bluuebunny/crossref_metadata_2025 huggingface dataset:
Numerical evaluation of the impact of laser preheat on interface structure and instability
Authors: Zhang Yongmin , Drake R. Paul , Glimm James
Published: June 2007

This paper presents a computational study of the impact of preheating, in advance of shock heating, on a structured interface and on the subsequent postshock instability evolution. The study was performed by applying a method, described previously, of evaluating radiative effects using a multidimensional, front-tracking hydrodynamic code with input from a one-dimensional, radiation-hydrodynamic code. The method is general and could be applied to a range of laser-driven shock experiments. Results of simulations are shown for both high and low levels of preheat, conducted using a robust front-tracking algorithm in the presence of a radiation energy source. In the low-preheat case, which represents the minimum to be anticipated in laboratory experiments, some impact of preheat on both preshock conditions and postshock evolution are observed. In the high-preheat case, which represents one potential result of preheating by increased radiation and/or energetic electrons, the preheat alters the spectral content of the interface structure. In this case, before the shock reaches the interface, higher-order harmonic modes are induced, the interface position is shifted, and the perturbation amplitude is reduced. Furthermore, the postshock evolution of the interface is affected by the amount of preheat and by whether radiative heating after the laser pulse is also included. Such a numerical assessment of preheating can be important to the design and analysis of laboratory experiments. The initial conditions for the interaction of any shock wave with structures in the target may be altered by the presence of preheating. This poses a challenge to the laser experimental study of fluid mixing. Numerical simulations can serve as a useful tool to guide decisions regarding control and/or measurement of this effect.
Comparative study on methodology in molecular dynamics simulation of nucleation
Authors: Julin Jan , Napari Ismo , Vehkamäki Hanna
Published: June 2007

Gas-liquid nucleation of 1000 Lennard-Jones atoms is simulated to evaluate temperature regulation methods and methods to obtain nucleation rate. The Berendsen and the Andersen thermostats are compared. The Berendsen thermostat is unable to control the temperature of clusters larger than the critical size. Independent of the thermostating method the velocities of individual atoms and the translational velocities of clusters up to at least six atoms are accurately described by the Maxwell velocity distribution. Simulations with the Andersen thermostat yield about two times higher nucleation rates than those with the Berendsen thermostat. Nucleation rate is extracted from the simulations by direct observation of times of nucleation onset and by the method of Yasuoka and Matsumoto [J. Chem. Phys. 109, 8451 (1998)]. Compared to the direct observation, the nucleation rates obtained from the method of Yasuoka and Matsumoto are higher by a factor of 3.
Study of SiNx and SiO2 passivation of GaN surfaces
Authors: Chevtchenko S. A. , Reshchikov M. A. , Fan Q. , Ni X. , Moon Y. T. , Baski A. A. , Morkoç H.
Published: June 2007

The optical properties of GaN films have been found to be sensitive to SiNx and SiO2 surface passivation. The main effect of such passivation on photoluminescence (PL) data is an increase of the PL intensity for near-band-edge emission. This effect is attributed to the removal of oxygen from the surface of GaN and the subsequent formation of a protective layer during passivation. The increase in PL intensity is more pronounced for samples passivated with SiO2, which demonstrate initially lower PL intensity and a lower equilibrium concentration of free electrons. A nearly constant band bending of approximately 1.0 eV at the surface has been observed for as-grown and passivated samples by scanning Kelvin probe microscopy (SKPM). This constant value is explained by pinning of the Fermi level at the surface. In addition, we have demonstrated that passivation of the GaN surface between the contacts of a Schottky diode leads to a reduction of the leakage current observed at reverse bias. It was found that the surface potential measured by SKPM increases as a function of distance from the Schottky contact much faster after SiNx passivation. We suggest that the passivation reduces the total density of surface states and therefore reduces surface recombination.
Robust output synchronization of phase planar systems
Authors: Rosas Almeida David I. , Alvarez Joaquin
Published: June 2007

We propose a technique to synchronize, under the master/slave synchronization scheme, two planar systems represented by phase state variables; we name them phase planar systems. The coupling signal has a discontinuous term that produces a closed-loop system having good characteristics of robustness with respect to bounded disturbances and parametric variations, and guarantees exponential convergence to the synchronization state. In general, the coupling signal needs the full state vector of both systems, but because we assume that only the system outputs are available, we include a robust observer. This observer also guarantees exponential convergence to the state of the plant in spite of the existence of bounded disturbances and parametric variations; this characteristic facilitates the stability analysis of the closed-loop system. The performance of the synchronization technique is illustrated with experimental results.
Comment on “Surface switching of rotating fluid in a cylinder” [Phys. Fluids 18, 101701 (2006)]
Authors: Vatistas Georgios H.
Published: June 2007

In a recent letter, T. Suzuki, M. Iima, and Y. Hayase [Phys. Fluids 18, 101701 (2006)] reported on a recurring interfacial symmetry breaking of a liquid vortex. Similar to the one reported by R. N. Jansson, M. P. Haspang, K. H. Jensen et al. [Phys. Rev. Lett. 96, 174502 (2006); 98, 049901(E) (2007)] quoted by the authors, the present intriguing manifestation is not new. Both events are sectors of a more encompassing interfacial symmetry breaking phenomenon first reported 16 years ago [G. H. Vatistas, J. Fluid Mech. 217, 241 (1990)].
The bimodal distribution structure of electron density brought about by the Hall effect in the electron diffusion region during magnetic reconnection
Authors: Huang F. C. , Sheng Z. M. , Ma Z. W.
Published: June 2007

The Hall effect is suggested to be responsible for decoupling of electrons and ions in plasma during magnetic reconnection. It leads to redistribution of charged particles during magnetic reconnection. In this paper, charge distribution brought about by the Hall effect during magnetic reconnection is studied by using compressible Hall magnetohydrodynamics numerical simulation. Decoupling of electrons and ions in plasma can be clearly shown in this numerical simulation result. The bimodal structure of electron distribution in the electron diffusion region as well as its motion during magnetic reconnection have been found and discussed.
Implications of a high dielectric constant in proteins
Authors: Lund Mikael , Jönsson Bo , Woodward Clifford E.
Published: June 2007

Solvation of protein surface charges plays an important role for the protonation states of titratable surface groups and is routinely incorporated in low dielectric protein models using surface accessible areas. For many-body protein simulations, however, such dielectric boundary methods are rarely tractable and a greater level of simplification is desirable. In this work, we scrutinize how charges on a high dielectric surface are affected by the nonpolar interior core of the protein. A simple dielectric model, which models the interior as a low dielectric sphere, combined with Monte Carlo simulations, shows that for small, hydrophilic proteins the effect of the low dielectric interior is largely negligible and that the protein (and solution) can be approximated with a uniform high dielectric constant equal to that of the solvent. This is verified by estimates of titration curves and acidity constants for four different proteins (BPTI, calbindin D9k, ribonuclease A, and turkey ovomucoid third domain) that all correlate well with experimental data. Furthermore, the high dielectric approximation follows as a natural consequence of the multipole expansion of the potential due to embedded protein charges in the presence of the low dielectric core region.
Conformational distribution of a ferroelectric liquid crystal revealed using fingerprint vibrational spectroscopy and the density functional theory
Authors: Korlacki R. , Steiner M. , Meixner A. J. , Vij J. K. , Hird M. , Goodby J. W.
Published: June 2007

The authors have investigated the conformational structure of the ferroelectric liquid crystal compound 4-3-methyl-2-chloropentanoyloxy-4″-hexyloxy-biphenyl also known under the abbreviations 3M2CPHOB and C6 using vibrational (IR and Raman) spectroscopy. The measured spectra exhibit two bands corresponding to the CO stretching vibration that are separated by 20cm−1. In contrast, the molecular structure comprises only one such group. They assigned the two bands to different conformers that coexist in a temperature range between 25 and 65°C covering the entire mesophase of this material. This assignment is strongly confirmed by calculated vibrational spectra based on the density functional theory.
Dependence of the electromigration flux on the crystallographic orientations of different grains in polycrystalline copper interconnects
Authors: Choi Zung-Sun , Mönig Reiner , Thompson Carl V.
Published: June 2007

Dual-damascene Cu electromigration test structures with unpassivated segments in the second level of metallization were tested under vacuum. About 20 immobile voids formed along 1000-μm-long structures. Crystallographic orientations of grains on anode and cathode sides of 158 voids were obtained. It is shown that electromigration diffusivities vary with crystallographic orientations of the grains, increasing in the order (1 1 1), (1 1 5), (7 5 13), (11 1 11), and other orientations not twin related to (1 1 1). Data of this type provide an improved basis for process development for optimized reliability and for extrapolation of test results for reliability projections.
Conditions for drastic evolution of magnetospheric current wedge
Authors: Ugai M.
Published: June 2007

Recently, it was demonstrated that magnetospheric current wedge drastically builds up as soon as the fast reconnection mechanism is triggered to cause three-dimensional magnetic field dipolarization in a finite extent in the east-west direction. The present paper further studies the current wedge dynamics for different values of kz, taken as a parameter, which specifies the effective east-west extent, inside which magnetic reconnection proceeds. It is demonstrated that only when the east-west width of the initial current sheet becomes 3–4 times larger than its thickness, the current wedge is allowed to drastically evolve; otherwise, no current wedge can occur. For the critical case, the temporal dynamics of fast reconnection and the associated current wedge evolution become more unsteady or impulsive; in particular, when the fast reconnection jet collides with the expanding magnetic loop, the sheet current, located at a considerable distance from the loop top, abruptly turns its direction to form a large-scale current wedge.
X-ray photoelectron spectroscopic study of the formation of catalytic gold nanoparticles on ultraviolet-ozone oxidized GaAs(100) substrates
Authors: Ghosh S. C. , Biesinger M. C. , LaPierre R. R. , Kruse P.
Published: June 2007

A comprehensive study of the oxide formed on GaAs by exposure to ultraviolet light and ozone (UV-ozone), and the evolution of Au nanoparticles on UV-ozone treated GaAs(100) substrates, has been carried out using x-ray photoelectron spectroscopy. The UV-ozone oxide has been found to consist of oxides of gallium and arsenic in addition to having a significant contribution from elemental arsenic concentrated mostly at the substrate-oxide interface. An intermediate suboxide species −AsxGa2O is found to make up a substantial part of the oxide layer, mostly near the surface region, whereas the interface to bulk GaAs is dominated by Ga2O3. As2O3 is found to be the dominant oxide in the initial UV-ozone oxide layer, while the oxide layer becomes rich in Ga2O3 as the sample is annealed at 500 °C for 300 s in a dry nitrogen environment. It is also found that gold does not alloy with Ga during the formation of gold nanoparticles by dewetting from the UV-ozone oxidized GaAs substrate.
Influence of Li-codoping on the radiation hardness of CsBr:Eu2+
Authors: Zimmermann J. , Hesse S. , von Seggern H. , Fuchs M. , Knüpfer W.
Published: June 2007

The poor radiation hardness of the otherwise excellent x-ray storage phosphor CsBr:Eu2+ constitutes a problem for its commercial application in medical diagnostics. X-ray induced vacancy centers such as M-centers enhance the diffusion of Eu2+ activators resulting in a formation of photostimulated luminescence (PSL) inactive europium clusters or second phases of europium compounds. The present study investigates the influence of Li-codoping on the radiation hardness of CsBr:Eu2+. It is reported that the integration of Li+ into the CsBr:Eu2+ suppresses the generation of M-centers during x-irradiation and thereby partially improves the radiation hardness.
Effect of substrate-induced strains on the spontaneous polarization of epitaxial BiFeO3 thin films
Authors: Zhang J. X. , Li Y. L. , Wang Y. , Liu Z. K. , Chen L. Q. , Chu Y. H. , Zavaliche F. , Ramesh R.
Published: June 2007

A single-domain thermodynamic theory is employed to predict the spontaneous polarizations of (001)c, (101)c, and (111)c oriented epitaxial BiFeO3 thin films grown on dissimilar substrates. The effects of various substrate-induced strains on the spontaneous polarization were studied. The dependences of the spontaneous polarization on film orientations and the types of substrate-induced strains were analyzed.
Multiple-channel mid-infrared optical parametric oscillator in periodically poled MgO:LiNbO3
Authors: Guo H. C. , Tang S. H. , Gao Z. D. , Qin Y. Q. , Zhu S. N. , Zhu Y. Y.
Published: June 2007

We report a compact all-solid-state mid-infrared optical parametric oscillator with wide tunability using periodically poled 5 mol % MgO-doped LiNbO3 with a multiple-channel structure design and pumped by a compact diode-pump solid-state laser operating at 1.064 μm. Wide tunability from 1.44 to 1.58 μm at the signal beam wavelength and from 3.28 to 4.11 μm at the idler beam wavelength was achieved by both varying the temperature and translating the crystal through the resonator and the pump beam with no realignment required. The tuning performance and its related factors were analyzed, and at the same time the output performance as well as the effect of mid-infrared absorption of idler beam was investigated.
Highly textured Bi2Te3-based materials for thermoelectric energy conversion
Authors: Ben-Yehuda O. , Shuker R. , Gelbstein Y. , Dashevsky Z. , Dariel M. P.
Published: June 2007

This work is concerned with Bi2Te3-based compounds known as being highly effective materials for thermoelectric applications near room temperature. These compounds are characterized by a remarkable anisotropy linked to their R3¯m crystal structure. Two textured p-type Bi0.4Sb1.6Te3 samples were prepared using a powder metallurgy approach, with the c axis parallel to the pressing direction. One sample was undoped while the second was doped with Pb which acts as an acceptor. The electrical conductivity, Hall coefficient, and magnetoresistivity were measured from room temperature down to 6K. The Seebeck coefficient α and electrical conductivity σ were measured along and perpendicular to the c axis from 300 up to 550K, and the thermal conductivity κ was measured at 300K. Different values of Seebeck coefficient were observed along and perpendicular to the c axis at temperatures above Ti, the beginning of intrinsic region in which the influence of the minority carriers becomes significant. Below Ti, the Seebeck coefficient was isotropic. The maximal power factor P=α2σ, calculated on the basis of the experimental results, was about 40μWcm−1K−2 for the direction perpendicular to the c axis. The thermal conductivity values for the temperature domain above 300K were calculated on the basis of a physical model and the measured values at 300K. The calculated values of the figure of merit Z=P∕κ were 3×10−3 and 2×10−3K−1 at 300 and 400K for the two samples, respectively. These values are comparable to those observed in Bi2Te3-based single crystals, thus making the powder metallurgy approach appropriate for thermoelectric conversion applications.
Effect of composition on the thermal stability of sputter deposited hafnium aluminate and nitrided hafnium aluminate dielectrics on Si (100)
Authors: Sivasubramani P. , Kim J. , Kim M. J. , Gnade B. E. , Wallace R. M.
Published: June 2007

Hafnium aluminate (HfAlO) and nitrided hafnium aluminate (HfAlON) dielectrics were sputter deposited on Si (100) substrate. The N2∕(N2+Ar) flow ratio was varied between 0 and 66.6% in order to vary the nitrogen concentration. The Hf/Al ratio was varied from 0.5 to 1. In situ x-ray photoelectron spectroscopy and electrical measurements were utilized to characterize the as-deposited HfAlO and HfAlON films. The thermal stability studies of the HfAlO and HfAlON thin films after a 1000 °C, 10 s argon rapid thermal anneal were performed using grazing incidence x-ray diffraction and backside secondary ion mass spectrometry. Suppression of crystallization and no detectible outdiffusion of hafnium and aluminum into the silicon substrate were seen for HfAlO and HfAlON thin films with a Hf/Al ratio of 0.5.
Quasisteady state interpulse plasmas
Authors: Bhattacharjee Sudeep , Dey Indranuj , Sen Abhijit , Amemiya Hiroshi
Published: June 2007

The generation of quasisteady state plasmas in the power off phase, by short pulses [pulse duration (τp)∼0.5–1.2μs] of intense (60–100kW) microwaves in the X band (9.45GHz) is observed experimentally. The steady state is sustained from a few to tens of microseconds and depends upon the ionization processes in the interpulse phase and the characteristic diffusion length. The results are explained by a model, which considers the electron acceleration effects by the large amplitude of the field, the energy losses, and the characteristic electromagnetic field decay time. The effects of wave frequency, microwave power density, and particle diffusion on the steady state are investigated. A striking difference with conventional afterglows of pulsed discharges is pointed out.
Vacuum ultraviolet-ultraviolet and x-ray excited luminescence properties of Ba3Gd(BO3)3:Ce3+
Authors: Han Bing , Liang Hong-bin , Lin Hui-hong , Zhong Jiu-ping , Su Qiang , Dorenbos Pieter , Danang Birowosuto M. , Zhang Guo-bin , Fu Yi-bing
Published: June 2007

The phosphors Ba3Gd(BO3)3:Ce3+ were prepared by a solid-state reaction technique at high temperature. The vacuum ultraviolet-ultraviolet and visible spectroscopic properties of the phosphors together with decay time curves are investigated and discussed. The spectroscopic properties are explained by occupancy of Ce3+ at two different Gd sites in the host lattice. The x-ray excited emission spectra of Ba3Gd(BO3)3:Ce3+ were studied and the number of photons emitted per unit of absorbed x-ray energy was calculated. The yield is rather poor and Ba3Gd(BO3)3:Ce3+ appears not a suitable x-ray phosphor.
Bichromatic laser emission from dipyrromethene dyes incorporated into solid polymeric media
Authors: Álvarez M. , Costela A. , García-Moreno I. , Amat-Guerri F. , Liras M. , Sastre R. , López Arbeloa F. , Bañuelos Prieto J. , López Arbeloa I.
Published: June 2007

Bichromatic laser emission from dipyrromethene-based solid-state dye lasers is reported. The dependence of this dual emission on different factors and its origin and causes are discussed in the light of different models proposed in the literature. Our experimental results indicate that the long-wavelength emission can be explained in terms of reabsorption/reemission effects and inhomogeneous broadening of the S0-S1 transition. The short-wavelength emission corresponds to the usual S0-S1 transition and dominates at low dye concentration.
Current effects on neck growth in the sintering of copper spheres to copper plates by the pulsed electric current method
Authors: Frei James M. , Anselmi-Tamburini Umberto , Munir Zuhair A.
Published: June 2007

The effect of a pulsed dc on the sintering of copper spheres to copper plates was investigated. It was shown that the current had a marked effect on neck growth between the spheres and the plates. The enhancement of sintering under the effect of the current was attributed to electromigration. Microstructural observations on fracture surfaces of necks formed under high currents showed considerable void formation. It was also observed that the current resulted in increased evaporation and the formation of bunched evaporation steps. Formation of these steps and their location relative to the neck were consistent with current density distributions. The results of this investigation provide direct evidence for the role of the current in the sintering in the pulse electric current sintering method.
Strong excitonic confinement effect in ZnS and ZnS:Mn nanorods embedded in polycarbonate membrane pores
Authors: Mandal S. K. , Mandal A. R. , Das S. , Bhattacharjee B.
Published: June 2007

We investigate here the optical properties of excitons in a large array of ordered semiconductor-insulator nanorods. A simple and elegant approach is described to obtain such semiconductor-insulator nanorod structures. Here, we investigate ZnS and ZnS:Mn nanorods of diameters ∼15–100nm crystallized within the pores of polycarbonate membrane. The nanorods confined in the dielectric matrix display strong ultraviolet (UV) excitonic absorption and emission bands in both undoped and Mn2+ doped samples. The data reveal that dielectric confinement rather than dimensional quantization has the significant effect in UV-visible optical absorption and photoluminescence (PL) processes in these semiconductor-insulator nanorods of sizes much wider than the Bohr radius. Interestingly, the emission band associated with Mn2+ transition (∼587–600nm) is also significantly affected by the size effect as well as dielectric discontinuity at the interface. A detailed investigation of PL emission from these embedded ZnS and ZnS:Mn nanorods is also reported.
Effect of dc electric field on conductivity and giant permittivity of KxTiyNi1−x−yO
Authors: Jana Pradip Kumar , Sarkar Sudipta , Chaudhuri B. K. , Sakata H.
Published: June 2007

Activation energies (Ea) for electrode and grain boundary (GB) conduction are found to be greater than that of the bulk conduction. This implies that both electrode and GB act as two insulating barriers affecting conductivity and dielectric relaxations in KxTiyNi1−x−yO. Decrease in Ea for electrode conduction above T>295K leads to relatively greater loss due to the dc-charge transportation between electrodes and dielectric. The Cole-Cole equation has been modified to explain ε′ over a wide range of frequency (10–107Hz). Impedance spectroscopic study has also been employed to distinguish electrode relaxation from GB relaxation.
Modeling the sequential electric field assisted diffusion into glass of two ion species
Authors: Oven R.
Published: June 2007

The sequential electric field assisted diffusion of two ion species into glass is modeled in this work. Using standard assumptions, two coupled nonlinear drift-diffusion equations are derived for the ion concentration distributions. These equations are solved numerically for the case where the two ion species each have a lower mobility than the indigenous sodium ions in the glass. It is shown that stationary state distributions form if the final species injected into the glass is the less mobile of the two. Analytical expressions are derived for the stationary state distributions. A possible application in integrated optics involving Ag+ and Cu+ ion diffusion is considered.
Correlation between structural, transport, and magnetic properties in Sm1−xAxMnO3 (A=Sr,Ca)
Authors: Hassen A. , Mandal P.
Published: June 2007

Transport, magnetic, and structural properties of Sm1−xAxMnO3, where A is Ca and Sr, have been investigated systematically over the range of doping 0⩽x⩽0.52. For x<0.30, both systems are ferromagnetic (FM) insulator, and the resistivity (ρ) can be described well by polaron hopping model. Above x=0.30, Sr-doped compounds exhibit large negative magnetioresistance (MR) in the FM metallic state and charge∕orbital ordering over a narrow doping range around x=0.50. On the other hand, Ca-doped system does not show any insulator to metal transition and MR, possibly due to the smaller average A-site cationic radius ⟨r⟩ than that for Sr-doped one. The magnetic ground state of Ca-doped system changes from FM to canted antiferromagnetic above x=0.30. A strong coupling between transport, magnetic, and structural properties has been established from this study. The results are summarized in (x−T) phase diagrams.
Dilute nitride GaInNAs and GaInNAsSb solar cells by molecular beam epitaxy
Authors: Jackrel David B. , Bank Seth R. , Yuen Homan B. , Wistey Mark A. , Harris James S. , Ptak Aaron J. , Johnston Steven W. , Friedman Daniel J. , Kurtz Sarah R.
Published: June 2007

Dilute nitride films with a roughly 1 eV band gap can be lattice-matched to gallium arsenide and germanium, and therefore could become a critical component in next-generation multijunction solar cells. To date most dilute nitride solar cells have been plagued with poor efficiency, due in large part to short diffusion lengths. This study focuses on two techniques aimed at improving the quality of dilute nitride films grown by molecular beam epitaxy: the utilization of biased deflection plates installed in front of the nitrogen plasma source, and the introduction of antimony during growth. Results from GaInNAs cells grown with and without deflection plates, and GaInNAsSb solar cells are reported. The use of biased deflection plates during GaInNAs growth improved every aspect of solar cell performance. For the GaInNAs devices grown with deflection plates, the dark current density, open-circuit voltage, and fill factor were the best of the devices studied. The GaInNAsSb cells had the highest quantum efficiency, almost 80% at maximum, mainly due to low background doping densities providing these devices with wide depletion widths. The GaInNAsSb materials also had quite narrow band gaps of 0.92 eV. Because of the high collection efficiency coupled with the narrow band gap, the sub-GaAs short-circuit current density produced by the GaInNAsSb cells is 14.8 mA∕cm2, which was the highest of the devices studied. This current is nearly 50% greater than the best dilute nitride solar cells in the literature, and is the first dilute nitride cell to produce enough current to current match the upper two subcells in a triple-junction device, composed of GaInP/InGaAs/GaInNAsSb.
Magnetotransport properties of AlxGa1−xN∕AlN∕GaN heterostructures grown on epitaxial lateral overgrown GaN templates
Authors: Biyikli N. , Ni X. , Fu Y. , Xie J. , Morkoç H. , Cheng H. , Kurdak Ç. , Vurgaftman I. , Meyer J.
Published: June 2007

We studied the low-temperature magnetotransport properties of AlxGa1−xN∕AlN∕GaN heterostructures with a two-dimensional electron gas (2DEG). Structures with different Al compositions were grown by metal-organic vapor-phase epitaxy on three types of templates: conventional undoped GaN, in situ epitaxial lateral overgrown GaN using a SiNx nanomask layer, and ex situ epitaxial lateral overgrown GaN (ELO-GaN) using a stripe-patterned SiO2 mask. All of the samples display Shubnikov–de Haas (SdH) oscillations that confirm the existence of 2DEGs. Field-dependent magnetoresistance and Hall measurements further indicate that the overgrown heterostructures have a parallel conducting layer in addition to the 2DEG. To characterize the parallel channel, we repeated the measurements after the 2DEG was etched away. 2DEG carrier density values were then extracted from the SdH data, whereas the zero-field 2DEG conductivity was determined by subtracting the parallel channel conductivity from the total. The quantitative mobility spectrum analysis could not be applied in some cases, due to a large contact resistance between the parallel channels. The resulting 2DEG mobility is about a factor of 2 higher in the ELO-GaN and SiN–GaN samples as compared to the standard control samples. The mobility enhancement is attributed to a reduction of threading dislocations by the two ELO techniques employed.
30° Si(g) partial dislocation mobility in nitrogen-doped 4H-SiC
Authors: Idrissi H. , Pichaud B. , Regula G. , Lancin M.
Published: June 2007

Well-controlled population of dislocations are introduced in 4H-SiC by bending in cantilever mode and annealing between 400 and 700°C. The introduced defects consist of double stacking faults, each bound by a pair of 30° Si(g) partial dislocations, and the expansion of which is asymmetric. The velocity of each individual 30° Si(g) pair is directly measured as a function of stress and temperature on the surface of samples etched after deformation. The activation energies of the 30° Si(g) partial dislocation pairs are strongly stress dependent, ranging between 1.25 and 1.7eV. These values are lower than the ones derived from plasticity experiments. This is probably because 30° Si(g) pairs and double stacking faults are generated in N-doped 4H-SiC (N=2×1018cm−3), with their development being promoted by quantum well action.
Polarization and lattice strains in epitaxial BaTiO3 films grown by high-pressure sputtering
Authors: Petraru A. , Pertsev N. A. , Kohlstedt H. , Poppe U. , Waser R. , Solbach A. , Klemradt U.
Published: June 2007

High-quality BaTiO3 films with thicknesses ranging from 2.9to175nm were grown epitaxially on SrRuO3-covered (001)-oriented SrTiO3 substrates by high-pressure sputtering. The crystal structure of these films was studied by conventional and synchrotron x-ray diffraction. The in-plane and out-of-plane lattice parameters were determined as a function of film thickness by x-ray reciprocal space mapping around the asymmetric (1¯03) Bragg reflection. BaTiO3 films were found to be fully strained by the SrTiO3 substrate up to a thickness of about 30nm. Ferroelectric capacitors were then fabricated by depositing SrRuO3 top electrodes, and the polarization-voltage hysteresis loops were recorded at the frequencies 1–30kHz. The observed thickness effect on the lattice parameters and polarization in BaTiO3 films was analyzed in the light of strain and depolarizing-field effects using the nonlinear thermodynamics theory. The theoretical predictions are in reasonable agreement with the measured thickness dependences, although the maximum experimental values of the spontaneous polarization and the out-of-plane lattice parameter exceed the theoretical estimates (43μC∕cm2 vs 35μC∕cm2 and 4.166Å vs 4.143Å). Possible origins of the revealed discrepancy between theory and experiment are discussed.
Circularly polarized waves in a plasma with vacuum polarization effects
Authors: Lundin J. , Stenflo L. , Brodin G. , Marklund M. , Shukla P. K.
Published: June 2007

The theory for large amplitude circularly polarized waves propagating along an external magnetic field is extended in order to also include vacuum polarization effects. A general dispersion relation, which unites previous results, is derived.
Induced increase in surface work function and surface energy of indium tin oxide-doped ZnO films by (NH4)2Sx treatment
Authors: Tsai Chia-Lung , Lin Yow-Jon , Wu Ping-Hsun , Chen Shu-You , Liu Day-Shan , Hong Jia-Huang , Liu Chia-Jyi , Shih Yu-Tai , Cheng Jie-Min , Chang Hsing-Cheng
Published: June 2007

The effects of (NH4)2Sx treatment on the surface electronic properties of the thin indium tin oxide (ITO)-doped ZnO films have been examined in this study. According to the experimental results, we found that the formation of S-metal bonds and the removal of oxygen vacancies near the (NH4)2Sx-treated ITO-doped ZnO surface could lead to an increase in the surface energy and the work function, meaning that (NH4)2Sx treatment might be more helpful to form the uniform deposition of the organic semiconductor on ITO-doped ZnO surfaces and improve the efficiency of ZnO-based organic devices.
Effects of magnetostatic coupling on stripe domain structures in magnetic multilayers with perpendicular anisotropy
Authors: Janicka K. , Burton J. D. , Tsymbal E. Y.
Published: June 2007

We investigate the effects of magnetostatic interlayer coupling on the formation of magnetic domains in exchange-coupled ferromagnetic films exhibiting perpendicular magnetic anisotropy. Using simple models for the stripe domain structures we investigate how the magnetostatic interlayer interaction affects the domain size. For antiferromagnetically exchange-coupled multilayers, we show that there is a domain overlap due to the magnetostatic interaction which exists only above a critical thickness and below a critical separation between the ferromagnetic films where it is scaled inversely proportional to the antiferromagnetic exchange coupling constant. We show that the magnetostatic coupling is responsible for the periodic “tiger tail” domain structure observed experimentally and demonstrate the dependence of the period of this structure on the interlayer exchange coupling.
Mesoporous matrices for quantum computation with improved response through redundance
Authors: Hodgson T. E. , Bertino M. F. , Leventis N. , D’Amico I.
Published: June 2007

We present a solid state implementation of quantum computation, which improves previously proposed optically driven schemes. Our proposal is based on vertical arrays of quantum dots embedded in a mesoporous material which can be fabricated with present technology. The redundant encoding typical of the chosen hardware protects the computation against gate errors and the effects of measurement induced noise. The system parameters required for quantum computation applications are calculated for II-VI and III-V materials and found to be within the experimental range. The proposed hardware may help minimize errors due to polydispersity of dot sizes, which is at present one of the main problems in relation to quantum dot-based quantum computation.
Mn L2,3 x-ray absorption spectra of a diluted Mn–Ge alloy
Authors: Ottaviano L. , Passacantando M. , Verna A. , D’Amico F. , Gunnella R.
Published: June 2007

An ion implanted Mn–Ge(100) alloy has been fabricated with a 30nm subsurface layer of disorderly diluted Mn atoms, which settle all in substitutional lattice sites after 400°C annealing in vacuum. The Mn L2,3 x-ray absorption spectroscopy of the disordered Mn implanted alloy is featureless, while the one taken on the alloy with complete substitutional Mn dilution shows a Mn2+ multiplet structure.
Field emission observed from metal-diamond junctions revealed by atomic force microscopy
Authors: Furkert S. A. , Wotherspoon A. , Cherns D. , Fox N. A. , Fuge G. M. , Heard P. J. , Lansley S. P.
Published: June 2007

A noncontact atomic force microscopy technique has been developed that enables sources of field emission to be detected and mapped in an air ambient. Areas as large as 900μm2 have been mapped. This new technique enables determination of the location and extent of the emission area on an individual emitting particle. Emission from nanodiamond particles is shown to occur not at the tip of the diamond, but from near the base where it forms a triple junction with the metal substrate. The reported observations should assist exploration of novel methods of controlling electron emission from devices constructed using diamond particles.
Scalable nanotemplate assisted directed assembly of single walled carbon nanotubes for nanoscale devices
Authors: Makaram Prashanth , Somu Sivasubramanian , Xiong Xugang , Busnaina Ahmed , Jung Yung Joon , McGruer Nick
Published: June 2007

The authors demonstrate precise alignment and controlled assembly of single wall nanotube (SWNT) bundles at a fast rate over large areas by combining electrophoresis and dip coating processes. SWNTs in solution are assembled on prepatterned features that are 80nm wide and separated by 200nm. The results show that the direction of substrate withdrawal significantly affects the orientation and alignment of the assembled SWNT bundles. I-V characterization is carried out to demonstrate electrical continuity of these assembled SWNT bundles.
Microstructure and properties of epitaxial antimony-doped p-type ZnO films fabricated by pulsed laser deposition
Authors: Guo W. , Allenic A. , Chen Y. B. , Pan X. Q. , Che Y. , Hu Z. D. , Liu B.
Published: June 2007

Antimony-doped p-type ZnO films epitaxially grown on (0001) sapphire substrates were fabricated by pulsed laser deposition at 400–600°C in 5.0×10−2Torr oxygen without postdeposition annealing. The films grown at 600°C have among the highest reported hole concentration of 1.9×1017cm−3 for antimony doping, Hall mobility of 7.7cm2∕Vs, and resistivity of 4.2Ωcm. Transmission electron microscopy reveals that the p-type conductivity closely correlates to the high density of defects which facilitate the formation of acceptor complexes and the compensation of native shallow donors. The thermal activation energy of the acceptor was found to be 115±5meV and the corresponding optical ionization energy is ∼158±7meV.
Ground state lasing at 1.34μm from InAs∕GaAs quantum dots grown by antimony-mediated metal organic chemical vapor deposition
Authors: Guimard D. , Arakawa Y. , Ishida M. , Tsukamoto S. , Nishioka M. , Nakata Y. , Sudo H. , Yamamoto T. , Sugawara M.
Published: June 2007

The authors report the fabrication of GaAs-based quantum dot (QD) lasers grown by metal organic chemical vapor deposition above 1.30μm. They fabricated a laser diode with five stacked InAs∕Sb:GaAs(100) QD layers, grown by antimony-surfactant-mediated growth. Ground state lasing was obtained at 1.34μm, with internal quantum efficiency of 62%, internal loss of 4.5cm−1 and ground state modal gain above 12cm−1. Lasing above 1.30μm could be achieved because of the beneficial effects of antimony on both the coherent InAs∕Sb:GaAs QD density and the suppression of the emission blueshift, usually observed for InAs∕GaAs QDs during postgrowth annealing at 600°C.
Nonequilibrium 1∕f noise in low-doped manganite single crystals
Authors: Wu X. D. , Dolgin B. , Jung G. , Markovich V. , Yuzhelevski Y. , Belogolovskii M. , Mukovskii Ya. M.
Published: June 2007

1 ∕ f noise in current biased La0.82Ca0.18MnO3 crystals has been investigated. The temperature dependence of the noise follows the resistivity changes with temperature, suggesting that resistivity fluctuations constitute a fixed fraction of the total resistivity, independently of the dissipation mechanism and magnetic state of the system. The noise scales as a square of the current as expected for equilibrium resistivity fluctuations. However, at 77K at bias exceeding some threshold, the noise intensity starts to decrease with increasing bias. The appearance of nonequilibrium noise is interpreted in terms of bias dependent multistep indirect tunneling.
Temperature-compensated high-stability silicon resonators
Authors: Melamud Renata , Kim Bongsang , Chandorkar Saurabh A. , Hopcroft Matthew A. , Agarwal Manu , Jha Chandra M. , Kenny Thomas W.
Published: June 2007

Composite micromechanical resonators were encapsulated in a hermetic environment using a wafer-scale encapsulation process compatible with complementary metal-oxide semiconductor processing. The resonator structure is comprised of single crystal silicon with a silicon dioxide coating and shows a frequency-temperature sensitivity that is comparable to uncompensated quartz crystal resonators. A frequency variation of less than 200ppm is achieved over a −40–125°C temperature range. The resonator exhibits a quadratic temperature behavior with a turnover temperature at which the frequency becomes insensitive to small temperature changes. The turnover temperature can be controlled for use in high precision frequency references.
Xenon excimer emission from pulsed high-pressure capillary microdischarges
Authors: Lee Byung-Joon , Rahaman Hasibur , Petzenhauser Isfried , Frank Klaus , Giapis Konstantinos P.
Published: June 2007

Intense xenon vacuum ultraviolet (VUV) emission is observed from a high-pressure capillary cathode microdischarge in direct current operation, by superimposing a high-voltage pulse of 50ns duration. Under stagnant gas conditions, the total VUV light intensity increases linearly with pressure from 400 to 1013mbar for a fixed voltage pulse. At fixed pressure, however, the VUV light intensity increases superlinearly with voltage pulse height ranging from 08to2.8kV. Gains in emission intensity are obtained by inducing gas flow through the capillary cathode, presumably because of excimer dimer survival due to gas cooling.
Relation between Al vacancies and deep emission bands in AlN epitaxial films grown by NH3-source molecular beam epitaxy
Authors: Koyama T. , Sugawara M. , Hoshi T. , Uedono A. , Kaeding J. F. , Sharma R. , Nakamura S. , Chichibu S. F.
Published: June 2007

Intensity ratios of characteristic deep cathodoluminescence (CL) bands at 4.6, 3.8, and 3.1eV to the near-band-edge emissions at 11K of AlN epilayers grown by NH3-source molecular beam epitaxy were correlated with the change in the S parameter of positron annihilation measurement, which represents the concentration or size of Al vacancies (VAl). Since the relative intensities of 3.1 and 3.8eV bands increased remarkably with lowering supply ratio of NH3 to Al (V/III ratio) and growth temperature (Tg), they were assigned to originate from VAl-O complexes. The VAl concentration could be decreased by adjusting V/III ratio and Tg, resulting in observation of fine excitonic features in the CL spectra. From the energy separation between the ground and first excited states, the binding energy of A exciton was determined to be 48meV.
Odd characteristics of Au film on pentacene
Authors: Ihm Kyuwook , Heo Hye-Eun , Chung Sukmin , Ahn Joung-Real , Kim Jung Hyun , Kang Tai-Hee
Published: June 2007

Anomalies of Au film formed on the pentacene surface are investigated as a counterpart of pentacene/Au structure. The Au film is found to contain pentacene derivatives originated from the pentacene layers, and it is composed of grains of various sizes formed as the Au thickness increases. The authors suggest that it is this abundance of peculiarities of the Au film that accounts for the attenuated density of states in the valence band. Deformation of Au grains is accompanied by the lift of the pentacene layers, which, in turn, brings about the device failure.
Controlled depinning of domain walls in a ferromagnetic ring circuit
Authors: Hara Masahiro , Kimura Takashi , Otani Yoshichika
Published: June 2007

The authors have investigated dynamics of paired domain walls in a ferromagnetic ring circuit, driven by a variable pulsed magnetic field. The magnetic reversal accompanied by the propagations of the domain walls depends not only on the amplitude but also on the rise time of the applied pulsed magnetic field. The threshold of the rise time is found comparable to the propagation time of a domain wall along a half-path of the ring. This finding may be useful for the high speed operation of the magnetic logic devices based on the domain wall motion.
Magnetic, magnetotransport, and optical properties of Al-doped Zn0.95Co0.05O thin films
Authors: Venkatesan M. , Stamenov P. , Dorneles L. S. , Gunning R. D. , Bernoux B. , Coey J. M. D.
Published: June 2007

Thin films of 5% Co-doped ZnO with a range of Al codoping exhibit a band-edge shift, which varies with carrier concentration as n2∕3. Carrier effective mass is 0.26me and mobility is ∼10cm2V−1s−1. The doped films, which contain coherent Co clusters of 4–8nm in size, exhibit a ferromagnetic moment of 0.3–1.0μB per cobalt. The magnetism is progressively destroyed by Al doping due to a reduction in Co-cluster formation. Magnetoresistance appears below 30K, but these materials cannot be regarded as dilute magnetic semiconductors.
Variable liquid crystal pretilt angles generated by photoalignment of a mixed polyimide alignment layer
Authors: Ho Jacob Y. L. , Chigrinov V. G. , Kwok H. S.
Published: June 2007

A method to generate any liquid crystal pretilt angle is proposed and demonstrated. This method is based on deep ultraviolet photoalignment of a layer consisting of nanostructured domains of horizontal and vertical alignment polyimides. Competition between the two types of domains produces the controlled pretilt angle. The obtained anchoring energy is reasonably large.
Gain characteristics of InAs∕InGaAsP quantum dot semiconductor optical amplifiers at 1.5μm
Authors: Kim N. J. , Oh J. M. , Kim M. D. , Lee D. , Pyun S. H. , Jeong W. G. , Jang J. W.
Published: June 2007

The authors have fabricated ridge waveguide quantum dot (QD) semiconductor optical amplifiers (SOAs) on InP substrates that operate in the 1.5μm region. The active layer consists of InAs∕InGaAsP QD layers with a high dot density, but which still have good isolation between dots in the lateral and vertical directions, as confirmed by time-resolved photoluminescence measurements. One of these QD SOAs exhibited a fiber-to-fiber gain of 22.5dB and a chip gain of 37dB at 1.51μm. The spectral gain shape was found to be maintained for variations of the peak gain from 12to22dB, reflecting the zero-dimensional density of states at room temperature.
Temperature-dependent brightening and darkening of photoluminescence from PbS quantum dots in glasses
Authors: Liu Chao , Kwon Yong Kon , Heo Jong
Published: June 2007

The authors report that the darkening and brightening of photoluminescence from PbS quantum dots can be controlled by adjusting the temperature and excitation intensity. Intensity of the photoluminescence from PbS quantum dots increased with time (photobrightening) when temperature was below 150K. On the other hand, darkening was observed at temperatures above 150K. It was also possible to make a reversible change between photobrightening and photodarkening from the same specimen through the careful control of the temperature and excitation intensity.
Terahertz dielectric assay of solution phase protein binding
Authors: Chen Jing-Yin , Knab J. R. , Ye Shuji , He Yunfen , Markelz A. G.
Published: June 2007

The authors demonstrate a method for rapid determination of protein-ligand binding on solution phase samples using terahertz dielectric spectroscopy. Measurements were performed using terahertz time domain spectroscopy on aqueous solutions below the liquid-solid transition for water. Small ligand binding sensitivity was demonstrated using triacetylglucosamine and hen egg white lysozyme with a decrease in dielectric response with binding. The magnitude of the change increases with frequency.
Targeted band gap creation using mixed sonic crystal arrays including resonators and rigid scatterers
Authors: Fuster-Garcia E. , Romero-García V. , Sánchez-Pérez J. V. , García-Raffi L. M.
Published: June 2007

Sonic crystals are periodic structures that have acoustic band gaps centred at frequencies depending on the lattice constant of the array and on the direction of the incident acoustic wave. To eliminate this dependence, this work presents designed mixed structures constructed with rigid scatterers and resonators embedded in air. Specifically, balloons filled with a blend of air and helium were used as resonators, showing experimental evidence about the resonant behavior of an array formed with these balloons. As a result, the authors obtain full band gaps in a predetermined range of frequencies desired.