bluuebunny/crossref_metadata_2025 huggingface dataset:
Free exciton emission and dephasing in individual ZnO nanowires
Authors: Zhang Yang , Chen Dai-Jang , Lee Ching-Ting
Published: October 2007

We report on systematic measurements of the dephasing of the free excitonic transition in individual ZnO nanowires with a diameter of about 70nm. The temperature-dependent intensity of the free exciton (FX) emission is well reproduced by a thermally activated behavior having two constant activation energies of 5.98 and 30.63meV. The FX linewidth broadening with temperature reveals the dephasing of FX scattered by acoustic phonons rather than longitudinal optical phonons. These findings indicate that the dephasing mechanism scattering from acoustic phonons in individual ZnO nanowires result from the compensation of acoustic phonons to the nonconservation of crystal momentum.
Transient separation of multicomponent liquid mixtures in thermogravitational columns
Authors: Haugen Kjetil B. , Firoozabadi Abbas
Published: October 2007

Transient separation of the individual components in a multicomponent liquid mixture in a thermogravitational column can be used to determine the thermal and molecular diffusion coefficients. Two models of the transient behavior are developed. First, a classical model where density only depends on temperature. Second, a general model where the compositional effect on density is also taken into account. Diffusion coefficients can be determined by fitting experimental data to either model. The procedure is demonstrated for a ternary liquid mixture. The results reveal that the classical model is very unreliable even though composition variation only contributes to the total buoyancy by one-third. Diffusion coefficients can be obtained reliably from the general model provided the experimental noise does not exceed ±1% of steady-state separation. This level of accuracy in composition measurements is achievable.
Molecular dynamics study of polarizable point dipole models for molten sodium iodide
Authors: Alcaraz Olga , Bitrián Vicente , Trullàs Joaquim
Published: October 2007

The structure, the ionic transport properties, and the dynamics of long-wavelength charge-density fluctuations, for two polarizable point dipole models of molten NaI, have been studied by molecular dynamics simulations. These models are based on a rigid ion potential to which the induced dipole polarization of the anions is added. The polarization is added in such a way that point dipoles are induced on the anions by both local electric field and short-range damping interactions that oppose the electrically induced dipole moments. The two polarizable ion models differ only in the range of the damping polarization interactions. The influence of the induced anion polarization on the different properties of simulated molten NaI is discussed.
Atomic force microscopy and photoemission electron microscopy study of the low-pressure oxidation of transition metal nitrides
Authors: Fleming A. J. , Surnev S. , Netzer F. P.
Published: October 2007

The oxidation of transition metal nitrides is examined by atomic force microscopy (AFM) and photoemission electron microscopy (PEEM) in the temperature range of 25–800°C in an ultrahigh vacuum study. The roughness of the surfaces of thin film coatings of vanadium (V), vanadium nitride (VN), titanium nitride (TiN), and TiAlVN, monitored as a function of (a) constant dose of 50000L (1L=10−6Torrs) O2 at various temperatures and (b) oxygen dose at a constant temperature of 800°C, is found to increase for all samples except TiAlVN. The morphological parameters (roughness and oxide grain volume) obtained from AFM are used to determine the extent of oxidation. The activation energy for the growth of oxide on the surface is calculated to be in the range of 0.4–0.5eV for all samples containing V atoms and 1.6eV for TiN. PEEM images of the pristine surfaces of TiN and TiAlVN show surface precipitates of the order of 5–50μm size. The workfunction of all surfaces and precipitates, obtained from photoemission spectra using PEEM, in combination with the effective activation energies measured by AFM, suggests that the main surface precipitate of TiAlVN is VN or VOxNy.
Size effect on electronic sputtering of LiF thin films
Authors: Kumar Manvendra , Khan S. A. , Rajput Parasmani , Singh F. , Tripathi A. , Avasthi D. K. , Pandey A. C.
Published: October 2007

Electronic sputtering in polycrystalline LiF thin film by 120MeV Ag25+ is investigated. The sputter yields of Li and F for the different thicknesses (10–265nm) of films are measured with online elastic recoil detection analysis technique. A reduction in sputter yield, from ∼2.3×106 to 2.2×104 atoms/ion, is observed with increase in the film thickness. The trend in the experimental results can be explained in terms of size effect in thin film following inelastic thermal spike model. The confinement of energy in the film having smaller grains and lower thickness results in higher temperature causing higher sputtering yield.
Residual strain and texture in free-standing nanoscale Cu-Nb multilayers
Authors: Aydıner C. C. , Brown D. W. , Misra A. , Mara N. A. , Wang Y.-C. , Wall J. J. , Almer J.
Published: October 2007

We investigate the residual strains in a free-standing Cu/Nb multilayer of 30 nm nominal layer thickness with synchrotron x-rays. This material system is characterized by columnar grains of Cu and Nb with incoherent interfaces and a sharp physical-vapor-deposition texture. High energy x-rays were used with an area detector along with multiple sample rotations to yield diffraction strain components in a very large number of directions. Due to the texture and the elastic anisotropy of constituents, observed diffraction strains cannot be derived from a single strain tensor (also known as linear sin2 ψ). Orientation-dependent diffraction strain modeling is utilized with a Vook–Witt micromechanical model. Obtained phase-resolved in-plane stress magnitudes are −515 MPa in Nb and +513 MPa in Cu, satisfying force equilibrium within experimental errors. The stresses of this magnitude will certainly influence the mechanical behavior of the multilayer upon further loading. The Vook–Witt model describes the Nb diffraction strains very well, and thereby provides information on the stress distribution in crystallites as a function of their orientation. On the other hand, the same level of agreement with the Vook–Witt model has not been achieved for Cu diffraction strains.
Molecular dynamics simulation of plastic deformation of nanotwinned copper
Authors: Cao A. J. , Wei Y. G.
Published: October 2007

The plastic deformation of polycrystalline Cu with ultrathin lamella twins has been studied using molecular dynamics simulations. The results of uniaxial tensile deformation simulation show that the abundance of twin boundaries provides obstacles to dislocation motion, which in consequence leads to a high strain hardening rate in the nanotwinned Cu. We also show that the twin lamellar spacing plays a vital role in controlling the strengthening effects, i.e., the thinner the thickness of the twin lamella, the harder the material. Additionally, twin boundaries can act as dislocation nucleation sites as they gradually lose coherency at large strain. These results indicate that controlled introduction of nanosized twins into metals can be an effective way of improving strength without suppression tensile ductility.
Dissipation and decay of fractal-generated turbulence
Authors: Seoud R. E. , Vassilicos J. C.
Published: October 2007

Space-filling fractal square grids fitted at the entrance of a wind tunnel’s test section generate unusually high Reynolds number homogeneous isotropic turbulence which decays locked into a single length-scale l. Specifically, during turbulence decay along the streamwise coordinate x, E11(k1,x)=u′2lf(k1l) over the entire range of wavenumbers, where l and the function f are about the same for all the grids tried here. As a result, this fractal-generated turbulence has the following properties which we have also observed in the decaying region: L∕λ is constant, independent of the x grid and Reλ; ϵ∼Reλ−1u′3∕Lu; and E11(k1)∼(u′3∕Lu)2∕3k1−5∕3 instead of E11(k1)∼ϵ2∕3k1−5∕3 in the observed range of wavenumbers where f(k1l)∼(k1l)−5∕3.
A note on the energy of relative equilibria of point vortices
Authors: Aref Hassan
Published: October 2007

Analytical formulas are derived for the energy of simple relative equilibria of identical point vortices such as the regular polygons, both open and centered, and the various known configurations consisting of nested regular polygons with or without a vortex at the center.
Surface plasmon excitation at second harmonic over a rippled surface
Authors: Singh D. B. , Tripathi V. K.
Published: October 2007

A high power laser of frequency ω incident on a rippled metal surface gives rise to oscillatory electron velocity v2ω at the second harmonic in the skin layer. Its coupling with the surface ripple (modeled as a density ripple) of suitable wave number produces a nonlinear current, driving a surface plasma wave at frequency 2ω.
Effects of the quantum dot ripening in high-coverage InAs∕GaAs nanostructures
Authors: Frigeri P. , Nasi L. , Prezioso M. , Seravalli L. , Trevisi G. , Gombia E. , Mosca R. , Germini F. , Bocchi C. , Franchi S.
Published: October 2007

We report a detailed study of InAs∕GaAs quantum dot (QD) structures grown by molecular beam epitaxy with InAs coverages θ continuously graded from 1.5 to 2.9 ML. The effect of coverage on the properties of QD structures was investigated by combining atomic force microscopy, transmission electron microscopy, x-ray diffraction, photoluminescence, capacitance-voltage, and deep level transient spectroscopy. In the 1.5–2.9 ML range small-sized coherent QDs are formed with diameters and densities that increase up to 15nm and 2×1011cm−2, respectively. For θ>2.4 ML large-sized QDs with diameters of 25nm and densities ranging from 2×108to1.5×109cm−2 coexist with small-sized QDs. We explain the occurrence of large-sized QDs as the inevitable consequence of ripening, as predicted for highly lattice-mismatched systems under thermodynamic equilibrium conditions, when the coverage of the epitaxial layer exceeds a critical value. The fraction of ripened islands which plastically relax increases with θ, leading to the formation of V-shaped defects at the interface between QDs and upper confining layers that propagate toward the surface. Island relaxation substantially affects the properties of QD structures: (i) free carrier concentration is reduced near the QD plane, (ii) the QD photoluminescence intensity is significantly quenched, and (iii) deep levels show up with typical features related to extended structural defects.
Activation energy and prefactor for surface electromigration and void drift in Cu interconnects
Authors: Choi Z.-S. , Mönig R. , Thompson C. V.
Published: October 2007

Surface electromigration rates on oxide-free surfaces of unpassivated damascene Cu interconnect segments have been determined through electromigration testing under vacuum. Electromigration-induced voids grew at the cathode end of the segments due to a flux divergence at refractory-metal-lined vias to the lead lines below the test segment. Copper diffusivity on a clean Cu surface was determined by measuring the size of the voids as a function of time and test temperature at a fixed current. An activation energy of 0.45±0.11eV and a prefactor of 3.35×10−12m2∕s were found for the product of the effective charge z* and the surface diffusivity Ds. This result is shown to be consistent with void drift rates measured in passivated interconnects.
Nanoindentation of the a and c domains in a tetragonal BaTiO3 single crystal
Authors: Park Young-Bae , Dicken Matthew J. , Xu Zhi-Hui , Li Xiaodong
Published: October 2007

Nanoindentation in conjunction with piezoresponse force microscopy was used to study domain switching and to measure the mechanical properties of individual ferroelectric domains in a tetragonal BaTiO3 single crystal. It was found that nanoindentation has induced local domain switching; the a and c domains of BaTiO3 have different elastic moduli but similar hardness. Nanoindentation modulus mapping on the a and c domains further confirmed such difference in elasticity. Finite element modeling was used to simulate the von Mises stress and plastic strain profiles of the indentations on both a and c domains, which introduces a much higher stress level than the critical value for domain nucleation.
Aggregate structure and percolation behavior in polymer/carbon black conductive composites
Authors: Liang Jizhao , Yang Quanquan
Published: October 2007

The percolation behavior and mechanisms of electrical conductivity in carbon black filled polymer composites were analyzed. An aggregate structure model of the carbon black particles was proposed based on the effective media theory (EMT) and the equivalent sphere of carbon black aggregate acts as a basic cell to describe the nonlinear conductive behavior. Then, a formula describing the relationship between the conductivity and the volume fraction of carbon black particles as well as an expression estimating percolation threshold of the composites were derived. This aggregate structure model overcomes the disadvantage of the EMT which cannot explain the phenomenon that percolation threshold is lower than 1∕3. Based on these expressions, the electrical conductivity and percolation threshold of several kinds of conductive polymeric composites were predicted and the results showed that the predictions were preferably consistent with the experimental data from the references.
Wave character of metrics and hyperbolic geometric flow
Authors: Kong De-Xing , Liu Kefeng
Published: October 2007

In this paper, we illustrate the wave character of the metrics and curvatures of manifolds and introduce a new understanding tool—the hyperbolic geometric flow. This kind of flow is new and very natural to understand certain wave phenomena in the nature as well as the geometry of manifolds. It possesses many interesting properties from both mathematics and physics. Several applications of this method have been found.
Improved N14 nuclear quadrupole resonance detection of trinitrotoluene using polarization transfer from protons to N14 nuclei
Authors: Lužnik Janko , Pirnat Janez , Jazbinšek Vojko , Apih Tomaž , Blinc Robert , Seliger Janez , Trontelj Zvonko
Published: October 2007

Combination of proton-nitrogen level crossing polarization transfer and pulsed spin-locking sequence makes N14 nuclear quadrupole resonance (NQR) in trinitrotoluene fast and sensitive enough to be used in routine detection of explosives. Enhancement factors for all three N14 NQR lines (the case with η≠0) were calculated and compared with experimental values. Good agreement between measured and calculated signal enhancement factors was observed. N14 NQR signals in a 15g trinitrotoluene sample of predominantly monoclinic modification were measured in 15s in different polarization magnetic fields. The conditions for optimal proton-nitrogen level crossing were determined.
The effect of a Si-doped protective layer on the discharge characteristics of an alternating current plasma display panel
Authors: Lee Don-Kyu , Park Chung-Hoo , Lee Hae June , Choi Joon-Young , Kim Dong-Hyun , Lee Ho-Jun
Published: October 2007

In order to improve the discharge characteristics such as discharge voltage, luminance, luminous efficacy, high-temperature misfiring, and lifetime, a small amount of Si was added to the MgO protective layer of a plasma display panel. The MgO protective layers were deposited by using an electron-beam evaporation method. The addition of Si onto a MgO protective layer can effectively increase the secondary electron emission coefficient so as to improve the discharge characteristics compared with using a pure MgO film. Both the surface characteristics of the protective layer and the electro-optical properties of 4in. test panels were investigated, such as the discharge voltage, luminance, luminous efficacy, high-temperature misfiring, and lifetime. The results show that Si doping of 0.03%–0.04% in MgO has the best performance on plasma display panels.
Using ion irradiation to make high-Tc Josephson junctions
Authors: Bergeal N. , Lesueur J. , Sirena M. , Faini G. , Aprili M. , Contour J. P. , Leridon B.
Published: October 2007

In this article we describe the effect of ion irradiation on high-Tc superconductor thin film and its interest for the fabrication of Josephson junctions. In particular, we show that these alternative techniques allow to go beyond most of the limitations encountered in standard junction fabrication methods, both in the case of fundamental and technological purposes. Two different geometries are presented: a planar one using a single high-Tc film and a mesa one defined in a trilayer structure.
Comment on “Analytic and numerical calculations of the dispersion characteristics of two-dimensional dielectric photonic gap structures” [J. Appl. Phys. 99, 063104 (2006)]
Authors: Szmulowicz Frank
Published: October 2007

The article by Samokhvalova et al. [J. Appl. Phys. 99, 026106 (2006)] introduces a solvable analytic model of two-dimensional dielectric photonic band gap structures in the form of rectangular dielectric rods. Here, it is shown that the model is simply a superposition of two one-dimensional models; that is, the authors have, in fact, solved a separable Helmholtz equation.
Molecular dynamics study of screening at ionic surfaces
Authors: Desai T. G.
Published: October 2007

Molecular dynamics simulations of NaCl fluid are used to understand the behavior of ionic fluid to screen the field generated by charges on the ionic crystal surfaces in absence of any external electric field. The NaCl fluid in the strongly coupled regime (corresponding to the melt) in contact with the charged octopolar (111) NaCl surface shows that the spatial correlations decay in an oscillatory manner, with a screening length λQ given by the envelope of the damped oscillations. By contrast to the Debye-Huckel theory, in the strongly coupled regime, λQ increases with increasing coupling strength (also seen in bulk ionic simulations). The NaCl fluid confined between neutral (100) NaCl surfaces also shows weak oscillatory charge decay near the surface. Similar oscillatory exponential decay was seen when the NaCl fluid was confined between two analytically smooth neutral walls. The origin of these oscillations was due to the difference in ion sizes. NaCl fluid confined between neutral octopolar (110) and dipolar (110) surface show stronger density oscillations than (100) surface but comparatively very weak charge oscillations. This paper shows that the strength of the charges on the crystal surfaces is enough to induce a characteristic spatial distribution of charges in the contacting fluid and the extent of distribution depends on the type of surface.
Influence of InAs, AlAs δ layers on the optical, electronic, and thermal characteristics of strain-compensated GaInAs∕AlInAs quantum-cascade lasers
Authors: Vitiello Miriam S. , Gresch Tobias , Lops Antonia , Spagnolo Vincenzo , Scamarcio Gaetano , Hoyler Nicolas , Giovannini Marcella , Faist Jérôme
Published: October 2007

We extracted the electronic temperatures, the thermal resistance (RL=11.5K∕W), the cross-plane thermal conductivity [k⊥=2.0±0.1W∕(Km)], and the thermal boundary resistance [TBR=(5–11)×10−10K∕Wm2] in strain-compensated Ga0.609In0.391As∕AlIn0.546As0.454 quantum-cascade lasers operating at 4.78μm in continuous wave up to 15°C and in pulsed mode up to 40°C. Submonolayer thick InAs and AlAs δ layers are included in the active region to increase the conduction band discontinuity. We found that potential interface broadening caused by the insertion of these δ layers allows for a 60% improvement of the thermal conductivity with respect to conventional lattice-matched GaInAs∕AlInAs heterostructures.
Atomic structure in Zr70Ni30 metallic glass
Authors: Yang L. , Yin S. , Wang X. D. , Cao Q. P. , Jiang J. Z. , Saksl K. , Franz H.
Published: October 2007

Atomic structure of Zr70Ni30 metallic glass (MG) was investigated by reverse Monte Carlo simulation combining with x-ray diffraction and Ni and Zr K-edge extended x-ray absorption of fine structure measurements. Distributions of coordination number (CN) and Voronoi clusters were analyzed by Voronoi tessellation method. The average CN of atoms was obtained to be 11.4 together with the average CN of Zr and Ni atoms of about 11.8 and 10.6, respectively. It is found that Z11 Kasper polyhedron and distorted icosahedra are mainly favored structural units in Zr70Ni30 MG. The discrepancy in atomic structure between Zr70Ni30 MG and its corresponding crystalline (or quasicrystalline) phases can explain the fact that Zr70Ni30 MG does not transform to neither icosahedral nor fcc Zr2Ni phase during crystallization process.
Influence of grain size on the tensile response of aluminum under plate-impact loading
Authors: Trivedi P. B. , Asay J. R. , Gupta Y. M. , Field D. P.
Published: October 2007

Plate-impact experiments were performed to examine the influence of grain size on the dynamic tensile (or spall) behavior of shocked polycrystalline aluminum. Ultrapure and commercially pure 1050 aluminum plates were cold rolled to 80% strain and heat treated under predetermined conditions to produce recrystallized samples with average grain sizes varying between 49 and 453μm. Well-characterized samples were subjected to plane wave loading at peak compressive stresses of 4 and 21GPa, and free-surface velocity profiles were obtained using velocity interferometry. At 4GPa, the observed pullback velocity, a characteristic feature of the spall response, was similar for different grain sizes of 1050 and ultrapure Al, suggesting that the preferential failure mode is intragranular. At 21GPa, the spall response (i.e., the pullback velocity and the signal structure) depended on the alloy content; the pullback velocity of ultrapure Al increased with increase in grain size, while it remained constant for 1050 Al. In addition, the structure of pullback signals showed a well-defined change in slope for different grain size samples in ultrapure Al, while no such feature was observed for 1050 Al. For the grain sizes examined, the σHEL was nearly independent of the grain size for 1050 Al and beyond a certain grain size for ultrapure Al.
Wave propagation in imperfectly bonded single walled carbon nanotube-polymer composites
Authors: Mitra Mira , Gopalakrishnan S.
Published: October 2007

In this paper, wave propagation in single walled carbon nanotube (SWNT) embedded polymer composites is studied, taking into account imperfect bonding between the SWNT and the matrix. The imperfect bonding is in terms of SWNT pull-out from the matrix and also partial interfacial shear stress transfer. The nanocomposite is considered as a continuum axisymmetric cylinder and modeled using the wavelet based spectral finite element technique which is especially suited for wave propagation analysis in finite dimension structures. This numerical scheme involves the Daubechies scaling function approximation in time and one spatial (axial) dimension. The effects of the SWNT pull-out and the partial interfacial shear stress transfer are incorporated in the model as constraints which are imposed using the penalty matrix method. Numerical experiments are performed to study the axial wave propagation in SWNT-polymer composites due to a broadband impulse load with frequency content in the terahertz range. The effects of the pull-out and the partial interfacial shear stress transfer on the wave characteristics are studied primarily for the axial wave propagation. These effects are also observed for the radial wave propagation due to axial-radial coupling arising from the finiteness of the nanocomposite structure.
Cross-sectional scanning tunneling microscopy study on II–VI multilayer structures
Authors: Wierts A. , Ulloa J. M. , Çelebi C. , Koenraad P. M. , Boukari H. , Maingault L. , André R. , Mariette H.
Published: October 2007

Cross-sectional scanning tunneling microscopy is used to study in the atomic scale the structural properties of ZnSeTe∕ZnTe multiple quantum wells and N:ZnTe delta-doped structures. Some peculiar effects are found on the cleaved (110) ZnTe surface plane, which have not been observed in III–V semiconductors. In particular, cleavage induced monatomic wide vacancy chains are always present on the Te sublattice. Furthermore, the semiconductor surface is manipulated when certain positive voltages are applied to the sample. Regarding the heterostructures, the ZnSeTe∕ZnTe quantum wells are found to have abrupt interfaces and the Se concentration is determined to be significantly larger than the nominal value.
Correlation between structural deformation and magnetoelectric response in (1−x) Pb(Zr0.52Ti0.48)O3–xNiFe1.9Mn0.1O4 particulate composites
Authors: Islam Rashed Adnan , Jiang Jiechao , Bai Feiming , Viehland Dwight , Priya Shashank
Published: October 2007

The ferroelectric, ferromagnetic, and magnetoelectric properties of (1−x) Pb(Zr0.52Ti0.48)O3–xNiFe1.9Mn0.1O4 (PZT-NFM) ceramic composites were found to be dependent upon postsinter annealing and aging. It was found on annealing and aging that (i) the size and density of the NFM phase is reduced, (ii) the PZT lattice constants changed from (a=3.87Å, c=4.07Å) to (a=4.07Å, c=4.09Å), (iii) the ferroelectric and ferromagnetic Curie temperatures decreased by 8 and 33°C, respectively, and (iv) the magnetoelectric coefficient increased by ∼50%.
A bistable liquid-crystal display mode based on electrically driven smectic A layer reorientation
Authors: Chen Hui-Yu , Shao Renfan , Korblova Eva , Lee Wei , Walba David , Clark Noel A.
Published: October 2007

We describe a device geometry using interdigital electrodes that enables the electrically driven reorientation of smectic A layers from a homeotropic state (dark between crossed polarizers) to a planar state (bright between crossed polarizers) and back again. The resulting electro-optic effect is strongly bistable and gives high contrast ratio and fast response, rendering it a potential candidate for flexible-display applications.
Boron carbide spherical particles encapsulated in graphite prepared by pulsed laser irradiation of boron in liquid medium
Authors: Ishikawa Yoshie , Shimizu Yoshiki , Sasaki Takeshi , Koshizaki Naoto
Published: October 2007

B 4 C submicron particles were obtained by laser irradiation of B particles in ethyl acetate under atmospheric pressure and room temperature. Absorbed laser energy brought about B melting and decomposition of surrounding liquid medium molecules, leading to a reaction between the melted B and carbon species to form B4C. Moreover, the obtained B4C particles were encapsulated in a graphite layer. Such a graphite surface layer is useful for medical functionalization of particles. Thus, obtained B4C particles encapsulated in graphite are expected to be promising agents for boron neutron capture therapy.
High Tc lead-free BaTiO3–(Bi1∕2Na1∕2)TiO3 positive temperature coefficient of resistivity ceramics with electrically heterogeneous structure
Authors: Xiang Ping-Hua , Takeda Hiroaki , Shiosaki Tadashi
Published: October 2007

A distinct positive temperature coefficient of resistivity effect has been observed in BaTiO3–(Bi1∕2Na1∕2)TiO3 (BT-BNT) ceramics sintered in a N2 flow with low O2 concentration. With the addition of BNT, the samples exhibit resistivity jumps of ∼103–105 starting at ∼190–210°C. X-ray diffraction results indicate that the BNT phase and BT phase formed a solid solution during sintering. An electrically heterogeneous structure, consisting of the grain interiors, outer grain shells, and grain boundaries, is revealed by the complex impedance analyses. The observed dc resistivity jump is attributed to the rapid resistivity rise in both grain boundaries and grain shells.
Nonlinear behavior of the energy gap in Ge1−xSnx alloys at 4K
Authors: Pérez Ladrón de Guevara H. , Rodríguez A. G. , Navarro-Contreras H. , Vidal M. A.
Published: October 2007

The optical energy gap of Ge1−xSnx alloys (x⩽0.14) grown on Ge substrates has been determined by performing transmittance measurements at 4K using a fast fourier transform infrared interferometer. The direct energy gap transitions in Ge1−xSnx alloys behave following a nonlinear dependence on the Sn concentration, expressed by a quadratic equation, with a so called bowing parameter b0 that describes the deviation from a simple linear dependence. Our observations resulted in b0RT=2.30±0.10eV and b04K=2.84±0.15eV, at room temperature and 4K, respectively. The validity of our fit is limited for Sn concentrations lower than 15%.
Product branching between reactive and nonreactive pathways in the collisional quenching of OH AΣ+2 radicals by H2
Authors: Dempsey Logan P. , Murray Craig , Lester Marsha I.
Published: October 2007

The collisional quenching of OH radicals in their excited AΣ+2 electronic state by molecular hydrogen is examined to determine the partitioning between reactive and nonreactive pathways. This is achieved using a pump-probe laser technique to compare the population prepared in the excited OH AΣ+2 state with that produced in the OH XΠ2 ground state from nonreactive quenching. Only a small fraction of the products, less than 15%, arise from nonreactive quenching; reactive quenching is the dominant product channel. The branching between the product channels provides a new dynamical signature of the conical intersection region(s) that couple the excited state potential for OH AΣ+2+H2 with OH XΠ2+H2 and H2O+H products.
Terahertz plasmon photoresponse in a density modulated two-dimensional electron channel of a GaAs∕AlGaAs field-effect transistor
Authors: Aizin G. R. , Fateev D. V. , Tsymbalov G. M. , Popov V. V.
Published: October 2007

We present a theory of dc photoresponse, change in device conductance, at the plasmon resonance in the density modulated two-dimensional electron channel of the grating-gated GaAs∕AlGaAs field-effect transistor irradiated by an electromagnetic wave of terahertz frequency. An equilibrium density modulation is shown to give rise to a specific mechanism of photoresponse due to a plasma electrostriction effect. In strongly modulated systems, this effect dominates the photoresponse and results in a strong increase of the resonant peak amplitudes and photoresponse sign reversal dependent on the modulation depth and the equilibrium density profile. These results are in good qualitative agreement with recent experiments.
Decoherence of exciton complexes in single InAlAs quantum dots measured by Fourier spectroscopy
Authors: Adachi S. , Yatsu N. , Kaji R. , Muto S. , Sasakura H.
Published: October 2007

We report the single-photon Fourier spectroscopy of exciton and exciton complexes in single self-assembled InAlAs∕AlGaAs quantum dots. A Michelson interferometer was inserted in the photoluminescence path for measuring the coherence length of the time-averaged emission of neutral excitons and their complexes. The effects of exchange interactions and fluctuations in the surrounding excess charges were estimated by comparing the coherence times and the excitation energies of the excitons and their complexes.
Wavelength selective quantum dot infrared photodetector with periodic metal hole arrays
Authors: Chang Chi-Yang , Chang Hsu-Yu , Chen Chia-Yi , Tsai Ming-Wei , Chang Yi-Tsung , Lee Si-Chen , Tang Shiang-Feng
Published: October 2007

In this letter, the effect of extraordinary transmission of periodic metal hole arrays is directly integrated into the quantum dot infrared photodetector with broadband response. It is found that the detector response is strongly modulated by the extraordinary transmission from the excitation of surface plasmon.
Charge storage in SnO2 nanoparticles: A method and mechanism for charge writing/erasing
Authors: Penny M. W. , Brown M. R. , Maffeïs T. G. G. , Rees P. , Wilks S. P. , Ferkel H. S.
Published: October 2007

In this work, we present and explain evidence of a charge confinement phenomena exhibited by SnO2 nanoparticles. By applying a voltage pulse from the scanning tunneling microscope (STM) tip or momentarily increasing the gap voltage, charged features can be written on the surface. The voltage dependence of the apparent height of the features, and current imaging tunneling spectroscopy experiments, supports the charge confinement theory put forward. This is substantiated by a numerical model illustrating how charge is confined in individual nanoparticles, and is used to explain the observations of STM experiments.
Anisotropic ambipolar carrier transport and high bipolar mobilities up to 0.1cm2V−1s−1 in aligned liquid-crystal glass films of oligofluorene
Authors: Chen Li-Yin , Ke Tung-Huei , Wu Chung-Chih , Chao Teng-Chih , Wong Ken-Tsung , Chang Ching-Chao
Published: October 2007

By taking advantage of the rich morphological characteristics of oligofluorenes and the mesophase-mediated molecular assembly/alignment, we studied carrier-transport properties of solid-state oligofluorenes in different morphologies and orientations. Compared to the amorphous phase, the uniaxially aligned liquid-crystal glass of oligofluorenes could give up to 100-fold enhancement in bipolar carrier-transport mobilities, bringing both hole and electron mobilities to nearly 0.1cm2∕Vs. Furthermore, significant anisotropic bipolar transport, i.e., preferential bipolar carrier transport along the oligomer chain direction is observed.
Thin film crystal growth template removal: Application to stress reduction in lead zirconate titanate microstructures
Authors: Gkotsis P. , Kirby P. B. , Saharil F. , Oberhammer J. , Stemme G.
Published: October 2007

A key issue for the design and reliability of microdevices is process related; residual stresses in the thin films from which they are composed, especially for sol-gel deposited Pb(Zrx,Ti1−x)O3 ceramics, where use of Pt as a template layer, though essential for the nucleation of the perovskite phase, results in structures with high levels of stress largely fixed by the thermal expansion coefficient mismatch between Pt and Si. Here a technique for the elimination of this stress is presented, involving the use of adhesive wafer bonding and bulk micromachining procedures to remove the Pt layer following the Pb(Zrx,Ti1−x)O3 deposition.
An economic assessment of forage options to improve the profitability of smallholder beef cattle enterprises in the Red Soils region of China
Authors: MacLeod N. D. , Wen S. , Hu M.
Published: October 2007

Beef cattle numbers in the People’s Republic of China have expanded dramatically in the past two decades. Most of this enlarged herd is carried as small numbers of animals on smallholder farms whose production focus has traditionally been on cropping, with very limited emphasis on the planting and use of forages and nutritious feed supplements. Limited plantings of specialised forages combined with a poor knowledge of appropriate feeding and husbandry practices for beef cattle by smallholder farmers remains a serious challenge to establishing genuinely sustainable beef enterprises for this sector. Public policy to further raise the level of beef cattle production on smallholder farms is focussing on encouraging the planting and use of new forage species and also on promoting research and extension for improved feeding practices that are centred on these forage species.
The paper presents some results from the economic component of an Australian Centre for International Agricultural Research (ACIAR) project that is exploring options for integrating the planting and feeding of improved forages within existing smallholder farming systems in the Red Soils region of south-central China. An economic model is used to examine these options using a synthetic case example of a smallholder farm enterprise located in Hunan Province. The impact of changing values for several critical profit drivers are reviewed along with the implications of these findings for the management of cattle feeding systems on smallholder farms in the Red Soils region. The model results identify the potential for cattle rearing activities based on producing and feeding improved forages to increase the economic welfare of smallholder households. Where cattle rearing is based on diets of poor quality feedstuffs, economic returns are negative, although positive net cash incomes may support continued commitment of resources to this activity in the short-term.
Germ cells are essential for sexual dimorphism in the medaka gonad
Authors: Kurokawa Hiromi , Saito Daisuke , Nakamura Shuhei , Katoh-Fukui Yuko , Ohta Kohei , Baba Takashi , Morohashi Ken-ichiro , Tanaka Minoru
Published: October 2007

To further elucidate the roles of germ cells in the sex differentiation of gonads, we have used the medaka, a teleost fish, to generate mutants that lack germ cells from the onset of gonadogenesis by the morpholino-mediated knockdown of
cxcr4
. The resulting germ-cell-deficient medaka show female-to-male sex reversal of their secondary sex characteristics, accompanied by increased levels of androgen and reduced levels of estrogen. A failure to maintain granulosa cells or estrogen-producing cells also occurs at early stages of sex differentiation in the
cxcr4
morphants, before the initiation of gonadal morphogenesis. In contrast, androgen-producing cells are unaffected in germ-cell-deficient medaka of either sex. In addition, a single tube-like gonad that expresses male-specific genes is formed in these mutants irrespective of the genetic sex. Significantly, each of these mutant phenotypes occurs in a somatic cell-autonomous manner, suggesting that gonadal somatic cells are predisposed toward male development in the absence of germ cells. This highlights the importance of germ cells in the sexual dimorphism of the gonads.
Terminally differentiated muscle cells are defective in base excision DNA repair and hypersensitive to oxygen injury
Authors: Narciso Laura , Fortini Paola , Pajalunga Deborah , Franchitto Annapaola , Liu Pingfang , Degan Paolo , Frechet Mathilde , Demple Bruce , Crescenzi Marco , Dogliotti Eugenia
Published: October 2007

The differentiation of skeletal myoblasts is characterized by permanent withdrawal from the cell cycle and fusion into multinucleated myotubes. Muscle cell survival is critically dependent on the ability of cells to respond to oxidative stress. Base excision repair (BER) is the main repair mechanism of oxidative DNA damage. In this study, we compared the levels of endogenous oxidative DNA damage and BER capacity of mouse proliferating myoblasts and their differentiated counterpart, the myotubes. Changes in the expression of oxidative stress marker genes during differentiation, together with an increase in 8-hydroxyguanine DNA levels in terminally differentiated cells, suggested that reactive oxygen species are produced during this process. The repair of 2-deoxyribonolactone, which is exclusively processed by long-patch BER, was impaired in cell extracts from myotubes. The repair of a natural abasic site (a preferred substrate for short-patch BER) also was delayed. The defect in BER of terminally differentiated muscle cells was ascribed to the nearly complete lack of DNA ligase I and to the strong down-regulation of XRCC1 with subsequent destabilization of DNA ligase IIIα. The attenuation of BER in myotubes was associated with significant accumulation of DNA damage as detected by increased DNA single-strand breaks and phosphorylated H2AX nuclear foci upon exposure to hydrogen peroxide. We propose that in skeletal muscle exacerbated by free radical injury, the accumulation of DNA repair intermediates, due to attenuated BER, might contribute to myofiber degeneration as seen in sarcopenia and many muscle disorders.
HOXB4's road map to stem cell expansion
Authors: Schiedlmeier Bernhard , Santos Ana Cristina , Ribeiro Ana , Moncaut Natalia , Lesinski Dietrich , Auer Herbert , Kornacker Karl , Ostertag Wolfram , Baum Christopher , Mallo Moises , Klump Hannes
Published: October 2007

Homeodomain-containing transcription factors are important regulators of stem cell behavior. HOXB4 mediates expansion of adult and embryo-derived hematopoietic stem cells (HSCs) when expressed ectopically. To define the underlying molecular mechanisms, we performed gene expression profiling in combination with subsequent functional analysis with enriched adult HSCs and embryonic derivatives expressing inducible HOXB4. Thereby, we identified a set of overlapping genes that likely represent “universal” targets of HOXB4. A substantial number of loci are involved in signaling pathways important for controlling self-renewal, maintenance, and differentiation of stem cells. Functional assays performed on selected pathways confirmed the biological coherence of the array results. HOXB4 activity protected adult HSCs from the detrimental effects mediated by the proinflammatory cytokine TNF-α. This protection likely contributes to the competitive repopulation advantage of HOXB4-expressing HSCs observed
in vivo
. The concept of TNF-α inhibition may also prove beneficial for patients undergoing bone marrow transplantation. Furthermore, we demonstrate that HOXB4 activity and FGF signaling are intertwined. HOXB4-mediated expansion of adult and ES cell-derived HSCs was enhanced by specific and complete inhibition of FGF receptors. In contrast, the expanding activity of HOXB4 on hematopoietic progenitors in day 4–6 embryoid bodies was blunted in the presence of basic FGF (FGF2), indicating a dominant negative effect of FGF signaling on the earliest hematopoietic cells. In summary, our results strongly suggest that HOXB4 modulates the response of HSCs to multiple extrinsic signals in a concerted manner, thereby shifting the balance toward stem cell self-renewal.
Enhanced phytoremediation of volatile environmental pollutants with transgenic trees
Authors: Doty Sharon L. , James C. Andrew , Moore Allison L. , Vajzovic Azra , Singleton Glenda L. , Ma Caiping , Khan Zareen , Xin Gang , Kang Jun Won , Park Jin Young , Meilan Richard , Strauss Steven H. , Wilkerson Jasmine , Farin Federico , Strand Stuart E.
Published: October 2007

Small, volatile hydrocarbons, including trichloroethylene, vinyl chloride, carbon tetrachloride, benzene, and chloroform, are common environmental pollutants that pose serious health effects. We have developed transgenic poplar (
Populus tremula
×
Populus alba
) plants with greatly increased rates of metabolism and removal of these pollutants through the overexpression of cytochrome P450 2E1, a key enzyme in the metabolism of a variety of halogenated compounds. The transgenic poplar plants exhibited increased removal rates of these pollutants from hydroponic solution. When the plants were exposed to gaseous trichloroethylene, chloroform, and benzene, they also demonstrated superior removal of the pollutants from the air. In view of their large size and extensive root systems, these transgenic poplars may provide the means to effectively remediate sites contaminated with a variety of pollutants at much faster rates and at lower costs than can be achieved with current conventional techniques.
Dissociating the neural mechanisms of memory-based guidance of visual selection
Authors: Soto David , Humphreys Glyn W. , Rotshtein Pia
Published: October 2007

Visual selection is influenced by items in working memory (WM) and priming from implicit memory when a stimulus is repeated across time. WM effects are typically held to be top-down in nature [Soto D, Heinke D, Humphreys GW, Blanco MJ (2005)
J Exp Psychol Hum Percept Perform
31:248–261], whereas implicit priming may operate in a bottom-up manner [Theeuwes J, Reimann B, Mortier K (2006)
Vis Cogn
14: 466–489]. How WM and implicit priming affects influence visual selection remains poorly understood, however. Here, we report functional MRI evidence that dissociates the neural mechanisms involved in these memory-based effects on selection. The reappearance of a stimulus held in WM enhanced activity in superior frontal gyrus, midtemporal, and occipital areas that are known to encode the prior occurrence of stimuli. In contrast, mere stimulus repetition elicited a suppressive response in the same regions. An additional finding was that a frontothalamic network was sensitive to the behavioral relevance of a match between the contents of WM and the visual search array, enhancing activity when the contents of WM matched the critical target of selection. Items held in WM influence selection by using neural coding distinct to effects of mere repetition.
Modeling river delta formation
Authors: Seybold Hansjörg , Andrade José S. , Herrmann Hans J.
Published: October 2007

A model to simulate the time evolution of river delta formation process is presented. It is based on the continuity equation for water and sediment flow and a phenomenological sedimentation/erosion law. Different delta types are reproduced by using different parameters and erosion rules. The structures of the calculated patterns are analyzed in space and time and compared with real data patterns. Furthermore, our model is capable of simulating the rich dynamics related to the switching of the mouth of the river delta. The simulation results are then compared with geological records for the Mississippi River.
The other side of the nearly neutral theory, evidence of slightly advantageous back-mutations
Authors: Charlesworth Jane , Eyre-Walker Adam
Published: October 2007

We argue that if there is a category of slightly deleterious mutations, then there should be a category of slightly advantageous back-mutations. We show that when there are both slightly deleterious and advantageous back-mutations, there is likely to be an increase in the rate of evolution after a population size expansion. This increase in the rate of evolution is short-lived. However, we show how its signature can be captured by comparing the rate of evolution in species that have undergone population size expansion versus contraction. We test our model by comparing the pattern of evolution in pairs of island and mainland species in which the colonization event was either island-to-mainland (population size expansion) or mainland-to-island (contraction). We show that the predicted pattern of evolution is observed.
Mitochondrial dysfunction due to long-chain Acyl-CoA dehydrogenase deficiency causes hepatic steatosis and hepatic insulin resistance
Authors: Zhang Dongyan , Liu Zhen-Xiang , Choi Cheol Soo , Tian Liqun , Kibbey Richard , Dong Jianying , Cline Gary W. , Wood Philip A. , Shulman Gerald I.
Published: October 2007

Alterations in mitochondrial function have been implicated in the pathogenesis of insulin resistance and type 2 diabetes. However, it is unclear whether the reduced mitochondrial function is a primary or acquired defect in this process. To determine whether primary defects in mitochondrial β-oxidation can cause insulin resistance, we studied mice with a deficiency of long-chain acyl-CoA dehydrogenase (LCAD), a key enzyme in mitochondrial fatty acid oxidation. Here, we show that LCAD knockout mice develop hepatic steatosis, which is associated with hepatic insulin resistance, as reflected by reduced insulin suppression of hepatic glucose production during a hyperinsulinemic-euglycemic clamp. The defects in insulin action were associated with an ≈40% reduction in insulin-stimulated insulin receptor substrate-2-associated phosphatidylinositol 3-kinase activity and an ≈50% decrease in Akt2 activation. These changes were associated with increased PKCε activity and an aberrant 4-fold increase in diacylglycerol content after insulin stimulation. The increase in diacylglycerol concentration was found to be caused by
de novo
synthesis of diacylglycerol from medium-chain acyl-CoA after insulin stimulation. These data demonstrate that primary defects in mitochondrial fatty acid oxidation capacity can lead to diacylglycerol accumulation, PKCε activation, and hepatic insulin resistance.