bluuebunny/crossref_metadata_2025 huggingface dataset:
Redistribution of Pb-rich phase during electromigration in eutectic SnPb solder stripes
Authors: Chou C. K. , Chen C. A. , Liang S. W. , Chen Chih
Published: March 2006

The microstructural evolution occurring within the eutectic SnPb solder during electromigration is investigated utilizing Blech specimens. Solder stripes of about 3μm in thickness were fabricated on Cu∕Ti metallization. It was found that the Pb-rich phase ripened and aligned along the direction of the electron flow following the current stressing of 9.7×103A∕cm2 at 80°C for 24h. As the stressing time or current density increased, the redistribution of the Pb-rich phase became more significant. Three-dimensional simulation was performed to examine the change in resistance and current-density distribution during electromigration. It was found that when the Pb-rich phase aligned along the direction of the electron flow, the total resistance of the solder stripe decreased. This reduction in total resistance may provide the driving force for the redistribution of the Pb-rich phase during electromigration. In addition, current crowding occurred in the vicinity of the interface of the Pb-rich and Sn-rich phases, creating a gradient of current density. This gradient might trigger the resistive Pb atoms to align along the direction of the electron flow.
Structural and mechanical properties of TaZrN films: Experimental and ab initio studies
Authors: Aouadi S. M.
Published: March 2006

This paper reports on the growth and characterization of the structural and mechanical properties of tantalum zirconium nitride films and the subsequent simulation of these properties using an ab initio calculation based on density functional theory (DFT) within the generalized gradient approximation. The films were deposited by reactive unbalanced magnetron sputtering and their physical and chemical properties were studied by means of x-ray diffraction (XRD), Rutherford backscattering (RBS), and nanoindentation. XRD revealed that these films formed a solid solution and that the lattice constant decreased linearly with Ta content. RBS provided the elemental composition of the films. Nanoindentation was used to evaluate the hardness and the elastic modulus. The hardness was found to have high values for a Ta∕(Ta+Zr) of 30% and 100%. The elastic modulus was found to increase monotonically with Ta content. The intrinsic elastic constants were calculated using DFT and the results were compared to the experimental values. A correlation between the computational and the experimental Young’s modulus was established. However, the trends observed for the measured hardness and the calculated shear modulus were not in agreement. This disagreement was due to the prominent extrinsic component of the hardness for these materials.
Au-catalyzed growth processes and luminescence properties of ZnO nanopillars on Si
Authors: Li Yujie , Feneberg Martin , Reiser Anton , Schirra Martin , Enchelmaier Rainer , Ladenburger Andreas , Langlois André , Sauer Rolf , Thonke Klaus , Cai Jun , Rauscher Hubert
Published: March 2006

ZnO nanopillars are often grown on various substrates by catalytic growth processes through a vapor-liquid-solid (VLS) mechanism. However, on silicon substrates, even with the catalyzed growth processes, it is still very difficult to obtain highly oriented ZnO nanopillar arrays. In this work, it was found that in most cases the actual growth process of ZnO on Si catalyzed by Au was not of real VLS character. In the initial growth stage, the substrate surface is partially melted and then oxidized into a very thin layer of SiO2. Zn-rich alloys instead of ZnO are first deposited on the SiO2∕Si substrates and form polycrystalline hillocks in an atmosphere with low O2 partial pressure. The difficulty for ZnO to nucleate on SiO2∕Si is another reason preventing ZnO nanopillars from growing epitaxially on the substrates. Defects, steps, and∕or stress on the substrate surfaces may support the nucleation process and thus may influence the initial growth stage and the control of the growth orientation of the pillars. With the help of scratches on Si substrates and by avoiding the formation of hillocks, well-aligned ZnO nanopillars were obtained both on Si (111) and (100) surfaces. Sharp photoluminescence (PL) peaks of bound exciton emissions and a free exciton emission were detected at low temperature. An unreported PL peak at about 3.367eV and in some cases a donor-acceptor-pair transition at about 3.308eV were also found. At higher temperature, a band-acceptor recombination process together with strong LO-phonon replicas occurred. A donor dopant concentration ND of about 5.7×1018cm−3 and an acceptor ionization energy of 126±2meV were evaluated from the spectra.
Analysis of super-radiant Smith-Purcell emission
Authors: Flory Curt A.
Published: March 2006

Smith-Purcell emission is studied for a system with no incident field or electromagnetic feedback mechanism other than what is provided by the grating itself. A model is developed that analyzes the interaction between the electron beam and the grating electromagnetic modes. The system of equations is solved numerically, demonstrating linear emitted power versus current at low current levels and super-radiant power emission at higher currents. These results are compared to the previously published experimental data, and the model is used to gain some understanding of the physical mechanisms at work.
Applicability range of Stoney’s formula and modified formulas for a film/substrate bilayer
Authors: Zhang Yin , Zhao Ya-pu
Published: March 2006

In addition to the layer thickness and effective Young’s modulus, the impact of the kinematic assumptions, interfacial condition, in-plane force, boundary conditions, and structure dimensions on the curvature of a film/substrate bilayer is examined. Different models for the analysis of the bilayer curvature are compared. It is demonstrated in our model that the assumption of a uniform curvature is valid only if there is no in-plane force. The effects of boundary conditions and structure dimensions, which are not (fully) included in previous models are shown to be significant. Three different approaches for deriving the curvature of a film/substrate bilayer are presented, compared, and analyzed. A more comprehensive study of the conditions regarding the applicability of Stoney’s formula and modified formulas is presented.
Tight-binding study of thermal expansions for Mo3Si
Authors: Ma Ning , Cooper Bernard R. , Kang Bruce S.
Published: March 2006

We implemented a tight-binding parameter extraction scheme that is suitable for the modeling of intermetallic alloy systems. Using Mo3Si as an example, we obtained the Slater-Koster tight-binding parameters directly from results of full-potential linear muffin-tin orbital calculation by using a modification of the approach of McMahan and Klepeis [Phys. Rev. B 56, 12250 (1997)]. The transferability and accuracy of these parameters were tested against ab initio results. Augmented by a fitted repulsive energy contribution that takes the form of embedded atom potential, the tight-binding total energy method was applied in Monte Carlo simulations to compute the coefficients of thermal expansion for Mo3Si.
Symmetrical threshold voltage in complementary metal-oxide-semiconductor field-effect transistors with HfAlOx(N) achieved by adjusting Hf∕Al compositional ratio
Authors: Kadoshima Masaru , Ogawa Arito , Ota Hiroyuki , Iwamoto Kunihiko , Takahashi Masashi , Mise Nobuyuki , Migita Shinji , Ikeda Minoru , Satake Hideki , Nabatame Toshihide , Toriumi Akira
Published: March 2006

We propose a method for restoring the symmetry in the threshold voltage (Vth) in complementary metal-oxide-semiconductor field-effect transistors (FETs) with a Hf-based high-k dielectric. This technique is based on the Al composition adjustment in the HfAlOx(N) dielectric film to achieve the symmetric Vth. The asymmetry of ∣Vth∣ in n and p metal-oxide-semiconductor FET (MOSFETs) due to the Fermi-level pinning at the interface between the poly-Si gate electrode and Hf-based high-k dielectric is considered to be induced independently by two kinds of interfacial dipoles. The adjustment of the Al content in HfAlOx(N) enables us to balance these dipoles. Vth values of n- and p-MOSFETs are shifted in the positive direction as the Al content in HfAlOx(N) increases. Symmetrical Vth values can be obtained for poly-Si and single fully silicided nickel gate electrodes when the Al contents are about 25 and 7at.% in HfAlOx(N), respectively.
Characteristics of nucleation layer and epitaxy in GaN/sapphire heterostructures
Authors: Narayan J. , Pant Punam , Chugh A. , Choi H. , Fan J. C. C.
Published: March 2006

We present the details of GaN nucleation layer grown on (0001) sapphire substrates below 600°C by metal organic chemical vapor deposition. These films have cubic (c-GaN) zinc blende structure which starts to transform into a hexagonal (h-GaN) wurtzite structure upon annealing around 650°C and above. The films deposited above 700°C by pulsed laser deposition directly on sapphire substrate showed the wurtzite structure. Both c-GaN and h-GaN films grow epitaxially on (0001) sapphire substrates via domain matching epitaxy, where integral multiples of planes match across the film-substrate interface. The c-GaN has the following epitaxial relationship: ⟨111⟩c-GaN‖⟨0001⟩sap, ⟨110⟩c-GaN‖⟨10-10⟩sap, and ⟨211⟩c-GaN‖⟨−2110⟩sap. In terms of planar matching, (220) planes of c-GaN match with (30-30) planes of sapphire, and 1∕3(4¯2¯2¯) planes of c-GaN match with (−2110) planes of sapphire in the perpendicular direction. The transformation from c-GaN into h-GaN involves the transformation of (220) planes of c-GaN into (−2110) planes of h-GaN and 1∕3(422) planes of c-GaN into (30-30) planes of h-GaN, and the epitaxial relationship changes to ⟨0001⟩h-GaN‖⟨0001⟩sap and ⟨−2110⟩h-GaN‖⟨10-10⟩sap. In terms of planar matching epitaxy, (−2110) planes of h-GaN match with (30-30) planes of sapphire, and, in the perpendicular direction, (30-30) planes of h-GaN match with (−2110) planes of sapphire. This epitaxial relationship is known as 30° or 90° rotation. It is interesting to note that relative spacing for c-GaN as well as h-GaN planes remains the same during this transformation because of a(c-GaN)=√2a(h-GaN)=√3c(h-GaN)∕2 equivalence between lattice constants of cubic and hexagonal structures. The transformation from cubic to hexagonal structure can occur via insertion or removal of stacking faults in {111} planes of c-GaN and {0001} planes of h-GaN. The hexagonal structure is preferred as a template for higher-temperature growth, however, the cubic structure, which is a defective hexagonal with stacking faults in alternate layers, can also provide a template for epitaxy. The role of Shockley partials terminating at the island edges and the dislocations associated with subgrain boundaries in the generation of threading dislocations is discussed.
Dielectric properties of hydrogen-terminated Si(111) ultrathin films
Authors: Nakamura Jun , Ishihara Shunsuke , Natori Akiko , Shimizu Tomo , Natori Kenji
Published: March 2006

Dielectric properties of Si(111) ultrathin films have been investigated using first-principles ground-states calculations in external electrostatic fields. With increasing thickness of Si(111) ultrathin films, the optical dielectric constant evaluated at the center of the slab converges to the experimental bulk dielectric constant at a thickness of only eight bilayers, while the energy gap of the slab is still larger than that of bulk Si. The converged theoretical dielectric constant for bulk Si is only 6.2% higher than the experimental one. Furthermore, spatial variations of the dielectric constant have also been evaluated using the position-dependent macroscopic field given by a clear-cut definition. The results show that the dielectric constant is reduced distinctly at the first few bilayers from the surface, which stems from the penetration of depolarized charges induced at the surface. Such an effective reduction of the depolarization field near the surface is one of the reasons for the decrease in optical dielectric constant for the ultrathin films.
Adhesive contact driven by electrostatic forces
Authors: Tang Tian , Hui Chung-Yuen , Jagota Anand
Published: March 2006

The Johnson-Kendall-Roberts (JKR) theory of contact [Proc. R. Soc. London, Ser. A. 324, 301 (1971)], which has been used to study adhesion of particles, is based on the assumption that the adhesive interactions are short ranged. This is not the case if the particles are charged. In this work, analytical and numerical (finite element) methods are used to analyze the adhesive contact of a spherical elastic insulating particle with uniformly distributed surface charge to a rigid conducting half space. Our results show that the contact radius can be accurately predicted by the JKR theory provided that the mechanically applied force in the JKR theory is supplemented by an appropriate electrostatic force.
Influence of short-term low current dc aging on the electrical and optical properties of InGaN blue light-emitting diodes
Authors: Rossi Francesca , Pavesi Maura , Meneghini Matteo , Salviati Giancarlo , Manfredi Manfredo , Meneghesso Gaudenzio , Castaldini Antonio , Cavallini Anna , Rigutti Lorenzo , Strass Uwe , Zehnder Ulrich , Zanoni Enrico
Published: March 2006

This work describes an experiment on degradation mechanisms of InGaN light-emitting diode (LED) test structures which do not fulfill the requirements of longlife products. We present a combined capacitance-voltage (C-V), deep level transient spectroscopy (DLTS), electroluminescence (EL), and cathodoluminescence (CL) study of short-term instabilities of InGaN∕GaN LEDs submitted to low current aging tests at room temperature. In the early stages of the aging tests, the EL and CL characterizations showed an optical power decrease, more prominent at low current levels. The C-V profiles indicated that the stress induced an apparent charge increase, well related to the deep level changes detected by DLTS and to the optical power decrease. It is supposed that the main cause of the degradation is the generation of nonradiative paths, due to the generation/propagation of defects activated by carrier transport.
Extended Klein-Gordon action, gravity and nonrelativistic fluid
Authors: Hassaïne Mokhtar
Published: March 2006

We consider a scalar field action for which the Lagrangian density is a power of the massless Klein-Gordon Lagrangian. The coupling of gravity to this matter action is considered. In this case, we show the existence of nontrivial scalar field configurations with vanishing energy-momentum tensor on any static, spherically symmetric vacuum solutions of the Einstein equations. These configurations in spite of being coupled to gravity do not affect the curvature of space-time. The properties of this particular matter action are also analyzed. For a particular value of the exponent, the extended Klein-Gordon action is shown to exhibit a conformal invariance without requiring the introduction of a nonminimal coupling. We also establish a correspondence between this action and a nonrelativistic isentropic fluid in one fewer dimension. This fluid can be identified with the (generalized) Chaplygin gas for a particular value of the power. It is also shown that the nonrelativistic fluid admits, apart from the Galileo symmetry, an additional symmetry whose action is a rescaling of the time.
Electric and magnetic behaviors of Li and Co codoped ZnO thin film on Si (100) substrate
Authors: Lin Yuan-Hua , Cai Jingnan , Nan Ce-Wen , Kobayashi M. , He Jinliang
Published: March 2006

Four kinds of Li and Co codoped ZnO (LCZO) thin films have been prepared by a sol-gel process technique. X-ray diffraction results revealed that the LCZO thin films possess single ZnO wurtzite structure and c-axis preferred orientation, and Li and Co ions substitute for the Zn sites without changing the wurtzite structure. The current density of the LCZO films indicated that Li ions play a role of acceptors to increase the samples’ resistivity. All of the LCZO thin films show ferromagnetic properties, and ferromagnetic components increase with increasing Li doping concentration at room temperature, which is ascribed to the induced defects by Li doping that make the ferromagnetic coupling interactions between the Co ions more effective.
High-peak-power pulsed operation of 2.0μm (AlGaIn)(AsSb) quantum-well ridge waveguide diode lasers
Authors: Eichhorn M. , Rattunde M. , Schmitz J. , Kaufel G. , Wagner J.
Published: March 2006

We have characterized 2.0μm (aluminium-gallium-indium)(arsenide-antimonide) quantum-well diode lasers in pulsed operation (20–60ns). A peak power of 1.25W could be achieved. The near-field distribution on the output facet and the spectral output have been analyzed. Single transverse mode operation can only be maintained at low pulse currents. Above a certain current limit higher order modes occur and fluctuations between these modes have been resolved on a 10ns time scale. The threshold for thermal and optical damage was investigated for ridge waveguide widths of 6, 8, and 16μm. No systematic damage threshold could be determined up to current densities as high as 200kA∕cm2.
Stress creation during Ni–Mn alloy electrodeposition
Authors: Hearne Sean J. , Floro Jerrold A. , Rodriguez Mark A. , Tissot Ralph T. , Frazer Colleen S. , Brewer Luke , Hlava Paul , Foiles Stephen
Published: March 2006

The stress evolution during electrodeposition of NiMn from a sulfamate-based bath was investigated as a function of Mn concentration and current density. The NiMn stress evolution with film thickness exhibited an initial high transitional stress region followed by a region of steady-state stress with a magnitude that depended on deposition rate, similar to the previously reported stress evolution in electrodeposited Ni [S. J. Hearne and J. A. Floro, J. Appl. Phys. 97, 014901–1 (2005)]. The incorporation of increasing amounts of Mn resulted in a linear increase in the steady-state stress at constant current density. However, no significant changes in the texture or grain size were observed, which indicates that an atomistic process is driving the changes in steady-state stress. Additionally, microstrain measured by ex situ x-ray diffraction increased with increasing Mn content, which was likely the result of localized lattice distortions associated with substitutional incorporation of Mn and∕or increased twin density.
Improving the light extraction efficiency of red-emitting conjugated polymer light emitting diodes
Authors: Adawi Ali M. , Connolly Liam G. , Whittaker David M. , Lidzey David G. , Smith Euan , Roberts Matthew , Qureshi Faisal , Foden Clare , Athanassopoulou Nicky
Published: March 2006

We demonstrate a significant improvement in the external electroluminescence efficiency of red-emitting polymer light emitting diodes (LEDs) by modifying the optical structure of the device. By using a cathode composed of a thin (5nm) film of calcium backed with an optically thick film of silver, we measured improvements in the external efficiency of polymer LEDs by a factor of 1.6 times compared to a device using a cathode composed of calcium backed with lower reflectivity aluminum. By incorporating the LED into a microcavity structure (to form a resonant cavity LED) it is possible to obtain additional (but rather smaller) improvements in external efficiency of the order of 1.15 times, compared to a standard LED utilizing the same cathode. By combining high reflectivity cathode∕mirror materials with a low finesse cavity structure, we show that the external efficiency of a LED can be improved by as much as 1.8 times compared to a standard (noncavity) LED. Our results are in good agreement with those of theoretical calculations and demonstrate the real improvements in device external efficiency that can be achieved by reducing optical losses within the LED structure.
Light induced electrostatic force spectroscopy: Application to local electronic transitions in InN epifilms
Authors: Su W. S. , Lu C. W. , Chen Y. F. , Lin T. Y. , Lin E. H. , Chang C. A. , Chen N. C. , Chang P. H. , Shih C. F. , Liu K. S.
Published: March 2006

A technique based on electrostatic force microscopy in which light is used to change the charge states of the local region in a solid is introduced and demonstrated. This technique provides a unique feature that it can be used to probe local electronic transitions of a solid in a submicron scale. As an illustration, it has been applied to study local electronic structure in InN epifilms. Combining with atomic force microscopy, it is found that surface state density in the dale region is larger than that in the pinnacle region and an electron accumulation layer does exist on the surface. In addition, the magnitude of the surface band bending obtained for the regions with different surface states is consistent with the result measured by other techniques. We point out that light induced scanning electrostatic force spectroscopy is a very useful tool to probe the local electronic transitions of a solid in a submicron scale with high sensitivity.
Electron injection and trapping in a laser wakefield by field ionization to high-charge states of gases
Authors: Chen Min , Sheng Zheng-Ming , Ma Yan-Yun , Zhang Jie
Published: March 2006

A scheme for electron injection into a laser wakefield is presented, which makes use of two orthogonally directed laser pulses and a gaseous medium with a moderate or high atomic number such as neon. A pump laser pulse ionizes the medium to its midcharge states to form underdense plasma and meanwhile excites a high amplitude wakefield firstly. Another ultrashort laser pulse with higher intensity is then injected transversely, which further ionizes the medium to high-charge states to produce new free electrons with certain energy. Part of these new-born electrons can be trapped and accelerated by the laser wakefield to high energies. Numerical simulations using a particle-in-cell code with field ionization included are conducted to verify the scheme.
Structural and vibrational properties of solid nitromethane under high pressure by density functional theory
Authors: Liu Hong , Zhao Jijun , Wei Dongqing , Gong Zizheng
Published: March 2006

The structural, vibrational, and electronic properties of solid nitromethane under hydrostatic pressure of up to 20GPa have been studied using density functional theory. The changes of cell volume, the lattice constants, and the molecular geometry of solid nitromethane under hydrostatic loading are examined, and the bulk modulus B0 and its pressure derivative B0′ are fitted from the volume-pressure relation. Our theoretical results are compared with available experiments. The change of electron band gap of nitromethane under high pressure is also discussed. Based on the optimized crystal structures, the vibrational frequencies for the internal and lattice modes of the nitromethane crystal at ambient and high pressures are computed, and the pressure-induced frequency shifts of these modes are discussed.
High-pressure synthesis and physical properties of an orthorhombic phase of chromium dioxide
Authors: Kuznetsov A. Yu. , de Almeida J. S. , Dubrovinsky L. , Ahuja R. , Kwon S. K. , Kantor I. , Kantor A. , Guignot N.
Published: March 2006

Chromium dioxide has been synthesized by direct combustion process between Cr and O2 in a diamond anvil cell at 14GPa using laser heating. Addition of nitrogen into the reactants’ environment substantially facilitated the oxidation of Cr and resulted in the formation of an orthorhombic CaCl2-structured phase of CrO2 after laser heating at 37GPa. High-pressure synchrotron x-ray diffraction and Raman spectroscopy measurements of quenched CrO2 samples at ambient temperature showed a tetragonal (rutile-type structure)-to-orthorhombic (CaCl2-type structure) phase transition at about 17GPa. The first principle calculations reproduce well the experimentally observed pressure domains of the stability of the tetragonal and orthorhombic phases of CrO2. The calculations show that the orthorhombic CrO2 is also half-metallic and that the ferromagnetic arrangement of spins of d electrons persists in orthorhombic phase of CrO2 up to 56GPa.
Correlation between electroluminescence efficiency and stability in organic light-emitting devices under pulsed driving conditions
Authors: Luo Yichun , Aziz Hany , Popovic Zoran D. , Xu Gu
Published: March 2006

We investigated the effect of using pulsed current (pc) versus direct current (dc) driving mode on the electroluminescence (EL) efficiency and operational stability of tris(8-hydroxy-quinoline)aluminum (AlQ3)-based organic light-emitting devices. The results show that the dependence of device stability on the driving mode correlates with the relative EL efficiency under the pc and dc driving modes, where the mode that gives higher EL efficiency at any given current density also gives higher operational stability regardless of the duty cycle.
Environmental stability and cryogenic thermal cycling of low-temperature plasma-deposited silicon nitride thin films
Authors: Martyniuk M. , Antoszewski J. , Musca C. A. , Dell J. M. , Faraone L.
Published: March 2006

Stress in low-temperature plasma-enhanced chemical vapor deposited silicon nitride (SiNx) thin films subject to cryogenic thermal cycling (100–323K) has been measured. It is observed that the SiNx deposition temperature strongly influences the thin film characteristics. For films deposited between 200 and 300°C, the thermal expansion coefficient is similar to that of silicon over the 180–323K temperature range. The room temperature thermal expansion coefficient of SiNx films is found to decrease sublinearly from 5.2×10−6to2.6×10−6K−1 as the temperature of the deposition process is increased from 50to300°C. The negative correlation between deposition temperature and thin film thermal expansion coefficient, and the positive correlation between deposition temperature and the thin film Young’s modulus inferred from nanoindentation are postulated to be associated with the local bonding environment within the thin film. The stress state of SiNx films deposited above 150°C is stable under atmospheric conditions, in contrast to SiNx films deposited below 100°C, which under atmospheric storage conditions become more tensile with time due to oxidation. In addition, SiNx thin films deposited below 100°C exhibit higher tensile stress values in vacuum than at atmospheric pressure, and vacuum annealing at 50°C of films deposited below 100°C introduces further tensile stress changes. These stress changes have been shown to be fully reversible upon reexposure to high purity nitrogen, helium, argon, oxygen, or laboratory atmosphere, and are likely to be associated with thin film porosity.
Size and temperature induced phase transition behaviors of barium titanate nanoparticles
Authors: Hoshina Takuya , Kakemoto Hirofumi , Tsurumi Takaaki , Wada Satoshi , Yashima Masatomo
Published: March 2006

High density, almost impurity-free and defect-free barium titanate (BaTiO3) fine particles with various sizes from 20to1000nm were prepared by the two-step thermal decomposition of barium titanyl oxalate and postheating treatment. The crystal structures of these particles were investigated as a function of the size and the temperature using synchrotron radiation x-ray diffraction (XRD) measurement. As a result, the size-induced ferroelectric (tetragonal-cubic) phase transition observed at around 30nm. Moreover, the temperature dependence of the crystal structures revealed that one phase transition at 135°C separated into two kinds of phase transition behaviors with decreased particle sizes, i.e., the tetragonal-cubic phase transition temperature was constant at 135°C despite particle sizes while the cell volume expansion temperature shifted to low temperature with decreasing particle sizes. Moreover, the temperature dependence of Raman scattering spectra clarified that the temperature at a discontinuous change of damping factor was almost consisted with the cell volume expansion temperature from the XRD measurement. These results suggested that the phase transition behavior of the BaTiO3 nanoparticles was quite different from that of the BaTiO3 single crystals.
Two-dimensional electronic spectra of symmetric dimers: Intermolecular coupling and conformational states
Authors: Szöcs V. , Pálszegi T. , Lukeš V. , Sperling J. , Milota F. , Jakubetz W. , Kauffmann H. F.
Published: March 2006

We study the information content of two-dimensional (2D) electronic photon-echo (PE) spectra, with special emphasis on their potential to distinguish, for waiting times T=0, between different conformations of electronically coupled symmetric dimers. The analysis is performed on the basis of an analytical formula for the frequency-domain 2D PE signal. The symmetric dimers are modeled in terms of two identical, energy-degenerate, excitonically coupled pairs of electronic states in the site representation. The spectra of conformationally weighted ensembles, composed of either two or four dimers, are compared with their one-dimensional linear absorption counterparts. In order to provide a realistic coupling pattern for the ensemble consisting of four dimers, excitonic couplings are estimated on the basis of optimized geometries and site-transition dipole moments, calculated by standard semiempirical methods for the bridged bithiophene structure 1,2-bithiophene-2-yl-ethane-1,2-dion (T2[CO]2). In the framework of our model, the highly readable 2D PE spectra can unambiguously identify spectral doublets, by relating peak heights and positions with mutual orientations of site-localized transition dipoles.
Methyl ethyl ketone+water+secondary butyl alcohol: A potential system for the exploration of a quadruple critical point
Authors: Madhavan Unni P. K.
Published: March 2006

We report preliminary experiments on the ternary-liquid mixture, methyl ethyl ketone (MEK)+water(W)+secondary butyl alcohol (sBA)—a promising system for the realization of the quadruple critical point (QCP). The unusual tunnel-shaped phase diagram shown by this system is characterized and visualized by us in the form of a prismatic phase diagram. Light-scattering experiments reveal that (MEK+W+sBA) shows near three-dimensional-Ising type of critical behavior near the lower critical solution temperatures, with the susceptibility exponent (γ) in the range of 1.217⩽γ⩽1.246. The correlation length amplitudes (ξo) and the critical exponent (ν) of the correlation length (ξ) are in the ranges of 3.536⩽ξo⩽4.611Å and 0.619⩽ν⩽0.633, respectively. An analysis in terms of the effective susceptibility exponent (γeff) shows that the critical behavior is of the Ising type for MEK concentrations in the ranges of 0.1000⩽X⩽0.1250 and X⩾0.3000. But, for the intermediate range of 0.1750⩽X<0.3000, the system shows a tendency towards mean-field type of critical behavior. The advantages of the system (MEK+W+sBA) over the system (3-methylpyridine+water+heavywater+potassiumIodide) for the realization of a QCP are outlined.
Giant enhancement of second harmonic generation in multiple photonic quantum well structures made of nonlinear material
Authors: Zhao Li-Ming , Gu Ben-Yuan
Published: March 2006

We present a general solution of second harmonic generation (SHG) in one-dimensional (1D) inhomogeneous systems. The structure consists of 1D multiple photonic quantum wells (PQWs) made of optical nonlinear material. The optimal arrangement of the polarization directions of the ferroelectric domains in multiple PQWs is determined by a simulated annealing method. We find that the conversion efficiency of SHG can be significantly enhanced when the fundamental wave frequency is aimed at one of the defect states. By tuning the polarization directions of the domains, the conversion efficiencies of “forward” and “backward” SHGs can be flexibly changed.
Ferroelectric domains in nitrobenzene-nitromethane solutions measured by hyper-Rayleigh scattering
Authors: Shelton David P.
Published: March 2006

Hyper-Rayleigh scattering (HRS) spectra were measured for liquid solutions of C6H5NO2 and CH3NO2 at T=300K. The depolarized HRS spectra at small frequency shift are dominated by two components due to reorientation of the nitrobenzene molecules. One is a Lorentzian with spectral width ν1=0.16−0.45cm−1 and corresponding orientation relaxation time τ=33–12ps. The second component is a narrow spike with spectral width <2MHz and corresponding relaxation time τ>80ns, attributed to HRS from slowly relaxing ferroelectric domains. The dipole order parameter g0=0.053±0.005, saturation parameter p=0.9±0.1, and volume V=20±6nm3 for these domains in nitromethane were determined from measurements of the nitrobenzene-concentration dependence of the intensity ratio for these two spectral components. Orientation of the 230 nitromethane molecules within each domain is inhomogenous but highly ordered.
An optical-optical double resonance probe of the lowest triplet state of jet-cooled thiophosgene: Rovibronic structures and electronic relaxation
Authors: Fujiwara Takashige , Lim Edward C. , Judge Richard H. , Moule David C.
Published: March 2006

The vibrational structure, rotational structure, and electronic relaxation of the “dark” T1A23(n,π*) state of jet-cooled thiophosgene have been investigated by two-color S2←T1←S0 optical-optical double resonance (OODR) spectroscopy, which monitors the S2→S0 fluorescence generated by S2←T1 excitation. This method is capable of isolating the T1 vibrational structure into a1, b1, and b2 symmetry blocks. The fluorescence-detected vibrational structure of the Tz spin state of T1 shows that the CS stretching frequency as well as the barrier height for pyramidal deformation are significantly greater in the A23(n,π*) state than in the corresponding A21(n,π*) state. The differing vibrational parameters of the T1 thiophosgene relative to the S1 thiophosgene can be attributed to the motions of unpaired electrons that are better correlated when they are in the excited singlet state than when they are in the triplet state of same electron configuration. A set of T1 structural parameters and the information concerning the T1 spin states have been obtained from least-square fittings of the rotationally resolved T1←S0 excitation spectrum. The nearly degenerate ∣x⟩ and ∣y⟩ spin states are well removed from ∣z⟩ spin component, indicating that T1 thiophosgene is a good example of case (ab) coupling. The decay of the ∣z⟩ spin state of T1 thiophosgene, obtained from time-resolved S2←T1←S0 OODR experiment, is characteristic of strong-coupling intermediate-case decay in which an initial rapid decay is followed by recurrences and/or a long-lived quasiexponential decay.
Noncoplanar organic field-effect transistor based on copper phthalocyanine
Authors: Di Chong-an , Yu Gui , Liu Yunqi , Xu Xinjun , Song Yabin , Wang Ying , Sun Yanming , Zhu Daoben , Liu Hongmin , Liu Xinyu , Wu Dexin
Published: March 2006

We present a method of fabricating noncoplanar channel organic field-effect transistors (OFETs) by a conventional photolithographic technique. Using this method, OFETs with micrometer critical features in slanting configurations and submicrometer critical features in vertical configurations were fabricated. The critical channel length over 1μm was controlled by the patterning technique, while the one of 0.5μm was defined by the thickness of an insulating layer between the drain and source electrodes. Also, we demonstrate that the OFETs containing two different metals as source and drain electrodes, respectively, are easily realized. All the OFETs based on copper phthalocyanine exhibit a high performance.
Linear and nonlinear drift waves in collisionless and collisional electron-positron-ion plasmas
Authors: Saleem H.
Published: March 2006

The linear and nonlinear drift waves in collisional and collisionless electron-positron-ion plasmas are studied in the classical limit. The possibilities of formation of solitons and shocks under different conditions are discussed. To estimate the growth rate of the linear drift dissipative instability in this case is not straightforward. However, in a limiting case it can be calculated exactly. Comparison of the results with the usual electron-ion plasmas is presented and the behavior of linear and nonlinear dynamics is pointed out for applications to laboratory and astrophysical situations.
Submicron gap capacitor for measurement of breakdown voltage in air
Authors: Hourdakis Emmanouel , Simonds Brian J. , Zimmerman Neil M.
Published: March 2006

We have developed a new method for measuring the value of breakdown voltage in air for electrode separations from 400nmto45μm. The electrodes used were thin film Au lines evaporated on sapphire. The resulting capacitors had an area of 80×80μm2. We demonstrate the ability to deduce the value of the separation of the plates by the value of the capacitance. The data acquired with this method do not agree with Paschen’s law for electrode separations below 10μm, as expected from previous experiments. Amongst the improvements of our method are the measurement of plate separation and the very small surface roughness (average of 6nm).
Theoretical analysis of cross-talking signals between counter-streaming electron beams in a vacuum tube oscillator
Authors: Shin Y. M. , Ryskin N. M. , Won J. H. , Han S. T. , Park G. S.
Published: March 2006

The basic theory of cross-talking signals between counter-streaming electron beams in a vacuum tube oscillator consisting of two two-cavity klystron amplifiers reversely coupled through input/output slots is theoretically investigated. Application of Kirchhoff’s laws to the coupled equivalent RLC circuit model of the device provides four nonlinear coupled equations, which are the first-order time-delayed differential equations. Analytical solutions obtained through linearization of the equations provide oscillation frequencies and thresholds of four fundamental eigenstates, symmetric/antisymmetric 0∕π modes. Time-dependent output signals are numerically analyzed with variation of the beam current, and a self-modulation mechanism and transition to chaos scenario are examined. The oscillator shows a much stronger multistability compared to a delayed feedback klystron oscillator owing to the competitions among more diverse eigenmodes. A fully developed chaos region also appears at a relatively lower beam current, ∼3.5Ist, compared to typical vacuum tube oscillators (10-100Ist), where Ist is a start-oscillation current.
Electron inertial effects on rotating magnetic field current drive
Authors: Ferrari Hugo E. , Farengo Ricardo , Clemente Roberto A.
Published: March 2006

The effect of finite electron mass on the formation and sustainment of a field reversed configuration (FRC) by rotating magnetic fields (RMF) is studied. The importance of inertial effects is measured by the ratio between the RMF frequency (ω) and the electron-ion collision frequency (ν). In the limit where this ratio is very small (ω∕ν→0), previous results corresponding to massless electrons are recovered. When ω∕ν increases there are significant changes in the value of the minimum external rotating field needed to sustain the FRC and the time required to reach a steady state. Since ν decreases with increasing temperature and decreasing density, these effects are expected to become more important as fusion relevant temperatures are approached.
Characteristics of metal-ferroelectric-insulator-silicon structures with ferroelectric (Pb0.8Ba0.2)ZrO3 thin films and (Ba0.5Sr0.5)TiO3 buffer layer
Authors: Liu Chien-Hung , Wu Jenn-Ming , Wu Lin-Jung
Published: March 2006

The results of the fabrication and characterization of ferroelectric (Pb0.8Ba0.2)ZrO3 (PBZ) thin films grown on nitrided silicon substrates with a (Ba0.5Sr0.5)TiO3 (BST) buffer layer by the rf-magnetron sputtering technique are reported. The PBZ thin films were used as the ferroelectric layer in the ferroelectric field effect transistors. The PBZ thin films were grown with highly (100) preferred orientation on BST buffer layers. The Auger electron spectroscopy depth profiles showed no significant interdiffusion between the PBZ and silicon components. The capacitance-voltage properties of Pt/PBZ (360nm)∕BST (30nm)∕Si structures demonstrated ferroelectric switching effect. The memory windows were about 0.6, 1.25, and 1.76V, respectively, for sweeping bias of ±3, ±4, and ±5V. The leakage current density was below 1×10−8A∕cm2 at ±4V.
Radio frequency rectifiers based on organic thin-film transistors
Authors: Rotzoll Robert , Mohapatra Siddharth , Olariu Viorel , Wenz Robert , Grigas Michelle , Dimmler Klaus , Shchekin Oleg , Dodabalapur Ananth
Published: March 2006

One important technical hurdle that has to be overcome for using organic transistors in radio-frequency identification tags is for these devices to operate at rf frequencies (typically 13.56MHz) in the front end. It was long thought that organic transistors are too slow for this. In this letter we show that organic transistor based full-wave rectifier circuits utilizing pentacene, a p-channel organic semiconductor, can operate at this frequency with a useful efficiency. In order to achieve such high-frequency operation, we make use of the nonquasistatic state of the transistors.
Electron and ion number densities in the space charge layer in thermal plasmas
Authors: Vishnyakov V. I.
Published: March 2006

The space charge layer near the charged surface in the thermal collision plasma has been studied. The analytical expressions describing the spatial distributions of electron and ion number densities, taking into account the nonequilibrium ionization, have been obtained. It has been shown that the nonequilibrium ionization in the space charge layer leads to the essential change of the distribution profile of these number densities.
In situ electrical probing and bias-mediated manipulation of dielectric nanotubes in a high-resolution transmission electron microscope
Authors: Golberg D. , Mitome M. , Kurashima K. , Zhi C. Y. , Tang C. C. , Bando Y. , Lourie O.
Published: March 2006

Boron nitride nanotubes filled with magnesium oxides [MgO,MgO2] and/or hydroxide [Mg(OH)2] are electrically probed and delicately manipulated inside a 300kV JEOL-3000F high-resolution transmission analytical electron microscope equipped with a side-entry “Nanofactory Instruments” piezoholder. At a low bias the nanotubes demonstrate truly insulating behavior. At a high bias of ±30V they show reversible breakdown current of several dozens of nA. Under 300kV electron beam irradiation the nanotubes are positively charged that allows us to perform on-demand manipulation with them through tuning of polarity and/or value of a bias voltage on a gold counterelectrode from −140 to +140V, owing to the prominent electrostatic nanotube-electrode interactions.
Equivalent circuit for an organic field-effect transistor from impedance measurements under dc bias
Authors: Jaiswal Manu , Menon Reghu
Published: March 2006

The output and forward transfer impedance of an organic field-effect transistor have been measured by a lock-in amplifier technique. The small-signal ac response of a pentacene FET, under dc bias, is used to construct the equivalent circuit. The output impedance parameters are numerically simulated using Bode plot analysis and the deviations at low frequency are modeled with contact impedance of the source-drain channel. The ac current generator at the output is estimated along with the gate capacitances.
Measurable entanglement
Authors: Klyachko Alexander , Öztop Barış , Shumovsky Alexander S.
Published: March 2006

The amount of entanglement carried by a quantum bipartite state is usually evaluated in terms of concurrence [S. Hill and W. K. Wootters, Phys. Rev. Lett. 78, 5022 (1997); P. Rungta, V. Bužek, C. M. Caves, M. Hillery, and G. J. Milburn, Phys. Rev. A 64, 042315 (2001)]. We give a physical interpretation of concurrence that reveals a way of its direct measurement and discuss possible generalizations.
Theory of transfer characteristics of nanotube network transistors
Authors: Kumar S. , Pimparkar N. , Murthy J. Y. , Alam M. A.
Published: March 2006

Carbon nanotubes (CNT) nanocomposites used for thin-film transistors (TFTs) provide one of the first technologically-relevant test beds for two-dimensional heterogeneous percolating systems. The characteristics of these TFTs are predicted by considering the physics of heterogeneous finite-sized networks and interfacial traps at the CNT/gate-oxide interface. Close agreement between our numerical results and different experimental observations demonstrates the capability of the model to predict the characteristics of CNT/nanowire-based TFTs. Such predictive models would simplify device optimization and expedite the development of this nascent TFT technology.
Effect of magnetic field on martensitic transition of Ni46Mn41In13 Heusler alloy
Authors: Oikawa K. , Ito W. , Imano Y. , Sutou Y. , Kainuma R. , Ishida K. , Okamoto S. , Kitakami O. , Kanomata T.
Published: March 2006

Magnetic and martensitic transition behaviors of a Ni46Mn41In13 Heusler alloy were investigated by differential scanning calorimetry and vibrating sample magnetometry. A unique martensitic transition from the ferromagnetic austenite phase to the antiferromagneticlike martensite phase was detected and magnetic-field-induced “reverse” transition was confirmed in a high magnetic field. In addition, a large positive magnetic entropy change, which reached 13J∕kgK at 9T, was observed to accompany reverse martensitic transition. This alloy shows promise as a metamagnetic shape memory alloy with magnetic-field-induced shape memory effect and as a magnetocaloric material.
Excitonic and biexcitonic decoherence in self-assembled GaAs quantum dots as observed by phase-locked interferography
Authors: Kuroda K. , Kuroda T. , Sakoda K. , Watanabe K. , Koguchi N. , Kido G.
Published: March 2006

We observe single-photon interferograms for emission of a single self-assembled GaAs quantum dot. A phase stabilizer is applied to the interferometer, enabling us to trace autocorrelations of weak emissions with a photon-counting level. At low excitation, where the average number of excitons in a dot is less than one, interferography reveals a single exponential decay, reflecting excitonic decoherence. At moderately high excitation, polarization interference between the exciton-biexciton transitions is found to appear on the interferogram. The decoherence time and the binding energy of biexcitons are determined simultaneously.
Thermal expansion of a glassy alloy studied using a real-space pair distribution function
Authors: Louzguine-Luzgin Dmitri V. , Inoue Akihisa , Yavari Alain R. , Vaughan Gavin
Published: March 2006

Thermal expansion of a glassy Cu55Hf25Ti15Pd5 alloy studied by using reciprocal space functions is verified using a real-space pair distribution function. The experimental results obtained by real-time diffraction during heating in a synchrotron beam and their Fourier transformation processing to derive radial distribution functions indicate that both reciprocal and real-space distribution functions give good agreement in the calculation of thermal expansion data. In addition to providing structural information, these findings indicate that the change in the average atomic nearest-neighbor distance evaluated from the variation of the position of the main broad diffraction maximum can provide good thermal expansion data for metallic glasses.