bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetic and electrical properties of amorphous CoFeB films
Authors: Jen S. U. , Yao Y. D. , Chen Y. T. , Wu J. M. , Lee C. C. , Tsai T. L. , Chang Y. C.
Published: March 2006

CoFeB films were deposited on glass substrate by the sputtering method. From x-ray-diffraction and electron-diffraction-ring patterns, the major phase in the as-deposited CoFeB film is amorphous (or nanocrystalline). However, we could also identify a minor CoFe(110) crystalline phase in the film. We have tried to suppress this crystalline phase by changing the Ar partial pressure (PAr) during deposition and found that the optimal condition is PAr=5×10−3Torr. Because the electrical resistivity value (ρ) of the film is in general larger than 100μΩcm, it also indicates that the amorphous phase is dominant. From the temperature coefficient of resistance measurement, we learn that the amorphous phase in the CoFeB film crystallizes in succession at two higher temperatures (Tcr1 and Tcr2) than the room temperature (RT). Besides the electrical properties, the film thickness (tf) dependence of saturation magnetization (Ms), saturation magnetostriction (λs), and coercivity (Hc) has also been discussed. From the Auger-depth profile analysis, it is found that there is one CoOx (with 0.4⩽x<1) oxide layer, about 15 Å in thickness, lying on the top surface of the CoFeB film, and another CoOx oxide layer, about 20 Å, lying near the CoFeB/glass interface. At RT CoOx is supposed to be paramagnetic. However, due to the proximity effect between CoOx and CoFeB, the CoOx layers may become ferromagnetic with the average magnetization Mox. By fitting the Ms data as a function of (1∕tf), we can show that the last conjecture is correct, and Mox is not zero. The CoOx layer plays an important role on Ms,λs, and Hc of the CoFeB films with tf ranging from 50 to 503 Å.
Magnetization reversal and microstructure of [(Nd2Fe14B)x∕Nb]n films
Authors: Tsai J. L. , Chin T. S. , Yao Y. D. , Kronmüller H.
Published: March 2006

This article correlated Nd2Fe14B single-layer and [(Nd2Fe14B)x∕Nb]n multilayer film magnetization reversal processes to the observed microstructure image. Coercivity temperature dependence was fitted to modified nucleation and pinning models. The microstructure parameter (αK) describes the deteriorating effect of Nd2Fe14B grain anisotropy on coercivity in the nucleation model. For a single layer with 400 nm thickness, the αK values change from 0.18 to 0.43 when annealing temperature increases from 550 to 903 K. The highest values of single- and multilayer films are 0.46 and 0.70, respectively. Film magnetization reversals were predominant and fitted the reverse domain nucleation model well. The Nb spacer layer enriched the grain boundaries of the Nd2Fe14B phase, which in turn inhibited the grain growth.
Vortex rotation control in Permalloy disks with small circular voids
Authors: Vavassori P. , Bovolenta R. , Metlushko V. , Ilic B.
Published: March 2006

We studied the magnetization reversal of Permalloy disks with a small circular void either concentric or decentered. In both systems the reversal takes place via the nucleation and annihilation of a magnetic vortex. By applying the diffracted magneto-optic technique combined with numeric micromagnetic simulations we retrieved the information about the sense of rotation of the magnetization in the vortex state. For the disks with the concentric void no preferential rotation has been observed. For the case of decentered void, the sense of rotation of all probed disks is deterministically controlled by appropriately choosing the direction of the externally applied field and the void position with respect to the disk center.
Spectroscopic analysis of defects in chlorine doped polycrystalline CdTe
Authors: Consonni V. , Feuillet G. , Renet S.
Published: March 2006

The effects of Cl doping of thick polycrystalline CdTe layers grown by close space sublimation on their crystalline structure and the compensation and passivation processes have been investigated. From an extensive low temperature photoluminescence study, it is shown that, in polycrystalline CdTe:Cl, the main compensation processes are similar to those occurring in monocrystalline CdTe:Cl. However, specific compensation processes are also evidenced in polycrystalline CdTe: Defect complexes can be formed between Cl atoms and impurities in the vicinity of extended defects such as dislocations, twin boundaries, and grain boundaries. Furthermore, using low temperature cathodoluminescence imaging, chlorine induced passivation processes are proposed for defects such as double positioning twin boundaries within the grains. On the structural side, a bimodal distribution of the grain size is found: While small grains (5μm in diameter) are predominant at low Cl doping (around 2×1017at.cm−3), larger grains (40μm in diameter) prevail at higher doping levels (around 6.2×1017at.cm−3), with the proportion of small versus large grain decreasing as the doping level increases. Interestingly, while the radiative emission of small grains is mainly attributed to compensating complexes implying chlorine and Cd vacancies, in larger grains, the luminescence comes from deeper levels most probably associated with compensating complexes implying residual impurities.
Positional disorder enhancement of exciton dissociation at donor∕acceptor interface
Authors: Koehler M. , Santos M. C. , da Luz M. G. E.
Published: March 2006

We investigate the dissociation of a Coulomb bounded electron-hole geminate pair at a disordered donor∕acceptor (D-A) heterojunction by extending a previous proposal in the literature [Arkhipov et al., Appl. Phys. Lett. 82, 4605 (2003)] and using Monte Carlo simulations. We show that the presence of a layer of randomly distributed dipoles at the D-A interface creates both a potential well that confines the hole and a repulsive barrier that prevents the geminate pair recombination, even when the effective mass of the hole is around the electron rest mass. Our calculations depend strongly on the heterojunction morphology. However, contrary to what is generally believed, we find that the disorder in the position of the dipoles along the D-A interface axis enhances the pair dissociation. Inhomogeneities in the acceptor concentration at the heterojunction can then create highly efficient centers for exciton dissociation. The model explains recent experimental results for organic D-A heterojunctions and has important consequences on the design of organic photovoltaic devices.
Irreversibility line and thermally activated flux flow in La1.6−xNd0.4SrxCuO4 films
Authors: Liu Y. , Li X. G.
Published: March 2006

The resistive transition broadening of the c-axis oriented La1.6−xNd0.4SrxCuO4 epitaxial films (x=0.1, 0.12, 0.14, and 0.16) has been systematically investigated under magnetic fields up to 14 T for both H⊥c and H‖c configurations. For H‖c, the irreversibility line corresponds to a melting line in high temperature and low field regions, whereas it shows a rapid increase following a decoupling theory in low temperature and high field regions. It is found that the in-plane resistivity ρab below the mean-field transition temperature Tc(H) follows Arrhenius-type thermally activated flux flow model ρ(T)=ρ0exp{−U0(H)[1−T∕Tc(H)]n∕kBT}, where n=3 for H‖c and n=1.5 for H⊥c. For H‖c, the field dependence of activation energy follows a power law, i.e., U0(H)∝H−α(α∼1), which can be interpreted in terms of thermal depinning of vortices in a quasi-two-dimensional superconductor. For H⊥c, a logarithmic dependence of activation energy is observed, which results from the plastic creep of flux lines associated with the intrinsic pinning in layered high-Tc superconductors.
Theoretical and experimental studies for nano-oxidation of silicon wafer by ac atomic force microscopy
Authors: Lin Jen Fin , Tai Chih Kuang , Lin Shuan Li
Published: March 2006

In the present study, all the oxidational experiments were carried out by using the function of the electrochemical chemical atomic force microscopy. The theory for the growth rate of the oxide film thickness is derived to be the combined effect of the alternating current (ac) voltage applied to the system and the charges entrapped in the oxide film formed on silicon wafer. Part of the space charges that accumulated in the oxide film can be removed by the negative part of the ac voltage. The residual proportion of the space charges was first determined by the force-displacement curves obtained from the experiments of the kelvin force microscopy. Then, the formula for the growth rate of the oxide film thickness was slightly modified by weighting this residual proportion at the term related to the space charges. The comparisons between the theoretical and experimental results in the oxide height as a function of different controlling factors were made. The behavior exhibited in the experimental results of the oxide height due to the change in the oxidation time is quite consistent with that predicted by the present model if the applied voltage is not high enough to oxidize the probe. The height of the oxide film was generally increased by elevating the positive electrical voltage. The increase in the modulation number (or the applied voltage frequency) was within a fixed time period of the oxide height.
Enhancement of Schottky barrier height on p-type GaN by (NH4)2Sx treatment
Authors: Lin Yow-Jon , You Chang-Feng , Lee Chi-Sen
Published: March 2006

Barrier height values of Ni contacts to (NH4)2Sx-treated p-type GaN (p-GaN) were obtained from current-voltage and x-ray photoelectron spectroscopy (XPS) measurements in this study. The induced deep level defect band through high Mg doping led to a reduction of the depletion layer width in the p-GaN near the interface and an increase in the probability of thermionic field emission (TFE). Furthermore, the calculated barrier height value of Ni contacts to (NH4)2Sx-treated p-GaN using the TFE model is close to the Schottky limit, which is in good agreement with the observed result by XPS measurements and suggests that (NH4)2Sx surface treatment leads to the removal of native oxides and the reduction of the surface state related to oxygen-induced and nitrogen-vacancy defects.
Monte Carlo simulation of damage and amorphization induced by swift-ion irradiation in LiNbO3
Authors: García G. , Agulló-López F. , Olivares-Villegas J. , García-Navarro A.
Published: March 2006

This paper presents a Monte Carlo (MC) simulation tool which is applied to describe the ion beam induced damage generated by electronic excitation in LiNbO3. Based on a previously published thermal spike based analytical model, the MC technique allows for a more flexible and accurate treatment of the problem. A main advantage of this approach with respect to the analytical one is the possibility of studying the role of statistical fluctuations, relevant at low fluences. The paper recalls the main features of the physical model, describes the MC algorithm, and compares simulation results to experimental data (irradiations of LiNbO3 using silicon ions at 5 and 7.5MeV and oxygen ions at 5MeV).
Transform method for semiconductor mobility
Authors: Johnson William B.
Published: March 2006

The components of the conductivity tensor are expressed in terms of Laplace and Fourier transforms of the electron and hole concentrations. The experimentally measured conductivity components are fitted to rational polynomial functions of the magnetic field, and these functions are used to find partial fraction expansions of the conductivity components. The partial fraction expansions are inverted using analytical formulas to derive the electron and hole concentrations.
Optical emission spectroscopy of metal-halide lamps: Radially resolved atomic state distribution functions of Dy and Hg
Authors: Nimalasuriya T. , Flikweert A. J. , Stoffels W. W. , Haverlag M. , van der Mullen J. J. A. M. , Pupat N. B. M.
Published: March 2006

Absolute line intensity measurements are performed on a metal-halide lamp. Several transitions of atomic and ionic Dy and atomic Hg are measured at different radial positions from which we obtain absolute atomic and ionic Dy intensity profiles. From these profiles we construct the radially resolved atomic state distribution function (ASDF) of the atomic and ionic Dy and the atomic Hg. From these ASDFs several quantities are determined as functions of radial position, such as the (excitation) temperature, the ion ratio Hg+∕Dy+, the electron density, the ground state, and the total density of Dy atoms and ions. Moreover, these ASDFs give us insight about the departure from equilibrium. The measurements show a hollow density profile for the atoms and the ionization of atoms in the center. In the outer parts of the lamp molecules dominate.
Electronic properties of the organic semiconductor interfaces CuPc∕C60 and C60∕CuPc
Authors: Molodtsova O. V. , Knupfer M.
Published: March 2006

We report on the electronic properties of the organic heterointerfaces between C60 and copper phthalocyanine (CuPc), studied by means of photoemission spectroscopy and the Kelvin-probe method. We found both interfaces, CuPc∕C60 and C60∕CuPc, to be nonreactive with pronounced shifts of the vacuum level pointing to the formation of an interfacial dipole mainly on the CuPc side of the heterojunctions. The dipole values are close to the difference of the work functions of the two materials. Important interface parameters and hole-injection barriers were obtained. It is shown that the sequence of deposition does not influence the electronic properties of the interfaces.
Laser ablation condensation of polymorphic ZrO2 nanoparticles: Effects of laser parameters, residual stress, and kinetic phase change
Authors: Tsai Meng-Hsiu , Chen Shuei-Yuan , Shen Roa-Pu (Claire) , Shen Pouyan
Published: March 2006

Nd-YAG laser ablation on a Zr target under optimum oxygen flow rate and a very high-temperature route was used to fabricate fluorite-type related ZrO2 nanoparticles, which are of monoclinic (m) and tetragonal (t) symmetries when the laser power density was below ca. 1×107W∕cm2, but t plus cubic (c) symmetry when above this threshold. Electron diffraction indicated that the dense c and t phases with specific size and residual stress were allowed to relax and/or kinetically phase change into lower-energy state as constrained by the intersections of the internal energy versus the cell volume plot of the two polymorphs. The m-ZrO2 can be rationalized as a derivative of the coarsened/coalesced nanoparticles with defects vulnerable to martensitic t−m transformation.
Phonon Knudsen flow in nanostructured semiconductor systems
Authors: Ziambaras E. , Hyldgaard P.
Published: March 2006

We determine the size effect on the lattice thermal conductivity of nanoscale wire and multilayer structures formed in and by some typical semiconductor materials, using the Boltzmann transport equation and focusing on the Knudsen-flow effect. For both types of nanostructured systems we find that the phonon transport is reduced significantly below the bulk value by boundary scattering off interface defects and/or interface modes. The Knudsen-flow effects are important for almost all types of semiconductor nanostructures but we find them most pronounced in Si and SiC systems due to the very large phonon mean free paths. We apply and test our wire thermal-transport results to recent measurements on Si nanowires. We further investigate and predict size effects in typical multilayered SiC nanostructures, for example, a doped-SiC∕SiC∕SiO2 layered structure that could define the transport channel in a nanosize transistor. Here the phonon-interface scattering produces a heterostructure thermal conductivity smaller than what is predicted in a traditional heat-transport calculation, suggesting a breakdown of the traditional Fourier analysis even at room temperatures. Finally, we show that the effective thermal transport in a SiC∕SiO2 heterostructure is sensitive to the oxide depth and could thus be used as an in situ probe of the SiC oxidation progress.
Size effect ordering in [FePt]100−xCrx nanoparticles
Authors: Srivastava C. , Thompson G. B. , Harrell J. W. , Nikles D. E.
Published: March 2006

A series of [FePt]100−xCrx nanoparticles (x=5, 10, and 16at.%) was chemically synthesized by two different techniques. In one method, the simultaneous chemical reduction of FeCl2∙4H2O, Pt-acetylacetonate, and Cr-acetylacetonate was used with 2, 4 hexadecanediol as the reducing agent and phenyl ether as the solvent. The as-prepared particles had a mean size of 1.5nm. In the second method, the simultaneous chemical reduction of Pt-acetylacetonate and Cr-acetylacetonate and the thermal reduction of Fe(CO)5 were used with adamantanecarboxylic acid as the reducing agent and hexadecylamine as the solvent. These as-prepared particles were 3.5nm in size. X-ray diffraction confirmed that the Cr formed a solid solution within the A1 FePt phase for both processes. Upon annealing, the Cr hindered sintered grain growth of FePt nanoparticle arrays. Consequently, we were able to use Cr as a means to tune the ordering temperature as a function of the size effect in FePt nanoparticles. The presence of Cr in the ordered FePt reduced the magnetic coercivity of the transformed nanoparticles.
Correlation between Zn vacancies and photoluminescence emission in ZnO films
Authors: Zubiaga A. , García J. A. , Plazaola F. , Tuomisto F. , Saarinen K. , Zuñiga Pérez J. , Muñoz-Sanjosé V.
Published: March 2006

Photoluminescence and positron annihilation spectroscopy have been used to characterize and identify vacancy-type defects produced in ZnO films grown on sapphire by metal-organic chemical-vapor deposition. The photoluminescence of the samples in the near band edge region has been studied, paying particular attention to the emission at 370.5nm (3.346eV). This emission has been correlated to the concentration of Zn vacancies in the films, which has been determined by positron annihilation spectroscopy.
Giant enhancement of optical nonlinearity in multilayer metallic films
Authors: Huang J. P. , Dong L. , Yu K. W.
Published: March 2006

The local electric field has been derived analytically inside multilayer metallic films with weak nonlinearity, and hence the effective linear dielectric constant and third-order nonlinear susceptibility of the layered structures are calculated. As the number of layers increases, the comparison with the previous results of graded metallic films shows a gradual transition from sharp peaks to a broad continuous band. These results give further evidence of the emerging absorption band reported recently [J. P. Huang and K. W. Yu, Appl. Phys. Lett. 85, 94 (2004)]. Thus, multilayer metallic films can serve as optical materials with a giant enhancement of optical nonlinearity and a large figure of merit.
Optimization criteria for an irreversible quantum Brayton engine with an ideal Bose gas
Authors: Wu Feng , Chen Lingen , Sun Fengrui , Wu Chih , Guo Fangzhong
Published: March 2006

The purpose of this paper is to study the optimal performance for an irreversible quantum Brayton engine consisting of two constant-frequency branches connected by two irreversible adiabatic branches. The solution of the generalized quantum master equation of a thermal system is obtained in the Heisenberg picture. The optimization region (or criteria) for an irreversible quantum Brayton engine is obtained. The relationship between the dimensionless power output P* versus efficiency η for the irreversible quantum Brayton engine with heat leakage and other irreversible losses are derived.
Grain-boundary magnetoresistance enhancement induced by network self-optimization
Authors: Wu Jianchun , Sun H. , Li Z. Y.
Published: March 2006

A random conductance network model is proposed to study the global magnetoresistance effects of granular systems of half-metallic oxides. Different transport mechanisms resulting from grain-boundary effects are considered and their contributions to the total magnetoresistance of the network are discussed. It is found that magnetoresistance can be enhanced obviously when the spatial current localization is large enough to produce a percolative conductance path along conductances of high magnetoresistance in the network.
Nondestructive microstructural diagnostic of integrated ferroelectric capacitor arrays: Correlation with electrical characteristics
Authors: Muller Ch. , Menou N. , Barrett R. , Save D.
Published: March 2006

Considering the critical role of the storage capacitor on the reliability of ferroelectric random access memories (FeRAM), it is of particular interest to analyze the electrical and microstructural characteristics of the ferroelectric capacitor after the integration steps. In addition to standard electrical testing, the architecture of memory cell arrays in 0.35μm complementary metal-oxide-semiconductor (CMOS) technology was characterized using an original approach combining microfocused synchrotron x-ray fluorescence and diffraction. A 1.5 kbyte FeRAM memory block was scanned with a step size of 0.2μm while simultaneously acquiring fluorescence spectra and diffraction patterns in order to obtain, respectively, chemical and crystallographic mapping of the thin films constituting the memory cell. The excellent agreement observed between measured data and memory cell design explains the good electrical characteristics of the analyzed FeRAM chips.
Ab initio study of the chemical bonding and mechanical properties of Li2SiZn
Authors: Sun Zhimei , Music Denis , Ahuja Rajeev , Schneider Jochen M.
Published: March 2006

Using ab initio calculations, we have investigated the chemical bonding and elastic properties of Li2SiZn (space groups P63∕mmc and P3¯m1). Both structures have been observed experimentally. These compounds exhibit a layered structure where Li–Zn layers are interleaved with Si layers. The most dominant chemical bonding is ionic, and the bulk moduli calculated are 42 and 22 GPa, respectively. While the layered nature thereof is similar to the so-called MAX phases (where M=transition metal, A=group element, and X=C or N) [M. W. Barsoum, Prog. Solid State Chem. 28, 201 (2000)], the elastic properties are rather different. This can be understood by analyzing the differences in chemical bonding between Li2SiZn and MAX phases. It is our ambition that these calculations may inspire future research on the Li2SiZn phases.
Valence-band splitting in InGaPN: Effects of epitaxial strain and atomic ordering
Authors: Lin K. I. , Wang T. S. , Lu Y. T. , Hwang J. S.
Published: March 2006

Both the epitaxial-strain and atomic-ordering effects in In0.54Ga0.46P1−yNy∕GaAs(y=0%−2.0%) heterostructures are characterized by high-resolution x-ray rocking curve measurements and photoreflectance (PR) spectra at various temperatures. The lattice constant of InGaPN follows a bowing effect instead of Vegard’s law. The valence-band splitting (VBS) and spin-orbit splitting of InGaPN are obtained from PR spectra. As the nitrogen concentration increases, the lattice mismatch (the compressive strain) between the epilayer and GaAs substrate decreases while the VBS increases, which implies an increase in the degree of ordering of InGaPN. In addition, the order parameter η of InGaPN is deduced from the VBS.
Surface-plasma generation of negative ions in gas discharges without cesium
Authors: Dudnikov Vadim , Farrell J. Paul
Published: March 2006

Surface-plasma generation (SPG) of negative ions in gas discharges is extremely sensitive to small admixture of uncontrolled impurities. An efficiency of SPG was greatly improved by injection into discharge a small amount of cesium or other substances with low ionization potential. For explanation of anomalistic H− generation in discharges without the addition of cesium it is important to identify the sources of important impurities. Generation of H− ion in different ion sources is analyzed. Sources of impurities catalyzing SPG are proposed. A higher probability of cold H− extraction is outlined and explained. A proposal for enhanced H− generation in discharges without cesium is presented. Further experiments for identification of the dominant mechanism of H− generation are discussed.
Exoelectron emission from silicon nanocrystals
Authors: Rosenman Gil , Aronov Daniel , Molotskii Michael , Roizin Yakov , Heiman Alexey , Mei Wan Yuet , de Blank Rene
Published: March 2006

We have observed a high-temperature thermostimulated exoelectron emission from charged silicon nanocrystals with nitrided surface embedded into the amorphous SiO2 matrix. The developed Auger model allows understanding thermostimulated exoelectron emission origin and estimating energy activation of traps responsible for charge retention in this type of flash memory based on Si nanocrystals. The high activation energy Et of the electrons trapped in the nanocrystals confirms high potential of Si nanocrystal materials for fabrication of semiconductor memories with enhanced retention.
Spectrophotometric determination of reflective liquid crystal cell parameters
Authors: Valyukh Sergiy , Valyukh Iryna , Skarp Kent
Published: March 2006

We introduce a method for the determination of the optical retardation, its wavelength dispersion, the cell twist angle, and the orientation of the input director in a reflective liquid crystal (LC) cell. These parameters are found from the extremes of a characteristic function defined as a sum of two spectral reflectivities of the LC cell placed between a pair of linear polarizers. The reflectivities are measured for two cell orientations, one of which is arbitrary and the other one is turned through 45°. Both theoretical analysis and experimental procedures are presented. Excellent agreement between the experiment and our theory has been found. The proposed method can be applied to the measurement of reflective LC cells with small and large cell gaps, as well as cells with small and large twist angles.
Electrical transport behavior of Ca3MnxCo4−xO9 (⩽x⩽1.28) at low temperatures
Authors: Li D. , Qin X. Y. , Gu Y. J. , Zhang J.
Published: March 2006

Electrical transport behavior of Ca3MnxCo4−xO9 (0⩽x⩽1.28), prepared by sol-gel process, was investigated at temperatures from 300K down to 5K. The results indicate that dc conductivity σ and carrier concentration decay exponentially with decreasing temperature below ∼75K and ∼100K for Ca3MnxCo4−xO9 with x=0 and 0.03, respectively. The increase of resistivity (T≲100K) of Ca3Mn0.03Co3.97O9 with decreasing temperature originates mainly from reduction in its carrier concentration due to weakening of thermal activation. Nevertheless, the temperature behavior of resistivity at T≲75K for Ca3Co4O9 is mainly governed by two factors: (1) reduction in its carrier concentration; and (2) rise of carrier mobility resulting presumably from reduced optical phonon scattering. In contrast, the temperature dependence of the resistivity for the heavily substituted Ca3MnxCo4−xO9 (x=0.57,0.9,1.28) displays semiconductor-like behavior in the whole temperature range investigated, but does not obey an exponential law. Instead, a relationship lnσ∝−(T0∕T)1∕3 is found to hold for these oxides, in agreement with Mott’s two-dimensional variable range hopping model, which suggests that Ca3Co4O9 is a quasi-two-dimensional transport system, and the hopping conduction comes from the disorder introduced by the substitution of Mn for Co in the conduction plane.
Effective ac response of graded colloidal suspensions
Authors: Wei En-Bo , Dong L. , Yu K. W.
Published: March 2006

The alternating current (ac) response has been investigated theoretically in colloidal suspensions consisting of suspended radially inhomogeneous graded particles having complex permittivity profiles under an external ac electric field. The gradation in the colloidal particles is modeled by physically motivated graded profiles as the dielectric function may only vary slightly along the radius, while the conductivity profile may vary rapidly along the radius. More precisely, the dielectric function is assumed to be a constant, while the conductivity has a power-law dependence on the radius variable r, namely, εi(r)=A+crk∕(iω). In previous attempts, this model was solved numerically via the differential effective dipole approximation. In this work, we will demonstrate the existence of exact analytical solutions of the local potentials in the graded particles in terms of the hypergeometric functions, and hence the effective ac response is calculated in the dilute limit. Our exact results will be applied to graded biological cell suspensions. Extensions to nonlinear ac response will be discussed.
Structural and magnetic properties of Nd0.7Ce0.3MnO3 thin films
Authors: Yanagida Takeshi , Kanki Teruo , Vilquin Bertrand , Tanaka Hidekazu , Kawai Tomoji
Published: March 2006

This paper reports on the structural and magnetic properties of Nd0.7Ce0.3MnO3 (NCeMO) epitaxial thin films lacking Ce-rich impurities with the intention being to investigate the effect of ionic radius of the A site cation on the microstructures and magnetic properties in comparison with (La,Ce)MnO3 (LCeMO) systems. The absence of Ce-rich impurities in NCeMO films was confirmed by high resolution transmission electron microscopy, which in fact differs from results of LCeMO films where such Ce-rich impurities are responsible for the emergence of extrinsic ferromagnetism. In addition, as the ambient atmosphere during film formation was reduced, not only the crystallinity tended to improve but also the ferromagnetic-like behavior was enhanced, unlike the case with LCeMO films.
Optical phonon modes and crystal structure of NaLaF4 single crystals
Authors: Lage Márcio Martins , Matinaga Franklin Massami , Gesland Jean-Yves , Moreira Roberto Luiz
Published: March 2006

Polarized Raman scattering and infrared reflectivity measurements have been used to investigate the crystal structure of Czochralski-grown NaLaF4 single crystals. The phonon symmetries, the simultaneous presence of polar modes in the infrared and Raman spectra, as well as the observation of piezoelectric resonance, helped us to identify the P6¯ group as the correct one for this crystal. This material belongs to a family of sodium lanthanide tetrafluorides (NaLnF4) crystals, whose photoluminescence efficiency is comparable to LiYF4. Therefore, NaLaF4 crystals may be important in the development of diode pumped up-conversion solid-state lasers. The number and behavior of the observed optical phonon modes were analyzed in terms of group theory predictions for the group symmetry found. A few anomalies in the phonon characteristics are discussed in terms of cationic disorder in the crystal lattice.
Effect of oxidation on the electronic structure of a Si29 quantum dot: Calculations of redshifts in energy gap
Authors: Nishida Masahiko
Published: March 2006

We calculate the electronic structure of the H-covered Si29 quantum dot (∼1nm across) for four different oxygen configurations self-consistently using the extended Hückel-type nonorthogonal tight-binding method. The redshift in energy gap can occur in all oxygen configurations studied, but the size of the redshift depends on the oxygen configuration and also on the oxidation level. The energy-gap redshifts calculated for both backbonded and double-bonded oxygen configurations are large enough to explain the ones observed in the photoluminescence (PL) spectra from porous Si. For bridge-bonded and inserted oxygen configurations, the calculated redshifts are too small to explain the observed ones. The lifetimes calculated for the double-bonded oxygen configuration are around 2μs independent of oxidation level and photon energy. In contrary, for the backbonded oxygen configuration the calculated lifetime increases with an increasing level of oxidation and with decreasing photon energy (from several to a hundred micro seconds), being on the same order of magnitude as lifetimes measured for PL in porous Si.
Structural transition from L1 phase to A1 phase in FePt films caused by ion irradiation
Authors: Hasegawa T. , Li G. Q. , Pei W. , Saito H. , Ishio S. , Taguchi K. , Yamakawa K. , Honda N. , Ouchi K. , Aoyama T. , Sato I.
Published: March 2006

The effect of ion irradiation (B+,Cr+,Ga+, and Nb+) on the crystalline structure and magnetic properties of L10 (face-centered tetragonal structure) FePt films was investigated. Irradiating with Cr+,Ga+, and Nb+ ions of less than 1 at.  % (1.6×1015ions∕cm2) dose yielded an almost ideal structural transition from the L10 to A1 (face-centered cubic structure) phase withoutdamage to the surface of the film. This structural transition was accompanied by a change in magnetic properties from a hard magnet with a coercivity Hc of ∼7kOe to a soft magnet with Hc<1kOe. A two-dimensional pattern composed of hard magnetic L10 and soft magnetic A1 phases was fabricated by using a focused Ga+ ion beam.
Dopant partitioning influence on the near-infrared emissions of Tm3+ in oxyfluoride glass ceramics
Authors: Lahoz F. , Almenara J. M. , Rodríguez-Mendoza U. R. , Martín I. R. , Lavín V.
Published: March 2006

The doping distribution of Tm3+ ions in a transparent oxyfluoride glass ceramic has been investigated. Optical absorption, luminescence, and excitation measurements have been performed in order to determine the environment in which Tm3+ ions and the infrared emissions they give rise to are located. An interesting result has been found: the main contribution to the 1465 nm emission (S band) is due to Tm3+ ions in the crystalline phase for low doping level. However, when the Tm3+ concentration is high the S-band emission comes from the small portion of Tm3+ that remains in the vitreous phase. It has been concluded that cross relaxation (CR) processes are responsible for the quenching of the S-band emission in the crystalline phase for high doping concentration. Lifetime measurements of the H43 level have also been taken and the probability of CR processes deduced.
Effect of nitrogen on the electronic properties of hafnium oxynitrides
Authors: Choi J. , Puthenkovilakam R. , Chang J. P.
Published: March 2006

First principles simulations are used to determine the electronic structures of hafnium oxynitrides with various nitrogen concentrations. The effect of nitrogen incorporation on the density of states and the band gaps of hafnium oxynitrides are examined by the plane-wave pseudopotential method in the framework of density functional theory. The band gap decreases with increasing N concentration due to the extension of N 2p states into the band gap of t-HfO2. The band alignment at the HfOxNy∕Si interface is calculated using the t-HfO2 based (t-HfO2)5(Hf3N4) and (t-HfO2)(Hf3N4) structures. The estimated valence-band offsets are 1.47 and 1.14 eV for (t-HfO2)5(Hf3N4) and (t-HfO2)(Hf3N4), respectively, which are significantly smaller than the reported value for HfO2∕Siat∼3.0eV.
Theoretical model for the optical gain coefficient of indirect-band-gap semiconductors
Authors: Tsai Chin-Yi
Published: March 2006

A theoretical model for the calculation of the optical gain coefficient of indirect-band-gap semiconductors is presented. An explicit formula for the calculation is derived from the second-order time-dependent perturbation theory of quantum mechanics, which incorporates all the eight possible processes of photon/phonon emission and absorption. Conditions for achieving the population inversion and optical gain are investigated. The results show that, under the assumptions of constant interaction matrix elements and a simplified band structure with equal energy gaps at the conduction band minimum and the valence band maximum, a rather simple result can be obtained, in which the conditions for achieving population inversion and optical gain are in phonon-emission-dominant processes, the Fermi energy difference between electrons and holes is required to be larger than the photon energy plus the phonon energy and in phonon-absorption-dominant processes, the Fermi energy difference between electrons and holes is required to be larger than the photon energy minus the phonon energy. Since phonon absorption and emission processes coexist in a real situation, the realistic conditions for achieving optical gain are far more complicated and should be calculated numerically. We calculated the optical gain coefficient of bulk crystalline silicon. The results show that, without the suppression of other second-order optical-loss processes (e.g., free-carrier absorption), it is unlikely that the magnitude of optical gain will overcome the optical loss to achieve the lasing condition for bulk crystalline silicon.
Ferroelastic domain switching behaviors of superionic conductor M3H(SO4)2 (M=K, Rb, and NH4) single crystals
Authors: Lim Ae Ran , Nam Si Wook , Chang Jin-Hae , Jeong Se-Young
Published: March 2006

We have measured the temperature dependences of the optical polarizing microscopy and the stress-strain hysteresis curves in the K3H(SO4)2,Rb3H(SO4)2, and (NH4)3H(SO4)2 crystals grown by using the slow evaporation method. From these temperature dependences, it was determined that the three single crystals undergo the ferroelastic phase transitions near Tc. We also studied the ferroelastic domain switching that occurs in the three single crystals due to external mechanical stress. The critical stress values for switching from the ferroelastic twin domain to the paraelastic single domain were obtained. The ferroelastic domain switching stress of Rb3H(SO4)2 crystals (0.7 MPa) was found to be higher than those of K3H(SO4)2 and (NH4)3H(SO4)2 crystals (0.3 and 0.25 MPa, respectively). (NH4)3H(SO4)2 and K3H(SO4)2 can therefore be more easily transformed by a weak external mechanical stress. This domain switching from the ferroelastic twin domain to the paraelastic single domain is explained by the restriction of the rotation of the SO4 tetrahedron by the external stress.
Carrier dynamics in α-Fe2O3 (0001) thin films and single crystals probed by femtosecond transient absorption and reflectivity
Authors: Joly Alan G. , Williams Joshua R. , Chambers Scott A. , Xiong Gang , Hess Wayne P. , Laman David M.
Published: March 2006

Femtosecond transient reflectivity and absorption are used to measure the carrier lifetimes in α-Fe2O3 thin films and single crystals. The results from the thin films show that initially excited hot electrons relax to the band edge within 300fs and then recombine with holes or trap within 5ps. The trapped electrons have a lifetime of hundreds of picoseconds. Transient reflectivity measurements from hematite (α-Fe2O3) single crystals show similar but slightly faster dynamics leading to the conclusion that the short carrier lifetimes in these materials are due primarily to trapping to Fe d-d states in the band gap. In the hematite single crystal, the transient reflectivity displays oscillations due to the formation of longitudinal acoustic phonons generated following absorption of the ultrashort excitation pulse.
Electrokinetic flow in a free surface-guided microchannel
Authors: Lee Jacky S. H. , Barbulovic-Nad Irena , Wu Zhemin , Xuan Xiangchun , Li Dongqing
Published: March 2006

The purpose of this study is to investigate electro-osmotic flow in a free surface-guided microchannel. Although multiphase microfluidics has attracted interests over the past few years, electro-osmotic flow involving free surfaces has yet to be studied in great detail. Several proposed theoretical models describing this type of electro-osmotic flow need to be verified by experiments. In this work, a surface-guided microchannel was fabricated using an innovative fabrication process. Because the liquid stream was confined by surface properties, solid sidewalls did not exist in this microchannel. Instead, the sidewalls were water-air or water-oil interfaces. Using this microchannel, two systems were investigated: water-air system and water-oil system. The experimental results were compared against three proposed models in order to gain more understandings on this type of electro-osmotic flow. Experimental results show that the liquid velocity near the liquid-fluid interface resembles a pluglike profile for both water-air and water-oil systems. Computer simulation results show that with the consideration of the electrical double layer and the surface charges, the electric forces inside the electrical double layer are counteracted by the surface forces at the liquid-fluid interface, also resulting in a pluglike velocity profile in the microchannel. Therefore, the model that considers both the electrical double layer and the surface charges at the liquid-fluid interface best describe the physical phenomenon observed in experiments.
Nonuniform charging effects on ion drag force in drifting dusty plasmas
Authors: Chang Dong-Man , Chang Won-Seok , Jung Young-Dae
Published: March 2006

The nonuniform polarization charging effects on the ion drag force are investigated in drifting dusty plasmas. The ion drag force due to the ion-dust grain interaction is obtained as a function of the dust charge, ion charge, plasma temperature, Mach number, Debye length, and collision energy. The result shows that the nonuniform charging effects enhance the momentum transfer cross section as well as the ion drag force. It is found that the momentum transfer cross section and the ion drag force including nonuniform polarization charging effects increase with increasing the Mach number and also the ion drag force increases with increasing the temperature. In addition, it is found that the ion drag force is slightly decreasing with an increase of the Debye length.
Optical and site-selective spectral studies of Eu3+-doped zinc oxyfluorotellurite glass
Authors: Babu P. , Jang Kyoung Hyuk , Seo Hyo Jin , Jayasankar C. K.
Published: March 2006

The Eu3+-doped zinc oxyfluorotellurite (TZLFE) glass of molar composition 59TeO2–20ZnO–20LiF–1Eu2O3 has been prepared and studied by broadband, Raman, and fluorescence-line narrowing (FLN) spectral techniques to investigate the local environments of Eu3+ ions in this glass. From the vibronic spectra, different TeO groups coupled to Eu3+ ions have been identified. From the broadband excitation and emission spectra at RT, energy levels of the Eu3+ ion and the atomic parameters have been calculated. The D05 level of Eu3+ ions in the TZLFE glass has been selectively excited within the inhomogeneously broadened F07→D05 excitation spectrum at 15K. The resulting D05→F17 fluorescence spectra and the lifetimes of the D05 level have been obtained as a function of the excitation wavelength. From the FLN spectra, three Stark levels have been identified and crystal-field (CF) analysis has been carried out to calculate second rank CF parameters, by assuming C2v orthorhombic symmetry, as a function of the excitation wavelength. From the CF parameters, CF strength parameter has been calculated. The observed behavior suggests the existence of a unique kind of site for all the environments of Eu3+ ions in the TZLFE glass. The local structure of Eu3+ ions in the TZLFE glass is almost similar to sodium borosilicate glass but with a shorter distribution of environments.
The formation of metallic nanoparticles in single crystal CaF2 under 157nm excimer laser irradiation
Authors: Cramer L. P. , Langford S. C. , Dickinson J. T.
Published: March 2006

Single crystal calcium fluoride (CaF2) is an important material for vacuum-ultraviolet optical components. Unfortunately, all metal halides tend to form defects when exposed to energetic particles and laser radiation, and these defects can degrade optical performance. Here we examine the consequences of exposing CaF2 to 157nm excimer laser radiation and show that several tens of thousands of pulses at fluences near 1J∕cm2 can color the material. Absorption spectra of the exposed material confirm the formation of metallic calcium nanoparticles similar to those produced by other forms of energetic radiation. The rate of nanoparticle formation depends on the bulk temperature and displays a local maximum near 50°C. Absorption measurements at 157nm display a transient absorption component that grows during prolonged irradiation and disappears on time scales of several minutes after irradiation ceases. The implications of these effects in optical components are discussed.
The multitime correlation functions, free white noise, and the generalized Poisson statistics in the low density limit
Authors: Pechen A. N.
Published: March 2006

In the present paper the low density limit of the nonchronological multitime correlation functions of boson number type operators is investigated. We prove that the limiting truncated nonchronological correlation functions can be computed using only a subclass of diagrams associated to noncrossing pair partitions and thus coincide with nontruncated correlation functions of suitable free number operators. The independent in the limit subalgebras are found and the limiting statistics is investigated. In particular, it is found that the cumulants of certain elements coincide in the limit with the cumulants of the Poisson distribution. An explicit representation of the limiting correlation functions and thus of the limiting algebra is constructed in a special case through suitably defined quantum white noise operators.
Hugoniot evaluation of the preheated metal from its principal Hugoniot
Authors: Dai Chengda , Tan Hua , Hu Jianbo , Yu Yuyin , Zheng Jun
Published: March 2006

The Hugoniot of preheated metal is related with its principal Hugoniot by means of the Grüneisen equation of state and the Rankine-Hugoniot equation. Modeling calculations show that the preheating lowers to a certain extent the shock impedance. The evaluated Hugoniot of preheated Mo is comparable with the available experimental data. Hugoniots of 6061-T6 Al flyers of different preheat temperatures in reported Hugoniot experiments on Z pinch [Knudson et al., J. Appl. Phys. 94, 4420 (2003)] are calculated, and the determined shock states of 6061-T6 Al using the impedance-match method are consistent with the reported results. This approach allows a reasonable evaluation for Hugoniot of the preheated metal mainly from its principal Hugoniot.
A theory of double hysteresis for ferroelectric crystals
Authors: Srivastava N. , Weng G. J.
Published: March 2006

A ferroelectric crystal is known to exhibit the usual single hysteresis below its Curie point TC, but above this temperature its electric displacement D versus electric field E plot tends to form double loops. We first point out that there is a fundamental difference in the formation of double loops from the single one: the single loop is formed solely by polar reorientation, but in the double loops the right branch of its upper loop is formed by phase transition and only the left branch is formed by polar reorientation (the process is reversed for the lower loop). In this study we take the view that both cubic→tetragonal phase transition and the polar reorientation of ferroelectric domain are thermodynamics-driving process and use this concept to develop a micromechanics-based thermodynamic model to simulate the double hysteresis behavior of the crystal. We first derive the thermodynamic driving force for both spontaneous polarization and domain switch at a given level of temperature, stress, electric field, and new domain concentration c1 and then establish the kinetic equations for domain growth. A dual-phase homogenization theory is then introduced to calculate the overall electric displacement and mechanical strain of the crystal. This approach differs from the classical Landau-Ginzburg-Devonshire theory in at least two significant aspects: (i) it is developed with a micromechanics-based thermodynamics principle, and (ii) it can provide the evolution of new domain concentration. The developed theory is applied to a BaTiO3 crystal. The calculated results show a single loop below its TC and double loops above it but with a diminishing width at higher temperature. Furthermore, the longitudinal strain ε vs E loop is found to exhibit the usual butterfly-shape relation below TC, but above it the loop shows a new, overlapping double-well picture. Good agreement with available test data is also observed.