bluuebunny/crossref_metadata_2025 huggingface dataset:
Relocation effect and polarization mixing of surface acoustic waves in AlN-GaN bilayers on Al2O3(0001) substrates
Authors: Takagaki Y. , Chilla E.
Published: June 2007

On the surface capped by an AlN-GaN bilayer, the particle displacements of surface acoustic waves (SAWs) are localized in the short-wavelength regime in the buried GaN layer instead of the overlaying AlN layer as a consequence of the significantly faster propagation of sound in AlN than in GaN. We numerically explore this relocation effect for Al2O3(0001) substrates, which are the commonly used substrate for group-III nitrides. We show that the critical wavelength for the relocation effect can be manipulated in a wide range by varying the thickness of the AlN layer owing to the moderate sound velocity in Al2O3. We also investigate the influences of the polarization mixing on the dispersion of SAWs. The SAW dispersion in the layered system is interpreted in terms of the anticrossing of relevant acoustic modes.
The role of proximity caps during the annealing of UV-ozone oxidized GaAs
Authors: Ghosh S. C. , Biesinger M. C. , LaPierre R. R. , Kruse P.
Published: June 2007

This study provides a deeper insight into the chemistry and physics of the common engineering practice of using a proximity cap, while annealing compound semiconductors such as GaAs. We have studied the cases of a GaAs proximity cap, a Si proximity cap, and no proximity cap. Using x-ray photoelectron spectroscopy, it has been found that annealing increases the gallium to arsenic ratio in the oxide layer in all cases. During the annealing of UV-ozone oxidized GaAs, it has been observed that GaAs proximity caps also serve as a sacrificial layer to accelerate the desorption of oxide species. In all cases surface deterioration due to pit formation has been observed, and the depth of pits is found to depend on the effective role played by the capping material. Energy dispersive x-ray analysis provides additional evidence that pits mainly consist of elemental As and gallium oxide, with most of the elemental As situated at the pit-substrate interface. Deposition of a thin layer of gold and subsequent annealing to 500°C for 300s under different capping conditions shows the use of a proximate cap to be practically insignificant in annealing Au deposited films.
Response to “Comment on ‘Surface switching of rotating fluid in a cylinder”' [Phys. Fluids 19, 069101 (2007)]
Authors: Suzuki Toshiyuki , Iima Makoto , Hayase Yumino
Published: June 2007

In our Letter [T. Suzuki, M. Iima, and Y. Hayase, “Surface switching of rotating fluid in a cylinder,” Phys. Fluids 18, 101701 (2006)], we reported a surface switching phenomenon of fluid in a cylinder driven by the endwall rotation. In the Comment by Vatistas [G. Vatistas, “Comment on ‘Surface switching of rotating fluid in a cylinder',” Phys. Fluids 19, 069101 (2007)], the author claims that a periodic sloshing phenomenon reported by Vatistas in 1990 is the same phenomenon as the surface switching. We clarify the difference between these phenomena in terms of the parameters, the phenomena, the boundary condition, and the physical mechanism.
Efficient binomial leap method for simulating chemical kinetics
Authors: Peng Xinjun , Zhou Wen , Wang Yifei
Published: June 2007

The binomial τ-leaping method of simulating the stochastic time evolution in a reaction system uses a binomial random number to approximate the number of reaction events. Theory implies that this method can avoid negative molecular numbers in stochastic simulations when a larger time step τ is used. Presented here is a modified binomial leap method for improving the accuracy and application range of the binomial leap method. The maximum existing population is first defined in this approach in order to determine a better bound of the number reactions. To derive a general leap procedure in chemically reacting systems, in this method a new sampling procedure based on the species is also designed for the maximum bound of consumed molecules of a reactant species in reaction channel. Numerical results indicate that the modified binomial leap method can be applied to a wider application range of chemically reacting systems with much better accuracy than the existing binomial leap method.
Theory of space-charge polarization for determining ionic constants of electrolytic solutions
Authors: Sawada Atsushi
Published: June 2007

A theoretical expression of the complex dielectric constant attributed to space-charge polarization has been derived under an electric field calculated using Poisson’s equation considering the effects of bound charges on ions. The frequency dependence of the complex dielectric constant of chlorobenzene solutions doped with tetrabutylammonium tetraphenylborate (TBATPB) has been analyzed using the theoretical expression, and the impact of the bound charges on the complex dielectric constant has been clarified quantitatively in comparison with a theory that does not consider the effect of the bound charges. The Stokes radius of TBA+(=TPB−) determined by the present theory shows a good agreement with that determined by conductometry in the past; hence, the present theory should be applicable to the direct determination of the mobility of ion species in an electrolytic solution without the need to measure ionic limiting equivalent conductance and transport number.
Semiclassical description of the nonlinear response of self-trapped vibrational excitations
Authors: Woutersen Sander
Published: June 2007

A simple description of the nonlinear response of self-trapped NH-stretch excitations is obtained by treating the NH-stretch modes as quantized, but the hydrogen bonds as classical modes. This approach provides both an intuitive picture of the self-trapping dynamics and a semiquantitative description of the experimentally observed transient-absorption spectrum.
Pair velocity correlations among swimming Escherichia coli bacteria are determined by force-quadrupole hydrodynamic interactions
Authors: Liao Qian , Subramanian Ganesh , DeLisa Matthew P. , Koch Donald L. , Wu Mingming
Published: June 2007

This Letter examines the pair velocity correlations among Escherichia coli (E. coli) bacteria swimming freely in a microfluidic channel. A large number of bacterial tracks are obtained using a particle tracking algorithm, and the longitudinal and transverse pair velocity correlation functions are evaluated. A theoretical analysis traces the origin of correlated motion between bacterial pairs to the translation of a bacterium driven by the force-quadrupole velocity disturbance caused by the swimming of neighboring bacteria. Both theory and experiments indicate that the longitudinal and transverse correlation functions are positive and negative, respectively.
Multichannel detector for ion temperature determination in vacuum ultraviolet spectrum
Authors: Daltrini A. M. , Machida M.
Published: June 2007

A vacuum ultraviolet spectrometer equipped with a charge coupled device and an open multichannel plate has been used to analyze the temperature of carbon and oxygen ions in the NOVA-UNICAMP tokamak. The detection system was optimized and aligned to minimize the instrumental broadening. Also, higher order diffractions of the emissions were analyzed, resulting in lower experimental errors. The ion temperature was monitored during the tokamak discharge, presenting values between 30 and 70eV.
Calibrating a tuning fork for use as a scanning probe microscope force sensor
Authors: Qin Yexian , Reifenberger R.
Published: June 2007

Quartz tuning forks mounted with sharp tips provide an alternate method to silicon microcantilevers for probing the tip-substrate interaction in scanning probe microscopy. The high quality factor and stable resonant frequency of the tuning fork allow accurate measurements of small shifts in the resonant frequency as the tip approaches the substrate. To permit an accurate measure of surface interaction forces, the electrical and piezoelectromechanical properties of a tuning fork have been characterized using a fiber optical interferometer.
Phases and domain structures in tungsten-doped Pb(Mg1∕3Nb2∕3)1−xTixO3 (x=0.35) crystal
Authors: Wang F.-T. , Tu C.-S. , Chien R. R.
Published: June 2007

Dielectric permittivity, electric polarization, and domain structure have been carried out as a function of temperature in 0.5 mol % WO3-doped Pb(Mg1∕3Nb2∕3)0.65Ti0.35O3 (PMNT35%) single crystal. Two first-order-type temperature-dependent phase transitions, monoclinic (rhombohedral)→monoclinic→cubic, were observed near 310 K (290 K) and 440 K (435 K) upon heating (cooling), respectively. “Monoclinic (rhombohedral)” represents that dominant monoclinic domains coexist with a small fraction of rhombohedral domains. Reorientations of monoclinic domains are mainly responsible for the low-temperature steplike dielectric anomaly. This study shows that WO3 dopant can enhance monoclinic domains whose dynamics play an important role in temperature-dependent structural transitions.
Optical transitions and upconversion fluorescence in Ho3+∕Yb3+ doped bismuth tellurite glasses
Authors: Wang Xueying , Lin Hai , Yang Dianlai , Lin Lin , Yue-Bun Pun Edwin
Published: June 2007

10 mol % of bismuth oxide was designed to add in making novel Ho3+ doped heavy metal tellurite glasses (LKBBT) with large refractive indices and glass densities. Based on optical absorption, Judd-Ofelt parameters Ω2, Ω4, and Ω6 have been derived to be 4.373×10−20, 1.906×10−20, and 1.451×10−20cm2, respectively, which indicate higher asymmetry and stronger covalent environment in LKBBT glasses compared with ZnF2–CdF2, oxyfluorobotate, and zirconium-barium-lanthanum-aluminum-sodium-fluoride glasses. Efficient visible upconversion emission bands peaked at 544, 657, and 753nm in Ho3+∕Yb3+ codoped LKBBT glass system have confirmed that two photons contribute to the upconversion processes under the pumping of a 974nm diode laser. Owing to large absorption and emission cross sections, Yb3+ ion is considered to be a preferable sensitizer for catching enough pumping energy and transferring considerable energy to Ho3+ in LKBBT glasses. Low maximum phonon energy and high spontaneous transition probabilities of Ho3+ are beneficial in obtaining efficient upconversion and IR emission in the glass system.
Characterization of mixed Nd:LuxGd1−xVO4 laser crystals
Authors: Yu H. H. , Zhang H. J , Wang Z. P. , Wang J. Y. , Yu Y. G. , Cheng X. F. , Shao Z. S. , Jiang M. H. , Ling Z. C. , Xia H. R.
Published: June 2007

A series of laser crystals Nd:LuxGd1−xVO4 (x=0.14,0.32,0.50,0.61,0.70,0.80) was grown by the Czochralski method. The thermal properties, including the average linear thermal expansion coefficients, thermal diffusion coefficients, specific heats, and thermal conductivities, of the mixed crystals were obtained. The material constants Ms for the thermal stress resistance figure were calculated and showed that the thermal fracture limits of the mixed crystals should be comparable with that of Nd:YVO4. The polarization absorption spectra from 240to1000nm were measured at room temperature and the absorption cross sections at 809nm were calculated. Using the Judd-Ofelt theory, the theoretical radiative lifetimes were calculated and compared with the experimental results. Continuous wave laser performances were achieved with the mixed crystals at the wavelength of 1.06μm when they were pumped by a laser diode. Thermal, optical, and laser properties have shown variation as a function of x and proved that the mixed crystals are good laser materials.
High-pressure Raman study of Ba doped type-III germanium clathrate Ba24Ge100 up to 26 GPa
Authors: Shimizu Hiroyasu , Fukushima Takumi , Kume Tetsuji , Sasaki Shigeo , Fukuoka Hiroshi , Yamanaka Shoji
Published: June 2007

High-pressure Raman measurements of type-III germanium clathrate Ba24Ge100 have been made up to 26 GPa at room temperature. We observed low-frequency vibrational (rattling) modes associated with guest Ba atoms at 25–50 cm−1, and host Ge framework vibrations around 50–250 cm−1, which are compared with those of silicon clathrate Ba24Si100. High-pressure phase transition was found at 3.2 GPa, which seems to be due to the structural distortion combined with the enhanced guest-host interactions. Ba24Ge100 becomes irreversibly amorphous at pressures above 22 GPa. This pressure is less than that of type-I Ba8Ge43 clathrate, indicating that type-III structure is less stable than type-I Ge clathrate under high pressures.
Composition dependent ultraviolet photoresponse in MgxZn1−xO thin films
Authors: Ghosh R. , Basak D.
Published: June 2007

Mg x Zn 1 − x O  (0.0⩽x⩽0.12) thin films have been deposited on glass substrates by sol-gel process. The hexagonal wurtzite structure of ZnO is retained in the Mg-substituted films. From the photoresponse measurements, it is observed that both ZnO and substituted films are UV sensitive and with increasing Mg content, the sensitivity shifts toward the shorter wavelength side with a gradual decrease in magnitude. The UV-to-visible current ratio remains almost constant up to x=0.08 and thereafter decreases. The decay time for 90% photocurrent gradually decreases to less than 4s for x=0.05 followed by an increase for higher values of x. The decay time is faster than the growth time for all the substituted films. The change in the photoconductivity with Mg content (x) is correlated to the microstructural change. A trap level, found to be involved in the photoconductivity, is located at ∼0.8eV below the conduction band. The photo-to-dark current ratio (Iph∕Id) gradually decreases from three orders of magnitude to one order with the increase in the Mg content.
Polarized emission lines from A- and B-type excitonic complexes in single InGaN/GaN quantum dots
Authors: Winkelnkemper M. , Seguin R. , Rodt S. , Schliwa A. , Reißmann L. , Strittmatter A. , Hoffmann A. , Bimberg D.
Published: June 2007

Cathodoluminescence measurements on single InGaN/GaN quantum dots (QDs) are reported. Complex spectra with up to five emission lines per QD are observed. The lines are polarized along the orthogonal crystal directions [112¯0] and [1¯100]. Realistic eight-band k⋅p electronic structure calculations show that the polarization of the lines can be explained by excitonic recombinations involving hole states which are formed either by the A or the B valence band.
Nanoscale subsurface imaging via resonant difference-frequency atomic force ultrasonic microscopy
Authors: Cantrell Sean A. , Cantrell John H. , Lillehei Peter T.
Published: June 2007

A scanning probe microscope methodology, called resonant difference-frequency atomic force ultrasonic microscopy (RDF-AFUM), has been developed. It employs an ultrasonic wave launched from the bottom of a sample while the cantilever of an atomic force microscope, driven at a frequency differing from the ultrasonic frequency by one of the contact resonance frequencies of the cantilever, engages the sample top surface. The nonlinear mixing of the oscillating cantilever and the ultrasonic wave in the region defined by the cantilever tip–sample surface interaction force generates difference-frequency oscillations at the cantilever contact resonance. The resonance-enhanced difference-frequency signals are used to create images of nanoscale near-surface and subsurface features. An analytical model is presented for assessing the RDF-AFUM phase signal resulting from near-surface variations in the sample contact stiffness and from the interaction of the bulk wave with subsurface structures. The application of the model to RDF-AFUM phase measurements of a 12.7μm thick film of LaRC™-CP2 polyimide polymer containing a monolayer of gold nanoparticles embedded 7μm below the specimen surface reveals variations in the Young modulus of the material of approximately 24% over regions roughly 10–35nm wide. The magnitude of the modulus variations suggests the occurrence of contiguous amorphous and crystalline phases within the bulk of the polymer. The RDF-AFUM micrograph indicates a preferential growth of the crystalline phase in the vicinity of the gold nanoparticles.
Particle momentum effects from the detonation of heterogeneous explosives
Authors: Frost D. L. , Ornthanalai C. , Zarei Z. , Tanguay V. , Zhang F.
Published: June 2007

Detonation of a spherical high explosive charge containing solid particles generates a high-speed two-phase flow comprised of a decaying spherical air blast wave together with a rapidly expanding cloud of particles. The particle momentum effects associated with this two-phase flow have been investigated experimentally and numerically for a heterogeneous explosive consisting of a packed bed of inert particles saturated with a liquid explosive. Experimentally, the dispersion of the particles was tracked using flash radiography and high-speed photography. A particle streak gauge was developed to measure the rate of arrival of the particles at various locations. Using a cantilever gauge and a free-piston impulse gauge, it was found that the particle momentum flux provided the primary contribution of the multiphase flow to the near-field impulse applied to a nearby small structure. The qualitative features of the interaction between a particle and the flow field are illustrated using simple models for the particle motion and blast wave dynamics. A more realistic Eulerian two-fluid model for the gas-particle flow and a finite-element model for the structural response of the cantilever gauge are then used to determine the relative contributions of the gas and particles to the loading.
One-dimensional analysis of N-on-p Pb1−xSnxSe compositionally graded heterojunction photodetectors
Authors: Elizondo S. L. , Shi Z.
Published: June 2007

This work presents a theoretical investigation of the performance of N-on-p Pb1−xSnxSe heterojunction photodiodes with a linear gradient in composition. We quantify the improvement of the quantum efficiency, R0A product, and detectivity for a graded heterojunction device as compared to a basic p-n homojunction photodetector. In addition to enhanced overall device performance, the graded heterojunction diode exhibits considerable improvements when surface recombination velocities are taken into account. For low surface recombination velocities the detectivity of the graded N-on-p heterostructure increases nearly two-fold in comparison to the p-n homojunction, while for high surface recombination velocities the detectivity of the graded heterostructure increases by as much as an order of magnitude.
Energies of the first row atoms from quantum Monte Carlo
Authors: Brown M. D. , Trail J. R. , López Ríos P. , Needs R. J.
Published: June 2007

All-electron variational and diffusion quantum Monte Carlo calculations of the ground state energies of the first row atoms (from Li to Ne) are reported. The authors use trial wave functions of four types: single-determinant Slater-Jastrow wave functions, multideterminant Slater-Jastrow wave functions, single-determinant Slater-Jastrow wave functions with backflow transformations, and multideterminant Slater-Jastrow wave functions with backflow transformations. At the diffusion quantum Monte Carlo level and using their multideterminant Slater-Jastrow wave functions with backflow transformations, they recover 99% or more of the correlation energies for Li, Be, B, C, N, and Ne, 97% for O, and 98% for F.
Role of ceramic matrix and Au fraction on the morphology and optical properties of cosputtered Au-ceramic thin films
Authors: Hazra S. , Gibaud A. , Sella C.
Published: June 2007

Surface sensitive x-ray scattering studies were carried out to understand the morphology of cermet thin films prepared by cosputtering metallic gold and ceramic materials on float glass substrates. It has been observed that the morphology of Au clusters in cermet thin films depends strongly on the matrix during growth, even if, all other conditions are kept identical. In particular, nearly isotropic growth of Au clusters, to form nanoparticles, is found in silica and alumina matrices, while anisotropic columnar-like growth of Au clusters, to form a nanorod-like shape, is found in a titanium oxide matrix. Thickness of the films was also found very different, which is likely to be related to the different sputtering yields of the ceramic materials. The volume fraction of Au estimated from the electron density profile shows that the total volume or the amount of Au is different in films of different ceramic matrices. This suggests that even the sputtering yield of Au is very different in the presence of different ceramic atmosphere, which is likely to be responsible for having a different morphology of Au clusters in different matrices. Optical absorption spectra of the films, on the other hand, show linear dependence of the absorption peak position with the volume fraction of Au and independent of both the ceramic matrix and morphology of Au clusters.
Giant magnetoresistance of Fe3O4-polymethylmethacrylate nanocomposite aligned fibers via electrospinning
Authors: Pavan Kumar V. S. , Jagadeesh Babu V. , Raghuraman G. K. , Dhamodharan R. , Natarajan T. S.
Published: June 2007

This paper describes the results of investigations on giant negative magnetoresistance (GMR) in nanocomposite aligned fibers prepared using electrospinning. The nanocomposite contains polymethylmethacrylate (PMMA) matrix and the nanoparticles of polymer (PMMA) grafted magnetite, Fe3O4 (PGM). Even for the low magnetic field (1T) a GMR of about 50% was observed for low loading (5wt%) of PGM at room temperature. The fibers were characterized using scanning electron microscope and high resolution transmission electron microscope. X-ray diffraction (XRD) patterns were recorded for clean PMMA fibers, PMMA granules, and PGM nanoparticles. The dc conductivity was calculated from the I-V characteristics for the fibers at room temperature.
Electronic structure and ferromagnetic properties of Cu-doped AlN from first principles
Authors: Jia Wei , Han Peide , Chi Mei , Dang Suihu , Xu Bingshe , Liu Xuguang
Published: June 2007

Using the first-principles method based on the density functional theory, we studied the ferromagnetic stability and electronic structure of (Al, Cu) N. The Cu dopants were found spin polarized and the calculated band structure suggested a 100% polarization of the conduction carriers. The ferromagnetic ground state in Cu-doped AlN can be explained in terms of p-d hybridization mechanism. Based on the analysis on Cu-doped ZnO [L. H. Ye et al., Phys. Rev. B 73, 033203 (2006)], Curie temperature (TC) higher than 350K can be expected in AlN doped with Cu. These results suggest that the Cu-doped wide band AlN may present a promising dilute magnetic semiconductor and find applications in the field of spintronics.
Transient study of self-heating effects in AlGaN/GaN HFETs: Consequence of carrier velocities, temperature, and device performance
Authors: Wu Yuh-Renn , Singh Jasprit
Published: June 2007

GaN transistors offer a unique combination of high speed and high power. Devices capable of operating at close to 180 GHz can also be biased so that each electron may emit up to 100 phonons as it transits from source to drain. Therefore, strong heating effects in GaN transistors are expected. In this article, we present a time dependent model to examine current, power and temperature in pulsed and direct current conditions. The studies show that self-heating effects are significant even for very short pulse widths (∼ a few nanoseconds). The results explain part of the reasons for the low velocity observed in the short pulse measurements. Our calculations also suggest that the device performance could improve more than 30% if we can efficiently remove the self-heating effects.
Efficient multiparticle sampling in Monte Carlo simulations on fluids: Application to polarizable models
Authors: Moučka Filip , Rouha Michael , Nezbeda Ivo
Published: June 2007

A novel Monte Carlo simulation scheme based on biased simultaneous displacements of all particles of the system has been developed. The method is particularly suited for systems with nonadditive interactions and its efficiency is demonstrated by its implementation for the polarizable Stockmayer fluid. Performance of the method is compared with both the standard one-particle move method and an unbiased multiparticle scheme by computing the mean squared displacements, rotation relaxation, and the speed of equilibration (translational order parameter). It is shown that the proposed biased method is about a factor of 10 faster, for the system considered, when compared with the other schemes.
Remnant magnetization of Fe8 high-spin molecules: X-ray magnetic circular dichroism at 300 mK
Authors: Letard Isabelle , Sainctavit Philippe , dit Moulin Christophe Cartier , Kappler Jean-Paul , Ghigna Paolo , Gatteschi Dante , Doddi Bruno
Published: June 2007

Fe8 high-spin molecules exhibit quantum spin tunneling at very low temperatures. Eight Fe3+ ions are sixfold coordinated and magnetically coupled through oxygen bridges. The net magnetization (MS=20 μB per molecule) results from competing antiferromagnetic interactions between the various Fe3+ ions (S=5/2). Because of the structural anisotropy of these molecules, the magnetization curve presents a hysteresis loop with staircases below 2 K. The staircases of the hysteresis loop are due to the quantum spin tunneling, which is temperature dependent for 400 mK<T<2 K and temperature independent below 400 mK. In order to reach temperatures low enough to observe a remnant magnetization, we developed a H3e−H4e dilution refrigerator. This device, that takes advantage of the thermodynamic particularities of the H3e−H4e mix, let us obtain x-ray magnetic circular dichroism (XMCD) spectra at 300 mK for this molecule. It has been possible to register an XMCD remnant signal, without magnetic field applied, at the iron L2,3 edges. XMCD coupled with ligand field multiplet calculations has allowed to determine the spin and orbit contributions to the magnetization of the Fe3+ ions.
Reactions of interstitial carbon with impurities in silicon particle detectors
Authors: Makarenko L. F. , Moll M. , Korshunov F. P. , Lastovski S. B.
Published: June 2007

We present deep level transient spectroscopy (DLTS) data measured on very high resistivity n-type float-zone silicon detectors after irradiation with 6MeV electrons. The carbon interstitial annealing kinetics is investigated as a function of depth in the detector structure and related to the inhomogeneous depth distribution of oxygen and carbon impurities in the devices. We compare our results with data published in previous works and point out some possible misinterpretation of DLTS data due to detector processing induced inhomogeneous distribution of impurities. Finally, we present a method to determine the absolute concentration of the oxygen and carbon impurities as functions of depth in devices by carefully analyzing the carbon interstitital annealing kinetics.
Structure and magnetic moments of mass-filtered deposited nanoparticles
Authors: Kleibert A. , Passig J. , Meiwes-Broer K.-H. , Getzlaff M. , Bansmann J.
Published: June 2007

Mass-filtered 3d transition metal nanoparticles have been produced by means of an ultrahigh vacuum compatible arc cluster ion source. High resolution transmission electron microscopy images of individual Fe, Co, and FeCo alloy particles with diameters of about 12 nm reveal the crystalline structure of the nanoparticles. X-ray absorption spectroscopy confirms the purity of the particles after in situ deposition. Analysis of the x-ray magnetic circular dichroism reveals bulklike total magnetic moments in all cases and strongly enhanced orbital moments for the iron nanoparticles. Furthermore, the data hint at a chemically ordered alloy in the case of FeCo particles.
Lattice thermal conductivity in a Si∕Ge∕Si heterostructure
Authors: Lü Xiang , Chu Junhao
Published: June 2007

We have developed a model of lattice thermal conductivity in a freestanding three-layered Si∕Ge∕Si heterostructure. This model takes into account the phonon confinement and interface scattering. Moreover, we derive an analytical expression of the interface scattering rate for phonon conduction in a semiconductor heterostructure. It is found that the phonon transport is reduced in the heterostructure. We further investigate the dependence of the thermal conductivity of the Ge well layer on the Si barrier layer thickness. We show that the thermal conductivity of the well layer can be modulated in a wide range by adjusting the barrier layer thickness. It is found that the thermal conductivity of the well layer increases with the barrier layer thickness in the case of highly specular scattering. The obtained results can be used for tuning the thermal conductivity in nanostructures.
Correlation between luminescence properties of AlxGa1−xAs∕GaAs single quantum wells and barrier composition fluctuation
Authors: Lourenço S. A. , da Silva M. A. T. , Dias I. F. L. , Duarte J. L. , Laureto E. , Quivy A. A. , Lamas T. E.
Published: June 2007

The luminescence mechanism at low temperatures in AlxGa1−xAs∕GaAs single quantum wells grown by molecular-beam epitaxy with different aluminum concentrations in the barrier has been studied in detail using the photoluminescence spectroscopy (PL) as function of temperature (8K⩽T⩽90K) combined with the excitation intensity. The asymmetry presented by the PL spectra at the low-energy side, the blueshift behavior of the PL peak energy, and the PL line broadening with increasing temperature are explained through the exciton localization in confinement potential fluctuations. The exciton localization effects on the PL spectra are progressively reinforced with the increase of the Al concentration in the barrier constituent material. The PL peak energy dependence on temperature has been fitted through the expression proposed by Pässler [Phys. Status Solidi B 200, 155 (1997)] adapted to systems with potential fluctuations, by subtracting the term σE2∕kBT, where σE is the standard deviation of the potential fluctuations. It was verified that σE increases systematically with the Al concentration in the barrier, according to the AlGaAs alloy compositional disorder theory.
Ambient pressure colossal magnetocaloric effect in Mn1−xCuxAs compounds
Authors: Rocco Daniel L. , de Campos Ariana , Carvalho Alexandre Magnus G. , Caron Luana , Coelho A. A. , Gama S. , Gandra Flávio C. G. , dos Santos Adenilson O. , Cardoso Lisandro P. , von Ranke Pedro J. , de Oliveira Nilson A.
Published: June 2007

Magnetic refrigeration is a good alternative to gas compression technology due to higher efficiency and environmental concerns. Magnetocaloric materials must exhibit large adiabatic temperature variations and a large entropic effect. MnAs shows the colossal magnetocaloric effect under high pressures or with Fe doping. In this work the authors introduce a class of materials—Mn1−xCuxAs—revealing a peak colossal effect of −175J∕(Kkg) for a 5T field variation at 318K and ambient pressure.
Metal-slotted polymer optical waveguide device
Authors: Kim Seong-Ku , Hung Yu-Chueh , Yuan W. , Cha DongHo , Geary K. , Fetterman H. R. , Ha Soon Mok , Pei Qibing , Luo Jindong , Zhou XingHua , Jen Alex K.-Y. , Kwon M.-S. , Steier W. H.
Published: June 2007

Metal-slotted optical waveguides (MSOWs) using an electro-optic polymer material have been experimentally demonstrated. The device consists of a three-layered slab waveguide in that the thin metal (gold) film strips are embedded on top of the lower cladding. The optical mode shapes and effective index of the propagation modes of the proposed waveguide structure were calculated using a simplified effective index method and a simulation tool. The fabrication and the device characteristics of a variable optical attenuator and an optical phase modulator based on MSOWs are discussed.
Two-dimensional neutron scattering study of the bulk microstructure of polytetrafluoroethylene under uniaxial stress
Authors: Noirez Laurence , Baroni Patrick
Published: June 2007

The effect of a uniaxial stress on the microstructural organization of the phase IV of the bulk polytetrafluoroethylene is examined. Due to the absence of hydrogen, this semicrystalline polymer is particularly adapted to neutron scattering. The experiment was performed using a pioneering two-dimensional neutron detector Barotron, illustrating this tool benefit in this research field. It will be shown that a high degree of alignment of the crystallites is obtained from lowest elongation rates. The two-dimensional analysis distinguished the structural contributions, affected or not by the mechanical deformation, and identified the amorphous from the crystalline components of the polymer.
Periodic nanohole arrays with shape-enhanced plasmon resonance as real-time biosensors
Authors: Lesuffleur Antoine , Im Hyungsoon , Lindquist Nathan C. , Oh Sang-Hyun
Published: June 2007

The authors report a combination of the enhanced transmission effect and shape resonances in a periodic array of nanoscale double-hole structures in a gold film to enhance the detection sensitivity of surface plasmon biosensors. Finite-difference time-domain calculations are used to quantify field enhancement at the apexes of the double-hole structure. The double-hole array was used to measure the formation of a self-assembled monolayer and for real-time sensing of protein adsorption onto a gold surface. This result demonstrates the potential to integrate propagating surface plasmons and localized shape resonances to improve real-time biosensors.
High-resolution transmission electron microscopy study on the growth modes of GaSb islands grown on a semi-insulating GaAs (001) substrate
Authors: Kim Y. H. , Lee J. Y. , Noh Y. G. , Kim M. D. , Oh J. E.
Published: June 2007

The initial growth behaviors of GaSb on a GaAs substrate were studied using a high-resolution electron microscope (HRTEM). Four types of GaSb islands were observed by HRTEM. HRTEM micrographs showed that strain relaxation mechanisms were different in the four types of islands. Although 90° misfit dislocations relieve misfit strain in the islands, additional mechanisms are required to relax the remaining strain. The existence of elastic deformation near the surface related to dislocations and intermediate layers between GaSb and GaAs were demonstrated in island growths. Finally, the generation of planar defects to relieve strain was observed in a specific GaSb growth.
Power consumption reduction scheme of magnetic microactuation using electroplated Cu–Ni nanocomposite
Authors: Huang Yu Wen , Chao Tzu-Yuan , Chen C. C. , Cheng Y. T.
Published: June 2007

This letter presents a power consumption reduction scheme of magnetic microactuation using Cu–Ni nanocomposite film which is electroplated in alkaline noncyanide based Ni colloidal copper plating solution at 40°C. The superconducting quantum interference device magnetometer measurements show that Cu film is modified from diamagnetism to ferromagnetism via the incorporation of Ni ferromagnetic nanoparticles into itself to form a Cu–Ni nanocomposite film. A magnetic microactuator made of the nanocomposite can have about 9% performance improvement in terms of actuation enlargement. In other words, the coil made of the Cu–Ni nanocomposite can exhibit better power efficiency for the same output displacement.
Annealing condition optimization and electrical characterization of amorphous LaAlO3∕GaAs metal-oxide-semiconductor capacitors
Authors: Choi Donghun , Harris James S. , Warusawithana Maitri , Schlom Darrell G.
Published: June 2007

The electrical properties of amorphous LaAlO3∕GaAs metal-oxide-semiconductor capacitors fabricated using molecular-beam deposition are investigated. The surface was protected during sample transfer between III-V and oxide molecular beam epitaxy chambers by a thick arsenic-capping layer. Amorphous LaAlO3 was deposited on c(4×4) and (2×4) reconstructed (100) GaAs surfaces. An annealing method, a low temperature-short time rapid thermal annealing (RTA) followed by a high temperature RTA, was developed, yielding extremely small hysteresis (∼30mV), frequency dispersion (∼60mV), and interfacial trap density (mid-1010eV−1cm−2).
Femtosecond laser writing of waveguides in periodically poled lithium niobate preserving the nonlinear coefficient
Authors: Osellame Roberto , Lobino Mirko , Chiodo Nicola , Marangoni Marco , Cerullo Giulio , Ramponi Roberta , Bookey Henry T. , Thomson Robert R. , Psaila Nicholas D. , Kar Ajoy K.
Published: June 2007

Optical waveguides have been inscribed in periodically poled lithium niobate by femtosecond laser pulses with the multiscan technique. Second harmonic generation experiments from a fundamental wavelength of 1567nm demonstrate that the nonlinear optical coefficient in the waveguides is preserved, yielding a conversion efficiency of 18%W−1.
Light enhancement by the formation of an Al oxide honeycomb nanostructure on the n-GaN surface of thin-GaN light-emitting diodes
Authors: Lin C. L. , Chen P. H. , Chan Chia-Hua , Lee C. C. , Chen Chii-Chang , Chang Jeng-Yang , Liu C. Y.
Published: June 2007

By using a micron polystyrene ball array as a template, an Al oxide honeycomb structure was produced on the n-GaN surface of a thin-GaN light-emitting diode (LED). The Al oxide honeycomb structure consists of the networking hexagonal Al oxide nanowall. With the Al oxide honeycomb nanostructure, the total lighting output of thin-GaN LED was enhanced by 35%. The authors believe that the networking nanowall of the Al oxide honeycomb structure acted as a waveguide to extract the light emitted to the outer medium effectively.
Effects of ion bombardment on the morphology and microstructure of carbon nanomaterials grown by microwave plasma chemical vapor deposition
Authors: Qin Yong , Xia Junhai , Staedler Thorsten , Jiang Xin
Published: June 2007

Effects of ion bombardment on the growth of carbon nanomaterials on Fe substrates in microwave plasma chemical vapor deposition were investigated. The product morphologies are directly correlated with the kinetic energy of bombarding ions and range from nanosheets, through cones, to aligned nanotubes for bias voltages from 0to−250V, respectively. A change in growth mechanism from a base growth to tip growth induced by the competition between etching and growth is responsible for the observed changes. These findings bring insights into the growth mechanism and provide a strong tool to control the structure of carbon nanomaterials.
Low coupling losses in YBa2Cu3O7 coated conductors with current sharing between the superconducting stripes
Authors: Takács S.
Published: June 2007

The authors calculate the coupling losses in striated YBa2Cu3O7 coated conductors covered partially with normal conducting layers in the form of segmented pieces, connecting the individual superconducting stripes. The authors propose a system of coating that fulfills the requirements of both acceptable coupling losses and sufficient current sharing between the stripes. This structure can be used in ac applications without twisting the superconducting stripes. The frequency range, in which the model and the calculations can be applied, is also given.
First-principles study of electronic and magnetic properties of BiNiO3
Authors: Cai M. Q. , Yang G. W. , Cao Y. L. , Yu W. H. , Wang L. L. , Wang Y. G.
Published: June 2007

First-principles calculations on the electronic and magnetic properties of BiNiO3 reveal that the G-type antiferromagnetic structure with the insulating ground state is more stable than other possible configurations. The hybridization of Ni–O and Bi–O leads to the reduction of the spin magnetic moment to 1.67μB in comparison with the Ni2+ d8 configuration of 2μB. The band gap of the antiferromagnetic insulating ground state is predicted to be 0.68eV. The antiferromagnetic interaction induces the localized spins to be S=1 in the G-antiferromagnetic structure of BiNiO3.
High transition temperature lead magnesium niobate–lead zirconate titanate single crystals
Authors: Amin Ahmed , Lee Ho-Yong , Kelly Brian
Published: June 2007

Recently developed relaxor lead magnesium niobate–ferroelectric lead zirconate titanate single crystals exhibit large length extensional k33 coupling, improved thermal stability, and a higher coercive field compared to morphotropic lead magnesium niobate–lead titanate single crystals. Experimental results and implications for sound projectors are presented and discussed in terms of the morphotropic phase boundary slope and width. Spontaneous polarizations derived from the calculated atom shifts in both crystals compared favorably with experimental values.
Enhanced outcoupling from organic light-emitting diodes using aperiodic dielectric mirrors
Authors: Agrawal Mukul , Sun Yiru , Forrest Stephen R. , Peumans Peter
Published: June 2007

Aperiodic dielectric stacks between the substrate and transparent anode in organic light-emitting diodes are used to improve the optical outcoupling efficiency. The authors demonstrate that a nine-layer SiO2∕SiNx aperiodic dielectric stack improves the brightness by 80% within a 60° viewing cone for a red-emitting organic light-emitting diode, while maintaining a Lambertian emission pattern. As the refractive index contrast between the two materials used in a two-component multilayer dielectric stack is increased, a brightness improvement of 170% in a 60° viewing cone is achievable while maintaining a Lambertian emission profile.
Structures and magnetic properties of (Mn, N)-codoped ZnO thin films
Authors: Yan Wensheng , Sun Zhihu , Liu Qinghua , Li Zhongrui , Shi Tongfei , Wang Feng , Qi Zeming , Zhang Guobin , Wei Shiqiang , Zhang Huawei , Chen Zhizhan
Published: June 2007

Zn 0.96 Mn 0.04 O and Zn0.96Mn0.04O:N thin films with wurtzite structure were grown by inductively coupled plasma enhanced chemical vapor deposition method. Although both samples exhibit ferromagnetism at room temperature, the saturation magnetic moment (1.4μB∕Mn) of the Mn- and N-codoped sample is much larger than that (0.3μB∕Mn) of the N-free one. The x-ray absorption near edge structure analysis reveals that the codoped Mn and N impurities can be substitutionally incorporated into the ZnO host in the Zn0.96Mn0.04O:N thin film. The first-principles calculations suggest that the N substitution for the O site in Mn-doped ZnO can change the interaction of neighboring Mn–Mn pairs from antiferromagnetic to ferromagnetic, and accordingly the effective magnetic moment per Mn is greatly enhanced.
Formation and luminescent properties of face-centered-cubic Si nanocrystals in silica matrix by magnetron sputtering with substrate bias
Authors: Lu Y. W. , Du X. W. , Sun J. , Hu S. L. , Han X. , Li H.
Published: June 2007

Si nanocrystals with face-centered-cubic (fcc) structure were fabricated using common sputtering technique, but applying bias on the substrate, the thermodynamic calculation indicates that the bombardment of Ar+ ions arising from the bias should be responsible for the crystallization and phase transition of Si nanocrystals. The silica films containing fcc-Si nanocrystals give strong blue and ultraviolet light emissions, which suggest that the band structure of Si material could be modified by changing the crystal structure.
Noninvasive monitoring of traumatic brain injury and post-traumatic rehabilitation with laser-induced photoacoustic imaging
Authors: Yang Sihua , Xing Da , Lao Yeqi , Yang Diwu , Zeng Lvming , Xiang Liangzhong , Chen Wei R.
Published: June 2007

A photoacoustic imaging system was used for noninvasive monitoring of traumatic mouse brain in vivo with high-quality reconstructed images. Traumatic lesions accompanying with hemorrhage in the mouse cortical surface were accurately mapped, and foreign bodies of two small copper wires inserted in the mouse brain were also detected. Furthermore, the time course of morphological changes of cerebral blood during rehabilitation process of a mouse brain with traumatic brain injury was obtained using a series of photoacoustic images. Experimental results demonstrate that photoacoustic technique holds the potential for clinical applications in brain trauma and cerebrovascular disease detection.
New result contradicts troubling old evidence of neutrino oscillation at small distances
Authors: Schwarzschild Bertram
Published: June 2007

A decade ago, the findings of an accelerator experiment at Los Alamos seemed to require the existence of sterile neutrinos immune to the weak nuclear force.
The challenge of nuclear weapons
Authors: Drell Sidney D.
Published: June 2007

To address the nuclear threat, argues the author, we will need to confront the reality of today’s nuclear stockpiles and work to create a future without nuclear weapons.
The perceptual basis for audio compression
Authors: Kohlrausch Armin
Published: June 2007

Because the human ear ignores small amounts of noise that accompany a strong signal, much of the information in audio files can be thrown away with little loss of fidelity.
Unbundling the State: Iraq and the ‘Recontainerization’ of Rule, Production, and Identity
Authors: Luke Timothy W
Published: June 2007

This analysis investigates the ‘recontainerization’ of state power, asking how the forms of ruling authority are being eclipsed around the world by ‘denationalization’, ‘detraditionalization’, or ‘destatalization’. The government of The Coalition Provisional Authority in Iraq is an excellent case study in the recontainerizing rule, production, and identity on a different political scale and pace than that characteristic of many nation-states. This odd transitional regime points toward some of the material underpinnings and mobile resistances that one finds in the recontainerization of rule in today's networks of globalist governance. It explores how various unbundled nonstatal organizations and postnational identities work through markets and multinational military interventions to displace the authority of current state structures in determining who rules whom, what is produced where, and how identities are maintained.
Are National Minorities in the EU Progressing towards the Acquisition of Universal Rights?
Authors: Markusse Jan D
Published: June 2007

This paper aims to acquire insights into processes and mechanisms behind the development of policies on universal citizenship rights at higher political scales. It considers politics on national minority rights at different scales in the context of thoughts on postnationalisation and denationalisation of citizenship, which are mainly based on globalisation theories, and in the context of thoughts on shifts in the scales of state activity from a regulation theoretical perspective. The development of common rules on the rights of minorities at the global and European scales is examined and confronted with policies of the individual states. From a systematic analysis of all national minorities in the European Union it appears that common rules at higher scales still allow for considerable diversity. The differences can be attributed to different characteristics of the minorities themselves as well as of the states concerned. Both reflect the vital legacy of strongly different historical paths of nation-state formation in different parts of Europe. The outcomes are better fitting in a regulation theoretical perspective than in a globalisation theoretical perspective on the development of common rules on minority rights in Europe.