (1) H+ proton generation in the high permittivity gate dielectric layer by the injected electrons and (2) transport of the H+ protons in the high permittivity material, resulting in bond breaking and trap generation. The threshold electron energy for H+ generation and the activation energy for H+ transport and bond breaking are extracted from fits to the experimental results.
Structure of nanocrystalline Re–Si thin film composites and their unusual thermoelectric properties
Authors: Pitschke W. , Hofman D. , Schumann J. , Kleint C. A. , Heinrich A. , Burkov A. T.
Published: July 2002

The structure of nanocrystalline rhenium–silicon composite ReSix films with 1.4<x<2.2 has been studied as a function of the time by means of high temperature x-ray diffraction and transmission electron microscopy. Simultaneously, the thermoelectric transport properties were measured during the heat treatment. The nanocrystallization was achieved by annealing of amorphous films deposited onto oxidized Si wafers by magnetron cosputtering. The crystallization process is characterized by a decreasing average crystallite size in the range between 7 and 19 nm. An unknown mechanism limits the grain growth after reaching a maximum size, which decreases with increasing Si content. In the final state the films contain only two phases: the amorphous phase and the nanocrystalline ReSi1.75 phase. The electrical conductivity and the thermoelectric power of these thin film composites show nonmonotonic dependence on the volume fraction of the nanocrystalline phase and depend on different parameters, which suggests a way to optimize the thermoelectric efficiency.
Effective-medium approximation for composite media: Realizable single-scale dispersions
Authors: Torquato S. , Hyun S.
Published: July 2002

It is known that the popular effective medium approximation (EMA) for the effective conductivity σe of a composite is exactly realizable by certain multiscale hierarchical microstructures. We have found a class of periodic, single-scale dispersions that achieve the EMA function at a given phase conductivity ratio for a two-phase, two-dimensional composite over all volume fractions. Moreover, to an excellent approximation (but not exactly), the same structures realize the EMA for almost the entire range of phase conductivities and volume fractions. The inclusion shapes are given analytically by the generalized hypocycloid, which in general has a nonsmooth interface. To find these structures, we utilized target optimization techniques and a theorem concerning the spectral function.
Far-infrared laser vibration–rotation–tunneling spectroscopy of water clusters in the librational band region of liquid water
Authors: Keutsch Frank N. , Fellers Ray S. , Viant Mark R. , Saykally Richard J.
Published: July 2002

We report the first high resolution spectrum of a librational vibration for a water cluster. Four parallel bands of (H2O)3 were measured between 510 and 525 cm−1 using diode laser vibration–rotation–tunneling (VRT) spectroscopy. The bands lie in the “librational band” region of liquid water and are assigned to the nondegenerate out of plane librational vibration. The observation of at least three distinct bands within 8 cm−1 originating in the vibrational ground state is explained by a dramatically increased splitting of the rovibrational levels relative to the ground state by bifurcation tunneling and is indicative of a greatly reduced barrier height in the excited state. This tunneling motion is of special significance, as it is the lowest energy pathway for breaking and reforming of hydrogen bonds, a salient aspect of liquid water dynamics.
Theoretical investigation of high temperature IV–VI compound continuous wave midinfrared vertical cavity surface emitting lasers
Authors: Khosravani S. , Shi Z.
Published: July 2002

Theoretical investigations on the optically pumped IV-VI mid-infrared vertical-cavity surface emitting lasers were made. Key parameters such as Auger recombination and heat dissipation were identified and maximum operating temperature, peak output power, and threshold pumping power were simulated. Unlike other band-to-band mid-IR laser materials, Auger recombination does not limit IV-VI diode lasers to operate at room temperature in continuous wave (cw) mode. However, insufficient heat dissipation is the dominant factor in preventing laser operation at room temperature. The calculated maximum cw operation temperature for a simple active layer design was 282 K and could be further improved for more advanced structures such as quantum well lasers. These results indicate that such lasers are promising for thermoelectrically cooled spectroscopic systems.
Spherically symmetric space–times with constant curvature scalar
Authors: Goenner Hubert , Havas Peter
Published: July 2002

In view of the geometrical importance of spaces with constant scalar curvature, a systematic study of spherically symmetric such space–time manifolds with respect to the eigenvalues of the Ricci tensor is made. The cases of two double or one quadruple eigenvalue are treated exhaustively. In the generic case of one double and two single eigenvalues, no conformally flat solutions, and only solutions with one arbitrary function of one variable are found. We also give all four-dimensional decomposable s.s. spaces with constant curvature scalar.
Electromigration of eutectic SnPb solder interconnects for flip chip technology
Authors: Lee T. Y. , Tu K. N. , Kuo S. M. , Frear D. R.
Published: July 2002

The electromigration of eutectic SnPb solder interconnects between a Si chip and a FR4 substrate was studied at 120 °C for up to 324 h with current stressing of 104 amp/cm2. Hillocks were observed at the anode and voids at the cathode. The dominant diffusing species was found to be Pb, confirmed by its accumulation at the anode. Diffusion markers were used to measure the electromigration flux and calculate the effective charge of atomic diffusion in the solder. Extensive microstructural evolution was also observed in the two-phase solder alloy that occurred by a ripening process.
Reduction in conduction noise of La–Ca–Mn–O polycrystalline films due to Ag addition
Authors: Khare Neeraj , Moharil U. P. , Gupta A. K. , Singh H. K. , Srivastava O. N.
Published: July 2002

Ag-added La0.67Ca0.33MnO3(Ag–LCMO) polycrystalline films have been prepared on SrTiO3 substrates by the ultrasonic spray pyrolysis technique and their properties are compared with the La0.67Ca0.33MnO3(LCMO) films prepared under similar conditions. Addition of Ag in LCMO films improves their surface morphology and reduces their lattice constant. During the synthesis process, Ag in Ag–LCMO film melts and segregates to grain boundaries. In the Ag–LCMO film, normalized voltage noise reduces by more than one order of magnitude, and peak temperature (Tp) and ferromagnetic transition temperature (Tc) increase by 10 and 20 K, respectively. Improvement in surface morphology and better oxygenation of the film due to the presence of Ag leads to reduction in conduction noise of the LCMO film.
Interaction of wide band gap single crystals with 248 nm excimer laser irradiation. VIII. Laser desorption of molecular ions from MgO
Authors: Kano S. , Langford S. C. , Dickinson J. T.
Published: July 2002

We present unambiguous evidence for direct, nonthermal laser desorption of molecular ions from the surface of an ionic wide band gap material, single crystal MgO. Using time resolved mass spectroscopy, we measure positive ions emitted during pulsed excimer laser irradiation at 248 nm. Species observed (besides singly and doubly charged Mg) include: Mg2+, MgO+, MgO2+ , Mg2O+ and (MgO)2+. The kinetic energies determined from time of flight measurements are species dependent, nearly independent of fluence, and range from 5 to 9 eV. The ion intensities display strongly nonlinear fluence dependencies, proportional to the fluence to the nth power, where n ranges from 3 to 4. These kinetic energies and fluence dependences support a defect mediated photoelectronic desorption mechanism. Emission is attributed to molecular ion precursors adsorbed at special sites associated with surface electron traps. Emission occurs when the electron trap is photoionized via the sequential absorption of 3–4 photons.
Effect of buffer layer preparation on GaN epilayers grown by gas-source molecular-beam epitaxy
Authors: Bousquet V. , Heffernan J. , Barnes J. , Hooper S.
Published: July 2002

Using gas-source molecular-beam epitaxy, and ammonia as a source of nitrogen, we have investigated the effect of buffer layer preparation on the electrical and optical properties of GaN epilayers. It is found that the buffer layer thickness and buffer anneal time have a dramatic effect on the Hall mobility and the background carrier density in the layers. With an optimized buffer layer, we have achieved mobility of ∼190 cm2/V s and have correlated this with a defect density as low as 2.1×109 cm−2 in the best layers. The variation of the intensity of defect related lines in photoluminescence spectra has been used to confirm the decrease of the defect density with increasing mobility in these layers.
Thermally tuning of the photonic band gap of SiO2 colloid-crystal infilled with ferroelectric BaTiO3
Authors: Zhou Ji , Sun C. Q. , Pita K. , Lam Y. L. , Zhou Y. , Ng S. L. , Kam C. H. , Li L. T. , Gui Z. L.
Published: July 2002

SiO 2 colloid crystal infilled with BaTiO3 was synthesized by a process of self-assembly in combination of a sol–gel technique. It is found that infilling of the fully crystallized BaTiO3 into the colloid assembly can enhance the photonic band gap significantly. In the vicinity of the ferroelectric phase transition point of BaTiO3 (100–150 °C), the photonic band gap of the assembly exhibits a strong temperature dependence. At the Curie point, the band gap has a 20 nm redshift, and the optical transmittance reaches its minimum. Such a temperature-tuning effect in the photonic band gap should be of high interest in device applications.
Spin canting and size effects in nanoparticles of nonstoichiometric cobalt ferrite
Authors: Ngo A. T. , Bonville P. , Pileni M. P.
Published: July 2002

The magnetic properties of nonstoichiometric cobalt ferrite nanocrystals differing by their sizes were investigated by magnetic measurements and Fe57 Mössbauer spectroscopy. We found that the spin-wave Bloch exponent increases with increasing particle size, in agreement with previous studies. Under a magnetic field of 7 T, we observed that the partial reorientation of the two ferrimagnetic Fe3+ sublattice magnetizations towards the field direction increases with increasing the particle size; we discuss the applicability of an intrinsic spin canting model as a function of the particle size. An evaluation of the relative amounts of cobalt and iron in each spinel site is given.
New insights into the fragility dilemma in liquids
Authors: Huang Dinghai , McKenna Gregory B.
Published: July 2002

A compilation of data for small molecule organic, polymeric, and inorganic glass-forming liquids shows that the original expectation, that there be a positive correlation between the thermodynamic measure of fragility Cpl/Cpg (or Cpl/Cpc) and the dynamic fragility index m, is not generally true. The results are consistent with three classes of behavior: (1) a decrease in m with increasing Cpl/Cpg for the polymeric glass formers; (2) a nearly constant value of m independent of Cpl/Cpc for small molecule organics and hydrogen bonding small molecules; (3) an increasing value of m with increasing Cpl/Cpc for inorganic glass formers as originally considered by Angell.
On the non-Gaussianity of chain motion in unentangled polymer melts
Authors: Smith Grant D. , Paul Wolfgang , Monkenbusch M. , Richter D.
Published: July 2002

We have investigated chain dynamics of an unentangled polybutadiene melt via molecular dynamics simulations and neutron spin echo experiments. Good short-time statistics allows for the first experimental confirmation of subdiffusive motion of polymer chains for times less than the Rouse time (τR) confirming behavior in this regime observed in simulations. Analysis of simulation trajectories obtained over several Rouse times reveals non-Gaussian segmental displacements for all time and length scales. These results, particularly non-Gaussian displacements on large time- and length scales, demonstrate the importance of intermolecular correlations on chain dynamics. Rouse-type analytical models fail to account for this non-Gaussianity leading to large deviations between the experimental dynamic structure factor and model predictions.
Adaptive notch filter-based signal processing method and system for vortex flowmeters
Authors: Xu Ke-Jun , Wang An-Min , Lv Xun-Hong
Published: July 2002

In this Note the vortex sensor signal is modeled as a narrow band process, and the frequency of the vortex sensor signal is measured by the means of adaptive notch filtering. A real time processing system based on a digital signal processor is developed. The testing results show that the adaptive notch filtering method and the processing system are effective for vortex flowmeters.
The collective dynamical properties of HCl: The transverse current correlations
Authors: Balucani U. , Pasqualini D. , Sutmann G. , Vallauri R.
Published: July 2002

Results are presented for the transverse current correlation functions and their spectra obtained by molecular dynamics simulation of liquid hydrogen chloride (HCl) at 201 K. To rationalize the results we analyze the data in the framework of the Mori–Zwanig theory and calculate the first order memory functions. This is done both in and exactly from the simulation data and by suitable approximations with one and two decay rates. It is found that the simple viscoelastic approximation with a single relaxation time is not sufficient to describe the dynamics of HCl whereas the extended model with two relaxation-times function accounts quite well for the simulation data in the low wave-number regime. Hydrogen bonding is found to play only a minor role in the dynamics of the liquid and the main features compare well with a simple liquidlike argon.