bluuebunny/crossref_metadata_2025 huggingface dataset:
On the polarization of the green emission of polyfluorenes
Authors: Yang X. H. , Neher D. , Spitz C. , Zojer E. , Brédas J. L. , Güntner R. , Scherf U.
Published: September 2003

An experimental and theoretical study of the anisotropic optical properties of polyfluorenes (PFs) bearing ketonic defects is presented. Polarized emission experiments performed on photooxidized aligned PF layers indicate that the transition dipole of the “green” CT π-π* transition of the keto-defect is oriented parallel to the chain direction. It is further observed that the polarization ratio of the green emission is slightly smaller than that of the blue emission component originating from undisturbed chains. Quantum mechanical calculations have been performed to support these observations. It is shown that the transition dipole moment of the CT π-π* transition of the defect is slightly misaligned with respect to the π-π* transition of the undisturbed PF chain, and that the angle between both depends on the chain conformation. For the most probably 5/2 helical conformation, this angle is, however, smaller than 5°. Further, polarized PL spectroscopy with polarized excitation has been performed to determine the extent of energy migration prior to emission from the keto-defect. For excitation at 380 nm, the polarization ratio of the green emission is essentially independent of the excitation polarization, indicating almost complete depolarization of the excitation before it is captured at a defect site. In contrast to this, energy migration after direct excitation of the keto-defect is inefficient or even absent.
Molecular dynamics studies of melting and liquid properties of ammonium dinitramide
Authors: Velardez Gustavo F. , Alavi Saman , Thompson Donald L.
Published: September 2003

The melting point and some liquid properties of ammonium dinitramide (ADN), NH4N(NO2)2, have been calculated using molecular dynamics calculations at various temperatures and pressures. The intramolecular potential for ADN was obtained from the AMBER 7 program and the intermolecular potential from Sorescu and Thompson [J. Phys. Chem. B 103, 6714 (1999)]. The simulations were performed for 3×2×4, 5×4×6, and 6×4×8 supercells of ADN. The 3×2×4 supercell was found to be adequate for predicting the melting point; however, the larger simulation cells were required to obtain converged results for the liquid properties. This model accurately predicts the temperature of the solid-to-liquid transition in ADN. The melting point of crystalline ADN has been determined by calculating the temperature dependence of the density, enthalpy, and radial distribution functions. The temperature dependence of the diffusion coefficient, calculated using equilibrium time-correlation functions, shows a discontinuity at the melting temperature and can also be used to determine the melting point. The value of the normal melting temperature of the perfect crystal calculated from the change in density is in the range 474–476 K, compared to the experimentally determined range 365–368 K. The difference is attributed to superheating of the perfect crystal. The superheating effect is eliminated by introducing voids in the crystal structure. Calculations of the temperature dependence of the density of a supercell with eight or more voids predict a melting temperature in the range 366–368 K, which is in excellent agreement with the experimental value. Melting temperatures have been calculated for pressures up to 0.8 GPa, which is the highest experimental pressure for ADN reported by Russell et al. [J. Phys. Chem. B 100, 3248 (1996)]. The computed dependence of the melting temperature on pressure is in excellent agreement with experiment. The temperature dependence of the diffusion and viscosity coefficient in the liquid temperature range were also calculated.
Molecular simulation of the vapor–liquid coexistence of mercury
Authors: Raabe Gabriele , Sadus Richard J.
Published: September 2003

The vapor–liquid coexistence properties of mercury are determined from molecular simulation using empirical intermolecular potentials, ab initio two-body potentials, and an effective multibody intermolecular potential. Comparison with experiment shows that pair-interactions alone are inadequate to account for the vapor–liquid coexistence properties of mercury. It is shown that very good agreement between theory and experiment can be obtained by combining an accurate two-body ab initio potential with the addition of an empirically determined multibody contribution. As a consequence of this multibody contribution, we can reliably predict mercury’s phase coexistence properties and the heats of vaporization. The pair distribution function of mercury can also be predicted with reasonable accuracy.
Entanglement molecular weight and frequency response of sliplink networks
Authors: Masubuchi Yuichi , Ianniruberto Giovanni , Greco Francesco , Marrucci Giuseppe
Published: September 2003

Brownian dynamics simulations of the linear viscoelastic response of entangled polymers have been performed, and compared quantitatively with some existing solution data at a fixed concentration and variable molecular weight. The model is a three-dimensional network where the nodes are sliplinks connecting chains in pair. The simulations make use of Langevin equations both for the node motion in space, and for the one-dimensional monomer sliding through sliplinks. Comparison with data is very satisfactory, but the molecular weight between entanglements that emerges from the model is unconventionally small.
Monte Carlo simulations and integral equation theory for the structure of telechelic polymers
Authors: Sung Bong June , Yethiraj Arun
Published: September 2003

The structure of telechelic polymers is investigated using off-lattice Monte Carlo simulations and the polymer reference interaction site model (PRISM) integral equation theory. The polymer molecules are modeled as tangent-sphere freely-jointed chains where all beads interact via a hard sphere potential and end beads interact via an additional short-ranged attractive potential. The static properties, i.e., conformational properties, end-bead aggregation, intermolecular pair correlations, and partial static structure factors are investigated as a function of density and temperature. For a given density, as the temperature is lowered, the chain ends aggregate to form multiplets. For a given temperature, this tendency is greater at higher densities. Predictions of the PRISM theory for the pair correlation functions and partial static structure factors are compared to the simulation results. Three different closure approximations, the reference-Molecular mean spherical approximation (R-MMSA), the reference-molecular Percus–Yevick closure with the high temperature approximation (R-MPY/HTA), and the linearized form of the R-MPY/HTA, are tested. At high temperatures, the linearized R-MPY/HTA and R-MMSA closures are in good agreement with the simulations, but the R-MPY/HTA closure underestimates the correlation between monomers. At low temperatures, when multiplets are formed, none of the closures are accurate for the pair correlation functions or the partial static structure factors.
A spectroscopic and computer simulation study of butanol vapors
Authors: Fanourgakis G. S. , Shi Y. J. , Consta S. , Lipson R. H.
Published: September 2003

Clusters of butanol formed above neat liquid samples were entrained in a supersonic jet and probed using 10.5 eV vacuum ultraviolet laser single-photon ionization/time-of-flight mass spectrometry. The four different isomers of butanol (n-butanol, sec-butanol, iso-butanol, and tert-butanol) were studied separately to assess the influence of the structure of the alkyl chain on the formation and stability of the hydrogen bonded clusters. Most of the higher mass features observed in the mass spectra could be assigned to protonated alcohol clusters, H(ROH)n+, n⩽3; R=C4H9, that arise from facile proton-alkoxy radical/alkoxide anion dissociation. Signals due to protonated trimers were only evident in the spectra of tert- and sec-butanol. Empirical force fields, density functional theory and ab initio methods were used to identify the geometries of all clusters up to the pentamers for the different isomers. Monte Carlo simulations established vapor-phase cluster distributions, while molecular dynamics was used to assess the relative stability of the isomeric tetramers. Together, these experimental and theoretical results suggest that butanol tetramers are “magic-number” structures, and that the protonated ion signals of size n could be correlated with the neutral cluster of size n+1, provided the vapor pressures sampled in the supersonic jet exceeded equilibrium values.
Hydrodynamic analysis of dynamical tunneling
Authors: Babyuk Dmytro , Wyatt Robert E. , Frederick John H.
Published: September 2003

The process of dynamical tunneling between local mode states is analyzed within the Bohmian approach to quantum mechanics. A number of quantum trajectories are followed for both a coupled Morse potential and the Barbanis potential. The superposition of nearly degenerate eigenstates leads to the formation of numerous permanent nodes concentrated near the potential valley. Around these nodes, vortices are evident on the flux map. Quantum trajectories can be attracted by a vortex and their velocity and kinetic energy will be significantly changed as they spiral around the vortex. However, trajectories starting remotely from vortices reflect more realistically the transition between a pair of localized states. These quantum trajectories can go through one or more regions where the potential energy is higher than the total energy. Hence, barrier tunneling accompanies dynamical tunneling.
Under-wrapped soluble proteins as signals triggering membrane morphology
Authors: Fernández Ariel , Scott L. Ridgway
Published: September 2003

This work identifies signals structurally encoded in soluble cytosolic proteins that induce an intricate or simple spanning of the inner membrane in a cellular compartment. Such signals are defined by the extent of intramolecular desolvation of backbone hydrogen bonds, a determinant factor in the interactivity of soluble proteins. The protein scaffolding of inner membranes varies widely but such differences do not explain a priori whether the membrane spanning will be simple or intricate. To address this problem, we show that a fluid phospholipid bilayer confining a water compartment for a soluble protein at 38 μM concentration is drawn to increase its interface area proportionally with the extent of intramolecular under-desolvation of the protein structure. We also predict and measure the optimal interface surface tension that enables such phenomenology. The in vitro kinetics of interface morphology development is autocatalytic, with an inhibitory mechanism switching on as the local concentration of protein molecules adsorbed on the bilayer reaches a threshold.
Multiphonon vibrational relaxation in liquids: An exploration of the idea and of the problems it causes for molecular dynamics algorithms
Authors: Ma Ao , Stratt Richard M.
Published: September 2003

The basic solid-state perspective on energy relaxation—that a solute transfers large amounts of energy to its surroundings by exciting overtones of the solid’s phonons—is sufficiently beguiling that it is tempting to try to apply it to high-frequency vibrational energy relaxation in liquids. We suggest that when the phonon concept is suitably adapted this picture does provide a surprisingly realistic and quantitative portrait of vibrational energy dispersal in solution. Within the nonlinear instantaneous-normal-mode/instantaneous-pair theory of vibrational relaxation, relaxation rates can be formally written as a sum over the contributions of successively higher overtones of fundamental solvent frequencies. However the presence of a significant width to the band of fundamental frequencies in the liquid state means that there could, in principle, be complex interferences between multiple contributing overtones, rendering the overtone picture no more than a formal construct. What we find is that such interferences do not occur. Despite the fact the band shape is log normal—with a relatively long band tail—the relaxation is invariably dominated by a single overtone. This same perspective also helps us understand one of the failings of the common velocity-Verlet molecular dynamics algorithm in predicting high-frequency energy relaxation.
Naturally formed graded junction for organic light-emitting diodes
Authors: Shao Yan , Yang Yang
Published: September 2003

In this letter, we report naturally-formed graded junctions (NFGJ) for organic light-emitting diodes (OLEDs). These junctions are fabricated using single thermal evaporation boat loaded with uniformly mixed charge transport and light-emitting materials. Upon heating, materials sublimate sequentially according to their vaporizing temperatures forming the graded junction. Two kinds of graded structures, sharp and shallow graded junctions, can be formed based on the thermal properties of the selected materials. The NFGJ OLEDs have shown excellent performance in both brightness and lifetime compared with heterojunction devices.
Monte Carlo simulation of organic light-emitting devices under alternating applied field
Authors: Paul Sharmistha , Pal Prabir , Pal Amlan J. , Talapatra G. B.
Published: September 2003

A Monte Carlo method has been employed to simulate electroluminescence (EL) from organic light-emitting devices (LEDs) under an alternating applied field. EL responses under forward and reverse bias modes have been simulated with different experimental parameters. Dependences of EL on the frequency of an applied field, electrode work function, band gap and film thickness of the active organic material, etc., have been studied. The origin of EL under alternating current (ac) mode has been explained in terms of radiative recombination of excitons formed via injected holes and electrons present from the previous cycle of ac voltage. The time response of EL intensity and its profile during forward and reverse bias half-cycles has been found to depend on carrier injection and also on their temporal and spatial distribution along the thickness of the emitting material. Efforts have been made to match a simulated EL response with representative experimental results. The Monte Carlo simulation results presented here provides a way to select certain parameters to fabricate efficient ac LEDs.
Compositional dependence of Raman scattering and photoluminescence emission in CuxGaySe2 thin films
Authors: Xue C. , Papadimitriou D. , Raptis Y. S. , Esser N. , Richter W. , Siebentritt S. , Lux-Steiner M. Ch.
Published: September 2003

Raman scattering and photoluminescence (PL) emission of CuxGaySe2 thin films, grown by metalorganic chemical vapor deposition (MOCVD) on GaAs (100) and by physical vapor deposition (PVD) on Glass/Mo substrates, were studied at room and low temperatures as a function of composition. Line width changes of Raman bands in the temperature range 20–300 K indicate the formation of a more disordered CuxGaySe2 phase with increasing Ga content. It is most likely that Raman bands observed at 193 and 199 cm−1 in the Ga-rich samples at low temperatures are associated with defect-related interface modes. The intensity increase of these bands and the photoluminescence intensity enhancement may be correlated to an increase in defect concentration in the Ga-rich phase. The radiative recombination in defect-rich Ga-rich samples with a higher degree of disorder apparently supports the PL emission. The observed changes are more pronounced in MOCVD than in PVD grown films.
Spin-polarized vacuum tunneling in field emission from Co-coated W(111) tips
Authors: Bryl Robert , Altman M. S.
Published: September 2003

Spin-polarized vacuum tunneling has been achieved in field emission from spontaneously and remanently magnetized ultrathin film Co-coated W(111) tips. The onset thickness of polarization at room temperature, equivalent to 3 Co (0001) layers, and the transverse spin-polarization directions are consistent with magnetic properties of Co films on macroscopic W(111) single-crystal surfaces. The orientation of the polarization vector in any of 12 transverse directions is due to crystallographic symmetry and the influence of tip morphology. Polarization of 10%–25% was obtained typically, while the highest recorded value was 48%. Manipulation of the polarization direction and magnitude were also achieved by application of transverse magnetic field pulses and demagnetization.
Temperature dependence of the fundamental band gap of InN
Authors: Wu J. , Walukiewicz W. , Shan W. , Yu K. M. , Ager J. W. , Li S. X. , Haller E. E. , Lu Hai , Schaff William J.
Published: September 2003

The fundamental band gap of InN films grown by molecular beam epitaxy have been measured by transmission and photoluminescence spectroscopy as a function of temperature. The band edge absorption energy and its temperature dependence depend on the doping level. The band gap variation and Varshni parameters of InN are compared with other group III nitrides. The energy of the photoluminescence peak is affected by the emission from localized states and cannot be used to determine the band gap energy. Based on the results obtained on two samples with distinctly different electron concentrations, the effect of degenerate doping on the optical properties of InN is discussed.
Exchange anisotropy determined by magnetic field dependence of ac susceptibility
Authors: Rodrı́guez-Suárez R. L. , Vilela Leão L. H. , de Aguiar F. M. , Rezende S. M. , Azevedo A.
Published: September 2003

ac susceptibility measurements of ferromagnetic/antiferromagnetic (FM/AF) bilayers are usually performed as a function of the temperature. In this work we describe measurements of transverse biased ac susceptibility (χt) of FM/AF bilayers as a function of the applied magnetic field H0. The measurements were carried out at room temperature by means of an ac magneto-optical Kerr effect susceptometer. The χt−1(H0) dependence, at the saturation magnetization regime, exhibits a linear behavior with the applied field parallel and perpendicular to the exchange bias direction. The linear extrapolation of χt−1 versus H0 cuts the abscissa at asymmetrical values of field due to the exchange bias coupling. The inverse susceptibility is calculated in the saturation regime by a model, which takes into account the free energy of both layers plus a term corresponding to the interfacial coupling. The exchange coupling field (HE) and uniaxial anisotropy (HU) are extracted from the best fit to the experimental results. The results obtained are crosschecked by those obtained from ferromagnetic resonance (FMR) and dc magnetometry. The measurements of the exchange bias and the uniaxial field in all of the three analyzed bilayers gave values that are consistently lower when measured by FMR than those obtained by ac and dc magnetometry. It is argued that the apparently discrepant values of HE and HU, obtained by different techniques, might be explained by existence of unstable AF grains at the AF/FM interface.
X-ray photoelectron spectroscopy and magnetic properties in Fe–SiO2 granular films
Authors: Honda S. , Shimizu T. , Une T. , Sakamoto M. , Kawabata K. , Tanaka T.
Published: September 2003

We have prepared single-layered Fe–SiO2 and multilayered Fe–SiO2/SiO2 granular films by rf magnetron sputtering and investigated their magnetic properties and x-ray photoelectron spectroscopy (XPS). The magnetization depends strongly on the film thickness for the single-layered films, and on SiO2 layer thickness for multilayers. In single-layered films, a lot of Fe granules near the film surface are oxidized by air that has penetrated from the surface, resulting in reduction of the net magnetization. With going to the inner layers, the oxidization is suppressed, but upon approaching the substrate considerable oxidation of Fe occurs again by consuming SiO2. In the multilayers, the energy shift of Si 2p and O 1s, seen in XPS occurs layer by layer, indicating that the Fe granules react with O and/or Si released from SiO2. The rate of oxidation of Fe becomes larger with increasing SiO2 layer thickness, especially at the layer boundaries. Thus, the change in magnetization can be explained by the oxidization of Fe granules.
Laser induced transformation of TiSi2
Authors: Lu L. , Lai M. O.
Published: September 2003

Transformation of TiSi2 on p-type (100) silicon substrates in two substrate conditions (crystalline-Si and 45 nm amorphous Si) induced by laser annealing was investigated. The formation of titanium silicides was characterized using x-ray diffraction and sheet resistance measurement. The first silicide phases observed were Ti5Si4 and C49 TiSi2 in crystalline-Si substrates, followed by transformation of C54 TiSi2 phase at higher laser energy densities of 1071 and 1250 J/cm2. In the case of the specimens using amorphous-Si substrates, the first silicide phase observed was Ti5Si3, followed by the coexistence of C49 TiSi2 and C54 TiSi2 at higher laser energy densities of 938 and 1000 J/cm2. It was noted that in general the formation of C54 phase occurred at a lower laser energy density on the amorphous-Si substrate than that on crystalline-Si substrate, indicating an easier C54 formation on the amorphous state. Sheet resistance measurements showed that the specimen containing only C54 silicide phase had the lowest resistivity.
Numerical investigation on the current mirror effect in a single-electron turnstile capacitively coupled to a one-dimensional array of small junctions
Authors: Mizugaki Yoshinao
Published: September 2003

We present numerical results of an examination of the current mirror effect in a single-electron turnstile capacitively coupled to a one-dimensional (1D) array of small tunnel junctions. We deal with a 20-junction 1D array and 4-junction turnstile coupled at their center electrodes via a coupling capacitor. A coupling parameter, Qc, and a current mirror index are introduced to define the strength of coupling and to evaluate the quality of the current mirror effect, respectively. Numerical results show that a finite gate charge of e/2 to the center electrodes of both the 1D array and the turnstile enhances the current mirror effect, where −e is the charge of an electron. To investigate the enhanced current mirror effect, charge propagation in the 1D array under a finite gate charge of e/2 is simulated. It is found that electrons temporarily trapped at the center electrode in the 1D array play an important role in the enhancement of the current mirror effect.
Optical properties of p-type In-doped SrTiO3 thin films
Authors: Guo Haizhong , Liu Lifeng , Fei Yiyan , Xiang Wenfeng , Lü Huibin , Dai Shouyu , Zhou Yueliang , Chen Zhenghao
Published: September 2003

We report the optical properties of p-type SrInxTi1−xO3 (x=0.1 and 0.2) films prepared by laser molecular beam epitaxy under different oxygen pressures. The results of x-ray diffraction and near-field scanning optical microscopy indicate that the films have high crystallinity, smoothness, and uniformity. Hall measurement confirms that the films are p-type semiconductors. The optical transmittance spectra reveal that the films are highly transparent in most of the visible region. A blueshift of the absorption edge with an increase in oxygen pressure during deposition has been observed and discussed. Moreover, the first-order Raman scattering peaks of the optical phonons in these In-doped SrTiO3 films are presented, and they are Raman inactive in bulk SrTiO3 single crystal.
Self-consistent calculation of subband occupation and electron–hole plasma effects: Variational approach to quantum well states with Hartree and exchange-correlation interactions
Authors: Ando Taro , Taniyama Hideaki , Ohtani Naoki , Nakayama Masaaki , Hosoda Makoto
Published: September 2003

We present a numerically stable computational method to calculate multiple subband states in quantum wells based on the variational minimization of the total Hamiltonian involving the nonlinear interaction terms such as the Hartree and exchange-correlation interactions. Application of a nonlinear multidimensional minimization procedure to the total Hamiltonian enables us to realize multiple state computations of improved convergence. As an example of the multiple state calculation, subband states of interacting electrons and holes in single quantum wells are computed. We demonstrate that the dependence of the overlap integral of electron and hole wave functions on the electron–hole density is significantly modified by the exchange-correlation interaction. Furthermore, we show the effects of the nonlinear interactions on the carrier population and chemical potential in multiple subbands at various temperatures and carrier concentrations. This method is expected to be a convenient tool for investigating the high-density behavior of carriers in semiconductor low-dimensional structures.
Line shape analysis of electron–hole plasma electroluminescence in fully strained SiGe epitaxial layers
Authors: Stoica T. , Vescan L.
Published: September 2003

The electroluminescence of p-i-n diodes with fully strained Si0.80Ge0.20/Si(001) is dominated by radiative recombination in an electron–hole plasma. The recombination mechanisms and the band gap renormalization have been studied experimentally and by modeling. In order to minimize the influence of the SiGe/Si interface regions and thus to study the intrinsic behavior of strained SiGe, electroluminescence diodes with thick layers of SiGe in a metastable strain state have been investigated. To explain the electroluminescence spectra, the band filling model for an electron–hole plasma system and different broadening procedures have been investigated. This line shape analysis allowed the determination of the dependence of the renormalized band gap on carrier density and comparison with theoretical predictions was done. The low-energy tails of the electroluminescence spectra correspond to a broadening of the initial electronic states of the recombination process in electron–hole plasma system. The experimental data obtained from the line shape analysis of the electroluminescence spectra are in good agreement with previous results on spectral photocurrent and quantum efficiency measurements.
InGaAsSbN: A dilute nitride compound for midinfrared optoelectronic devices
Authors: Li W. , Héroux J. B. , Wang W. I.
Published: September 2003

The growth of the antimony-rich dilute nitride alloys GaSbN, InGaSbN, and InGaAsSbN on GaSb substrates by solid-source molecular-beam epitaxy is reported. Bulk GaSbN layers are characterized by x-ray diffraction and the nitrogen incorporation is estimated to be close to 1%. A nitrogen-induced redshift of the photoluminescence peak wavelength as large as 110 meV is observed in In.15Ga.85Sb1−xNx/GaSb quantum wells. Photoluminescence emission at 77 K for an In0.3Ga0.7As0.1Sb0.9−xNx/Al0.25Ga0.75As0.02Sb0.98 multiple quantum well structure shows a 66 meV redshift due to nitrogen incorporation (2.33 μm emission wavelength), demonstrating the potential of this compound for midinfrared optoelectronic device applications.
Infrared spectroscopic study on the Langmuir–Blodgett films of merocyanine dye–arachidic acid–n-octadecane ternary system
Authors: Morita Shin-ichi , Miura Yasuhiro F. , Sugi Michio , Hirano Yoshiaki
Published: September 2003

We have studied the mobility and the orientation of the hydrocarbon chain in the Langmuir–Blodgett films of merocyanine dye (MS)–arachidic acid (C20)–n-octadecane (AL18) with the molar mixing ratio [MS]:[C20]:[AL18]=1:2:x (x=0,1,2,3,4,5) by means of infrared spectroscopy. Deuterated C20 (C20-d) and deuterated AL18 (AL18-d) have been used to separate the infrared bands due to the hydrocarbon chain of MS. The bandwidths of CH2 antisymmetric and CH2 symmetric stretching modes decrease in the range from x=0 up to 2, indicating that the motion of the hydrocarbon chain of MS is hindered by mixing the AL18-d molecules as the third component. From x=0 to 2, the orientation of the long axis of the MS hydrocarbon chain is found to change. The change in orientation is associated with the decrease in the mobility of the MS hydrocarbon chain. It is indicated that the J aggregate is accompanied by the thermal motion of the hydrocarbon chain of MS, and that the H aggregate is accompanied by the highly packed state of the hydrocarbon chains of MS and AL18-d, suggesting that the aggregate formation is controlled by the state of hydrocarbon chain packing.
Specific heat and magnetic properties of Nd0.5Sr0.5MnO3 and R0.5Ca0.5MnO3 (R=Nd, Sm, Dy, and Ho)
Authors: López J. , de Lima O. F.
Published: September 2003

Magnetization and specific heat measurements of Nd0.5Sr0.5MnO3, Nd0.5Ca0.5MnO3, Sm0.5Ca0.5MnO3, Dy0.5Ca0.5MnO3, and Ho0.5Ca0.5MnO3 samples were performed. Near the charge ordering (CO) and ferromagnetic transition temperatures, the specific heat curves showed peaks superimposed to the characteristic response of the lattice oscillations. These peaks allowed us to estimate the entropy variation for each phase transition. The entropy variation corresponding to the CO transition was higher than the one corresponding to the ferromagnetic transition. Furthermore, specific heat measurements in presence of a 9 T magnetic field showed that this field was not strong enough to affect the specific heat in the CO phase transition region. Our results suggest that the CO phase is very stable and almost independent of magnetic fields.
Effects of a thin AlAs layer on InAs quantum dot electronic structure
Authors: Movilla J. L. , Climente J. I. , Planelles J.
Published: September 2003

The influence of a thin AlAs layer (1 nm) located at different positions inside a GaAs matrix on the electronic states of nearby deposited InAs quantum dots is theoretically investigated. Calculations are performed within the three-dimensional one-band effective mass model including mass dependence on energy and position. In order to provide a realistic description of the wave function diffusion outside the dot, finite confinement barriers are considered. It is shown that the presence of the AlAs high potential barrier can account for the reported experimental blueshift when the AlAs layer is grown as a capping material, but this is not the case when the AlAs layer is grown in the substrate. An alternative explanation of the experimental data [Kim et al., J. Appl. Phys. 91, 5055 (2002)] based on our calculations is proposed.
Raman study of Mg, Si, O, and N implanted GaN
Authors: Katsikini M. , Papagelis K. , Paloura E. C. , Ves S.
Published: September 2003

The effect of Mg, Si, N, and O ion implantation (with doses in the range 5×1013–1×1018 cm−2), in epitaxially grown GaN samples has been studied using Raman spectroscopy. It is found that implantation increases the static disorder and activates modes that were not allowed in the as-grown material. More specifically it causes the appearance of three additional Raman peaks at 300, 420, and 670 cm−1. It is found that the position of these peaks does not depend on the type of the implant and thus they do not correspond to local vibrational modes. They are attributed to disorder activated Raman scattering (300 cm−1) and/or to implantation induced N and Ga vacancies or interstitials (420 and 670 cm−1). Finally, ion implantation causes a marginal increase of the build-in hydrostatic stress.
Measurements of cooling by room-temperature thermionic emission across a nanometer gap
Authors: Hishinuma Yoshikazu , Geballe Theodore H. , Moyzhes Boris Y. , Kenny Thomas W.
Published: September 2003

We have completed an investigation of cooling at room temperature by thermionic emission. The use of a small nm-sized gap lowered the vacuum barrier between the electrodes, enabling emission from surfaces with work functions of ∼1 eV at room temperature. We utilized a microfabricated cantilever with a cesiated metal coating on the tip, and an integrated thermometer to initiate and control an emission current of 1–10 nA, and to detect the resulting temperature changes. Using a lock-in technique, temperature changes of 0.1–1.0 mK were observed, corresponding to cooling power of 1–10 nW. The amplitude of this signal and its dependence on emission current and bias voltage are in good agreement with our model. Possible applications for cooling and energy conversion are discussed.
Reversible electrostatic control of micromechanical structure tunneling characteristics
Authors: Teh W. H. , Smith C. G.
Published: September 2003

We report on a submicrometer thick, surface nanomachined curvature-free metallic cantilever with an insulated contact tip applied as a micromechanical contact tunneling device. A technique for modulating the tunneling current characteristics based on an elastic compressible contact surface model is demonstrated by changing the effective tunneling area for asperities via change in electrostatic pressure. The electrostatic pressure comes from the excess potential applied on the gate after the cantilever has been switched on. This allows the gate voltage to independently modulate the characteristics of the tunneling barrier that the electrons must cross in a reversible manner. Preliminary measurements show a reasonably good agreement with the Hertzian contact force law.
Improved depth profiling with slow positrons of ion implantation-induced damage in silicon
Authors: Fujinami M. , Miyagoe T. , Sawada T. , Akahane T.
Published: September 2003

Variable-energy positron annihilation spectroscopy (VEPAS) has been extensively applied to study defects in near-surface regions and buried interfaces, but there is an inherent limit for depth resolution due to broadening of the positron implantation profile. In order to overcome this limit and obtain optimum depth resolution, iterative chemical etching of the sample surface and VEPAS measurement are employed. This etch-and-measure technique is described in detail and the capabilities are illustrated by investigating the depth profile of defects in Si after B and P implantations with 2×1014/cm2 at 100 keV followed by annealing. Defect tails can be accurately examined and the extracted defect profile is proven to extend beyond the implanted ion range predicted by the Monte Carlo code TRIM. This behavior is more remarkable for P ion implantation than B, and the mass difference of the implanted ions is strongly related to it. No significant difference is recognized in the annealing behavior between B and P implantations. After annealing at 300 °C, the defect profile is hardly changed, but the ratio of the characteristic Doppler broadening, S, a parameter for defects, to that for the bulk Si rises by 0.01, indicating that divacancies, V2, are transformed into V4. Annealing at more than 500 °C causes diffusion of the defects toward the surface and positron traps are annealed out at 800 °C. It is proved that this resolution-enhanced VEPAS can eliminate some discrepancies in defect profiles extracted by conventional means.
Remote hydrogen plasma processing of ZnO single crystal surfaces
Authors: Strzhemechny Yuri M. , Nemergut John , Smith Phillip E. , Bae Junjik , Look David C. , Brillson Leonard J.
Published: September 2003

We have studied the effects of remote hydrogen plasma treatment on the defect characteristics in single crystal ZnO. Temperature-dependent (9–300 K) and excitation intensity-dependent photoluminescence spectra reveal that H-plasma exposure of ZnO effectively suppresses the free-exciton transition and redistributes intensities in the bound-exciton line set and two-electron satellites with their phonon replicas. The resultant spectra after hydrogenation exhibit a relative increase in intensity of the I4 (3.363 eV) peak, thought to be related to a neutral donor bound exciton, and a peak feature at 3.366 eV with a distinctly small thermal activation energy. Hydrogenation also produces a violet 100 meV wide peak centered at ∼3.15 eV. Remote plasma hydrogenation yields similar changes in room-temperature depth-dependent cathodoluminescence spectra: the emission intensity increases with hydrogenation mostly in the violet and near-ultraviolet range. Subsequent annealing at 450 °C within the same plasma environment completely restores both the photoluminescence and cathodoluminescence spectra in the subband gap range. The appearance of another bound-exciton feature at 3.366 eV and a relative intensity increase of the donor-bound exciton at line I4 with H-plasma exposure, and the reversibility with annealing of the spectral changes, indicate a direct link between hydrogen indiffusion and appearance of a shallow donor.
Electrochemical kinetics of hydrogen intercalation in gadolinium switchable mirrors
Authors: Di Vece M. , Swart I. , Kelly J. J.
Published: September 2003

Potential-step experiments on gadolinium thin-film electrodes provide current and optical transmission transients, whose time constants are related to effective hydrogen diffusion coefficients according to a standard diffusion model. The diffusion coefficients and switching kinetics depend on temperature, hydrogen concentration, potential, and film thickness. A thickness dependence of the kinetics is observed in which the diffusion coefficient and optical switching time are linearly dependent.
Luminescence energy and carrier lifetime in InGaN/GaN quantum wells as a function of applied biaxial strain
Authors: Shapiro N. A. , Feick H. , Hong W. , Cich M. , Armitage R. , Weber E. R.
Published: September 2003

Continuous-wave and time-resolved photoluminescence of InGaN quantum wells are measured as a function of applied biaxial strain, which provides a unique means of altering the built-in polarization field in these structures. The direction and magnitude of the shift of the luminescence-peak energy are quantitatively analyzed within an analytical carrier separation model. It is found that the presently used piezoelectric coefficients of InGaN are not entirely consistent with our experimental results. Instead, consistent interpretation of our data requires the e13 and e33 piezoelectric coefficients of InN to be ∼15% larger than the commonly accepted values. Our analysis allows the assignment of an effective carrier-separation parameter to each investigated quantum-well sample, which quantifies the shift of the luminescence peak energy with the change in the polarization field. The effective carrier separation is found to be zero for narrow quantum wells (<1.5 nm) and asymptotically approaches the full quantum well width for increasing well width. However, heavy doping or increased indium content are found to reduce the effective carrier separation, which is ascribed to screening of the polarization field or localization effects, respectively. A reduction of the carrier lifetime with the application of strain supports the carrier separation model and allows the derivation of a quantity related to the change of the wave function shape with the polarization field.
In situ measurement of cavity length variation across the transverse section of broad-area vertical-cavity surface-emitting lasers
Authors: Barland Stéphane , Marino Francesco , Giudici Massimo , Tredicce Jorge , Balle Salvador
Published: September 2003

We present a method to measure the cavity length variations of a broad-area vertical-cavity surface-emitting laser. The method is based on the observation of an inhomogeneous nonlinear response of the system under coherent optical injection. Due to the presence of a cavity resonance frequency gradient in the system, the boundary of a modulational instability manifests itself as a separation of the device in two distinct spatial regions. The observation of the position of this boundary depending on parameters reveals the degree of homogeneity of the sample and allows a direct measurement of the spatial variations of the cavity resonance frequency.
Carbazole-functionalized europium complex and its high-efficiency organic electroluminescent properties
Authors: Xin H. , Li F. Y. , Guan M. , Huang C. H. , Sun M. , Wang K. Z. , Zhang Y. A. , Jin L. P.
Published: September 2003

A complex tris(dibenzoylmethanato){1-ethyl-2-(N-ethyl-carbazole-yl-4) imidazo[4,5-f]1,10phenanthroline} europium(III) [Eu(DBM)3phencarz] functionalized by a hole-transport group carbazole was synthesized. Devices using this complex as emitter showed greatly enhanced performance benefited from the increased hole-transport properties and efficient energy transfer from carbazole to the central ions. A 58-nm-single-layer device gave a brightness of 20 cd/m2 at 15 V. The highest power efficiency of 2.7 lm/W at 5 V and 0.5 cd/m2 and the luminance exceeding 2000 cd/m2 at 20 V was obtained from a device with the configuration of ITO/TPD(20 nm)/Eu(DBM)3phencarz(40 nm)/BCP(20 nm)/AlQ(40 nm)/Mg0.9Ag0.1(200 nm)/Ag(80 nm).
Giant magnetoresistance in Mn2−xCuxSb (x⩽0.4) compounds
Authors: Zhang Y. Q. , Zhang Z. D. , Xiong D. K. , Li W. F. , He J.
Published: September 2003

Magnetic and magnetotransport properties of Mn2−xCuxSb (x=0, 0.1, 0.2, and 0.4) compounds have been studied. A phase transition from a ferrimagnetic to an antiferromagnetic state occurs in the Cu-substituted compounds as the temperature is reduced. A magnetic field-driven transition takes place in the Cu-containing compounds, changing the spin configuration from antiferromagnetic to ferromagnetic. The giant magnetoresistance effect is observed in these compounds, accompanying the transition. The largest magnetoresistance ratio of −40% is observed for Mn1.9Cu0.1Sb at 170 K in a magnetic field of 5 T.
Spin injection at Heusler/semiconductor interfaces: First-principles determination of potential discontinuity and half-metallicity
Authors: Picozzi S. , Continenza A. , Freeman A. J.
Published: September 2003

Technologically relevant properties, such as potential discontinuity and half-metallic behavior, are determined by means of an accurate first-principles approach for Co2MnGe/GaAs and Co2MnGe/Ge interfaces. Interface states appear in both sides of the junctions, so that half-metallicity, typical of bulk Co2MnGe, is locally lost. As for the potential discontinuity, the character of the contact is dramatically affected by the semiconductor side: In Co2MnGe/GaAs, irrespective of the atomic termination, the Fermi level is pinned within the energy band gap, whereas it lies close to the valence-band maximum in the case of Co2MnGe/Ge. This gives rise to a rectifying contact in the first case and ohmic for holes in the second case, both potentially useful for spin–injection purposes. Finally, we investigate the effects of possible Co–Mn antisites in bulk Co2MnGe and find that half-metallicity is only locally destroyed, since defect-induced gap states are shown to be screened in a couple of nearest-neighbor atomic shells.
Temperature and frequency characteristics of the interfacial capacitance in thin-film barium–strontium–titanate capacitors
Authors: McAneney J. , Sinnamon L. J. , Bowman R. M. , Gregg J. M.
Published: September 2003

Thin film Au/Ba0.5Sr0.5TiO3/SrRuO3 capacitor structures, with a thickness of dielectric varying between ∼70 and ∼950 nm, were deposited on {001} MgO single-crystal substrates using pulsed laser deposition. Low-field dielectric measurements were performed as a function of temperature and frequency. At all temperatures and frequencies, the dielectric response as a function of thickness was found to adhere reasonably well to the so-called “series capacitor model,” from which nominal “bulk” and “interfacial” capacitance components could be extracted. The bulk component showed weak frequency dependence but strong temperature dependence, with a peak in permittivity and dielectric loss around 250 K and 150 K, respectively. Well above 250 K, reasonable Curie–Weiss behavior was evident. Overall, the extracted bulk component behaved much as would be expected in real bulk ceramics or single crystals lending confidence as to the general applicability of the series capacitor model. The functional behavior of the extracted interfacial capacitance was rationalized as being due to a combination of a thermally independent background, and thermally activated space charge. The activation energy of the space charge (∼0.6 eV) is commensurate with the detrapping of electrons from shallow level traps associated with oxygen vacancies. Importantly, since this component acts in series with the bulk component, the functional analysis performed here implies that the oxygen vacancies lie in a plane parallel to the electrodes.
Domain structure in Joule-heated CoFeSiB glass-covered amorphous microwires probed by magnetoimpedance and ferromagnetic resonance
Authors: da Silva R. B. , Carara M. , de Andrade A. M. H. , Severino A. M. , Sommer R. L.
Published: September 2003

Impedance spectra (100 kHz⩽f⩽1.8 GHz) were measured at different magnetic fields, and magnetoimpedance and magnetization measurements were performed in as-produced and Joule-heated Co70.4Fe4.6Si10B15 glass-covered amorphous microwires. From the magnetization curves and ferromagnetic resonance features obtained from the impedance spectra, it was possible to propose a domain structure in the as-produced microwires, as well as to follow its evolution with the annealing current. The inner core and outer shell domain structure were observed. The outer shell domains evolve from a circumferential, in the as-cast sample, to a helical structure in the case of the annealed samples. On the other hand, the inner core evolves from a domain structure typical of a radial anisotropy to a longitudinal one. This domain structure evolution is explained in terms of the combined effects of the stress, promoted by the annealing temperature, and the magnetic field caused by the annealing current.
Near-field light emission from nano- and micrometric complex structures
Authors: Pieruccini M. , Savasta S. , Girlanda R. , Iotti R. C. , Rossi F.
Published: September 2003

We propose a general theoretical scheme for the investigation of light emitted from nano- and micrometric structures of arbitrary shape and composition. More specifically, the proposed fully three-dimensional approach allows to derive the light-intensity distributions around the emitting structures and their modifications in the presence of nearby scattering objects. Our analysis allows to better identify the nontrivial relationship between near-field images and fluorescent objects.
Time-resolved measurement of the refractive index for photopolymerization processes
Authors: Dorkenoo Kokou , van Wonderen A. J. , Bulou Hervé , Romeo Michelangelo , Crégut Olivier , Fort Alain
Published: September 2003

A double-interferometer technique is employed to examine the dynamics of a photopolymerization process. The dye molecule is eosine Y. The refractive index and the thickness of the photopolymerizable film are measured as a function of time. During the photopolymerization process, the first quantity increases by 2%, while the second quantity decreases by more than 4%. Therefore, the refractive index cannot be measured by means of single-interferometer techniques. By fitting our experimental curves to a rate equation, the quantum yield and the absorption coefficient of the sample can be determined with good accuracy.
Optical detection of quasi-static actuation of nanoelectromechanical systems
Authors: Meyer Christine , Lorenz Heribert , Karrai Khaled
Published: September 2003

An all optical method designed to test the functionality of nanoelectromechanical systems is presented. Silicon tweezers consisting of freestanding nanometer-sized prongs are prepared using electron beam lithography. Images of the tweezers structures are taken by scanning confocal microscopy while the prongs are electrostatically actuated under a low frequency ac voltage. The images, which are demodulated at the actuation frequency and its higher harmonics, clearly resolve the actuating parts of the tweezers. An actuation amplitude down to 6 pm (rms)/Hz can be detected.