bluuebunny/crossref_metadata_2025 huggingface dataset:
Preparation and quantitative magnetic studies of single-domain nickel cylinders
Authors: O’Barr R. , Lederman M. , Schultz S. , Xu Weihua , Scherer A. , Tonucci R. J.
Published: July 2002

For a complete experimental and theoretical explanation of the magnetic processes in an interacting collection of submicron magnetic particles, a fundamental understanding of the magnetic properties of individual single-domain particles must first be achieved. We have prepared elongated Ni columns ranging in diameter from 0.15 to 1.0 μm by electroplating into specially prepared Al2O3 and glass channeled pore membranes. We have also prepared controlled arrays of Ni columns using e-beam lithography, subsequently electroplating into the written patterns. Using transmission electron microscopy, we have characterized the shape, size, morphology, and crystal structure of the columns. Magnetic force microscopy has been used to determine the switching field Hs versus the applied field angle of the columns. Although the switching field data can be fit to the functional form for nucleation by curling in an infinite cylinder, the observed weak dependence of Hs on column diameter is inconsistent with that expected for curling, particularly for columns of diameter ≳0.3 μm.
Device and material characterization of Cu(InGa)Se2 solar cells with increasing band gap
Authors: Shafarman William N. , Klenk Reiner , McCandless Brian E.
Published: July 2002

Thin-film solar cells have been fabricated from Cu(InGa)Se2 films which were deposited by four-source elemental evaporation with [Ga]/([In]+[Ga]) from 0.27 to 0.69 corresponding to a band gap from 1.16 to 1.45 eV. The films were intentionally deposited with no grading of the Ga and In to avoid gradients in their electrical and optical properties. X-ray diffraction, energy-dispersive x-ray spectroscopy, and Auger electron spectroscopy show that the films have uniform composition with no change in structure and morphology. Glass/Mo/Cu(InGa)Se2/CdS/ZnO devices have open-circuit voltage increasing over the entire band gap range to 788 mV and 15% total area efficiency for band gap less than 1.3 eV, or [Ga]/([In]+[Ga]) less than 0.5. A decrease in device efficiency with higher Ga content is caused primarily by a lower fill factor. Analysis of current–voltage and quantum efficiency measurements show that this results from a voltage-dependent current collection.
Magnetic domain structure in ultrathin Cu/Ni/Cu/Si(001) films (invited)
Authors: Hug Hans J. , Stiefel B. , Moser A. , Parashikov I. , Klicznik A. , Lipp D. , Güntherodt H.-J. , Bochi Gabriel , Paul D. I. , O’Handley R. C.
Published: July 2002

We present a series of magnetic force microscope (MFM) images of epitaxial magnetic thin films. The films studied, Ni/Cu/Si(001) capped by 2 nm of Cu, exhibit perpendicular anisotropy over an exceptionally broad thickness range, 2 nm<h<14 nm. The magnetic domain structure of the as-grown films shows a sharp transition to a finer length scale above a finite critical thickness of 12 nm. Micromagnetic theory provides the first quantitative description for these general but previously unexplained phenomena. Further we discuss MFM data obtained on films with a thickness larger than 14 nm. These films show a pronounced in-plane anisotropy.
Magnetic field dependence of the binding energy of a hydrogenic impurity in a spherical quantum dot
Authors: Xiao Zhigang , Zhu Jiqian , He Fengai
Published: July 2002

The binding energy of the ground state of a hydrogenic off-center donor in a spherical quantum dot in a magnetic field is calculated by a variational method within the effective-mass approximation. The effects of location on a hydrogenic donor in a magnetic field on the binding energy of the ground state are studied. It is found that in the case of an on-center donor, the magnetic field tends to enhance the strength of the Coulomb interaction and the binding energy of the ground state, and in the case of an off-center donor, the effect of the magnetic field on the binding energy of the ground state is somewhat complicated.
A Monte Carlo study on the electron-transport properties of high-performance strained-Si on relaxed Si1−xGex channel MOSFETs
Authors: Roldán J. B. , Gámiz F. , López-Villanueva J. A. , Carceller J. E.
Published: July 2002

We have studied the electron-transport properties of strained-Si on relaxed Si1−xGex channel MOSFETs using a Monte Carlo simulator adapted to account for this new heterostructure. The low-longitudinal field as well as the steady- and nonsteady-state high-longitudinal field transport regimes have been described in depth to better understand the basic transport mechanisms that give rise to the performance enhancement experimentally observed. The different contributions of the conductivity-effective mass and the intervalley scattering rate reduction to the mobility enhancement as the Ge mole fraction rises have been discussed for several temperature, effective, and longitudinal-electric field conditions. Electron-velocity overshoot effects are also studied in deep-submicron strained-Si MOSFETs, where they show an improvement over the performance of their normal silicon counterparts.
Physics and applications of organic microcavity light emitting diodes
Authors: Dodabalapur A. , Rothberg L. J. , Jordan R. H. , Miller T. M. , Slusher R. E. , Phillips Julia M.
Published: July 2002

The important changes produced on the electroluminescence characteristics of organic materials due to planar microcavity effects are examined in detail. The photon density of states is redistributed such that only certain wavelengths, which correspond to allowed cavity modes, are emitted in a given direction. This enables us to realize color selectivity over a large wavelength (and color coordinate) range with broadband emitters such as 8-hydroxyquinoline aluminum (Alq), and intensity enhancement in narrow band emitters. The intensity enhancement in Alq-based cavity light emitting diodes (LEDs) is extensively evaluated both experimentally and theoretically. The design considerations for and device characteristics of a novel multiple emissive layer LED are also described.
Size-distribution and annealing behavior of end-of-range dislocation loops in silicon-implanted silicon
Authors: Pan G. Z. , Tu K. N. , Prussin A.
Published: July 2002

A study of end-of-range (EOR) dislocation loops in silicon implanted with 50 keV 1016 Si/cm2 was carried out by using transmission electron microscopy. Two kinds of post-implantation anneals were performed, furnace anneals at 850 °C and rapid thermal anneals at 1000 °C. We observed the ripening for two types of EOR dislocation loops. They were faulted Frank dislocation loops and perfect prismatic dislocation loops. By separating their size distribution profiles, we found that their distribution profiles are different from that of conventional Ostwald ripening for precipitates. A long tail distribution profile was formed for perfect prismatic dislocation loops. We analyzed the distribution profiles and found that the size distribution profile of faulted Frank dislocation loops could be well fitted by a normal Gaussian probability function and that of perfect prismatic dislocation loops by a log-normal Gaussian probability function. Measurement of the total number of interstitials within both types of loops shows that the ripening of EOR dislocation loops is conservative. Knowing the size-distribution profiles of the EOR dislocation loops, it was possible to perform an analysis of the ripening behavior of the two types of dislocation loops.
Canted structures in the Mn3+/Mn4+ perovskites
Authors: Jirák Z. , Hejtmánek J. , Pollert E. , Maryško M. , Dlouhá M. , Vratislav S.
Published: July 2002

Neutron diffraction and magnetization measurements have been performed on numerous systems with the divalent and monovalent substitution, Pr1−xAx2+Mn1−x3+Mnx4+O3 (A=Ca, Sr, Ba) and Pr1−x/2Ax/2+Mn1−x3+Mnx4+O3 (A=Na, K). The study shows that the transition from the Ay-type (layer) antiferromagnetism in pure PrMnO3 to the ferromagnetism in the mixed-valence systems is not continuous. In a range of about 5%–15% of Mn4+ (x∼0.05–0.15), canted magnetic arrangements of the AyFz type are observed. The interlayer coupling is changed gradually from θ=180° for x=0–0.05 to θ∼90° for x=0.15. For about x=0.15–0.20, the canted phase with 90° coupling coexists with a purely ferromagnetic arrangement of the Fy type. The observed behavior is related to an interplay of the magnetic superexchange and the double exchange.
New production method of 100-μm-thick grain-oriented 3% silicon steel sheets
Authors: Nakano M. , Ishiyama K. , Arai K. I. , Fukunaga H.
Published: July 2002

We report a new method of preparing highly oriented 3% silicon steel sheets with a thickness of 100 μm, which enables us to realize the mass production with existing manufacturing facility. In our new method, a hot-rolled silicon steel is thinned to 100 μm by a three-step rolling process with intermediate annealings and the (110)[001] is created by the final annealing taking advantage of surface energy. The obtained 100-μm-thick sheets had the Goss texture and superior soft magnetic properties (the magnetic induction at 800 A/m, B8⋅=⋅1.9 T, the coercive force <4 A/m). In addition, the iron loss W 15/50 at 1.5 T, 50 Hz under the applied tensile strength of 2 kg/mm2 was 0.53 W/kg. This value is less than that of the conventional (300 μm) grain-oriented silicon steels by about 37%.
Appropriate analytical description of the temperature dependence of exciton peak positions in GaAs/AlxGa1−xAs multiple quantum wells and the Γ8v−Γ6c gap of GaAs
Authors: Pässler R. , Oelgart G.
Published: July 2002

We have performed a detailed numerical reinvestigation of the photoluminescence peak position data given by G. Oelgart et al. [J. Appl. Phys. 74, 2742 (1993)] for the ground state heavy-and light-hole excitons in a high quality molecular beam epitaxially grown GaAs/Al0.3Ga0.7As multiple quantum well structure. Appropriate fittings of the measured temperature dependencies of exciton peak positions from 4.2 up to 340 K are shown to be provided by a novel analytical four-parameter representation developed recently by one of the authors for the gap shrinkage effect in semiconductors. The magnitude of the limiting (T→∞) shrinkage coefficient, α=0.475 meV/K, and the associated average phonon temperature, Θ=222.4 K, have been determined. Characteristic qualitative differences and basic deficiencies of earlier three-parameter models are discussed and illustrated numerically by comparisons with various experimental observations from low to high temperatures.
Linear dependence of both the hardness and the elastic modulus of pulsed laser deposited a-SiC films upon their Si–C bond density
Authors: El Khakani M. A. , Chaker M. , O’Hern M. E. , Oliver W. C.
Published: July 2002

Amorphous a-SiC films exhibiting excellent hardness and elastic modulus mechanical properties, as determined by nanoindentation, have been deposited by means of the pulsed laser deposition (PLD) technique onto either Si(100) or fused quartz substrates, at deposition temperatures ranging from 20 to 650 °C. The increase of the deposition temperature of PLD a-SiC films (from 20 to 650 °C) markedly enhances both their hardness and their elastic modulus. PLD a-SiC films with hardness and elastic modulus characteristics as high as 50 and 380 GPa, respectively, are obtained at 650 °C deposition temperature. On the microstructural level, the increase of the substrate deposition temperature (from 20 to 650 °C) favors the formation of Si–C bonds, leading thereby to a substantial increase of the Si–C bond density in PLD a-SiC films, as evidenced by Fourier-transform infrared analysis. This work clearly reinforces the concept that the Si–C bond density (NSi–C) is the dominant microstructural parameter that determines the variation of the hardness and elastic modulus of a-SiC films. Indeed, a constant-plus-linear dependence for both the hardness and the elastic modulus of a-SiC films upon their Si–C bond density was established over an NSi–C range as large as (4–24)×1022 bond cm−3.
Experimental study of nonstationary electron transport in sub-0.1 μm metal–oxide–silicon devices: Velocity overshoot and its degradation mechanism
Authors: Mizuno Tomohisa , Ohba Ryuji
Published: July 2002

We have experimentally studied the electron velocity overshoot and the mechanism of its degradation in the inversion layer of sub-0.1 μm metal–oxide–silicon (MOS) field-effect transistors. Both silicon-on-insulator (SOI) and bulk structures were studied. At low transverse electric fields, that is, for low carrier densities in SOI devices under low gate drive conditions, it is possible to achieve electron velocity overshoot due to nonstationary transport in the sub-0.1 μm region. However, it is very difficult in MOS structures to improve electron velocity at high surface electron densities because of the reduced electron mobility in high transverse fields. Moreover, the surface electron density of MOS structures is reduced when a low channel impurity concentration is chosen to improve low field mobility; this results from the expanded inversion layer width. These results indicate the physical limitations of scaled MOS structures with regards to the realization of higher current capabilities.
Band gap energies of bulk, thin-film, and epitaxial layers of CuInSe2 and CuGaSe2
Authors: Chichibu S. , Mizutani T. , Murakami K. , Shioda T. , Kurafuji T. , Nakanishi H. , Niki S. , Fons P. J. , Yamada A.
Published: July 2002

Band gap and excitonic resonance energies of high-quality bulk single crystals, polycrystalline thin films, and epitaxial layers of CuInSe2 and CuGaSe2 were determined as a function of temperature by means of photoreflectance, optical absorption (OA), and photoluminescence measurements. OA spectra were fit including excitonic absorption from low temperature up to room temperature (RT). The band gap energy of 1.032 eV and free exciton (FE) resonance energy of 1.024 eV were obtained at RT for strain-free CuInSe2 giving an exciton binding energy of 7.5 meV. The band gap energy of both CuInSe2 and CuGaSe2 was found to be essentially independent of the molar ratio of Cu to group-III atom (Cu/III) for near-stoichiometric and Cu-rich samples. The disappearance of the FE absorption in the In-rich (Cu/In<0.88) CuInSe2 thin films was explained by plasma screening of Coulomb interactions. A slight decrease in the band gap energy of the In-rich films was attributed to a degradation of film quality such as high-density defects, grains, and structural disordering. The fundamental band gap energy in strained CuInSe2 and CuGaSe2 epilayers was shown to decrease due to in-plane biaxial tensile strain.
Temperature induced diffusion in Mo/Si multilayer mirrors
Authors: Voorma H.-J. , Louis E. , Koster N. B. , Bijkerk F.
Published: July 2002

To optimize the growth process of Mo/Si multilayers, the effect of an elevated substrate temperature during deposition has been studied in the temperature range between 300 K and 550 K. Multilayer properties, such as interface roughness, d-spacing, and structure of the layers, have been investigated during deposition and cool-down, after cool-down, and during heating. A number of techniques have been used: small-angle, near-normal incidence, and in situ reflectivity measurements. It is found that the increased substrate temperature changes the interface roughness to a minimum value for samples produced at 488 K. Also, a change of the d-spacing as a function of time and temperature is observed and is explained by annihilation of free volume of the Si layer. The atomic structures of the layers deposited at different temperatures have been analyzed with high resolution electron microscopy (TEM), which shows that both materials are amorphous for the entire temperature range investigated. At the extremes of the temperature range investigated irregular layer structures, such as ripples and V-shaped structures, have been observed by TEM.
Variable range hopping conduction in multiwalled carbon nanotubes
Authors: Yosida Yositaka , Oguro Isamu
Published: July 2002

An experimental study of electrical transport phenomena in magnetic-fields H up to 5 T in pressed samples of a porous system composed of multiwalled carbon nanotubes is reported. The exp[(T0/T)1/4] temperature dependence in the zero-field dc electrical resistance R(0) is found at low temperatures. Below 50 K, transverse magnetoresistance (MR), [R(H)−R(0)]/R(0), is negative. In the weak-field region (H<0.1 T), the negative MR is quadratic in H, and becomes linear in the field range 0.1–0.3 T. The linear and negative MR shows T−0.65±0.06 temperature dependence. The temperature dependence of R(0) is described in terms of Mott’s law for three-dimensional variable range hopping (3D VRH) conduction. The low-field magnetotransport features support a predicted quantum interference effect on 3D VRH conduction.
Electronic structures of organic molecular materials for organic electroluminescent devices studied by ultraviolet photoemission spectroscopy
Authors: Sugiyama Kiyoshi , Yoshimura Daisuke , Miyamae Takayuki , Miyazaki Takafumi , Ishii Hisao , Ouchi Yukio , Seki Kazuhiko
Published: July 2002

Electronic structures of evaporated films of five organic light-emitting and carrier-injecting materials for organic electroluminescent devices were studied by ultraviolet photoemission spectroscopy. The compounds examined were (i) light-emitting materials tris(8-hydroxyquinolino) aluminum (Alq3) and 1,2,3,4,5-pentaphenylcyclopentadiene, (ii) a hole-injecting material N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine, and (iii) electron-injecting materials N,N′-diphenyl-1,4,5,8-naphthyletracarboxyldiimide and 1,3,5-tris(5-phenyl-1,3,4-oxadiazol-2-yl)benzene. The spectral features corresponding to the top parts of the valence states, which dominate the electric properties of the materials, were assigned by the comparison with the simulated density of states obtained from PM3 molecular orbital calculations. Using these calculations, the evolution of the electronic structure of each molecule from those of constituent parts was discussed. The characters of the unoccupied states obtained by these calculations were also presented. Using these data, the correlation between the ionization threshold energies determined by ultraviolet photoemission spectroscopy and the carrier-injecting and light-emitting properties were discussed.
The effect of the electron irradiation on the electrical properties of thin polycrystalline CdS layers
Authors: Ruxandra V. , Antohe S.
Published: July 2002

The electrical properties of nonirradiated and electron irradiated structures, containing a polycrystalline thin layer of CdS, sandwiched between two gold electrodes, were investigated. The thin films of CdS, obtained through thermal-vacuum evaporation on the glass substrate at a temperature of 220 °C, were subjected to two sessions of irradiation with 7 MeV electrons to the fluences of 4×1015 and 6×1015 e/cm2, respectively. In the case of nonirradiated structures, under low voltages, the Ohm’s law is followed with a thermally activated electron concentration of n0≅3×1016 cm−3, an electron mobility μ0≅0.1 cm2/V s and a room temperature electrical conductivity σ0≅4×10−4 Ω−1 cm−1. In this range of voltage the electron irradiation induces a small increase in the activation energy of mobility, determining, of course, a small decreasing of the mobility. At high-applied voltage, there is a space-charge-limited conductivity controlled by a single trap level having the depth of Ec−Et≅0.086 eV and the total trap concentration Nt≅8.9×1015 cm−3. In this range of voltage, the electron irradiation modifies the trap distribution. After the first session of irradiation a uniform trap distribution appears and after the second session, an exponential trap distribution was induced in the band gap of CdS layer. All the induced trap distributions are characterized and their effect on the charge transport mechanisms is discussed.
Effective conductivities of rectangular arrays of aligned spheroids
Authors: Lu Shih-Yuan
Published: July 2002

The effective conductivities (σeff) of rectangular arrays of aligned spheroids are accurately determined. The array structure is governed by the array aspect ratio (ra), inclusion aspect ratio (ri), and inclusion volume fraction ( f ). By varying these three parameters, a wide range of array structure can be generated and the effects of the three parameters on the effective conductivities of the array are investigated. The present array is found to be a more efficient structure in utilizing the enhancing or reducing effect of the inclusion, as compared to the simple cubic array of spheres. For cases of equal ra and ri, the effective parallel conductivity (σeff=) increases, while the effective perpendicular conductivity (σeff+) decreases with increasing aspect ratio, but both are only a weak function of the aspect ratio if the reduced conductivity of the inclusion (σ) is within the range of 0.1–10. At a fixed ri, σeff= decreases while σeff+ increases with increasing ra. If ra is fixed, σeff= increases while σeff+ decreases with increasing ri. For prolate systems (ri>1), σeff= is greater than σeff+ for all permissible f if ri⩾ra, but the magnitude order switches at higher f when ri<ra. As to oblate systems (ri<1), σeff= is less than σeff+ for all permissible f if ri⩽ra, but the magnitude order switches at higher f when ri>ra.
Modeling of the transient enhanced diffusion of boron implanted into preamorphized silicon
Authors: Lampin E. , Senez V. , Claverie A.
Published: July 2002

We present a physically based modeling of the transient enhanced diffusion (TED) of boron implanted into preamorphized silicon. We start by describing the nucleation and growth of a supersaturation of Si interstitial atoms into dislocation loops. Our modeling of the nucleation and growth of the dislocation loops is divided into three distinct stages: the nucleation, the “pure growth,” and the Ostwald ripening. The implementation of this modeling into the process simulator IMPACT-4 allows one to correctly predict the size and density evolutions of the dislocation loops observed by transmission electron microscopy for a variety of annealing times and temperatures. This simulation also gives access to the concomitant behavior of the free Si interstitials atoms responsible for TED. Implementation of this model into IMPACT-4 shows that TED in preamorphized Si can be simulated for a variety of experimental conditions by assuming boron diffusion occurs through the coupling of boron atoms with this fast evolving supersaturation. It is shown that while not affecting too much the defect evolution itself, the value of the recombination velocity at the surface is a crucial parameter to correctly estimate the amount of TED after a given annealing. Best fits are obtained for recombination lengths of about 16 nm for annealing under Ar gas.
Thermoluminescence of ZrO2 doped with Er2O3 induced by ultraviolet and x-ray radiation
Authors: Lai Lee-Jene , Su Ching-Shen
Published: July 2002

This work investigates the thermoluminescent (TL) glow of Er2O3-doped ZrO2 sample, after separate irradiation with UV and synchrotron x-ray radiations. Experimental results indicate that, in addition to a major TL peak at 98 °C and a minor one at 205 °C, another minor peak appears at 135 °C. The intensity of two minor peaks, particularly the one at 135 °C, increases rapidly with the photon energy. Initial rise method is used to obtain the activation energies of 0.96 and 0.99 eV for 135 and 205 °C TL peaks, respectively. Moreover, the trap depth is theoretically estimated by invoking a HHe+2 like model in which an electron is trapped by an effective positive charge localized on an oxygen site but perturbed by two effective positive charges localized on a cation site. Derived theoretical trap depths, 0.95 and 0.97 eV, are in excellent agreement with experimental values.
Theoretical investigation of the influence of space charges on ferroelectric properties of PbZrTiO3 thin film capacitor
Authors: Baudry L.
Published: July 2002

During ion etching (IE) processes used for making a Pt electrode in the fabrication of ferroelectric capacitors, the film is exposed to radiation and energetic ion bombardment. The influence of such processes on polarization-electric field characteristics of lead zirconate titanate thin film capacitor has been reported by E. G. Lee [Appl. Phys. Lett. 69, 1223 (1996)]. A large field shift and a constriction in the hysteresis loop are experimentally observed. For these authors, this behavior is probably due to the existence of space charges trapped near electrodes. For a better understanding of these phenomena, we have developed a model of ferroelectric capacitor based on the phenomenological Landau–Devonshire theory. A two-dimensional lattice of dipoles is assumed to describe the film. By solving Poisson’s equation, the electric-field distribution inside the film is calculated, which allows us to locally determine the evolution of the polarization by minimizing free energy. IE effects and probably the existence of space charges in the film are introduced by means of a doping layer in the film near the electrode. The influence of important parameters such as doping level, and thickness of the doped layer on the hysteresis loop are investigated. Main experimental thin film electric behaviors are well reproduced and explained considering dipoles switched inside the lattice. Results are interpreted by splitting the hysteresis loop deformation into two effects with different origins: a constriction effect related to the number of impurities inside the film and a shift effect related to the asymmetrical impurities distribution.
Photostimulated luminescence process in the x-ray storage phosphor CsBr:Ga+
Authors: Rogulis U. , Schweizer S. , Assmann S. , Spaeth J.-M.
Published: July 2002

CsBr doped with Ga+ has previously proved to be an efficient X-ray storage phosphor with a figure of merit comparable to that of the commercially used BaFBr:Eu2+. By measuring the magnetic circular dichroism of the optical absorption (MCDA), the MCDA-detected electron paramagnetic resonance and the photostimulated luminescence (PSL), the paramagnetic defect centers generated upon x-irradiation involved in the storage and read-out process were investigated and found to be particularly simple: The electron traps are F centers, the hole trap centers are Ga2+ centers with a hyperfine interaction with Ga69 of A69=6.5 GHz. No superhyperfine interaction was resolved. It is proposed that the PSL-active Ga2+ center is an isolated Ga2+ on a Cs+ site.
Magnetic properties of mixed cobalt–zinc ferrite nanoparticles
Authors: Hochepied J. F. , Pileni M. P.
Published: July 2002

Solid solutions of nonstoichiometric mixed cobalt–zinc ferrite nanoparticles Co0.73yZn0.73(1−y)Fe2.18□0.09O4, were prepared in order to study their magnetic properties as a function of cobalt content y. The saturation magnetization changes with increasing y due to the various occupancies of cations in tetrahedral and octahedral sites and/or to an increase in the disorder of the ferrimagnetic structure. The cobalt content and particle size effects on the coercive field were studied. The increase in the ratio between remanent and saturation magnetizations indicates that, with increasing y, a transition takes place from uniaxial to cubic anisotropy. Field cooled (FC) curves drastically change with the cobalt content. At zero cobalt, this curve is characterized by a well-defined peak that disappears progressively with increasing y. This is attributed to the particles themselves and neither to their interactions nor to experimental procedures. The fit of the zero FC (ZFC) curve indicates an unrealistically high simulated saturation magnetization, consistent with a drastic field-dependent behavior of the ZFC-FC curves.
Curie temperature of multiphase nanostructures
Authors: Skomski R. , Sellmyer D. J.
Published: July 2002

The Curie temperature and the local spontaneous magnetization of ferromagnetic nanocomposites are investigated. The macroscopic character of the critical fluctuations responsible for the onset of ferromagnetic order means that there is only one Curie temperature, independent of the number of magnetic phases present. The Curie temperature increases with the grain size and is, in general, larger than predicted from the volume averages of the exchange constants. However, the Curie-temperature enhancement is accompanied by a relative reduction of the spontaneous magnetization. Due to the quadratic dependence of the permanent-magnet energy product on the spontaneous magnetization, this amounts to a deterioration of the magnets performance. The length scale on which an effective intergranular exchange coupling is realized (coupling length) depends on the Curie-temperature difference between the phases and on the spacial distribution of the local interatomic exchange. As a rule, it is of the order of a few interatomic distances; for much bigger grain sizes the structures mimic an interaction-free ensemble of different ferromagnetic materials. This must be compared to the magnetic-anisotropy coupling length, which is of the order of 10 nm. The difference is explained by the nonrelativistic character of the Curie-temperature problem.
Localized-state distributions in molecularly doped polymers determined from time-of-flight transient photocurrent
Authors: Nagase Takashi , Naito Hiroyoshi
Published: July 2002

Localized-state distributions have been studied in a molecularly doped polymer (MDP) system of a polymer binder (polycarbonate) doped with charge-transporting [N, N′-diphenyl-N, N′-bis(3-methylphenyl)(1,1′-biphenyl)-4,4′-diamine (TPD)] and trap-forming molecules [1-phenyl-3-(p-diethylaminostyryl)-5-(p-diethylaminophenyl)pyrazoline (PRA)] simultaneously by means of the conventional time-of-flight (TOF) transient photocurrent measurements. The existence of a transport energy in the MDP system is experimentally shown by comparing Gaussian distributions of localized states deduced by the Gaussian disorder model, due to Bässler and co-workers [H. Bässler, Phys. Status Solidi B 175, 15 (1993)], with localized-state distributions determined from the analysis of the TOF transient photocurrent data, based on the trap-controlled band transport [H. Naito, J. Ding, and M. Okuda, Appl. Phys. Lett. 64, 1830 (1994)]. The transport energy is found to be located at the center of the Gaussian distribution due to the host TPD molecules. It is also found that at 0.1 mol % PRA addition, the Gaussian distribution of localized states due to TPD molecules is broadened through the random electrostatic potential generated by dipoles of PRA, and at 1 mol % PRA addition, the localized-state structure due to PRA molecules, as well as the further broadening of the Gaussian distribution, are observed. The energy level of the structure is determined to be 0.54 eV above the transport energy, which is almost equal to the difference in the ionization potential between PRA and TPD.
Surface enhancement factors for Raman scattering at silver electrodes. Role of adsorbate–surface interactions and electronic structure
Authors: Weaver Michael J. , Farquharson Stuart , Tadayyoni M. A.
Published: July 2002

The integrated Raman band intensities for internal vibrational modes of various adsorbates at roughened silver electrodes are combined with surface concentrations obtained electrochemically to yield surface Raman scattering cross sections. These data together with the corresponding bulk-phase Raman intensities provide estimates of the degree of enhancement of the vibrational bands at the silver surface with respect to the bulk medium, i.e., the ‘‘surface enhancement factors,’’ SEF. For surface-attached thiocyanate, isothiocyanate-bridged Cr(III) complexes, and also for Cr(III) and Co(III) hexaammines that are electrostatically attracted to chloride-coated silver, SEF values are obtained that are relatively insensitive to both the vibrational mode and the adsorbate, even though substantial (up to 500 fold) variations are observed in the Raman cross sections. This finding applies to stretching modes involving nonbridging ammine groups as well as to bridging thiocyanate modes, although the frequencies of the latter, but not of the former, are perturbed strongly by the silver surface. Comparisons were also made between Raman cross sections, surface enhancement factors, and excitation profiles for pyrazinepentaammineosmium(III) and (II) to explore the influence of electronic resonance since the Os(II) form displays an intense charge-transfer band in the vicinity of the laser wavelengths (460–650 nm) employed. A pronounced peak in the Os(II) excitation profile, absent for Os(III), is obtained that is markedly (∼100 nm) red shifted for the adsorbed vs bulk complex.
Feshbach resonances in electron–molecule scattering by the complex multiconfiguration SCF and configuration interaction procedures: The 1Σ+g autoionizing states of H2
Authors: Yabushita Satoshi , McCurdy C. William
Published: July 2002

The complex resonance energies (positions and widths) and wave functions for the lowest 1Σ+g doubly excited autoionizing states of H2 are directly computed by using the multiconfiguration self-consistent field (MCSCF) method and the configuration interaction (CI) method within the context of the complex basis function technique. These autoionizing states are Feshbach resonances (as opposed to shape resonances), and single-configuration self-consistent field calculations provide no information about the lifetimes of such states. All of these methods rely on the existence of a complex variational principle for complex resonance energies. It is shown that by using a small orbital space the MCSCF method can give essentially the same complex energies as the full CI method. Numerical results are in good agreement with previous theoretical results, especially with the optical potential calculation employing a diffuse basis set by Schneider and Collins [Phys. Rev. A 28, 166 (1983)], indicating that poor agreement with previous complex coordinate results by Moiseyev and Corcoran [Phys. Rev. A 20, 814 (1979)] is attributable to their small basis set. A detailed study of basis set and correlation effects on the complex energies is also presented.
Semiconductor cluster beams: One and two color ionization studies of Six and Gex
Authors: Heath J. R. , Liu Yuan , O’Brien S. C. , Zhang Qing-Ling , Curl R. F. , Tittel F. K. , Smalley R. E.
Published: July 2002

Supersonic beams of clusters of Si and Ge atoms have been produced by laser vaporization followed by supersonic expansion in a helium carrier. The cluster beams were characterized by F2(7.9 eV) and ArF(6.4 eV) excimer laser ionization accompanied by time-of-flight mass analysis. In addition, the feasibility of a resonant two-photon ionization (R2PI) spectroscopic study was explored by two-color experiments involving initial excitation with the second (2.36 eV) and third (3.54 eV) harmonics of the Nd:YAG followed by excimer laser ionization. All two-photon ionization processes were found to produce extensive fragmentation of the larger clusters. The observed fragmentation pattern for the silicon and germanium clusters were remarkably similar to each other, but drastically different from that seen for metal clusters in the same apparatus. Unlike metal clusters, which tend to lose one atom at a time, these semiconductor clusters appear to fragment by a fission process, the daughter ions falling almost exclusively in the size range from 6 to 11 atoms. Time delay studies in the two-color experiments established that clusters of both Si and Ge have excited electronic states with lifetimes of approximately 100 ns. This again is dramatically different from the behavior found with metal clusters, and indicates the feasibility of R2PI spectroscopy on these cold semiconductor particles. The existence of such long-lived excited states indicates that there is probably an energy gap between the band of electronic states being excited and the ground electronic state.
Noncrystalline structure of argon clusters. II. Multilayer icosahedral structure of ArN clusters 50<N<750
Authors: Farges J. , de Feraudy M. F. , Raoult B. , Torchet G.
Published: July 2002

In a preceding paper, we have described the polyicosahedral structure of small argon clusters containing less than about 50 atoms which were produced in a free jet expansion. Going on with this study, we describe presently the multilayer icosahedral structure of larger argon clusters containing up to about 750 atoms, a value for which the transition to the fcc crystalline bulk structure is found to occur. Cluster models are used in order to study the third icosahedral layer construction, and the transition from a twin to a regular surface arrangement, which is expected to take place around 75 atoms, is observed on experimental patterns. The good agreement between experimental and calculated diffraction functions leads to an estimate of mean cluster sizes, cluster size distributions, and temperature (32±2 K) of clusters containing several hundreds of atoms.
Temperature-dependent resonance Raman profiles of β-carotene in carbon disulfide
Authors: Lee S. A. , Chan C. K. , Page J. B. , Walker C. T.
Published: July 2002

Resonance Raman (RR) profiles of the 1005, 1155, and 1525 cm−1 modes of β-carotene dissolved in carbon disulfide have been measured at room temperature and at 172 K. Previous studies, based upon room temperature measurements, have indicated that inhomogeneous (i.e., site) broadening may be important for this system. Our measurements are the first RR data for this system at two temperatures. Such data are necessary in order to study the relative importance of inhomogeneous broadening and thermal broadening. Using previously developed transform techniques, we analyze our RR data by calculating profile line shapes directly from our measured optical absorption data for each temperature. The assumptions underlying this analysis do not include inhomogeneous broadening, and the calculations yield profile line shapes which are in quite good overall agreement with the measured profile line shapes for all three modes at both temperatures. We have also extended the transform calculations in order to incorporate inhomogeneous broadening. However, the agreement between the measured and calculated RR profile line shapes is not substantially improved by the inclusion of inhomogeneous broadening in the transform analysis.
Ground-state properties of alkali dimers XY (X, Y=Li to Cs)
Authors: Igel-Mann G. , Wedig U. , Fuentealba P. , Stoll H.
Published: July 2002

Pseudopotential configuration interaction (CI) calculations using large basis sets have been performed for all homonuclear and heteronuclear alkali dimers XY (X,Y=Li to Cs). Results are given for ground-state spectroscopic constants. The maximum deviations from accurate experimental data are 0.03 Å for Re, 0.02 eV for De, 4 cm−1 for ωe, 0.02 eV for ionization energies, and 0.1 D for dipole moments. Predictions are made for a number of experimentally uncertain or unknown values.
On the modified moment method and irreversible thermodynamics
Authors: Eu Byung Chan
Published: July 2002

The modified moment method for the Boltzmann equation is reexamined and improved. There now is only one consistency condition instead of a set of consistency conditions, and it is a necessary and sufficient condition for the extended Gibbs relation. The condition is seen to be a partial differential equation for the entropy density. A strategy to determine the entropy from the data on transport processes is presented. By this strategy measurements of transport coefficients amount to a measurement of nonequilibrium entropy just as measurements of heat capacity, compressibility, etc., yield an equilibrium entropy. A relative measure of probability for a pair of steady states is given in terms of the entropy productions at the steady states. According to the measure a steady state of lower entropy production is more favorable.
Fermi resonance phase space structure from experimental spectra
Authors: Kellman Michael E. , Lynch Eric D.
Published: July 2002

The method of algebraic resonance dynamics is developed to extract semiclassical dynamics of molecules from the fit to the experimental spectrum of bend and stretch normal modes with 2:1 Fermi resonance. The Heisenberg correspondence principle is used to determine phase space profiles for resonance polyads. Each quantum level in a polyad corresponds to a trajectory in action-angle phase space. Profiles for experimental spectra of CO2 and a series of substituted methanes show a common phase space structure with four distinct types of semiclassical motion.
Electron energy loss characterization of NO on Rh(111). I. NO coordination and dissociation
Authors: Root T. W. , Fisher Galen B. , Schmidt L. D.
Published: July 2002

Electron energy loss spectroscopy (EELS) has been used to study the adsorption of nitric oxide on Rh(111). NO adsorbs in twofold bridge sites at all coverages. The dissociation rate of NO on this surface at low coverage has been measured directly using EELS. The first order dissociation rate parameters are Ea =19.2±0.3 kcal/mol and ν(1)0 =1×1014±0.3 s−1 at θNO ≊0.2. Ordered low energy electron diffraction (LEED) patterns are observed only for partially dissociated NO adlayers and not for NO adsorbed molecularly at 95 K. Because NO adsorbs in twofold bridge sites at all coverages, the N–O stretching frequency exhibits a 150 cm−1 shift from zero to saturation coverage.
Electron energy loss characterization of NO on Rh(111). II. Coadsorption with oxygen and CO
Authors: Root T. W. , Fisher Galen B. , Schmidt L. D.
Published: July 2002

High resolution electron energy loss spectroscopy has been used to examine the effects of coadsorbed oxygen and carbon monoxide on the bonding of nitric oxide adsorbed on the Rh (111) surface. NO is converted from twofold bridge sites (1500–1600 cm−1) to linear sites (1840 cm−1) when annealed to 275 K in the presence of coadsorbed oxygen. Terminally bound linear NO does not dissociate, but desorbs molecularly, a finding consistent with the commonly observed oxygen inhibition of NO dissociation over noble metals. Coadsorbed CO does not significantly inhibit or promote the dissociation of a low coverage of nitric oxide compared to the clean Rh (111) surface. CO and NO appear to segregate and form islands of each species at 95 K if CO is adsorbed first. Annealing this adlayer above 250 K disperses the islands and mixes the adsorbate layer at temperatures below where NO dissociation and reaction with CO are significant. The Rh–CO bond is weakened in well-mixed CO+NO adlayers, indicating that the NO–CO repulsive interaction acts through the Rh substrate rather than directly between the adsorbed molecules.
Hydrogen transfer in double minimum potential: Kinetic properties derived from quantum dynamics
Authors: Meyer R. , Ernst R. R.
Published: July 2002

The interaction of hydrogen transfer in a double minimum potential with a condensed phase environment is studied. For a symmetric double minimum system, the tunneling motion in the vibrational ground state is retarded efficiently by fluctuation as well as by rearrangement of the lattice consisting of harmonic oscillators. Environmental displacements with large inertia cause dynamic asymmetry by failing to cooperate with the transfer motion and favor a thermally activated process even at low temperatures. To describe such processes, an effective Hamiltonian is derived, which consists of a leading term referring to a one-dimensional transfer motion along an asymmetric potential profile and of a random perturbation term linear in the transfer coordinate. The power spectral density is derived for the perturbation given as a superposition of the time-dependent quantum mechanical expectation values of the vibrational displacements in the environment. A master equation treatment is proposed to describe the kinetic properties and is applied to a model for benzoic acid dimers in the crystalline state. The model reproduces the full temperature dependence of the observed NMR T1 data for (C6H5COOH)2 and (C6H5COOD)2 with plausible parameters and relates the temperature-dependent apparent activation energy to the energy level scheme of the transfer motion.
Brownian dynamics simulations of a three-subunit and a ten-subunit worm-like chain: Comparison of results with trumbell theory and with experimental results from DNA
Authors: Lewis Roger J. , Allison Stuart A. , Eden Don , Pecora R.
Published: July 2002

Brownian dynamics simulations of a three-bead and a ten-bead model of a flexible linear macromolecule in solution are performed. Dynamic forward depolarized light scattering autocorrelation functions are calculated from these dynamic simulations to a delay time of 30 μs and are analyzed by the program contin, yielding the simplest distribution of decay times that is consistent with the simulated data. The distribution of decay times obtained from the simulations of the three-bead model are compared with those obtained from the calculations of Roitman and Zimm on the dynamic behavior of a three-bead trumbell. The simulations and the theory of Roitman and Zimm show excellent agreement. The distribution of decay times obtained from the simulations of the ten-bead model with a persistence length of 600 Å is compared with previously published transient electric birefringence data from a 367 base pair monodisperse DNA fragment, again showing excellent agreement. Results from dynamic simulations of the ten-bead model with varying persistence length are then favorably compared with new transient electric birefringence data from the 367 base pair DNA fragment, obtained at varying ionic strength. The simulations of a model with a persistence length of 900 Å agree with the transient electric birefringence results from the DNA in a 1.4 mM ionic strength solution, showing the behavior of a slightly flexible rod. Simulations of a model with persistence length of 400 Å agree with the transient electric birefringence results from the DNA in a 4.4 mM ionic strength solution, showing behavior more indicative of a semistiff worm-like chain. Simulations of ensembles of rigid three-bead and ten-bead models yield dynamics quite different from their flexible counterparts. This raises concerns about the use of ensembles of rigid shapes as models for rotational dynamics of flexible molecules. These studies independently confirm the behavior predicted by the Roitman–Zimm calculations for the trumbell and demonstrate that Brownian dynamics simulations of a ten-bead model of DNA reproduce the dynamic decay spectrum and the dependence of that decay spectrum on chain stiffness, as measured by transient electric birefringence.
The relative kinetic energy distribution of the hydrogen atoms formed by the dissociation of the electronically excited H2 molecule
Authors: Engel Volker , Metiu Horia
Published: July 2002

We develop a model describing the relative kinetic energy distribution of the hydrogen atom pair formed by spontaneous photon emission in a transition from the bound state g 3Σ+g of H2 to the repulsive state b 3Σ+u. The results are compared to the experiments of Koot, van der Zande, and Los. The calculations are based on a time dependent theory of the dissociation process.
Spectra of isotopically mixed benzene trimers
Authors: Börnsen K. O. , Lin S. H. , Selzle H. L. , Schlag E. W.
Published: July 2002

Isotopically mixed jets of benzene produced important new detailed structural results from the various possible isotopic benzene trimers in a supersonic jet. From different isotopic spectral shifts, a detailed and consistent model of the interaction between the three benzene molecules can be inferred. The splitting of the 0–0 transition of isotopically mixed trimers can be predicted from this model. This model is consistent with a ‘‘zig–zag’’ structure for the trimer, not a cyclic structure. In contrast to the dimer spectra, van der Waals modes were observed. We also discovered an important, new exciton splitting in the van der Waals modes. This splitting demonstrates for the first time that the van der Waals modes are uncoupled from molecular vibrations. Furthermore, it is a sensitive indicator of the identity of the molecules so coupled. The spectra of higher benzene clusters (dimer to pentamer) have also been measured.