bluuebunny/crossref_metadata_2025 huggingface dataset:
Dy
3
+
- and Eu3+-doped LaGaO3 nanocrystalline phosphors for field emission displays
Authors: Liu Xiaoming , Lin Jun
Published: December 2006

Nanocyrstalline LaGaO3 and Dy3+- and Eu3+-doped LaGaO3 were prepared through a Pechini-type sol-gel process. X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), photoluminescence, cathodoluminescence spectra, and lifetimes were utilized to characterize the samples. XRD reveals that the samples begin to crystallize at 900°C and pure LaGaO3 phase can be obtained at 1000°C. FE-SEM images indicate that the Dy3+- and Eu3+-doped LaGaO3 samples are both composed of aggregated spherical particles with sizes ranging from 40to80nm. Under the excitation of ultraviolet light and low voltage electron beams (1–5kV), the undoped LaGaO3 sample shows a strong blue emission peaking at 433nm, and the Dy3+- and Eu3+-doped LaGaO3 samples show their characteristic emissions of Dy3+ (F9∕24-H15∕26 and F9∕24-H13∕26 transitions) and Eu3+ (D0,1,25-F1,2,3,47 transitions), respectively. The relevant luminescence mechanisms are discussed.
Ab initio study of semiconducting carbon nanotubes adsorbed on the Si(100) surface: Diameter- and registration-dependent atomic configurations and electronic properties
Authors: Barraza-Lopez Salvador , Albrecht Peter M , Romero Nichols A. , Hess Karl
Published: December 2006

We present an ab initio study of semiconducting carbon nanotubes adsorbed on an unpassivated Si(100) surface. Despite the usual gap underestimation in density functional theory, a dramatic reduction of the semiconducting gap for these hybrid systems as compared with the electronic gaps of both their isolated constitutive components has been found. This is caused by the changes in the electronic structure as the surface reconstructs due to tube’s proximity, the concomitant electronic charge transfer from the nanotubes, and the band hybridization with silicon and carbon states resulting in the appearance of states within the energy gap of the formerly isolated nanotube. Furthermore, it is determined that semiconducting nanotubes exhibit weaker adsorption energies and remain at a greater distance from the Si(100) surface as compared to metallic nanotubes of similar diameter. This effect may be useful for the solid-state separation of metallic and semiconducting nanotubes.
First-principles study of the work function of nitrogen doped molybdenum (110) surface
Authors: Black-Schaffer Annica M. , Cho Kyeongjae
Published: December 2006

The electronic properties of nitrogen doped Mo(110) surfaces were investigated using the first-principles pseudopotential method within the local density approximation in order to determine the effect of doping on the work function. Nitrogen doping was modeled by adsorbing N in both surface and subsurface positions. Surface adsorption of nitrogen was found to increase the work function by as much as 2eV due to the negative surface dipole induced by the electronegativity of nitrogen. Subsurface doping of nitrogen is energetically similar to surface adsorption, but has a small effect on the work function and only when within the first two to three surface Mo layers.
Evaluation of the OH− influence on visible and near-infrared quantum efficiencies of Tm3+ and Yb3+ codoped sodium aluminophoshate glasses
Authors: Terra Idelma A. A. , de Camargo Andrea S. S. , Nunes Luiz Antonio de O. , Carvalho Renê A. , Li Máximo Siu
Published: December 2006

This paper presents a throughout spectroscopic investigation of Yb3+∕Tm3+ codoped sodium aluminophosphate glasses in the visible and near-infrared spectral regions. Two sets of samples, prepared in air and nitrogen atmospheres, were characterized by transmission, absorption, luminescence, excited state lifetimes, and upconversion. The data are discussed in terms of OH− contents in the samples, and their influence on Tm3+ fluorescence quantum efficiencies. Besides, a theoretical analysis of radiative properties, by the Judd-Ofelt [Phys. Rev. 127, 750 (1962)]; J. Chem. Phys. 37, 511 (1962)] theory, is also presented in comparison to experimental data. Results indicate that samples prepared in N2 atmosphere, with lower OH− content, present superior properties than those prepared in air. Among these properties are a 30% more extensive infrared transmission of the host matrix, higher emission intensities, and decreased quenching effects, characterizing them as potential materials for high power laser generation, as well as efficient upconversion.
Hole current impedance and electron current enhancement by back-contact barriers in CdTe thin film solar cells
Authors: Pan Jun , Gloeckler Markus , Sites James R.
Published: December 2006

The combined effects of a significant back-contact barrier and a low absorber carrier density frequently alter the current-voltage (J-V) characteristics of CdTe solar cells. This combination leads to two competing mechanisms that can alter the J-V characteristics in two different ways. One is a majority-carrier (hole) limitation on current in forward bias that reduces the fill factor and efficiency of the solar cell. The second is a high minority-carrier (electron) contribution to the forward diode current that results in a reduced open-circuit voltage. CdTe solar cells are particularly prone to the latter, since the combination of a wide depletion region and impedance of light-generated holes at the back contact increases the electron injection at the front diode. The overlap of front and back space-charge regions will generally enhance the electron current, but is not a requirement for substantially increased forward current. The simulated J-V curves, illustrating the two major effects, are in good agreement with the experimental curves that have been observed in recent years.
Controlled in situ boron doping of diamond thin films using solution phase
Authors: Roy M. , Dua A. K. , Nuwad J. , Girija K. G. , Tyagi A. K. , Kulshreshtha S. K.
Published: December 2006

Controlled boron doping of diamond film using nontoxic reagents is a challenge in itself. During the present study, attempts have been made to dope diamond films in situ with boron from a solution of boric acid (H3BO3) in methanol (CH3OH) using a specially designed bubbler that ensured continuous and controlled flow of vapors of boron precursors during deposition. The samples are thoroughly characterized using a host of techniques comprising of x-ray photoelectron spectroscopy, Raman, x-ray diffraction, and current-voltage measurements (I-V). Cross-sectional micro-Raman spectroscopy has been used to obtain depth profile of boron in diamond films. Boron concentration ([B]) in the films is found to vary linearly on a semilog scale with molarity (M) of H3BO3 in CH3OH. Lattice constant of our samples is smaller than the reported American society for testing and materials (ASTM) values due to oxygen incorporation and it increases with [B] in the diamond samples. Heavily boron doped samples exhibit Fano deformation of the Raman line shape and negative and/zero activation barrier in temperature dependent I-V measurements that indicate the formation of metallic phase in the samples. The present study illustrates the feasibility of safe and controlled boron doping of diamond films using a solution of H3BO3 in CH3OH over a significant range of [B] from semiconductor to metallic regime but with a little adverse effect due to unintentional but unavoidable incorporation of oxygen.
Clifford representations in integrable systems
Authors: Wehefritz-Kaufmann Birgit
Published: December 2006

In this paper we analyze integrable systems from a Clifford algebra point of view. This approach allows us to give a clear representation theoretic exposition of techniques used in spin systems, thereby showing their naturality. We then extend this approach to the analysis of the XX-model with nondiagonal boundaries which is among others related to growing and fluctuating interfaces and stochastic reaction-diffusion systems. With this rationale, it is possible to diagonalize the system and find new hidden conservation laws.
Bohmian mechanics with complex action: A new trajectory-based formulation of quantum mechanics
Authors: Goldfarb Yair , Degani Ilan , Tannor David J.
Published: December 2006

In recent years there has been a resurgence of interest in Bohmian mechanics as a numerical tool because of its local dynamics, which suggest the possibility of significant computational advantages for the simulation of large quantum systems. However, closer inspection of the Bohmian formulation reveals that the nonlocality of quantum mechanics has not disappeared—it has simply been swept under the rug into the quantum force. In this paper we present a new formulation of Bohmian mechanics in which the quantum action, S, is taken to be complex. This leads to a single equation for complex S, and ultimately complex x and p but there is a reward for this complexification—a significantly higher degree of localization. The quantum force in the new approach vanishes for Gaussian wave packet dynamics, and its effect on barrier tunneling processes is orders of magnitude lower than that of the classical force. In fact, the current method is shown to be a rigorous extension of generalized Gaussian wave packet dynamics to give exact quantum mechanics. We demonstrate tunneling probabilities that are in virtually perfect agreement with the exact quantum mechanics down to 10−7 calculated from strictly localized quantum trajectories that do not communicate with their neighbors. The new formulation may have significant implications for fundamental quantum mechanics, ranging from the interpretation of non-locality to measures of quantum complexity.
Nonlinearities and piezoelectric fields in AlN∕GaN wurtzite heterostructures
Authors: Willatzen M. , Lassen B. , Lew Yan Voon L. C.
Published: December 2006

A one-dimensional model for examining the combined influence of lattice mismatch, spontaneous polarization, piezoelectricity, strain, and nonlinear permittivity effects is described. Nonlinear permittivity effects in two-layer AlN∕GaN structures used in, e.g., heterojunction field effect transistors are shown to have a significant influence (approximately 20%) in the absence of interface charge screening due to strong electric fields in the thin-film AlN layer. Considerable reductions occur, however, in the electric field when interface charge screening is effectively reducing the importance of nonlinear permittivity effects. Determination of dynamic strain characteristics is performed for the case of a strong dc electric field in the AlN layer (due to short-circuit dc voltage conditions over the full structure) superimposed by a small ac voltage and it is shown that resonance frequencies exist being weakly affected by permittivity nonlinearities. As a corollary, it is found that a nonlinear permittivity analysis indicates that the material sound speed becomes electric field dependent due to piezoelectric coupling.
A paradigm for interconnect geometry to reduce grain boundary resistance
Authors: Josell D.
Published: December 2006

Interconnects with nonuniform width are proposed for control of grain boundary density; the grain boundaries will be driven toward the narrowest locations to reduce the free energy of the system. The positive impact on electrical transport of the reduced grain boundary density is weighed against the negative impact of local constrictions and reduced line packing, with practical geometrical parameters quantified through consideration of sidewall roughness. It is found that technologically relevant sub-100-nm interconnects could benefit from use of this line geometry.
Excess Si and passivating N and F atoms near the pyrolytic-gas-passivated ultrathin silicon oxide film/Si(100) interface
Authors: Yamada Hiroshi
Published: December 2006

Number densities of Si, O, N, and F atoms near the 3.5–6.5-nm-thick silicon oxide film/Si(100) interface produced by a recently proposed in-situ passivation method [pyrolitic-gas passivation (PGP)] that uses a little pyrolytic N2O and NF3 gases were determined. It was found that the generation of excess Si atoms relative to the stoichiometric SiO2 composition near the interface is effectively inhibited by the localized passivating N and F atoms. Moreover, the number of excess Si decreases while those of N and F increase with decreasing humidity. These PGP effects can be confirmed only at a humidity of less than 1ppb. It is therefore believed that N and F passivations effectively contribute to compensate the residual inconsistent-state bonding sites near the interface that still remain through an extreme dehydration.
Sensitivity of photoelectron circular dichroism to structure and electron dynamics in the photoionization of carvone and related chiral monocyclic terpenone enantiomers
Authors: Harding Chris J. , Powis Ivan
Published: December 2006

The photoelectron circular dichroism that arises in the angular distribution of photoelectrons emitted from the carbonyl group in randomly oriented pure enantiomers of carvone, and a number of carvone derivatives, is investigated by continuum multiple scattering calculations. Core ionization of carbonyl C 1s orbitals is examined for six different isopropenyl tail conformations of carvone. These show clear differences of behavior both between axial and equatorial conformations, and between the three rotational conformers of each. The pronounced dependence of the dichroism on orientation of a tail grouping, itself remote from the localized initial C 1s site, indicates the presence of long range final state photoelectron scattering effects. Analogous data for the outermost valence orbital, partially localized on the carbonyl group, are also presented. The apparently enhanced sensitivity of the dichroism exhibited in this work is discussed in terms of the particular dependence on photoelectron interference effects that is probed by the dichroism measurement and is contrasted with the usual β parameter and cross section determinations.
Texture profile and aspect ratio influence on the front reflectance of solar cells
Authors: Llopis Francisco , Tobías Ignacio
Published: December 2006

Diffraction gratings and reactive ion etching textures have been proposed as a way to enhance the optical performance of solar cells. Extensive calculations have been carried out to investigate the influence of depth-to-period ratio of triangular and rectangular gratings on front reflectance. Losses are quantified by means of the reduction of short-circuit current due to reflection. It is concluded that gratings with triangular profile can feature very low reflectance when the depth-to-period ratio is very large. If periods are very small compared to light wavelength, however, light does not become tilted and other means to confine it in the cell must be additionally engineered.
Electron dynamics and harmonics emission spectra due to electron oscillation driven by intense laser pulses
Authors: Tian Youwei , Yu Wei , He Feng , Xu Han , Senecha Vinod , Deng Degang , Wang Yi , Li Ruxin , Xu Zhizhan
Published: December 2006

The dynamics and harmonics emission spectra due to electron oscillation driven by intense laser pulses have been investigated considering a single electron model. The spectral and angular distributions of the harmonics radiation are numerically analyzed and demonstrate significantly different characteristics from those of the low-intensity field case. Higher-order harmonic radiation is possible for a sufficiently intense driving laser pulse. A complex shifting and broadening structure of the spectrum is observed and analyzed for different polarization. For a realistic pulsed photon beam, the spectrum of the radiation is redshifted for backward radiation and blueshifted for forward radiation, and spectral broadening is noticed. This is due to the changes in the longitudinal velocity of the electron during the laser pulse. These effects are much more pronounced at higher laser intensities giving rise to even higher-order harmonics that eventually leads to a continuous spectrum. Numerical simulations have further shown that broadening of the high harmonic radiation can be limited by increasing the laser pulse width. The complex shifting and broadening of the spectra can be employed to characterize the ultrashort and ultraintense laser pulses and to study the ultrafast dynamics of the electrons.
Characterizing electron temperature gradient turbulence via numerical simulation
Authors: Nevins W. M. , Candy J. , Cowley S. , Dannert T. , Dimits A. , Dorland W. , Estrada-Mila C. , Hammett G. W. , Jenko F. , Pueschel M. J. , Shumaker D. E.
Published: December 2006

Numerical simulations of electron temperature gradient (ETG) turbulence are presented that characterize the ETG fluctuation spectrum, establish limits to the validity of the adiabatic ion model often employed in studying ETG turbulence, and support the tentative conclusion that plasma-operating regimes exist in which ETG turbulence produces sufficient electron heat transport to be experimentally relevant. We resolve prior controversies regarding simulation techniques and convergence by benchmarking simulations of ETG turbulence from four microturbulence codes, demonstrating agreement on the electron heat flux, correlation functions, fluctuation intensity, and rms flow shear at fixed simulation cross section and resolution in the plane perpendicular to the magnetic field. Excellent convergence of both continuum and particle-in-cell codes with time step and velocity-space resolution is demonstrated, while numerical issues relating to perpendicular (to the magnetic field) simulation dimensions and resolution are discussed. A parameter scan in the magnetic shear, s, demonstrates that the adiabatic ion model is valid at small values of s (s<0.4 for the parameters used in this scan) but breaks down at higher magnetic shear. A proper treatment employing gyrokinetic ions reveals a steady increase in the electron heat transport with increasing magnetic shear, reaching electron heat transport rates consistent with analyses of experimental tokamak discharges.
Effect of the Earth’s Coriolis force on the large-scale circulation of turbulent Rayleigh-Bénard convection
Authors: Brown Eric , Ahlers Guenter
Published: December 2006

We present measurements of the large-scale circulation (LSC) of turbulent Rayleigh-Bénard convection in water-filled cylindrical samples of heights equal to their diameters. The orientation of the LSC had an irregular time dependence, but revealed a net azimuthal rotation with an average period of about 3days for Rayleigh numbers R≳1010. On average there was also a tendency for the LSC to be aligned with upflow to the west and downflow to the east, even after physically rotating the apparatus in the laboratory through various angles. Both of these phenomena could be explained as a result of the coupling of the Earth’s Coriolis force to the LSC. The rate of azimuthal rotation could be calculated from a model of diffusive LSC orientation meandering with a potential barrier due to the Coriolis force. The model and the data revealed an additional contribution to the potential barrier that could be attributed to the cooling system of the sample top that dominated the preferred orientation of the LSC at high R. The tendency for the LSC to be in a preferred orientation due to the Coriolis force could be canceled by a slight tilt of the apparatus relative to gravity, although this tilt affected other aspects of the LSC that the Coriolis force did not.
The influence of the ion polarization current on magnetic island stability in a tokamak plasma
Authors: Fitzpatrick R. , Waelbroeck F. L. , Militello F.
Published: December 2006

The influence of the ion polarization current on the stability of a constant-ψ magnetic island in a tokamak plasma is investigated numerically using a reduced two-fluid model in two-dimensional slab geometry. The polarization current is found to be negligibly small when the island is either too narrow or too wide. However, under certain circumstances, there exists an intermediate regime in which the polarization current is appreciable, and has a stabilizing influence on the island. This effect may account for the metastable nature of neoclassical tearing modes in tokamak plasmas.
Cooperative doping effects of Ti and C on critical current density and irreversibility field of MgB2
Authors: Zhao Y. , Feng Y. , Shen T. M. , Li G. , Yang Y. , Cheng C. H.
Published: December 2006

Carbon and titanium concurrently doped MgB2 alloys with a composition of Mg1−xTixB2−yCy have been prepared by an in situ solid state reaction method. The combination effects of carbon and Ti doping on structure, microstructure, irreversibility field (Hirr), and critical current density (Jc) of MgB2 have been investigated. It is found that doping Ti in MgB2 does not reduce the solubility of carbon in MgB2, but, on the contrary, it eliminates the porosity present in the carbon-doped MgB2, resulting in an improved intergrain connectivity. As a result, Jc and Hirr of MgB2 have been significantly improved, sharply contrasted to the situation of C-only-doped or Ti-only-doped MgB2 bulks. Our results clearly reveal that carbon and Ti codoping are largely cooperative in improving the performance of MgB2 in the high magnetic fields (>3T) and at high temperature (∼20K).
Influence of the annealing treatments on the luminescence properties of SiO∕SiO2 multilayers
Authors: Jambois O. , Rinnert H. , Devaux X. , Vergnat M.
Published: December 2006

The formation of silicon nanocrystals (Si-ncs) and their room temperature photoluminescence (PL) properties were investigated in samples elaborated by the evaporation method. Silicon oxide (SiO) single layer and SiO∕SiO2 multilayers with different SiO layer thicknesses from 1to5nm were prepared and annealed at different temperatures up to 1050°C. The structure and the formation of Si-nc were studied by transmission electron microscopy (TEM) and by Fourier transform infrared (FTIR) absorption spectroscopy. It is demonstrated that the Si-ncs appear by the phase separation process from SiO due to the annealing treatments. Contrary to the SiO single layer, the multilayers are a powerful system to obtain highly luminescent Si-nc and to control the Si-nc size for SiO layer thicknesses lower than or equal to 4nm. It is clearly shown that, in agreement with the quantum confinement theory, the PL energy is a decreasing function of the Si-nc size. However, thanks to the correlation between FTIR, TEM, and PL results, it is demonstrated that the PL energy is also strongly dependent on the quality of the matrix in which the Si-nc are embedded. A model based on the existence of a SiOx shell surrounding the Si-nc is proposed to explain the PL results.
Test wave harmonics generation in an inhomogeneous plasma with an electron beam and a return current
Authors: Ferrante G. , Uryupin S. A. , Batani D.
Published: December 2006

Generation of second and third harmonics of a pump wave, taking place during electron beam propagation in a plasma with a return current, is investigated. Polarization properties and generation efficiency are specifically addressed. Analytical dependencies of the investigated physical values are reported for both the cases when the generation mechanism is provided by collisions of beam electrons with plasma ions, and when it is provided by collisions of bulk electrons creating the return current.
Complex self-consistent field and multireference single- and double-excitation configuration interaction calculations for the Πg2 resonance state of N2−
Authors: Honigmann Michael , Buenker Robert J. , Liebermann Heinz-Peter
Published: December 2006

Self-consistent field and multireference single- and double-excitation configuration interaction calculations employing the complex basis function technique are carried out for the Πg2 resonance state of the N2− molecule as well as several other anionic resonance states in the neighboring energy region. The results of calculations employing the same method for the S1 (2s2) state of the He atom and the Σg+1 (σu2) state of the H2 molecule are found to be in good agreement with those of earlier work. The present theoretical treatment has succeeded for the first time in satisfying the rigorous criterion of the complex variational principle in computing the N2− resonance states, namely, a cusp in the plots of real versus imaginary components of the corresponding complex energies has been located at each internuclear distance. On this basis, it is found that the open-shell orbital in the lowest-energy adiabatic N2− resonance state of Πg2 symmetry changes its character from quite compact at large internuclear distance to relatively diffuse for r<2.3a0. This is in contrast to all previous theoretical treatments of this system that have not rigorously satisfied the complex variational principle in their determination of this wave function.
Role of effective atomic masses in memory function-based models for liquids: A simulation study of liquid water
Authors: Calandrini Vania , Sutmann Godehard , Deriu Antonio , Kneller Gerald R.
Published: December 2006

In a simulation study of liquid water, the authors show that the single molecule dynamics up to 40THz is well described by a rigid-body model for the second order memory function of the intermediate self-scattering function.
( 00 l ) epitaxial AgTaO3 and AgNbO3 thin films on (001)SrRuO3∕(001)LaAlO3 substrates by chemical solution deposition
Authors: Telli M. B. , Bharadwaja S. S. N. , Biegalski M. D. , Trolier-McKinstry S.
Published: December 2006

( 00 l ) epitaxial AgTaO3 and AgNbO3 thin films were prepared on (001)SrRuO3∕(001)LaAlO3 substrates by chemical solution deposition. The dielectric constants and loss of ∼300nm thick films were 110±10 and 0.025±0.005 for AgTaO3 and 550±55 and 0.020±0.005 for AgNbO3 at room temperature. In both films, the temperature coefficient of capacitance is smaller than that for bulk ceramics of the same composition. The capacitance changes from −20to120°C were ⩽1.2% for AgTaO3 and ⩽3.6% for AgNbO3. The tunability of the AgTaO3 film was 1.6% at 230kV∕cm field, while 21% tunability was measured for AgNbO3 at 190kV∕cm.
Control of the non-Markovian dynamics of a qubit
Authors: Pötz W.
Published: December 2006

Solid-state implementations of a qubit based on semiconductor quantum dots or Josephson junctions are studied within a driven spin-boson model with Ohmic spectral function. It is shown that direct or indirect optimization allows selection of simple control fields which drive the dissipative system from a given initial state to a selected target state provided that the system’s intrinsic minimal response times are respected. The control field can be used to establish interference between the system-bath and system-control interactions, leading to control over effective transition rates and the dissipative action of the environment.
kHz stimulated Brillouin spectroscopy
Authors: Ohno S. , Sonehara T. , Tatsu E. , Koreeda A. , Saikan S.
Published: December 2006

We have developed a high precision stimulated Brillouin spectrometer. Since this spectrometer is based on the stimulated light scattering, Brillouin measurements are possible without any signal reduction even at extremely low temperatures. The use of stable lasers for the pump and probe waves and a real-time frequency-monitor gives a high frequency resolution reaching 20 kHz, which is, to the best of authors knowledge, the highest resolution so far reported for Brillouin spectrometers. The frequency-monitor has been achieved through the measurement of the beat frequency between the two lasers by using a microwave frequency counter. With this spectrometer, we have succeeded to measure Brillouin spectra in single crystals of TeO2 from room temperature to liquid helium temperature. Furthermore, in a sample with [110] surfaces, we have observed a spectral splitting due to the multireflection of phonons at crystal surfaces.
Simple setup to measure electrical properties of polymeric films
Authors: Hiremath R. K. , Rabinal M. K. , Mulimani B. G.
Published: December 2006

A simple method to measure electrical conductivity of conducting organic films has been described. A setup, based on four-probe technique, is specifically designed and fabricated for nondestructive electrical conductivity measurements of freestanding thin films. The current-voltage and temperature dependent characteristics of thin films of polyethylenedioxythiophene and polypyrrole and thick wafers of germanium have been used to test the setup. The results obtained are highly reproducible and are in good agreement with the reported values in the literature, employing different techniques.
Effect of oxygen on characteristics of nickel oxide∕indium tin oxide heterojunction diodes
Authors: Chang Hung-Lu , Lu T. C. , Kuo H. C. , Wang S. C.
Published: December 2006

p -nickel oxide (NiOx)∕n-indium tin oxide heterostructure p-n junction diodes were fabricated on glass substrates and showed rectifying characteristics or negative differential resistance (NDR) characteristics depending on the content of oxygen in the NiOx films. After annealing the heterojunction diodes at 450°C in air for about 30min, the characteristics of NDR disappeared and transparent rectifying diodes were observed. The oxygen content could be observed by inspecting the characteristics of the NiOx films before and after annealing using x-ray photoelectron spectrum and thermogravimetric analysis as well as atomic force microscopy. The released oxygen in the NiOx films could be responsible for the disappearance of NDR characteristics and the change of the nonideal rectifying diode characteristics.
Wave front reversal with frequency conversion in a nonreciprocal medium
Authors: Melkov G. A. , Vasyuchka V. I. , Lazovskiy V. V. , Tiberkevich V. S. , Slavin A. N.
Published: December 2006

The phenomenon of wave front reversal in a nonreciprocal medium has been discovered and studied experimentally. The phenomenon has been investigated on the example of parametric wave front reversal of nonreciprocal surface spin waves in a ferrite film with asymmetric surface metallization. It was found that the carrier frequency of the reversed wave packet is not equal to the carrier frequency of the incoming wave packet, and the magnitude of this frequency difference increases linearly with the decrease of the bias magnetic field. The discovered effect can be used for manipulation of pulsed signals in a microwave frequency range.
Unusual pulsating states in hydrocarbon-oxygen premixed flames
Authors: Gorman M. , Perrollier S.
Published: December 2006

An unusual type of pulsating state has been observed using a new ignition protocol for heavy hydrocarbon-oxygen premixed flames on a circular porous plug burner. The shape and motion of these states are quasicircular, luminous, pulsating regions of M (M=1, 2, 3, or 4) lobes that increase in size as the flame propagates outward. As the lobes expand, they break apart near their midpoints and form counterpropagating spiral-like arms. These spiral arms rotate, “collide” with arms generated by adjacent lobes, and are extinguished. We will describe the unusual characteristics of the dynamics of these states.
Structure and spectral features of H+(H2O)7: Eigen versus Zundel forms
Authors: Shin Ilgyou , Park Mina , Min Seung Kyu , Lee Eun Cheol , Suh Seung Bum , Kim Kwang S.
Published: December 2006

The two dimensional (2D) to three dimensional (3D) transition for the protonated water cluster has been controversial, in particular, for H+(H2O)7. For H+(H2O)7 the 3D structure is predicted to be lower in energy than the 2D structure at most levels of theory without zero-point energy (ZPE) correction. On the other hand, with ZPE correction it is predicted to be either 2D or 3D depending on the calculational levels. Although the ZPE correction favors the 3D structure at the level of coupled cluster theory with singles, doubles, and perturbative triples excitations [CCSD(T)] using the aug-cc-pVDZ basis set, the result based on the anharmonic zero-point vibrational energy correction favors the 2D structure. Therefore, the authors investigated the energies based on the complete basis set limit scheme (which we devised in an unbiased way) at the resolution of the identity approximation Møller-Plesset second order perturbation theory and CCSD(T) levels, and found that the 2D structure has the lowest energy for H+(H2O)7 [though nearly isoenergetic to the 3D structure for D+(D2O)7]. This structure has the Zundel-type configuration, but it shows the quantum probabilistic distribution including some of the Eigen-type configuration. The vibrational spectra of MP2/aug-cc-pVDZ calculations and Car-Parrinello molecular dynamics simulations, taking into account the thermal and dynamic effects, show that the 2D Zundel-type form is in good agreement with experiments.
Insertion of rare gas atoms into BF3 and AlF3 molecules: An ab initio investigation
Authors: Jayasekharan T. , Ghanty T. K.
Published: December 2006

The structure, stability, charge redistribution, and harmonic vibrational frequencies of rare gas inserted group III-B fluorides with the general formula F–Rg–MF2 (where M=B and Al; Rg=Ar, Kr, and Xe) have been investigated using ab initio quantum chemical methods. The Rg atom is inserted in one of the M–F bond of MF3 molecules, and the geometries are optimized for ground as well as transition states using the MP2 method. It has been found that Rg inserted F–Rg–M portion is linear in both F–Rg–BF2 and F–Rg–AlF2 species. The binding energies corresponding to the lowest energy fragmentation products MF3+Rg (two-body dissociation) have been computed to be −670.4, −598.8, −530.7, −617.0, −562.1, and −494.0kJ∕mol for F–Ar–BF2, F–Kr–BF2, F–Xe–BF2, F–Ar–AlF2, F–Kr–AlF2, and F–Xe–AlF2 species, respectively. The dissociation energies corresponding to MF2+Rg+F fragments (three-body dissociation) are found to be positive with respect to F–Rg–MF2 species, and the computed values are 56.3, 127.8, and 196.0kJ∕mol for F–Ar–BF2, F–Kr–BF2, and F–Xe–BF2 species, respectively. The corresponding values for F–Ar–AlF2, F–Kr–AlF2, and F–Xe–AlF2 species are also found to be positive. The decomposition of F–Rg–MF2 species into the MF3+Rg (two-body dissociation) channel typically proceeds via a transition state involving F–Rg–M out-of-plane bending mode. The transition state barrier heights are 35.5, 62.7, 89.8, 22.0, 45.6, and 75.3kJ∕mol for F–Ar–BF2, F–Kr–BF2, F–Xe–BF2, F–Ar–AlF2, F–Kr–AlF2, and F–Xe–AlF2 species, respectively. The calculated geometrical parameters and the energy values suggest that these species are metastable and may be prepared and characterized using low temperature matrix isolation techniques, and are possibly the next new candidates for gas phase or matrix experiments.
Solvation and proton transfer in polar molecule nanoclusters
Authors: Kim Hyojoon , Kapral Raymond
Published: December 2006

Proton transfer in a phenol-amine complex dissolved in polar molecule nanoclusters is investigated. The proton transfer rates and mechanisms, as well as the solvation of the complex in the cluster, are studied using both adiabatic and nonadiabatic dynamics. The phenol-amine complex exists in ionic and covalent forms and as the size of the cluster increases the ionic form gains stability at the expense of the covalent form. Both the adiabatic and nonadiabatic transfer reaction rates increase with cluster size. Given a fixed cluster size, the stability of the covalent state increases with increasing temperature. The proton transfer rates do not change monotonously with an increase in temperature. A strong correlation between the solvent polarization reaction coordinate and the location of the phenol-amine complex in the cluster is found. The ionic form of the complex strongly prefers the interior of the cluster while the covalent form prefers to lie on the cluster surface.
Influence of Prandtl number on stability of mixed convective flow in a vertical channel filled with a porous medium
Authors: Bera P. , Khalili A.
Published: December 2006

Buoyancy opposed mixed convection is considered in a vertical channel filled with an isotropic, porous medium, in which the motion of an incompressible fluid is induced by external pressure gradients and buoyancy forces. The Brinkman-Wooding-extended Darcy model has been used to study the instability mechanisms of the basic flow and its dependence on the Prandtl number (Pr) of the fluid. The stability analysis indicated that for the same Reynolds number (Re), the fully developed base flow was highly unstable for a fluid with high Pr. For a porous medium with a Darcy number (Da) of 10−6 and Pr⩾0.7, two different types of instability, Rayleigh-Taylor (R-T) and buoyant instability, are observed. The R-T instability mode is observed for relatively small values of Re. Further, the results show that for Da=10−5 and Pr<1, the spectrum of the energy profile is abrupt and sudden, whereas the same is smooth when Da=10−6. In the case of R-T instability, the critical value of Ra at low Re is given by −2.47∕Da. Though the R-T mode of instability is independent of Pr, the range of Re that sustains the R-T mode is a function of Pr. It has been found that enhancement of Pr reduces the Re range mentioned above. In contrast to the case of a purely viscous fluid, where the effect of Pr is not significant, in isotropic porous media Pr plays a significant role in characterizing the flow stability. The instability characteristics of zero temperature flux perturbation (BC-I) and zero heat flux perturbation (BC-II) on the boundaries differ significantly in the case of the R-T stability mode. However, both conditions lead to similar results for buoyant stability, except at small values of Re.
Cusp type electrostatic analyzer for measurements of medium energy charged particles
Authors: Kasahara S. , Asamura K. , Saito Y. , Takashima T. , Hirahara M. , Mukai T.
Published: December 2006

We have developed a new electrostatic analyzer which enables medium energy (≲200 keV∕q) plasma particle measurements with full solid angle coverage. The design of the test model realizes the uppermost measurement energy of ∼200 keV∕q with applied high voltages of ±5 kV. Laboratory experiments with the test model analyzer show that its performance agrees with numerical simulations. The test model design is well suited for combination with a mass analysis unit, while our new design can also be applied to medium energy electron measurements. Medium energy ion/electron sensors with this new design will surely be appreciated for upcoming space missions that will observe hot/energetic plasma structures in the regions such as the inner magnetosphere or reconnection region.
Phase modulation atomic force microscope with true atomic resolution
Authors: Fukuma Takeshi , Kilpatrick Jason I. , Jarvis Suzanne P.
Published: December 2006

We have developed a dynamic force microscope (DFM) working in a novel operation mode which is referred to as phase modulation atomic force microscopy (PM-AFM). PM-AFM utilizes a fixed-frequency excitation signal to drive a cantilever, which ensures stable imaging even with occasional tip crash and adhesion to the surface. The tip-sample interaction force is detected as a change of the phase difference between the cantilever deflection and excitation signals and hence the time response is not influenced by the Q factor of the cantilever. These features make PM-AFM more suitable for high-speed imaging than existing DFM techniques such as amplitude modulation and frequency modulation atomic force microscopies. Here we present the basic principle of PM-AFM and the theoretical limit of its performance. The design of the developed PM-AFM is described and its theoretically limited noise performance is demonstrated. Finally, we demonstrate the true atomic resolution imaging capability of the developed PM-AFM by imaging atomic-scale features of mica in water.
Does Mn doping play any key role in tailoring the ferromagnetic ordering of TiO2 thin films?
Authors: Hong Nguyen Hoa , Sakai Joe , Ruyter Antoine , Brizé Virginie
Published: December 2006

Mn was doped into TiO2 to clarify the real effects of Mn doping on magnetic and structural properties of TiO2 thin films. Mn doping does not play any key role in introducing ferromagnetism (FM) in the system. When the dopant concentration is small (below 5%), the Mn atoms can still be substituted for Ti atoms and maintain the TiO2 structure, so that it enhances the magnetic moment of the TiO2 host to some extent. When the dopant content increases to larger than 10%, along with a destruction of the anatase structure, the Mn doping consequently degrades and then destroys the FM of the host.
Equilibriumizing all food chain chaos through reproductive efficiency
Authors: Deng Bo
Published: December 2006

The intraspecific interference of a top-predator is incorporated into a classical mathematical model for three-trophic food chains. All chaos types known to the classical model are shown to exist for this comprehensive model. It is further demonstrated that if the top-predator reproduces at high efficiency, then all chaotic dynamics will change to a stable coexisting equilibrium, a novel property not found in the classical model. This finding gives a mechanistic explanation to the question of why food chain chaos is rare in the field. It also suggests that high reproductive efficiency of top-predators tends to stabilize food chains.
Magnetization reversal in the anisotropy-dominated regime using time-dependent magnetic fields
Authors: Rivkin K. , Ketterson J. B.
Published: December 2006

The authors study magnetization reversal using various rf magnetic pulses. The authors show numerically that switching is possible with simple sinusoidal pulses; however, the optimum approach is to use a frequency-swept (chirped) rf magnetic pulse, the shape of which can be derived analytically. Switching times of the order of nanoseconds can be achieved with relatively small rf fields, independent of the anisotropy’s strength.
Highly power efficient organic light-emitting diodes with a p-doping layer
Authors: Chang Chan-Ching , Hsieh Ming-Ta , Chen Jenn-Fang , Hwang Shiao-Wen , Chen Chin H.
Published: December 2006

In this letter, the authors demonstrate p-i-n organic light-emitting diodes (OLEDs) incorporating a p-doped transport layer which comprises tungsten oxide (WO3) and 4,4′,4″-tris(N-(2-naphthyl)-N-phenyl-amino)triphenylamine (2-TNATA) to replace the volatile tetrafluro-tetracyanoquinodimethane. The authors propose the 2-TNATA:WO3 composition functions as a p-doping layer which significantly improves hole injection and conductivity of the device that leads to the fabrication of tris(8-quinolinolato)aluminum based p-i-n OLEDs with long lifetime, low driving voltage (3.1V), and high power efficiency (3.5lm∕W) at 100cd∕m2.
Control and improvement of p-type conductivity in indium and nitrogen codoped ZnO thin films
Authors: Chen L. L. , Ye Z. Z. , Lu J. G. , Chu Paul K.
Published: December 2006

p -type ZnO thin films have been fabricated by In and N codoping. The carrier type in the In–N codoped ZnO can be controlled by adjusting the growth conditions and good p-type conductivity is obtained at temperatures between 490 and 580°C. The p-type behavior is improved when a buffer layer is used. The lowest reliable room temperature resistivity is found to be 7.85Ωcm in the presence of a buffer layer. The ZnO-based homostructural p-n junctions comprising a n-ZnO:In layer on a p-ZnO:(In,N) layer on a buffer layer display apparent electrical rectification in the authors’ repeated measurements.
Line narrowing of terahertz vibrational modes for organic thin polycrystalline films within a parallel plate waveguide
Authors: Melinger Joseph S. , Laman N. , Harsha S. Sree , Grischkowsky D.
Published: December 2006

Waveguide terahertz time-domain spectroscopy is used to demonstrate the narrowing of vibrational lines for a thin polycrystalline film of 1,2-dicyanobenzene in a parallel plate metal waveguide. When compared to corresponding linewidths for 1,2-dicyanobenzene in a pellet sample at room temperature, the linewidths for the waveguide film are found to be significantly sharper. For measurements near 77K, a dramatic line narrowing is observed for the waveguide film, yielding linewidths as much as five times sharper than found in the pellet. These effects are attributed to much smaller inhomogeneous broadening in the waveguide film and result in a more informative spectrum.
High-power, continuous-wave operation of distributed-feedback quantum-cascade lasers at λ∼7.8μm
Authors: Darvish S. R. , Zhang W. , Evans A. , Yu J. S. , Slivken S. , Razeghi M.
Published: December 2006

The authors present high-power continuous-wave (cw) operation of distributed-feedback quantum-cascade lasers. Continuous-wave output powers of 56mW at 25°C and 15mW at 40°C are obtained. Single-mode emission near 7.8μm with a side-mode suppression ratio of ⩾30dB and a tuning range of 2.83cm−1 was obtained between 15 and 40°C. The device exhibits no beam steering with a full width at half maximum of 27.4° at 25°C in cw mode.
Mo
O
x
modified Ag anode for top-emitting organic light-emitting devices
Authors: Cao Jin , Jiang XueYin , Zhang ZhiLin
Published: December 2006

Efficient top-emitting organic light-emitting devices (TOLEDs) using a thin MoOx layer modified Ag as the effective hole-injection anode are demonstrated. With tris-(8-hydroxy quinoline)aluminum as emitting layer and trilayer LiF∕Al∕Ag as semitransparent cathode, the Ag∕MoOx based TOLED shows a tune-on voltage of 2.67V and a maximum current efficiency of 7.27cd∕A, which are much better than those (3.92V, 6.12cd∕A) obtained from Ag∕Ag2O based TOLED and those (5.25V, 3.5cd∕A) obtained from the corresponding bottom-emitting organic light-emitting devices. Contact potential difference measurement shows that the work function of Ag∕MoOx is higher than those of Ag∕Ag2O and ozone-treated indium tin oxide, leading to a stronger hole injection. The good performance of Ag∕MoOx based TOLED is attributed to the efficient hole injection from the Ag∕MoOx anode as well as a microcavity effect.
Assessment of a long-range corrected hybrid functional
Authors: Vydrov Oleg A. , Scuseria Gustavo E.
Published: December 2006

Common approximate exchange-correlation functionals suffer from self-interaction error, and as a result, their corresponding potentials have incorrect asymptotic behavior. The exact asymptote can be imposed by introducing range separation into the exchange component and replacing the long-range portion of the approximate exchange by the Hartree-Fock counterpart. The authors show that this long-range correction works particularly well in combination with the short-range variant of the Perdew-Burke-Ernzerhof (PBE) exchange functional. This long-range-corrected hybrid, here denoted LC-ωPBE, is remarkably accurate for a broad range of molecular properties, such as thermochemistry, barrier heights of chemical reactions, bond lengths, and most notably, description of processes involving long-range charge transfer.
Structure and dielectric properties of barium titanate thin films grown by sol-gel-hydrothermal process
Authors: Xu Jinbao , Zhai Jiwei , Yao Xi
Published: December 2006

Barium titanate (BT) thin films have been prepared on Pt∕Ti∕SiO2∕Si substrates through the use of a low temperature process. This technique combines the conventional sol-gel process and hydrothermal methods. X-ray diffraction analysis showed that the BT thin films had a pure perovskite structure. Au∕BT∕Pt capacitors have been fabricated and the dielectric properties have been discussed for SGHT-prepared BT films treated at 180 and 500°C, respectively. It was found that the BT films annealed at 500°C had much better ferroelectric properties. This improvement of ferroelectric properties through a heat treatment is probably associated with the change of density of hydroxyl groups in the films.