bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetic and microwave properties of nanocomposite films on the basis of Fe–Co–Ni particles of various shapes
Authors: Meshcheryakov V. F. , Fetisov Y. K. , Stashkevich A. A. , Viau G.
Published: September 2008

The problem of the physical mechanisms of collective magnetic behavior, both static and dynamic, of an assembly of ferromagnetic particles comprising a nanocomposite material has been addressed. Detailed measurements of magnetization curves and microwave absorption spectra of films prepared from chemically synthesized (polyol technique) Fe–Ni–Co particles of various shapes and sizes have been performed. It has been shown that the dipole-dipole interparticle interactions play an important role in the magnetic behavior of such systems. An original method, relying on the shift of the magnetization curves in parallel and perpendicular geometry due to the planar anisotropy, allowing estimation of the concentration of magnetic particles has been proposed. The measurements of microwave absorption spectra at 9.5 GHz have revealed the features typical of local resonances localized on individual nanoparticles (Kittel mode and the lowest SWR) as well as those that can be attributed to collective behavior due to dipole-dipole interactions. It has been demonstrated that magnetization reversal is accompanied by a microwave hysteresis.
Electron transport in silicon nanowires: The role of acoustic phonon confinement and surface roughness scattering
Authors: Ramayya E. B. , Vasileska D. , Goodnick S. M. , Knezevic I.
Published: September 2008

We investigate the effects of electron and acoustic phonon confinements on the low-field electron mobility of thin, gated, square silicon nanowires (SiNWs), surrounded by SiO2. We employ a self-consistent Poisson–Schrödinger–Monte Carlo solver that accounts for scattering due to acoustic phonons (confined and bulk), intervalley phonons, and the Si/SiO2 surface roughness. The wires considered have cross sections between 3×3 and 8×8 nm2. For larger wires, the dependence of the mobility on the transverse field from the gate is pronounced, as expected. At low transverse fields, where phonon scattering dominates, scattering from confined acoustic phonons results in about a 10% decrease in the mobility with respect to the bulk phonon approximation. As the wire cross section decreases, the electron mobility drops because the detrimental increase in both electron-acoustic phonon and electron-surface roughness scattering rates overshadows the beneficial volume inversion and subband modulation. For wires thinner than 5×5 nm2, surface roughness scattering dominates regardless of the transverse field applied and leads to a monotonic decrease in the electron mobility with decreasing SiNW cross section.
Substituting a copper atom modifies the melting of aluminum clusters
Authors: Cao Baopeng , Starace Anne K. , Neal Colleen M. , Jarrold Martin F. , Núñez Sara , López José M. , Aguado Andrés
Published: September 2008

Heat capacities have been measured for Aln−1Cu− clusters (n=49–62) and compared with results for pure Aln+ clusters. Aln−1Cu− and Aln+ have the same number of atoms and the same number of valence electrons (excluding the copper d electrons). Both clusters show peaks in their heat capacities that can be attributed to melting transitions; however, substitution of an aluminum atom by a copper atom causes significant changes in the melting behavior. The sharp drop in the melting temperature that occurs between n=55 and 56 for pure aluminum clusters does not occur for the Aln−1Cu− analogs. First-principles density-functional theory has been used to locate the global minimum energy structures of the doped clusters. The results show that the copper atom substitutes for an interior aluminum atom, preferably one with a local face-centered-cubic environment. Substitution does not substantially change the electronic or geometric structures of the host cluster unless there are several Aln+ isomers close to the ground state. The main structural effect is a contraction of the bond lengths around the copper impurity, which induces both a contraction of the whole cluster and a stress redistribution between the Al–Al bonds. The size dependence of the substitution energy is correlated with the change in the latent heat of melting on substitution.
Effect of the nonlocal exchange on the performance of the orbital-dependent correlation functionals from second-order perturbation theory
Authors: Schweigert Igor V. , Bartlett Rodney J.
Published: September 2008

Adding a fraction of the nonlocal exchange operator to the local orbital-dependent exchange potential improves the many-body perturbation expansion based on the Kohn–Sham determinant. The effect of such a hybrid scheme on the performance of the orbital-dependent correlation functional from the second-order perturbation theory (PT2H) is investigated numerically. A small fraction of the nonlocal exchange is often sufficient to ensure the existence of the self-consistent solution for the PT2H potential. In the He and Be atoms, including 37% of the nonlocal exchange leads to the correlation energies and electronic densities that are very close to the exact ones. In molecules, varying the fraction of the nonlocal exchange may result in the PT2H energy closely reproducing the CCSD(T) value; however such a fraction depends on the system and does not always result in an accurate electronic density. We also numerically verify that the “semicanonical” perturbation series includes most of the beneficial effects of the nonlocal exchange without sacrificing the locality of the exchange potential.
Experiments and numerical simulations on high-density magnetohydrodynamic electrical power generation
Authors: Murakami Tomoyuki , Okuno Yoshihiro
Published: September 2008

We describe experiments and numerical simulations on high-density energy conversion using a compact closed-cycle magnetohydrodynamic electrical power generator, where shock-tube-based experiments and quasi-three-dimensional numerical simulations are fully coupled. The temporal plasma-fluid behavior, the one- and two-dimensional plasma-fluid structures, the enthalpy-entropy diagram, the Hall voltage-Hall current characteristics, and the quality of the energy conversion efficiency are investigated. The slightly divergent channel configuration, the application of high- and uniform-density magnetic flux to the entire generator, the high electrical conductivity, the symmetric plasma structure and stable plasma behavior, and the sufficient pressure gradient used to drive the fluid overcome the disadvantages of the generator due to its compactness, and markedly improve its energy conversion performance, namely, a power density of 0.76GW∕m3, an isentropic efficiency of 51%, and an enthalpy extraction ratio of 17.0%.
Dielectric relaxation and electronic structure of double perovskite Sr2FeSbO6
Authors: Dutta Alo , Sinha T. P. , Shannigrahi Santiranjan
Published: September 2008

The dielectric property and the electronic structure of a double perovskite, Sr2FeSbO6 (SFS) synthesized by solid state reaction technique are investigated. The x-ray diffraction of the sample taken at room temperature shows cubic phase. The scanning electron micrograph of the sample also confirms the formation of the single phase of the material. We have measured the capacitance and conductance of SFS in a frequency range from 50 Hz to 1 MHz and in a temperature range from 163 to 463 K. A relaxation is observed in the entire temperature range as a gradual decrease in ϵ′(ω) and as a broad peak in ϵ″(ω). The frequency dependent electrical data are analyzed in the framework of conductivity and electric modulus formalisms. The frequencies corresponding to the maxima of the imaginary electric modulus at various temperatures are found to obey an Arrhenius law with an activation energy of 0.74 eV. The Cole–Cole model is used to study the dielectric relaxation of SFS. The scaling behavior of imaginary part of electric modulus suggests that the relaxation describes the same mechanism at various temperatures. The frequency dependent conductivity spectra follow the universal power law. The electronic structure of the SFS is studied by x-ray photoemission spectroscopy (XPS). Its valence band consists mainly of the oxygen 2p-states hybridized with the Fe 3d-states. The XPS spectra are investigated by the first principles full potential linearized augmented plane wave method. The angular momentum projected total and partial density of states obtained from first principles calculation are used to analyze the XPS results of the sample. The calculated electronic structures of SFS are qualitatively similar to those of the XPS spectra in terms of spectral features, energy positions, and relative intensities. The electronic structure calculation reveals that the electrical properties of SFS are dominated by the interaction between transition-metal and oxygen ions.
Unified and stable scattering matrix formalism for acoustic waves in piezoelectric stacks
Authors: Zhang Victor Y. , Laude Vincent
Published: September 2008

This paper presents a unified and full scattering matrix (s-matrix) formalism for modeling of acoustic waves in piezoelectric multilayered structures. A stable recursive algorithm is derived for computation of the total s-matrix of a stack in terms of the interface s-matrix, both referring to the eigenmode amplitudes. The derivation is direct and succinct, the deduced expressions of the s-matrix are terse and concise, and the recursion algorithm is efficient and convenient for implementation. The total s-matrix recursion scheme differs from the previously published partial matrix algorithms in that the recursions are conducted once for all independent of the stack boundary conditions and so the same results apply for any post-specified boundary condition. Numerical examples are given to show its numerical features that are superior to other currently used matrix formalisms, such as unconditional stability for both large and small thicknesses, being pole-free and branch point-sensitive, constant mean magnitude with stable phase, involving only dimensionless elements.
One-dimensional tunneling calculations in the imaginary-frequency, rectilinear saddle-point normal mode
Authors: Wang Yimin , Bowman Joel M.
Published: September 2008

We present tunneling calculations using the reaction path Hamiltonian in the zero-curvature approximation and a one-dimensional Hamiltonian in the imaginary-frequency, rectilinear normal mode of a saddle point, neglecting the vibrational angular momentum terms. This latter Hamiltonian was recently introduced and applied to the tunneling splitting in full-dimensional malonaldeyde [Y. Wang et al., J. Chem. Phys. 128, 224314 (2008)]. The results using the latter method are shown to be much more accurate than those using the former one for the ground-state tunneling splittings for H and D-transfer in malonaldehyde and for the D+H2 reaction in three dimensions for zero total angular momentum.
Carrier recombination effects in strain compensated quantum dot stacks embedded in solar cells
Authors: Alonso-Álvarez D. , Taboada A. G. , Ripalda J. M. , Alén B. , González Y. , González L. , García J. M. , Briones F. , Martí A. , Luque A. , Sánchez A. M. , Molina S. I.
Published: September 2008

In this work we report the stacking of 50 InAs/GaAs quantum dot layers with a GaAs spacer thickness of 18 nm using GaP monolayers for strain compensation. We find a good structural and optical quality of the fabricated samples including a planar growth front across the whole structure, a reduction in the quantum dot size inhomogeneity, and an enhanced thermal stability of the emission. The optimized quantum dot stack has been embedded in a solar cell structure and we discuss the benefits and disadvantages of this approach for high efficiency photovoltaic applications.
Mixed time-dependent density-functional theory/classical trajectory surface hopping study of oxirane photochemistry
Authors: Tapavicza Enrico , Tavernelli Ivano , Rothlisberger Ursula , Filippi Claudia , Casida Mark E.
Published: September 2008

We present a mixed time-dependent density-functional theory (TDDFT)/classical trajectory surface hopping (SH) study of the photochemical ring opening in oxirane. Previous preparatory work limited to the symmetric CC ring-opening pathways of oxirane concluded that the Tamm-Dancoff approximation (TDA) is important for improving the performance of TDDFT away from the equilibrium geometry. This observation is supported by the present TDDFT TDA/SH calculations which successfully confirm the main experimentally derived Gomer-Noyes mechanism for the photochemical CO ring opening of oxirane and, in addition, provide important state-specific information not easily accessible from experiments. In particular, we find that, while one of the lowest two excited states is photochemically relatively inert, excitation into the other excited state leads predominantly to rapid ring opening, cyclic-C2H4O→C•H2CH2O•. This is followed by hopping to the electronic ground state where hot (4000K) dynamics leads to further reactions, namely, C•H2CH2O•→CH3CHO→C•H3+C•HO and CH4+CO. We note that, in the dynamics, we are not limited to following minimum energy pathways and several surface hops may actually be needed before products are finally reached. The performance of different functionals is then assessed by comparison of TDDFT and diffusion Monte Carlo potential energy curves along a typical TDDFT TDA/SH reaction path. Finally, although true (S0,S1) conical intersections are expected to be absent in adiabatic TDDFT, we show that the TDDFT TDA is able to approximate a conical intersection in this system.
Effect of impurity incorporation on emission wavelength in cathodoluminescence spectrum image study of GaN pyramids grown by selective area epitaxy
Authors: Song D. Y. , Chandolu A. , Stojanovic N. , Nikishin S. A. , Holtz M.
Published: September 2008

Metal-organic chemical vapor deposition has been used for selective epitaxy of GaN pyramids ranging in size from over 1μm to 500 nm in length at the base. Pyramids are terminated by {11¯01} crystal facets. The optical properties of the pyramids are investigated by cathodoluminescence (CL) in a scanning electron microscope. CL spectrum imaging reveals the pyramid apices to emit light at ∼363.4 nm corresponding to the emission wavelength of relaxed GaN. As the CL excitation is moved away from the apex a systematic redshift is observed. The redshift is ∼4 nm for pyramids with 3 μm base dimension and ∼2 nm for the 500 nm pyramids. The shift is attributed to diffusion of silicon and oxygen into the GaN pyramids due to SiO2 mask decomposition with negligible contribution from stress. The observations are backed by finite element simulations of diffusion and stress.
Thermodynamic and hydrodynamic interaction in concentrated microgel suspensions: Hard or soft sphere behavior?
Authors: Eckert Thomas , Richtering Walter
Published: September 2008

The colloidal phase behavior, structure factors, short-time collective diffusion coefficients, and hydrodynamic interactions of concentrated poly(N-isopropylacryamide) (PNiPAM) microgels in dimethylformamide suspensions were measured with simultaneous static and dynamic three-dimensional cross-correlated light scattering. The data are interpreted through comparison with hard sphere theory. The fluid-crystal transition and the static structure factors can be described consistently by the hard sphere approximation. On the other hand, collective diffusion and hydrodynamic interaction cannot be described satisfactorily by the hard sphere model. The microgel structure is different from hard spheres, as the cross-link density decreases with the distance from the particle center leading to a “fuzzy” particle surface with dangling polymer chains. These seem to affect the hydrodynamic interaction much more as compared to direct thermodynamic interaction.
A flexible luminescent probe to monitor fast ion losses at the edge of the TJ-II stellarator
Authors: Jiménez-Rey D. , Zurro B. , Guasp J. , Liniers M. , Baciero A. , García-Muñoz M. , Fernández A. , García G. , Rodríguez-Barquero L. , Fontdecaba J. M.
Published: September 2008

A mobile luminescent probe has been developed to detect fast ion losses and suprathermal ions escaping from the plasma of the TJ-II stellarator device. The priorities for its design have been flexibility for probe positioning, ease of maintenance, and detector sensitivity. It employs a coherent fiber bundle to relay, to the outside of the vacuum chamber, ionoluminescence images produced by the ions that impinge, after entering the detector head through a pinhole aperture, onto a screen of luminescent material. Ionoluminescence light detection is accomplished by a charge-coupled device camera and by a photomultiplier, both of which are optically coupled to the in-vacuum fiber bundle head by means of a standard optical setup. A detailed description of the detector, and the first results obtained when operated close to the plasma edge, are reported.
The effect of salt on the melting of ice: A molecular dynamics simulation study
Authors: Kim Jun Soo , Yethiraj Arun
Published: September 2008

The effect of added salt (NaCl) on the melting of ice is studied using molecular dynamics simulations. The equilibrium freezing point depression observed in the simulations is in good agreement with experimental data. The kinetic aspects of melting are investigated in terms of the exchange of water molecules between ice and the liquid phase. The ice/liquid equilibrium is a highly dynamic process with frequent exchange of water molecules between ice and the liquid phase. The balance is disturbed when ice melts and the melting proceeds in two stages; the inhibition of the association of water molecules to the ice surface at short times, followed by the increased dissociation of water molecules from the ice surface at longer times. We also find that Cl− ions penetrate more deeply into the interfacial region than Na+ ions during melting. This study provides an understanding of the kinetic aspects of melting that could be useful in other processes such as the inhibition of ice growth by antifreeze proteins.
Thermoluminescence in pure LiF crystals: Glow peaks and their connection with color centers
Authors: Baldacchini G. , Montereali R. M. , Nichelatti E. , Kalinov V. S. , Voitovich A. P. , Davidson A. T. , Kozakiewicz A. G.
Published: September 2008

Nominally pure LiF crystals were irradiated with the same dose (0.85 106 R) of gamma rays at ambient and low temperatures (−60 °C) and the resulting thermoluminescence (TL) is reported. Various optical and thermal treatments were applied in order to change the concentration of color centers (CCs). The effect of such treatments on the glow curves is observed. Knowing the coloration from optical transmission and photoluminescence measurements made on the same samples, we attribute many of the glow peaks (GPs) to the annealing of F center aggregates. For the present conditions of irradiation and dose, TL processes begin with decay of F3+ centers that display a GP at 164 °C. F3(R) centers follow and are responsible for GPs at 193 and 228 °C. A GP at 263 °C is ascribed to F2 centers. Several peaks at temperatures in the range of 280–380 °C are associated with impurity perturbed F centers. A GP at 410 °C is associated with a complex of aggregated F and H centers. These attributions are accomplished by means of TL spectra, optical transmission spectra, and annealing procedures, and are critically discussed. The experimental data confirm the general trend of thermal stability of CCs, which decreases by moving from simple F centers to more complex ones, and the existence of exchange dynamics among CCs.
Boundary integral simulations of liquid emptying from a model gravure cell
Authors: Hoda Nazish , Kumar Satish
Published: September 2008

We perform boundary integral simulations to understand the removal of Newtonian liquids from a model gravure cell. Two different configurations are considered. In the first configuration, there is a free surface and an outlet boundary, and the liquid is driven out of a cavity by a combination of horizontal substrate motion and an imposed pressure gradient; a similar model was used by Powell et al. [Trans. IChemeE, Part C 78, 61 (2000)]. The percentage of liquid remaining in the cavity Vr is influenced by the capillary number Ca, cavity depth D, and contact angle θ. We found that Vr decreases with a decrease in Ca or D, consistent with prior studies, and for a shallow enough cavity, almost all of the liquid can be removed. Additionally, Vr decreases with an increase in θ. In the second configuration, there are two free surfaces, and the liquid is driven out of the cavity by moving the substrate both horizontally and vertically. Our simulations suggest that Vr decreases with an increase in the extensional velocity V, and in some cases the entire cavity can be emptied when V is greater than a critical value. The present work sheds light on the roles that surface wettability, cavity size, substrate kinematics, and free-surface dynamics play in surface-tension-driven liquid emptying from tiny cavities.
The glass-liquid transition of water on hydrophobic surfaces
Authors: Souda Ryutaro
Published: September 2008

Interactions of thin water films with surfaces of graphite and vitrified room-temperature ionic liquid [1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6])] were investigated using time-of-flight secondary ion mass spectrometry as a function of temperature and annealing time to elucidate the glass-liquid transition of water at the molecular level. Surface diffusion of water occurs at temperatures higher than 120K, thereby forming three-dimensional clusters (a two-dimensional layer) on the [bmim][PF6] (graphite) surface. The hydrophobic effect of the surface decreases with increasing coverage of water; the bulklike properties evolve up to 40 ML, as evidenced by the occurrence of film dewetting at around the conventional glass transition temperature (140K). Results also showed that aging is necessary for the water monolayer (a 40 ML water film) to dewet the graphite ([bmim][PF6]) surface. The occurrence of aging is explainable by the successive evolution of two distinct liquids during the glass-liquid transition: low density liquid is followed by supercooled liquid water. The water monolayer on graphite is characterized by the preferred orientation of unpaired OH groups toward the surface; this structure is arrested during the aging time despite the occurrence of surface diffusion. However, the water monolayer formed on the [bmim][PF6] surface agglomerates immediately after the commencement of surface diffusion. The structure of low density liquid tends to be arrested by the attractive interaction with the neighbors.
Correlation between doping induced disorder and superconducting properties in carbohydrate doped MgB2
Authors: Kim J. H. , Dou S. X. , Oh Sangjun , Jerčinović M. , Babić E. , Nakane T. , Kumakura H.
Published: September 2008

A comprehensive study of the effects of carbohydrate doping on the superconductivity of MgB2 has been conducted. In accordance with the dual reaction model, more carbon substitution is achieved at lower sintering temperature. As the sintering temperature is lowered, lattice disorder is increased. Disorder is an important factor determining the transition temperature for the samples studied in this work, as evidenced from the correlations among the lattice strain, the resistivity, and the transition temperature. It is further shown that the increased critical current density in the high field region can be understood by a recently-proposed percolation model [M. Eisterer et al., Phys. Rev. Lett. 90, 247002 (2003)]. For the critical current density analysis, the upper critical field is estimated from a correlation that was reported in a recent review article [M. Eisterer, Supercond. Sci. Technol. 20, R47 (2007)], where a sharp increase in the upper critical field by doping is mainly due to an increase in lattice disorder or impurity scattering. On the other hand, it is shown that the observed reduction in self-field critical current density is related to the reduction in the pinning force density by carbohydrate doping.
Negative signals of photoinduced current transient and thermally stimulated current spectroscopy in GaAs due to negative-U properties of oxygen
Authors: Kühnel Günter
Published: September 2008

Bulk GaAs grown under high oxygen potential shows an intermediate resistivity (ρ∼3×103 Ω cm at room temperature) with an electron activation energy E=0.42 eV. The relevant defect has been identified in literature as off-centered substitutional oxygen with three different charge states. Photoelectrical measurements on such GaAs material exhibit distinctly anomalous signals. Photoinduced current transient spectroscopy (PICTS) shows a signal with a negative sign, whereas thermally stimulated currents (TSCs) are clearly lower than the dark current. By means of a set of coupled differential equations, the carrier exchange between the defect levels and the conduction band is described. Solving these rate equations, the PICTS and TSC curves are modeled. In this way, it could be demonstrated that the anomalous PICTS and TSC signals are caused by the negative-U behavior of the off-center oxygen. This is in agreement with results of infrared spectroscopy.
Influence of applied electric fields on the absorption coefficient and subband energy distances of intersubband transitions in AlN/GaN coupled double quantum wells
Authors: Cen L. B. , Shen B. , Qin Z. X. , Zhang G. Y.
Published: September 2008

The influence of applied electric fields on the absorption coefficient and subband energy distances of intersubband transitions (ISBTs) in AlN/GaN coupled double quantum wells (CDQWs) has been investigated by solving the Schrödinger and Poisson equations self-consistently. It is found that the absorption coefficient of the ISBT between the ground state and the second excited state (1odd–2odd) can be equal to zero when the electric fields are applied in AlN/GaN CDQWs, which is related to the applied electric field induced symmetry recovery of these states. Meanwhile, the energy distances between 1odd–2odd and 1even–2odd subbands have different relationships from each other with the increase of applied electric fields due to the different polarization-induced potential drops between the left and right wells. The results indicate that an electrical-optical modulator operated within the optocommunication wavelength range can be realized in spite of the strong polarization-induced electric fields in AlN/GaN CDQWs.
Single-narrow-band red upconversion fluorescence of ZnO nanocrystals codoped with Er and Yb and its achieving mechanism
Authors: Liu Yunxin , Yang Qibin , Xu Changfu
Published: September 2008

ZnO nanocrystals doped with Er3+ and Yb3+ ions were synthesized by the chemical-combustion process. The single-narrow-band red fluorescence (centered at 662 nm) in ZnO:Er, Yb was observed under 115 mW excitation of a 980 nm laser diode. The full width at half maximum of the red emission band is equal to 30 nm, which is the narrowest emission band ever reported in Er3+ doped systems. This narrower emission band in ZnO:Er, Yb nanocrystal is beneficial for improving the precision and sensitivity of the indicators in fluorescent label detection. The mechanisms resulting in the single-narrow emission band were analyzed by combining X-ray diffraction, Raman spectra, fluorescence spectra, and a simple model.
Elastic and viscoelastic effects in rubber/air acoustic band gap structures: A theoretical and experimental study
Authors: Merheb B. , Deymier P. A. , Jain M. , Aloshyna-Lesuffleur M. , Mohanty S. , Berker A. , Greger R. W.
Published: September 2008

The transmission of acoustic waves through centimeter-scale elastic and viscoelastic two-dimensional silicone rubber/air phononic crystal structures is investigated theoretically and experimentally. We introduce a finite difference time domain method for two-dimensional elastic and viscoelastic composite structures. Elastic fluid-solid phononic crystals composed of a two-dimensional array of cylindrical air inclusions in a solid rubber matrix, as well as an array of rubber cylinders in an air matrix, are shown to behave similarly to fluid-fluid composite structures. These systems exhibit very wide band gaps in their transmission spectra that extend to frequencies in the audible range of the spectrum. This effect is associated with the very low value of the transverse speed of sound in rubber compared to that of the longitudinal polarization. The difference in transmission between elastic and viscoelastic rubber/air crystals results from attenuation of transmission over a very wide frequency range, leaving only narrow passing bands at very low frequencies. These phononic crystals demonstrate the practical design of elastic or viscoelastic solid rubber/air acoustic band gap sound barriers with small dimensions.
Evolution of magnetic correlations in single crystalline La1.2Sr1.8Mn2O7
Authors: Nithya R. , Xie B. X. , Lin J. G. , Chou F. C. , Han T. C.
Published: September 2008

The electron-spin-resonance measurement for the double perovskite, La1.2Sr1.8Mn2O7 single crystal is carried out from 80 to 450 K. Magnetic phase transitions from the paramagnetic to a mixed state are identified at Tp∼335 K and from mixed to a ferromagnetic state at Tc∼117 K. Two intermediate lines with 2<g<4 are observed within a narrow temperature range (280–340 K), which could be attributed to the magnetic interactions within the double layered perovskite via the interlayer (Mn–O–Mn) and interbilayer (Mn–O–O–Mn) couplings. On the other hand, a strong ferromagnetic resonance line with g=3–6 persists at a wide range of temperature from 340 to 100 K, which linearly varies with temperature and thus attributed to the intralayer (Mn–O–Mn) interaction.
Boosting the on-current of a n-channel nanowire tunnel field-effect transistor by source material optimization
Authors: Verhulst Anne S. , Vandenberghe William G. , Maex Karen , Groeseneken Guido
Published: September 2008

Tunnel field-effect transistors (TFETs) are potential successors of metal-oxide-semiconductor FETs because of the absence of short-channel effects and of a subthreshold-slope limit. As a solution to the low on-currents of silicon-based TFETs, the incorporation of silicon-germanium at the source-channel interface has been considered. However, the understanding of the band-to-band-tunneling mechanism at heterojunctions is incomplete. We have investigated through device simulations and modeling the impact of source-material modifications on the tunnel current in n-channel nanowire TFETs. Our modeling work includes the development of a semi-analytical model, which determines the tunnel probability along the dominant tunnel path in two-dimensional TFETs. In particular, we have analyzed the impact of the bandgap, electron affinity, effective mass, dielectric constant, and density of states of both source and channel material. We show that a small-bandgap source material and a large positive electron-affinity offset at the source-channel interface boost the tunnel current of n-channel TFETs.
High-pressure x-ray diffraction study on the structure and phase transitions of the defect-stannite ZnGa2Se4 and defect-chalcopyrite CdGa2S4
Authors: Errandonea D. , Kumar Ravhi S. , Manjón F. J. , Ursaki V. V. , Tiginyanu I. M.
Published: September 2008

X-ray diffraction measurements on the sphalerite-derivatives ZnGa2Se4 and CdGa2S4 have been performed upon compression up to 23 GPa in a diamond-anvil cell. ZnGa2Se4 exhibits a defect tetragonal stannite-type structure (I4¯2m) up to 15.5 GPa and in the range from 15.5 to 18.5 GPa the low-pressure phase coexists with a high-pressure phase, which remains stable up to 23 GPa. In CdGa2S4, we find that the defect tetragonal chalcopyrite-type structure (I4¯) is stable up to 17 GPa. Beyond this pressure a pressure-induced phase transition takes place. In both materials, the high-pressure phase has been characterized as a defect-cubic NaCl-type structure (Fm3¯m). The occurrence of the pressure-induced phase transitions is apparently related with an increase in the cation disorder on the semiconductors investigated. In addition, the results allow the evaluation of the axial compressibility and the determination of the equation of state for each compound. The obtained results are compared to those previously reported for isomorphic digallium sellenides. Finally, a systematic study of the pressure-induced phase transition in 23 different sphalerite-related ABX2 and AB2X4 compounds indicates that the transition pressure increases as the ratio of the cationic radii and anionic radii of the compounds increases.
Experimental evidence of strain relaxed domain structure in (100)/(001)-oriented epitaxial lead titanate thick films grown by metal organic chemical vapor deposition
Authors: Nakaki Hiroshi , Kim Yong Kwan , Yokoyama Shintaro , Ikariyama Rikyu , Funakubo Hiroshi , Streiffer S. K. , Nishida Ken , Saito Keisuke , Gruverman Alexei
Published: September 2008

Epitaxial (100)/(001)-oriented PbTiO3 films with thickness of 2.8 μm were grown on Nb-doped (100) SrTiO3 substrates by pulsed metal organic chemical vapor deposition. Complex domain structures consisting of c-domains (c1) and three types of a-domains (a1, a2, and a3) were observed by piezoresponse force microscopy in conjunction with high-resolution x-ray diffraction analysis. The obtained results suggest that (a2/a3) and (a1/c1) domain configurations represent mostly strain-relaxed structures. This conclusion is in good agreement with theoretical predictions made for thick films [Phys. State., Solidi 37, 329 (1976)].
Monomode low loss optical waveguide in KTiOPO4 formed by combining ion implantation with ion exchange
Authors: Wang Liang-Ling , Wang Ke-Ming , Lu Fei , Shi Bo-Rong , Wang Xue-Lin , Wang Lei , Lu Qing-Ming
Published: September 2008

Monomode KTiOPO4 waveguide was formed by ion implantation with 500 keV O+ ions at a dose of 1×1015 ions cm−2 and the following ion exchange in pure RbNO3 at 340 °C for 45 min. Results indicate that it is an effective method to modulate the waveguide modes by combining ion exchange with ion implantation. Positive changes in both nx and ny refractive indices occur in the waveguide region. Continuous and homogenous field pattern of the propagation light in the waveguide was collected and studied using the end-coupling method. The irradiation damage as well as the Rb distribution was analyzed by means of the Rutherford backscattering spectrometry technique. The concentration profiles of Rb and K in KTiOPO4 were measured by the time of flight secondary ion mass spectrometry. Results show that the lattice damages formed by ion implantation act as a diffusion barrier, which stop the Rb ion and K ion exchange in the deeper depth.
Thermal tweezers for manipulation of adatoms and nanoparticles on surfaces heated by interfering laser pulses
Authors: Mason Daniel R. , Gramotnev Dmitri K. , Gramotnev Galina
Published: September 2008

We conduct the detailed numerical investigation of a nanomanipulation and nanofabrication technique—thermal tweezers with dynamic evolution of surface temperature, caused by absorption of interfering laser pulses in a thin metal film or any other absorbing surface. This technique uses random Brownian forces in the presence of strong temperature modulation (surface thermophoresis) for effective manipulation of particles/adatoms with nanoscale resolution. Substantial redistribution of particles on the surface is shown to occur with the typical size of the obtained pattern elements of ∼100 nm, which is significantly smaller than the wavelength of the incident pulses used (532 nm). It is also demonstrated that thermal tweezers based on surface thermophoresis of particles/adatoms are much more effective in achieving permanent high maximum-to-minimum concentration ratios than bulk thermophoresis, which is explained by the interaction of diffusing particles with the periodic lattice potential on the surface. Typically required pulse regimes including pulse lengths and energies are also determined. The approach is applicable for reproducing any holographically achievable surface patterns, and can thus be used for engineering properties of surfaces including nanopatterning and design of surface metamaterials.
A new multicomponent CKP hierarchy and solutions
Authors: Wu Hongxia , Zeng Yunbo , Fan Tianyou
Published: September 2008

Based on the eigenfunction symmetry constraints, we first propose a new multicomponent C-type subhierarchy for Kadomtsev-Petviashvili (mcCKP) hierarchy and its Lax representation, which provides an effective way to find two types of new CKP equation with self-consistent sources along with their related Lax representations. Also it admits reductions to k-constrained CKP hierarchy and to a (1+1)-dimensional soliton hierarchy with self-consistent sources, which gives rise to two types of new Kaup–Kuperschmidt (KK) equation with self-consistent sources and two types of new bidirectional Kaup–Kuperschmidt equation with self-consistent sources. By using the solutions of the CKP and KK equations and their corresponding eigenfunctions, N-soliton solutions for the first type of CKP self-consistent source (CKPSCS) and of KKSCS are constructed by means of method of variation in constant, respectively.
Local droplet etching of nanoholes and rings on GaAs and AlGaAs surfaces
Authors: Stemmann A. , Heyn Ch. , Köppen T. , Kipp T. , Hansen W.
Published: September 2008

We study the formation of nanoholes and rings on GaAs and AlGaAs surfaces by local droplet etching (LDE) with gallium and indium. The nanohole properties are tuned by variation in etching temperature and time as well as by the etchant. Nanoholes fabricated by In LDE are larger and have an about ten times lower density compared to Ga LDE, which allows the fabrication of nanoholes with ultralow density of less than 5×106 cm−2. Furthermore, the nanohole borders are surrounded by distinct walls. The walls are crystallized from droplet material and serve as quantum rings with tunable size and band gap.
Lipid flip-flop driven mechanical and morphological changes in model membranes
Authors: Ramachandran Sanoop , Kumar P. B. Sunil , Laradji Mohamed
Published: September 2008

We study, using dissipative particle dynamics simulations, the effect of active lipid flip-flop on model fluid bilayer membranes. We consider both cases of symmetric as well as asymmetric flip-flops. Symmetric flip-flop leads to a steady state of the membrane with an effective temperature higher than that of the equilibrium membrane and an effective surface tension lower than that of the equilibrium membrane. Asymmetric flip-flop leads to transient conformational changes in the membrane in the form of bud or blister formation, depending on the flip rate.
A comparison between different semiclassical approximations for optical response functions in nonpolar liquid solution. II. The signature of excited state dynamics on two-dimensional spectra
Authors: Shi Qiang , Geva Eitan
Published: September 2008

Optical response functions are known to reflect quantum dynamics in a superposition state and as such, lack a well-defined classical limit. In a previous paper we considered the importance of accounting for the quantum nature of the dynamics by comparing the linear absorption spectrum and homodyne-detected time-integrated two-pulse photon-echo signal as calculated via the semiclassical forward-backward approach, linearized semiclassical approach, and standard approach which is based on equilibrium ground state dynamics [Shi and Geva, J. Chem. Phys. 122, 064506 (2005)]. In the present paper, we extend the comparison to the case of heterodyne-detected and time-resolved nonlinear time-domain rephasing and nonrephasing signals generated in three-pulse experiments and the corresponding frequency-domain two-dimensional spectra. The comparison is performed in the context of a two-state chromophore solvated in a nonpolar liquid. It is shown that the inherent insensitivity of the standard method to the nonequilibrium dynamics on the excited state potential surface gives rise to two-dimensional spectra which are symmetrical relative to the diagonal. In contrast, accounting for the effect of nonequilibrium excited state dynamics, as is the case within the forward-backward and linearized semiclassical methods, is found to give rise to two-dimensional spectra that become increasingly asymmetrical relative to the diagonal as the waiting time between the second and third pulses becomes larger. It is argued that the emergence of the asymmetry provides a useful probe of nonequilibrium solvation on the excited state potential surface.
Acoustic reflectivity goniometry of bounded ultrasonic pulses: Experimental verification of numerical models
Authors: Bouzidi Youcef , Schmitt Douglas R.
Published: September 2008

The angle of incidence reflectivity of a bounded acoustic pulse from water loaded elastic solids is tested using an experimental configuration with a large transmitter and a small pointlike receiver. The experimental response, affected by well-known nonspecular effects near critical angles, is modeled fully by propagation of the full bounded pulse using a phase advance technique. This allows for visualization of the evolution of the bounded pulse with time and upon reflection. The modeled and the observed reflectivities and phase rotations, as measured along the specular path, agree well with observations even at post Rayleigh critical angles of incidence. Moreover, small amplitude deviations are accurately reproduced. The longitudinal and Rayleigh critical angles are obtained from the observed reflectivity curves and used to calculate the material elastic wave speeds, which are in good agreement with those measured directly. The density is also obtained by inversion of portions of the reflectivity curves where they do not significantly differ from the plane wave response. These tests validate use of the full phase propagation method to study such reflection problems.
On the simulated scaling based free energy simulations: Adaptive optimization of the scaling parameter intervals
Authors: Zheng Lianqing , Yang Wei
Published: September 2008

Recently, we developed a generalized ensemble based free energy simulation technique, the simulated scaling (SS) method [Li et al., J. Chem. Phys. 126, 024106 (2007)]. In the SS simulations, random walks in the scaling parameter space are realized and free energy values can be conveniently estimated based on trial biasing weights. To improve free energy convergence in the SS simulations, we adopt a recent adaptive algorithm to systematically optimize the scaling parameter intervals; here, the optimization target is the round-trip rate between two end chemical states. As demonstrated in our model studies on the solvation of chloride ion and methane, free energy convergence can be greatly improved when the round-trip rates are accelerated.
Bi-Hamiltonian aspects of a matrix Harry Dym hierarchy
Authors: Fontanelli Laura , Lorenzoni Paolo , Pedroni Marco , Zubelli Jorge P.
Published: September 2008

We study the Harry Dym hierarchy of nonlinear evolution equations from the bi-Hamiltonian view point. This is done by using the concept of an S-hierarchy. It allows us to define a matrix Harry Dym hierarchy of commuting Hamiltonian flows in two fields that projects onto the scalar Harry Dym hierarchy. We also show that the conserved densities of the matrix Harry Dym equation can be found by means of a Riccati-type equation.
Nondistributive algebraic structures derived from nonextensive statistical mechanics
Authors: Cardoso Pedro G. S. , Borges Ernesto P. , Lobão Thierry C. P. , Pinho Suani T. R.
Published: September 2008

We propose a two-parametric nondistributive algebraic structure that follows from (q,q′)-logarithm and (q,q′)-exponential functions. Properties of generalized (q,q′)-operators are analyzed. We also generalize the proposal into a multiparametric structure (generalization of logarithm and exponential functions and the corresponding algebraic operators). All n-parameter expressions recover (n−1)-generalization when the corresponding qn→1. Nonextensive statistical mechanics has been the source of successive generalizations of entropic forms and mathematical structures, in which this work is a consequence.
Dielectric spectroscopy of silver nanoparticle embedded soda glass
Authors: Bahniwal Suman , Sharma Annu , Aggarwal Sanjeev , Deshpande S. K.
Published: September 2008

Silver-soda glass nanocomposites have been synthesized by using ion exchange technique followed by thermal annealing. Ultraviolet-visible absorption spectroscopy and transmission electron microscopy of the ion exchanged and annealed samples confirmed the presence of silver nanoparticles embedded inside the glass matrix. The size of silver nanoparticles has been found to increase with annealing temperature. The frequency response of the dielectric constant (ε′) and dielectric loss (tan δ) has been studied in both the soda glass and the silver nanoparticle embedded soda glass samples. The dielectric constant increases from 0.9 to 1.89 in the frequency range from 900 kHz to 70 MHz with increasing silver nanoparticle size, which in turn is enhanced as a result of increasing annealing temperature. Possible Coulomb blockade effects have been discussed to explain this novel dielectric behavior.
Recombination processes in midinfrared AlxIn1−xSb light-emitting diodes
Authors: Mirza B. I. , Nash G. R. , Smith S. J. , Buckle L. , Coomber S. D. , Emeny M. T. , Ashley T.
Published: September 2008

Emission characteristics, spectral properties, and quantum efficiencies of AlxIn1−xSb light-emitting diodes, with aluminum compositions between 0% and 8.75%, have been investigated as a function of temperature from 25 to 300 K, and a function of current from 1 to 100 mA. As both current and temperature are varied a change in the dominant recombination mechanism is observed as indicated by changes in the measured emission. An analysis of the light-current characteristics shows that Auger processes become important in all devices at temperatures above 100 K, implying an activation energy of approximately 7–13 meV depending on the aluminum composition.
Sensitized holmium upconversion emission in LiNbO3 triply doped with Ho3+, Yb3+, and Nd3+
Authors: Li Ai-Hua , Zheng Zhi-Ren , Lü Qiang , Sun Liang , Xu Yu-Heng , Liu Wei-Long , Wu Wen-Zhi , Yang Yan-Qiang , Lü Tian-Quan
Published: September 2008

Observed is 15 times enhancement of Ho3+ green upconversion (UC) emission in Ho3+(0.75 mol %)/Nd3+(0.75 mol %) doped lithium niobate crystal after incorporating with 4 mol % of Yb3+ under 800 nm femtosecond laser excitation. Yb3+ acting as bridging ion increases the energy transfer efficiency from Nd3+ F43/2 level to Ho3+ in these triply doped systems. The nonradiative energy transfer efficiency from Nd3+ F43/2 to Yd3+ F25/2 level can reach to 56%. The UC emission mechanisms are proposed based on spectral, kinetic, and pump power dependence analyses. The green UC emission is accomplished through a multi-ion interaction involving ground state absorption by the Nd3+ (I49/2→H29/2,F45/2) followed by three successive energy transfer processes involving one Nd3+-Yb3+ (F43/2+F27/2→I49/2+F25/2) and two Yb3+-Ho3+ pairs (F25/2+I58→F27/2+I56 and F25/2+I56→F27/2+S52,F54); whereas the red is dominated by excited state absorption of the Ho3+ (I57→F55) following two successive energy transfer processes involving the Nd3+-Yb3+ (F43/2+F27/2→I49/2+F25/2) and Yb3+-Ho3+ pairs (F25/2+I58→F27/2+I56) followed by the Ho3+ nonradiative relaxation to I57.