bluuebunny/crossref_metadata_2025 huggingface dataset:
Direct measurement of current-induced fieldlike torque in magnetic tunnel junctions
Authors: Devolder T. , Kim Joo-Von , Chappert C. , Hayakawa J. , Ito K. , Takahashi H. , Ikeda S. , Ohno H.
Published: June 2009

We present a direct and quantitative method to measure the current-induced fieldlike torques in current perpendicular to plane magnetic multilayers. We illustrate this method on nanopillars patterned from MgO-based magnetic tunnel junctions. By measuring how the Stoner astroids are displaced, shrinked, and distorted under the influence of a bias current, we separate the contributions of the current-induced fieldlike torque, the current-induced heating, and the current-induced spin torque. In contrast to high frequency based measurement methods that are better suited to analyze the spin torque than the fieldlike term, our measurement is direct and quantitative for the current-induced fieldlike term and the heating induced reduction in shape anisotropy, while only qualitative for the spin torque.
A generalized equivalent circuit theory for the electric and magnetic resonances of metallic wire networks
Authors: Zhang Weiyi , Chui S. T.
Published: June 2009

We generalize Kirchoff’s law for multiply connected wire networks to finite frequencies. We focus on the boundary conditions not present in the conventional Kirchoff’s law at joints when more than three wires come together, which is absent in our previous “circuit theory” for the finite frequency properties of metallic wire networks for singly connected structures. These boundary conditions at the joints involve introducing localized boundary electric fields, in addition to the electric fields of inductive and capacitive origins. The boundary fields act as natural “Lagrange multipliers” for imposing the boundary conditions on the circuit currents. In this way the number of equations is the same as the number of unknowns. The eigenmodes determine not only the circuit current and charge profiles, but also the boundary electric fields which supplement such profiles. The application to T- and H-shape metallic wire networks suggests that the basic types of resonances are mainly controlled by the symmetry and the wire dimensions of the networks. The low frequency modes form along the longest connected paths of the wire network while the high frequency modes can be generated via succeedingly adding more nodes along these various wire paths. The characteristic behavior of the electric and magnetic responses can be inferred from the circuit current profile of a given mode, which offers a simple physical picture on circuit design with particular electromagnetic parameters.
Transport properties near the metal to insulator transition in samarium substituted (Bi,Pb)-2212 system
Authors: Shabna R. , Sarun P. M. , Vinu S. , Syamaprasad U.
Published: June 2009

The electrical transport properties of insulating and superconducting samples of Bi1.7Pb0.4Sr2−xSmxCa1.1Cu2.1O8+δ system is studied across the metal to insulator transition (MIT) region by varying the x values from 0.5 to 1.0 in steps of 0.1. X-ray diffraction analysis, scanning electron microscopy, energy dispersive x-ray analysis (EDAX), and electrical resistivity measurements have been employed for the characterization of all samples. The x-ray and EDAX analyses indicate that samarium (Sm) atoms are incorporated into the crystalline structure of Bi1.7Pb0.4Sr2Ca1Cu2O8+δ [(Bi,Pb)-2212]. Samples with x≤0.6 undergo superconducting transitions while those with 0.7≤x≤1.0 exhibit semiconducting behavior. The MIT is observed at 0.6<x≤0.7 with a suppression in superconductivity. The results show that with increase in temperature, the transport mechanism of the semiconducting samples transits from two dimensional variable range hopping to thermally activated conduction. A scaling model is presented for the MIT and the results are interpreted on the basis of hole filling and disorder due to the substitution of Sm at the strontium site of (Bi,Pb)-2212 system.
Correlation between YBa2Cu3O7 nuclei density and the grain orientation of the CeO2 buffered Ni–W template of the second-generation superconducting wire
Authors: Solovyov Vyacheslav F. , Abraimov Dmytro , Miller Dean , Li Qiang , Wiesmann Harold
Published: June 2009

The prime goal of the second-generation superconducting wire technology is to grow high-quality epitaxial layers of YBa2Cu3O7 (YBCO) superconductor using high-rate deposition on low-cost, kilometer-long substrates. We analyze the influence of Ni–W RABiTS™ substrate grain misalignment on nucleation of epitaxial YBCO during metal-organic ex situ processing. Electron backscatter diffraction orientation maps are correlated with YBCO nuclei density obtained from scanning-electron microscopy. A critical Ni–W grain tilt misorientation angle of 8.5° was identified above which the YBCO nuclei density was observed to be extremely low, approaching zero. A proposed model explains the reduction in nuclei density as being due to the absence of (001) substrate terraces wide enough to accommodate the critical size for YBCO nuclei. This study emphasizes the strong effect of the out-of-plane tilt of substrate grains on superconducting properties of YBCO layers produced by metal-organic deposition.
A universal theoretical approach for examining the efficiency of doping processes in semiconductors
Authors: Gavaza G. M. , Yu Z. G. , Wu P.
Published: June 2009

The efficiency of doping crystalline semiconductors is often associated with the solubility of the dopant. However, the scope of doping is usually a high density of extrinsic carriers, which is not necessarily guaranteed by a high solubility of the dopant but by the high concentrations of a certain type of defects. We present a universal approach to relate the concentrations of defects resulting in extrinsic carriers in crystalline semiconductors as a function of the doping process experimental parameters. Based on quantum multiple scattering, our method features two main theoretical novelties: the transition operators are calculated by iteratively “switching on” the interaction potentials and the temperature effects are introduced as semiclassical vibrations. The bulk and doped configurations are characterized by their ground state electronic structures obtained from first-principles calculations. The dependency on substrate temperature of the carrier concentration calculated with our method is shown to reproduce well the experimental results not only for well-known processes (the n-type doping of ZnO with Al and the p-type doping of GaAs with Be) but also for challenging processes such as the p-type doping of ZnO with P. This proves that our approach is reliable and that it can become a powerful tool in the search for optimal growth condition.
Extended x-ray absorption fine structure study of phase transitions in the piezoelectric perovskite K0.5Na0.5NbO3
Authors: Kodre A. , Tellier J. , Arčon I. , Malič B. , Kosec M.
Published: June 2009

Following an x-ray diffraction study of phase transitions of the piezoelectric perovskite K0.5Na0.5NbO3 the structural changes of the material are studied using extended x-ray absorption fine structure analysis, whereby the neighborhood of Nb atom is determined in the temperature range of monoclinic, tetragonal, and cubic phases. Within the entire range Nb atom is displaced from the center of the octahedron of its immediate oxygen neighbors, as witnessed by the splitting of Nb–O distance. The model shows high prevalence of the displacement in the (111) crystallographic direction of the simple perovskite cell. The corresponding splitting of the Nb–Nb distance is negligible. There is no observable disalignment of the linear Nb–O–Nb bonds from the ideal cubic arrangement, judging from the intensity of the focusing of the photoelectron wave on the Nb–Nb scattering path by the interposed oxygen atom. As a general result, the phase transitions are found as an effect of the long-range order, while the placement of the atoms in the immediate vicinity remains largely unaffected.
Double barrier strained quantum well infrared photodetectors for the 3–5μm atmospheric window
Authors: Guériaux Vincent , Nedelcu Alexandru , Bois Philippe
Published: June 2009

We present a detailed study of double barrier strained Al0.35Ga0.65As/AlAs/GaAs/In0.2Ga0.8As quantum well infrared photodetectors on GaAs substrate. Measurements were made on four different well widths active layers and on several mesa pixels with different optical coupling structures. We obtained responses peaked in the spectral range 3.6–4.6 μm. Based on the experimental results, we show that in the background limited regime, the impact ionization is the restrictive transport process for midwave detectors. We also demonstrate a 4 μm structure with the high background limited detectivity of 2×1011 Jones at 77 K and 9×1010 Jones at 110 K (2π field of view, 300 K background).
Room temperature ferromagnetism of pure ZnO nanoparticles
Authors: Gao Daqiang , Zhang Zhaohui , Fu Junli , Xu Yan , Qi Jing , Xue Desheng
Published: June 2009

We report the room temperature ferromagnetism (RTF) of pure ZnO nanoparticles, which were prepared by coprecipitation method. Magnetization measurement indicates that the ZnO nanoparticles annealed in air at 450, 550, 650, and 800 °C exhibit the RTF and the decrease in the ferromagnetism is performed with the increase in annealed temperature. Selected area electron diffraction, x-ray diffraction, and x-ray photoelectron spectroscopy measurements show that all the samples possess a typical wurtzite structure and no other impurity phases are observed. The results of the Raman spectra indicate that there are lots of defects existing in the fabricated samples. It is also found that the ferromagnetism of ZnO nanoparticles increases after annealing in vacuum condition and decreases after annealing in a rich-oxygen atmosphere. These results confirm that the oxygen vacancies play an important role in introducing ferromagnetism for the ZnO nanoparticles in our case.
X-ray diffraction and infrared multiple-angle incidence resolution spectroscopic studies on the crystal structure and molecular orientation of zinc-porphyrin thin films on a SiO2/Si substrate
Authors: Ryuzaki S. , Hasegawa T. , Onoe J.
Published: June 2009

The crystal structure and molecular orientation of zinc-octaethylporphyrin [Zn(OEP)] thin films deposited on a SiO2/Si(111) substrate have been investigated using x-ray diffraction (XRD) and infrared multiple-angle incidence resolution spectroscopy (MAIRS), respectively. XRD results show that the Zn(OEP) thin films have two kinds of crystallites with a diffraction plane (2θ=7.9°) parallel to the substrate and with that (2θ=7.1°) inclined mainly by 60°–70° to the substrate. Comparison with simulated XRD results of the Zn(OEP) single crystal indicates that the diffraction angle of 2θ=7.9° is consistent with that of the (011¯) plane of the single crystal, while the angle of 2θ=7.1° is slightly different from that of 7.3° corresponding to the (010) plane of the single crystal. On the other hand, MAIRS results demonstrate that Zn(OEP) molecules in the crystallite corresponding to 2θ=7.9° are inclined by 52.2±0.9° to the substrate, which is in a good agreement with that of ca. 50° estimated from the Zn(OEP) single crystal structure. In addition, these crystalline sizes are estimated to be approximately 20 nm and independent of film thickness. Accordingly, it can be concluded from the XRD and MAIRS results that the Zn(OEP) films have two kinds of grains. One has the single crystal structure, in which Zn(OEP) molecules are inclined by 52.2°±0.9° to the substrate. On the other hand, the other has a crystal structure close to the single crystal, in which Zn(OEP) molecules would be inclined by 10°–20° to the substrate.
On the relationship between nonlinear equations integrable by the method of characteristics and equations associated with commuting vector fields
Authors: Zenchuk A. I.
Published: June 2009

It was shown recently that Frobenius reduction of the matrix fields reveals interesting relations among the nonlinear partial differential equations (PDEs) integrable by the inverse spectral transform method (S-integrable PDEs), linearizable by the Hopf–Cole substitution (C-integrable PDEs), and integrable by the method of characteristics (Ch-integrable PDEs). However, only two classes of S-integrable PDEs have been involved: soliton equations (such as Korteweg–de Vries, nonlinear Schrödinger, Kadomtsev–Petviashvili, and Davey–Stewartson equations) and GL(N,C) self-dual-type PDEs (such as Yang–Mills equation). In this paper we consider the simple five-dimensional nonlinear PDE from another class of S-integrable PDEs, namely, scalar nonlinear PDE which is the commutativity condition of the pair of vector fields. We show its origin from the (1+1)-dimensional hierarchy of Ch-integrable PDEs after certain composition of Frobenius type and differential reductions imposed on the matrix fields. Matrix generalization of the above scalar nonlinear PDE will be derived as well.
Symmetries and reduction techniques for dissipative models
Authors: Ruggieri Marianna , Valenti Antonino
Published: June 2009

Symmetries and reduction techniques are applied to a mathematical model describing one-dimensional motions in nonlinear dissipative media. Reduced equations through the optimal system of subalgebras are performed and some exact invariant solutions are shown in a physical application.
Biased-probe-induced water ion injection into amorphous polymers investigated by electric force microscopy
Authors: Knorr Nikolaus , Rosselli Silvia , Miteva Tzenka , Nelles Gabriele
Published: June 2009

Although charging of insulators by atomic force microscopy (AFM) has found widespread interest, often with data storage or nanoxerography in mind, less attention has been paid to the charging mechanism and the nature of the charge. Here we present a systematic study on charging of amorphous polymer films by voltage pulses applied to conducting AFM probes. We find a quadratic space charge limited current law of Kelvin probe force microscopy and electrostatic force microscopy peak volumes in pulse height, offset by a threshold voltage, and a power law in pulse width of positive exponents smaller than one. We interpret the results by a charging mechanism of injection and surface near accumulation of aqueous ions stemming from field induced water adsorption, with threshold voltages linked to the water affinities of the polymers.
Selenium-loaded zeolite (ZSM-5): I. Structures and fundamental properties
Authors: Saitoh Akira , Tanaka Keiji
Published: June 2009

Atomic structures of Se clusters impregnated into ZSM-5 single crystals have been investigated through structural and optical experiments. Raman-scattering spectroscopy suggests that Se in the zeolite takes single chain structures. X-ray diffraction implies that the chains form fourfold (not threefold as in trigonal Se) helices. The chain length is estimated from weight and electron-spin resonance measurements at 102 atoms. The material shows an optical absorption edge at ℏω≃2.2 eV, which accompanies dichroism, suggesting chain alignment in the zeolite pores.
Selenium-loaded zeolite (ZSM-5): II. Photoinduced phenomena
Authors: Saitoh Akira , Tanaka Keiji
Published: June 2009

Photoinduced phenomena in Se-loaded ZSM-5 single crystals have been studied through electron-spin resonance, optical absorption, and photoluminescence spectroscopy. Marked exposure effects appear at room temperature. Upon illumination of visible light, an electron-spin signal at g≈2.00 increases by an order, subgap absorption of 103 cm−1 appears, and photoluminescence at subgap regions weakens to a half. Timescales of these photoinduced changes including recoveries are not the same, which suggests different origins for the three changes. The spin increase evinces bond scission in single Se chains.
Electronic and chemical properties of cathode structures using 4,7-diphenyl-1,10-phenanthroline doped with rubidium carbonate as electron injection layers
Authors: Chen Mei-Hsin , Chen Yu-Hung , Lin Chang-Tin , Lee Guan-Ru , Wu Chih-I , Leem Dong-Seok , Kim Jang-Joo , Pi Tun-Wen
Published: June 2009

The electronic properties and chemical interactions of cathode structures using 4,7-diphenyl-1, 10-phenanthroline (Bphen) doped with rubidium carbonate (Rb2CO3) as electron injection layers were investigated. Current-voltage characteristics reveal that the devices with Bphen/Rb2CO3/Al as cathode structures possess better electron injection efficiency than those with cathode structures of Bphen/LiF/Al. Ultraviolet and x-ray photoemission spectroscopy shows that n-type doping effects resulting from Rb2CO3 and the gap states created by aluminum deposition are both keys to the improved carrier injection efficiency. Moreover, theoretical calculation indicates that the chemical reaction between aluminum and the nitrogen atoms in Bphen is the origin of the gap states.
Computer simulation of the light yield nonlinearity of inorganic scintillators
Authors: Kerisit Sebastien , Rosso Kevin M. , Cannon Bret D. , Gao Fei , Xie YuLong
Published: June 2009

To probe the nature of the physical processes responsible for the nonlinear scintillation light yield of inorganic scintillators, we have combined an ab initio based Monte Carlo code for calculating the microscopic spatial distributions of electron-hole pairs with an atomistic kinetic Monte Carlo (KMC) model of energy-transfer processes. In the present study, we focus on evaluating the contribution of an annihilation mechanism between self-trapped excitons (STE) to the scintillation response of pure CsI and Ce-doped LaBr3. A KMC model of scintillation mechanisms in pure CsI was developed previously and we introduce in this publication a similar model for Ce-doped LaBr3. We show that the KMC scintillation model is able to reproduce both the kinetics and efficiency of the scintillation process in Ce-doped LaBr3. Relative light output curves were generated at several temperatures for both scintillators from simulations carried out at incident γ-ray energies of 2, 5, 10, 20, 100, and 400 keV. These simulations suggest that STE-STE annihilation can account for the initial rise in relative light yield with increasing incident energy for both types of materials. This is due to the fact that the proportion of high-density regions decreases as the incident energy increases, thus reducing the likelihood for STE-STE encounter. In addition, the simulations clearly show a lack of temperature dependence of the relative light output, in agreement with a majority of experimental work on the temperature dependence of nonlinearity in inorganic scintillators. The collective modeling tool is a fundamental advance over phenomenological modeling approaches because it has its foundation in first-principles physics of scintillation. While the KMC simulations here are parametrized largely by empirically derived rate constants, this study suggests that combining ab initio based electron-hole pair distributions with ab initio derived rate constants for a select set of energy transfer processes is in principle sufficient to detach this tool from experiment entirely, yielding a holistic predictive simulation framework useful for exploring a wide range of scintillator performance characteristics.
The effects of oxygen concentration and light intensity on the photostability of zwitterionic chromophores
Authors: Raymond S. G. , Williams G. V. M. , Lochocki B. , Bhuiyan M. D. H. , Kay A. J. , Quilty J. W.
Published: June 2009

Photostability measurements at different oxygen partial pressures and light intensities have been made on host-guest films containing amorphous polycarbonate and an organic chromophore with a high second order nonlinear optical figure of merit. We find that the photodegradation quantum efficiency dramatically increases with increasing oxygen partial pressure. At very low oxygen partial pressures (8×10−6 bar) the average number of photons required to photodegrade a chromophore is as high as 2×108 at 655 nm. The photodegradation quantum efficiency in air is observed to decrease with increasing optical intensity. We show that this is due to a reduced oxygen content in the film caused by chromophore photodegradation rather than ground state bleaching. There is an anomalous increase and then decrease in the photoluminescence intensity that cannot easily be explained.
Influence of surface coating on the upconversion emission properties of LaPO4:Yb/Tm core-shell nanorods
Authors: Ghosh Pushpal , de la Rosa Elder , Oliva Jorge , Solis David , Kar Arik , Patra Amitava
Published: June 2009

A solution based hydrothermal method is used for the preparation of LaPO4:Yb–Tm doped and LaPO4:Yb/Tm core-shell upconverted nanorods. The morphologies, structure, formation mechanism, and upconversion emission properties of these nanocrystals are investigated in detail. Tensile strain is observed for LaPO4:Yb–Tm doped material but compressive strain is obtained for LaPO4:Yb/Tm core-shell material. Analysis suggests that the lattice strain plays an important role on the upconversion emission. Bright blue (475 nm) and red (650 nm) upconversion emissions associated to G14→H36 and F32→H36 transitions were observed and a significant higher in upconversion emission intensity is observed in core-shell nanorods. The effective decay time calculated are 230 and 110 μs for core-shell and doped nanorods, respectively, indicating the removal of surface defects due to surface coating. It reveals that the core-shell structure shows better upconversion emission efficiency.
Peculiarities of holes injection into organic molecular solids
Authors: Averkiev N. S. , Zakrevskii V. A. , Rozhansky I. V. , Sudar N. T.
Published: June 2009

A microscopic mechanism of holes injection from metallic electrode into organic molecular solids (OMSs) in high electric field is proposed. A case is considered of ionization energy of the molecules exceeding work function of the metal. It is shown that the main contribution to the injection current comes from direct isoenergetic transitions (without interaction with phonons) from localized states in OMS to empty states in the metal. Strong dependence of the injection current on applied voltage originates from variation of the number of empty states available in the metal rather than by modification of the interface barrier shape.
Design of solar beam collectors consisting of multilayer optical waveguide films for integrated solar energy conversion systems
Authors: Shioya Rina , Yoshimura Tetsuzo
Published: June 2009

To reduce consumption of semiconductor materials, to provide wide-angle solar beam collecting capability, and to make systems flexible and compact, we propose integrated solar energy conversion systems with waveguide-type solar beam collectors having film structures. We design structures of solar beam collectors consisting of multilayer optical waveguides with vertical mirrors and estimate the solar beam collecting efficiencies using the beam propagation method and the finite difference time domain method. The efficiencies for vertical incident light beams of 600 nm wavelength increase from 62% to 87% with increasing the layer counts from 1 to 3. The efficiency for incident angle of 45° is 1.5 times larger in solar beam collectors with 45° and 30° mirrors than in those with only 45° mirrors. Almost the same light-beam-collecting characteristics are obtained for wavelengths of 400 and 800 nm, achieving overall light beam collecting efficiency of more than 60%. These results suggest that the solar beam collector consisting of multilayer optical waveguides with mirrors of different angles enables wide-angle light beam collecting.
Strong enhancement of terahertz emission from GaAs in InAs/GaAs quantum dot structures
Authors: Estacio Elmer , Pham Minh Hong , Takatori Satoru , Cadatal-Raduban Marilou , Nakazato Tomoharu , Shimizu Toshihiko , Sarukura Nobuhiko , Somintac Armando , Defensor Michael , Awitan Fritz Christian B. , Jaculbia Rafael B. , Salvador Arnel , Garcia Alipio
Published: June 2009

We report on the intense terahertz emission from InAs/GaAs quantum dot (QD) structures grown by molecular beam epitaxy. Results reveal that the QD sample emission was as high as 70% of that of a p-type InAs wafer, the most intense semiconductor emitter to date. Excitation wavelength studies showed that the emission was due to absorption in strained undoped GaAs, and corresponds to a two order-of-magnitude enhancement. Moreover, it was found that multilayer QDs emit more strongly compared with a single layer QD sample. At present, we ascribe the intense radiation to huge strain fields at the InAs/GaAs interface.
Experimental verification of quasiscarred resonance mode
Authors: Kim Chil-Min , Lee Sang Hun , Oh Kwang Ryung , Kim Jong Hoi
Published: June 2009

We have experimentally verified the lasing of a quasiscarred resonance mode (QSRM) in a spiral-shaped InGaAsP microcavity laser by pumping the cavity boundary with current injection. To confirm the lasing of a QSRM, a far field pattern is obtained and six laser emission directions are found; one corresponds to the whispering gallery type modes and the other to a QSRM. By observing the spectrum and lasing thresholds, the lasing characteristics of the QSRM are analyzed.
A dose dependence study of O2 adsorbed on large Ar clusters
Authors: Rander T. , Lindblad A. , Lundwall M. , Tchaplyguine M. , Öhrwall G. , Svensson S. , Björneholm O.
Published: June 2009

An investigation of the behavior of O2 molecules in and on O2-doped large (⟨N⟩≃8000) Ar host clusters has been performed by means of core and valence photoelectron spectroscopy. Data from pure O2 and Ar clusters, as well as from O2-doped Ar clusters, are presented. The experimental data together with calculations of the binding energy shifts of oxygen molecular ions in and on the surface of a large host Ar cluster show that the diffusion behavior has a strong dependence on the doping pressure. We conclude that the oxygen molecules in the doped Ar host do not partake in band formation, since there is clear vibrational resolution in the spectral features stemming from screened O2+ ions. This implies that valence photoelectron spectroscopy can be used to determine the geometrical structure of this and certain, similar, cluster systems.
Light-cone matrix product
Authors: Hastings M. B.
Published: June 2009

We show how to combine the light-cone and matrix product algorithms to simulate quantum systems far from equilibrium for long times. For the case of the XXZ spin chain at Δ=0.5, we simulate to a time of ≈22.5. While part of the long simulation time is due to the use of the light-cone method, we also describe a modification of the infinite time-evolving bond decimation algorithm with improved numerical stability, and we describe how to incorporate symmetry into this algorithm. While statistical sampling error means that we are not yet able to make a definite statement, the behavior of the simulation at long times indicates the appearance of either “revivals” in the order parameter as predicted by Hastings and Levitov (e-print arXiv:0806.4283) or of a distinct shoulder in the decay of the order parameter.
Electromagnetically induced transparency-like effect in a single polydimethylsiloxane-coated silica microtoroid
Authors: Xiao Yun-Feng , He Lina , Zhu Jiangang , Yang Lan
Published: June 2009

We study both experimentally and theoretically the coupling between a fiber taper and two whispering-gallery modes, which are simultaneously excited in a single pilydimethyl-siloxane-coated silica microtoroid system. The transmission spectrum of the fiber-coupled two-mode microresonator shows a sharp electromagnetically induced transparency-like window within the resonant absorption region. This line shape results from destructive interference between two optical pathways associated with two distinct coexisting modes in a single resonator. The sharp transparency peak has great potential applications in light modulation and highly sensitive biochemical sensors.
Phase tunable holographic fabrication for three-dimensional photonic crystal templates by using a single optical element
Authors: Xu Di , Chen Kevin P. , Harb Ahmad , Rodriguez Daniel , Lozano Karen , Lin Yuankun
Published: June 2009

This paper demonstrates a phase tunable holographic fabrication of three-dimensional photonic lattice structures using a single optical element. A top-cut four-side prism is employed to generate five-beam three-dimensional interference patterns. A silica glass slide is inserted into the optical path to adjust the phase of one interfering beam relative to other four beams. The phase control of the interfering laser beam renders the lattice of the interference pattern from a face-center tetragonal symmetry into a high contrast, interconnecting diamondlike symmetry. This method provides a flexible approach to fabricating three-dimensional photonic lattices with improved photonic band structures.
Electric field dependence of trap-assisted-tunneling current in strained SiGe source/drain junctions
Authors: Gonzalez M. Bargallo , Simoen E. , Vissouvanadin B. , Eneman G. , Verheyen P. , Loo R. , Claeys C.
Published: June 2009

This paper evaluates the impact of the local electric field on the leakage current of strained Si1−xGex source/drain junctions. The difference in the SiGe and Si lattice constants creates a biaxial in-plane compressive stress in the epilayer and a tensile expansion on the top of the underlying silicon substrate. Current-voltage (I-V) and capacitance-voltage (C-V) measurements were employed to further investigate the stress-induced leakage current when field-assisted mechanisms such as trap-assisted-tunneling and band-to-band-tunneling are dominant, owing to the presence of high electric fields in the highly doped silicon depletion region.
High permittivity SrHf0.5Ti0.5O3 films grown by pulsed laser deposition
Authors: Yan L. , Suchomel M. R. , Grygiel C. , Niu H. J. , McMitchell S. R. C. , Bacsa J. , Clark J. H. , Werner M. , Chalker P. R. , Rosseinsky M. J.
Published: June 2009

High permittivity SrHf0.5Ti0.5O3 films (k=62.8) have been deposited on (001) Nb–SrTiO3 single crystal conducting substrates by pulsed laser deposition. The SrHf0.5Ti0.5O3 films grow epitaxially with atomically smooth surfaces (root mean square roughness 4.8 Å) and a c-axis orientation parallel to the substrate. The measured band gap of SrHf0.5Ti0.5O3 is 3.47 eV compared with 3.15 eV in SrTiO3. Under an applied electric field of 600 kV/cm, the leakage current density of the SrHf0.5Ti0.5O3 films is 4.63×10−4 A/cm2. These attractive dielectric properties and enhanced band gap values make SrHf0.5Ti0.5O3 a promising candidate for high-k dielectric applications in silicon-based integrated circuits.
Breakdown phenomena of Al-based high-k dielectric/SiO2 stack on 4H-SiC
Authors: Kumta A. , Rusli  , Xia J. H.
Published: June 2009

In this report we present the breakdown phenomena of Al-based high-k dielectrics and thermal SiO2 intermediate layer stack on 4H-SiC. The Al-based dielectrics, namely, sputter deposited aluminum nitride (AlNx) and hydrogenated aluminum nitride (AlNy:H), have been used to form metal-insulator-semiconductor (MIS) capacitors and field-plate terminated Schottky barrier diodes (SBDs) on 4H-SiC. Multistep breakdown modes visible on these devices have been investigated using measurements of dielectric relaxation currents on MIS capacitors.
A time-gating scintillation detector for the measurement of laser-induced fast neutrons
Authors: Lee Sungman , Park Sangsoon , Yea Kwon-hae , Cha Hyungki
Published: June 2009

A time-gating scintillation detector, in which a fast high voltage switch is used for gating a channel photomultiplier, was developed for a measurement of laser-induced fast neutrons. The x rays generated from the intense femtosecond laser and the solid target interactions were suppressed selectively and a time-of-flight signal of a laser-generated fast neutron was measured effectively. The detector was used successfully to measure the neutron yield of a femtosecond, deuterated, polystyrene plasma.
A passive long-wavelength infrared microscope with a highly sensitive phototransistor
Authors: Kajihara Yusuke , Komiyama Susumu , Nickels Patrick , Ueda Takeji
Published: June 2009

A passive scanning confocal microscope in the long-wavelength infrared (LWIR) region has been developed for sensitive imaging of spontaneous LWIR radiation by utilizing an ultrahighly sensitive detector, called the charge-sensitive infrared phototransistor (CSIP). The microscope consisted of room-temperature components including a Ge objective lens and liquid helium temperature components including a confocal pinhole, Ge relay lenses, and CSIP detector. With the microscope, thermal radiation (wavelength of 14.7 μm) spontaneously emitted by the object was studied with a spatial resolution of 25 μm. Clear passive LWIR imaging pictures were obtained by scanning a sample consisting of glass, Al foil, Ag paste, and Au. Clear passive LWIR image was also obtained even when the sample surface was covered by a GaAs or Si plate. This work suggests usefulness of CSIP detectors for application of passive LWIR microscopy.
Nanostructured black silicon and the optical reflectance of graded-density surfaces
Authors: Branz Howard M. , Yost Vernon E. , Ward Scott , Jones Kim M. , To Bobby , Stradins Paul
Published: June 2009

We fabricate and measure graded-index “black silicon” surfaces and find the underlying scaling law governing reflectance. Wet etching (100) silicon in HAuCl4, HF, and H2O2 produces Au nanoparticles that catalyze formation of a network of [100]-oriented nanopores. This network grades the near-surface optical constants and reduces reflectance to below 2% at wavelengths from 300 to 1000 nm. As the density-grade depth increases, reflectance decreases exponentially with a characteristic grade depth of about 1/8 the vacuum wavelength or half the wavelength in Si. Observation of Au nanoparticles at the ends of cylindrical nanopores confirms local catalytic action of moving Au nanoparticles.
Real-time millimeter-wave shadowgraphy using the visible continuum from a slab of the Cs–Xe dc discharge
Authors: Gitlin M. S. , Tsvetkov A. I.
Published: June 2009

We have demonstrated video-rate millimeter-wave (MMW) shadowgraphy using a slab of the positive column of Cs–Xe dc discharge as a two-dimensional real-time MMW sensor. MMW images were converted into visible images using the visible continuum from the plasma slab. Near-field shadow projection MMW images of amplitude and phase objects have been obtained using pulsed watt-scale 35.4 GHz radiation for object illumination. The single-frame signal-to-noise ratio was about 13 dB for an exposure time of 1 ms. Time-averaged MMW intensity could be about several mW/cm2 in the case of video-rate imaging.
Space charge effects in rf traps: Ponderomotive concept and stroboscopic analysis
Authors: Shah Kushal , Ramachandran Harishankar
Published: June 2009

Exact solutions for one-dimensional (1D) plasma dynamics in an rf trap are known when space charge effects are neglected [K. Shah and H. S. Ramachandran, Phys. Plasmas 15, 062303 (2008)]. In this work, weak space charge effects in an rf trap are considered. An analytic expression for the time varying distribution function of the 1D plasma is obtained. It is shown that the plasma is a Maxwellian up to the lowest order in nonlinearity and that the spatially constant temperature periodically oscillates in time at the same rate as the rf frequency. It was shown by Krapchev [Phys. Rev. Lett. 42, 497 (1979)] that the time averaged distribution function is double humped with respect to velocity beyond a certain threshold in space. The time average of the complete time varying distribution function is obtained and some of the predictions of Krapchev are recovered, while also finding discrepancies. The relationship between stroboscopic orbits and the time averaged ponderomotive orbit are obtained for such traps.
Electron spectroscopy using two-dimensional electron detection and a camera in a single electron counting mode
Authors: Vos M. , Went M. R. , Weigold E.
Published: June 2009

A brief description is given of an economical implementation of the read out of a two-dimensional detector in an electron spectrometer by a charge coupled device camera, using a pulse counting mode. Count rates up to 10 kHz can be handled in this way. A comparison with results obtained using a resistive anode detector is given for the case of electron scattering from Xe atoms. Good agreement was obtained between both detection techniques, establishing the validity of the method described here.
Coupled translation-rotation eigenstates of H2 in C60 and C70 on the spectroscopically optimized interaction potential: Effects of cage anisotropy on the energy level structure and assignments
Authors: Xu Minzhong , Sebastianelli Francesco , Gibbons Brittney R. , Bačić Zlatko , Lawler Ronald , Turro Nicholas J.
Published: June 2009

We have developed a quantitatively accurate pairwise additive five-dimensional (5D) potential energy surface (PES) for H2 in C60 through fitting to the recently published infrared (IR) spectroscopic measurements of this system for H2 in the vibrationally excited ν=1 state. The PES is based on the three-site H2–C pair potential introduced in this work, which in addition to the usual Lennard-Jones (LJ) interaction sites on each H atom of H2 has the third LJ interaction site located at the midpoint of the H–H bond. For the optimal values of the three adjustable parameters of the potential model, the fully coupled quantum 5D calculations on this additive PES reproduce the six translation-rotation (T-R) energy levels observed so far in the IR spectra of H2@C60 to within 0.6%. This is due in large part to the greatly improved description of the angular anisotropy of the H2-fullerene interaction afforded by the three-site H2–C pair potential. The same H2–C pair potential spectroscopically optimized for H2@C60 was also used to construct the pairwise additive 5D PES of H2 (ν=1) in C70. This PES, because of the lower symmetry of C70 (D5h) relative to that of C60 (Ih), exhibits pronounced anisotropy with respect to the direction of the translational motion of H2 away from the cage center, unlike that of H2 in C60. As a result, the T-R energy level structure of H2 in C70 from the quantum 5D calculations on the optimized PES, the quantum numbers required for its assignment, and the degeneracy patterns which arise from the T-R coupling for translationally excited H2 are all qualitatively different from those determined previously for H2@C60 [M. Xu et al., J. Chem. Phys. 128, 011101 (2008).
High dielectric loss and its monotonic dependence of conducting-dominated multiwalled carbon nanotubes/silica nanocomposite on temperature ranging from 373 to 873 K in X-band
Authors: Song Wei-Li , Cao Mao-Sheng , Hou Zhi-Ling , Fang Xiao-Yong , Shi Xiao-Ling , Yuan Jie
Published: June 2009

The dielectric properties of multiwalled carbon nanotubes/silica (MWNTs/SiO2) nanocomposite with 10 wt % MWNTs are investigated in the temperature range of 373–873 K at frequencies between 8.2 and 12.4 GHz (X-band). MWNTs/SiO2 exhibits a high dielectric loss and a positive temperature coefficient (PTC) of dielectric effect that complex permittivity increases monotonically with increasing temperature. The PTC effect on the dielectric constant is ascribed to the decreased relaxation time of interface charge polarization, and the PTC effect on the dielectric loss is mainly attributed to the increasing electrical conductivity. The loss tangent strongly supports the dominating contribution of conductance to the dielectric loss.
Structural and magnetic properties of ε-Fe1−xCoxSi thin films deposited via pulsed laser deposition
Authors: Manyala Ncholu , Ngom Balla D. , Beye A. C. , Bucher Remy , Maaza Malik , Strydom Andre , Forbes Andrew , Johnson A. T. Charlie , DiTusa J. F.
Published: June 2009

We report pulsed laser deposition synthesis and characterization of polycrystalline Fe1−xCoxSi thin films on Si (111). X-ray diffraction, transmission electron, and atomic force microscopies reveal films to be dense, very smooth, and single phase with a cubic B20 crystal structure. Ferromagnetism with significant magnetic hysteresis is found for all films including nominally pure FeSi films in contrast to the very weak paramagnetism of bulk FeSi. For Fe1−xCoxSi this signifies a change from helimagnetism in bulk, to ferromagnetism in thin films. These ferromagnetic thin films are promising as a magnetic-silicide/silicon system for polarized current production, manipulation, and detection.
Post-fabrication fine-tuning of photonic crystal quantum well infrared photodetectors
Authors: Schartner S. , Kalchmair S. , Andrews A. M. , Klang P. , Schrenk W. , Strasser G.
Published: June 2009

Photonic crystal (PC) devices require high fabrication accuracy for on demand positioning of resonances. We describe post-fabrication fine-tuning of a PC quantum well infrared photodetector (QWIP) by sidewall-deposition of silicon nitride. The PC resonance was shifted over a bandwidth of 43 cm−1. From photoresponse measurements we calculated a tuning coefficient of ∂ν/∂dSiN=−0.06 cm−1/nm. The QWIP responsivity did not suffer from nitride absorption while the PC resonance increased by a factor of 1.6. This shows that post-fabrication tuning by dielectric deposition with, e.g., silicon nitride is a feasible method to achieve precise implementations of PC devices.
Origin of second-order nonlinear optical response of polarity-controlled ZnO films
Authors: Park J. S. , Yamazaki Y. , Takahashi Y. , Hong S. K. , Chang J. H. , Fujiwara T. , Yao T.
Published: June 2009

Polarity-controlled ZnO thin films grown on different buffer layers were investigated as nonlinear optical materials for second harmonic generation. The effective nonlinear optical coefficient (deff) of ZnO grown on Cr-compound buffer layers showed a higher value than that of ZnO grown on MgO buffer layers. The correlations among the grain size and surface roughness with the values of deff were found to be strong. The deff of the ZnO film increased with decrease in the grain size and surface roughness. This relationship could be explained by the increase in reaction interface and the decrease in scattering probability at the surface as decrease in grain size and surface roughness, respectively.
Crystal and electronic structure of lithiated nanosized rutile TiO2 by electron diffraction and electron energy-loss spectroscopy
Authors: Wang C. M. , Yang Z. G. , Thevuthasan S. , Liu J. , Baer D. R. , Choi D. , Wang D. H. , Zhang J. G. , Saraf L. V. , Nie Z. M.
Published: June 2009

The crystal and electronic structure of the lithiated nanosized rutile TiO2 were studied using electron diffraction and electron energy-loss spectroscopy (EELS) in a transmission electron microscopy. EELS reveals the Li K-edge at the energy-loss position of ∼61 eV. After lithiation, the t2g-eg crystal-field splitting on both Ti L2,3-edge and O K-edge decreases, the O K-edge shifts toward a higher energy-loss position and the separation between the pre-edge peak and main peak on the O K-edge decreases, suggesting that the lithiation of rutile TiO2 was accompanied by the reduction in Ti ion, indicating a charge transfer from Li to Ti.
Orientation-dependent potential barriers in case of epitaxial Pt–BiFeO3–SrRuO3 capacitors
Authors: Pintilie L. , Dragoi C. , Chu Y. H. , Martin L. W. , Ramesh R. , Alexe M.
Published: June 2009

The leakage current in epitaxial BiFeO3 capacitors with bottom SrRuO3 and top Pt electrodes, grown by pulsed laser deposition on SrTiO3 (100), SrTiO3 (110), and SrTiO3 (111) substrates, is investigated by current-voltage (I-V) measurements in the 100–300 K temperature range. It is found that the leakage current is interface-limited and strongly dependent on the orientation of the substrate. The potential barriers at the electrode interfaces are estimated to about 0.6, 0.77, and 0.93 eV for the (100), (110), and (111) orientations, respectively.
Fabrication of high-Q polydimethylsiloxane optical microspheres for thermal sensing
Authors: Dong C.-H. , He L. , Xiao Y.-F. , Gaddam V. R. , Ozdemir S. K. , Han Z.-F. , Guo G.-C. , Yang L.
Published: June 2009

Polydimethylsiloxane (PDMS) optical microspheres are fabricated and whispering gallery modes with quality factors of 106 in the 1480 nm band are demonstrated. The dependence of the resonance shifts on the input power is investigated in both the transient (blueshift) and the steady-state (redshift) regimes. Moreover, we demonstrate that such high-Q PDMS optical resonators can be used as highly sensitive thermal sensors with temperature sensitivity of 0.245 nm/°C, which is one order of magnitude higher than conventional silica microsphere resonators. The estimated thermal resolution of the sensor is 2×10−4 °C.
Realization of high efficiency microcavity top-emitting organic light-emitting diodes with highly saturated colors and negligible angular dependence
Authors: Wang Qiang , Deng Zhaoqi , Ma Dongge
Published: June 2009

An alternative way to optimize the emission characteristics of a microcavity top-emitting organic light-emitting diode (TOLED) based on a simple device structure is demonstrated via combining a comprehensive theoretical analysis in the microcavity effects with the experimental modification in the carrier injection of both electrodes. It can be seen that the resulting TOLED exhibits much higher efficiencies and a more saturated color than those of the corresponding conventional bottom-emitting device, as well as hardly detectable color shift with viewing angles. Such a strategy may be more feasible in practical application for active-matrix organic light-emitting diode displays.
Temporal behavior of cold atmospheric plasma jet
Authors: Shashurin A. , Shneider M. N. , Dogariu A. , Miles R. B. , Keidar M.
Published: June 2009

Temporally resolved evolution of parameters in atmospheric plasma jet is studied by means of microwave scattering, fast photographing, and measuring of jet currents. It is observed that streamer (“plasma bullet”) propagating along with gas flow is generated immediately after the breakdown. It is demonstrated that an afterglow plasma column remains on the way of streamer passing. Lifetime of the afterglow plasma column is 3–5 μs, which is longer than that of the streamer.
Magnetization profile in the MnIr/CoFe exchange bias system
Authors: Doi Shuuichi , Awaji Naoki , Nomura Kenji , Hirono Toko , Nakamura Tetsuya , Kimura Hiroaki
Published: June 2009

The element-specific magnetization profile in the Mn75Ir25/Co70Fe30 exchange bias system was studied by x-ray resonant magnetic reflectometry using circular polarized soft x rays. From the analysis of reflectivities obtained at Co L3 and Mn L3 absorption peak, we found the uncompensated Mn spins located at the MnIr/CoFe interface and in the MnIr bulk region which couples antiferromagnetically and ferromagnetically to the Co spin, respectively. A “pinned” Mn spin which does not rotate with the Co spin was observed at the MnIr/CoFe interface.
Frequency-dependent complex conductivity of an organic thin-film transistor
Authors: Lenski Daniel R. , Southard Adrian , Fuhrer Michael S.
Published: June 2009

We measure the complex impedance between source/drain electrodes and the gate electrode of a pentacene thin-film transistor (TFT) at frequencies 50 Hz<ω/2π<20 kHz. Modeling the TFT as a distributed resistor and capacitor (RC) network (RC transmission line), we find that the data cannot be explained by a model including only a real, frequency-independent sheet conductivity. Instead, we use the RC transmission line model to extract the frequency-dependent complex sheet conductivity σ(ω)=σ′(ω)+jσ″(ω) of the pentacene film. At high frequencies, σ(ω) increases with frequency, σ′(ω) and σ″(ω) become similar in magnitude, and the on/off ratio is significantly reduced.