bluuebunny/crossref_metadata_2025 huggingface dataset:
Evaluation of the characteristic dimensions for porous sound-absorbing materials
Authors: Henry Michel , Lemarinier Pavel , Allard Jean F. , Bonardet Jean L. , Gedeon Antoine
Published: July 2002

The concept of characteristic thermal dimension was recently introduced to predict the behavior at high frequencies of the bulk modulus of fluids saturating a porous frame. This thermal dimension is closely related to the specific surface of the solid porous frame. It is shown that the standard Brunauer, Emmett, and Teller method (BET) of measuring the specific surface and acoustical measurement of the bulk modulus are in good agreement, and that the viscous dimension can be evaluated unambiguously from the BET method plus an acoustical measurement.
Dynamic double layer model: Description of time dependent charging phenomena in insulators under electron beam irradiation
Authors: Melchinger A. , Hofmann S.
Published: July 2002

A theoretical model is presented which describes time dependent charging phenomena in insulators irradiated by an electron beam. This so called dynamic double layer model (DDM) is based on the simplified assumption of charge development in two axi-symmetric cylindrical volumes at the insulator surface and at a certain depth below the surface, respectively. Important physical interaction mechanisms like the electrostatic interaction between the surface potential and emitted secondary electrons, the transport characteristics of internally generated secondary electrons in a positively charged insulator surface, as well as radiation induced electrical conductivity are included in the DDM model. Experimentally determined stationary and time dependent charging behaviour of Al2O3 single crystals (sapphire) irradiated with an electron beam between 1 and 15 keV energy can be qualitatively explained by the DDM model.
Study of shear force as a distance regulation mechanism for scanning near-field optical microscopy
Authors: Durkan C. , Shvets I. V.
Published: July 2002

Results of the investigation into the suitability of the shear-force distance regulation mechanism for scanning near-field optical microscopy (SNOM) are presented. It is shown that there is a range of relative sizes of surface features to tip size that gives rise to malfunctions of the shear-force distance regulation mechanism. If the size of the tip is comparable to the size of any depressions on the sample, the corresponding shear-force image may show contrast reversal. It is also shown that the resolution obtained when imaging a surface with the shear-force scanning microscope may differ for the two perpendicular lateral directions. The resolution along the oscillation axis is usually lower than the one perpendicular to it. This has implications for SNOM images. Therefore, the interpretation of shear force, and hence SNOM images, may become complex when imaging any sample with significant surface roughness.
Kirkendall voids and the formation of amorphous phase in the Al-Pt thin-film system prepared by high-temperature successive deposition
Authors: Radi Zs. , Barna P. B. , Lábár J.
Published: July 2002

We report on the amorphization processes taking place in the Al-Pt thin-film system prepared by high-temperature successive deposition. The formation of Kirkendall voids in the Al film was directly proved and intensively studied in relation to the percentage of Al and Pt components. An estimation of the composition of the amorphous phase is given, and the model proposed for the amorphization process is discussed. The films were analyzed by transmission electron microscopy, cross-sectional transmission electron microscopy, selected area electron diffraction, and energy dispersive x-ray microanalysis.
High coercivity rare earth–cobalt films
Authors: Malhotra S. S. , Liu Y. , Shan Z. S. , Liou S. H. , Stafford D. C. , Sellmyer D. J.
Published: July 2002

Rare earth–cobalt (RCo, R=Sm, Pr) films with thicknesses from 30 to 700 nm have been prepared with and without a Cr underlayer by dc magnetron sputtering from a R2Co7 composite target. The as-deposited SmCo films with a Cr underlayer (SmCo∥Cr) have magnetic coercivities of about 500–2800 Oe and the PrCo∥Cr films have coercivities of about 100–300 Oe, but after annealing at 500 °C coercivities as high as 31 kOe for SmCo∥Cr films and 10 kOe for PrCo∥Cr films were observed. The as-deposited PrCo films are composed mostly of an amorphous phase with about 30 vol % of crystallites but after annealing at 500 °C the film is transformed completely to crystallites of about 10 nm diameter as revealed by high-resolution transmission electron microscopy (HRTEM). Nanodiffraction and HRTEM studies show that the crystallites have a closed-packed hexagonal structure. HRTEM study also shows that the annealed SmCo films with a Cr underlayer have grain sizes of about 20 nm and the SmCo films without the Cr underlayer have grain sizes of about 10 nm. The large increase in coercivity for the annealed films is due to the growth of the crystallites.
Radical and film growth kinetics in methane radio-frequency glow discharges
Authors: Dagel D. J. , Mallouris C. M. , Doyle J. R.
Published: July 2002

The gas kinetics of a 30 mTorr radio-frequency methane glow discharge are studied as a function of methane depletion including conditions suitable for hard carbon thin-film deposition. Mass spectrometry is used to measure the partial pressures of the species C2H6, C2H4, C2H2, C3H8, C3H6, and C3H4. Net film growth was calculated using mass balance and corroborated by direct measurements of deposition rate. Using a combination of static and flowing discharge measurements, the net yields of C2H6, C2H4, and C2H2 are described using a simple analytic model. C2H6 is modeled as a production from CH3 recombination, and the production of C2H4 is modeled as reaction of CH with CH4 where the CH can be produced both by direct electron collisional dissociation of CH4 as well as reaction of CH2 with H. C2H2 production is modeled as arising principally from C2H4 depletion. The principal dissociation mechanism of these molecules appears to be electron collisional dissociation. The CH3 radical densities deduced from this analysis are in good agreement with threshold ionization radical measurements reported in the literature. In addition, the methane electron collisional dissociative branching is inferred to be approximately 68% CH3 and 32% CH2+CH. The results of this analysis when compared to the observed film yield imply that the CHn radicals do not make a significant direct contribution to amorphous carbon film growth. Rather, the film appears to result from the depletion products of discharge-produced gas molecules such as C2H6, C2H4, and C2H2.
Small-signal capacitance and conductance of biased a-Si structures
Authors: Furlan J. , Skubic I. , Smole F. , Popović P. , Topič M.
Published: July 2002

Small-signal capacitance and conductance of experimental samples of a-Si n-i-n structures were measured in a wide frequency range under various bias conditions. The measured capacitance at low frequencies greatly exceeds the expected value derived from the ΔQ/ΔV ratio, where ΔQ is a change of the trapped charge corresponding to a change ΔV of the applied voltage. This capacitance increases with the steady-state bias and decreases with the frequency of the measuring signal. The measured low-frequency small-signal conductance equals the differential conductance obtained from the steady-state current–voltage characteristics, but it increases with the rising frequency of the measuring signal. A small-signal analytical model of an a-Si n-i-n structure is developed which agrees well with the experimental results. With this model, the high capacitive effect of the n-i-n device at low frequencies is explained on the basis of a phase shift which arises from the delayed capture–emission mechanism of carriers in the localized states. Using this model, it is shown that an increasing frequency of small-signal excitation moves the energy region of gap states engaged in the delaying action toward the conduction band, resulting in a decreasing capacitance and an increasing conductance of the a-Si n-i-n structure.
Formation and growth of black spots in organic light-emitting diodes
Authors: McElvain J. , Antoniadis H. , Hueschen M. R. , Miller J. N. , Roitman D. M. , Sheats J. R. , Moon R. L.
Published: July 2002

We report electroluminescence (EL) degradation studies of thin-film organic light-emitting diodes under ambient conditions. Bilayer organic ITO/TPD/Alq3/Mg/Ag devices were studied via EL and photoluminescence (PL) microscopy. In situ imaging of device luminescing areas and measurement of sample luminance were performed, allowing for a detailed study of black spot formation and luminance reduction under constant voltage stress conditions. Post-stress devices were further characterized using PL microscopy, and it was found that black spots result from delamination of the metal at the Alq3/Mg interface initiated by pinholes on the cathode, caused by substrate defects.
Temperature-dependent dispersion relation of ferroelectric lithium tantalate
Authors: Abedin Kazi Sarwar , Ito Hiromasa
Published: July 2002

The temperature-dependent dispersion equation is derived to predict the ordinary and extraordinary refractive indices of lithium tantalate in the region of 0.4–4 μm and for a temperature range of 25 °C–300 °C. The Sellmeier equation is compared with the measured data.
Quantitative model of volume hologram formation in photopolymers
Authors: Colvin V. L. , Larson R. G. , Harris A. L. , Schilling M. L.
Published: July 2002

A quantitative model is presented to describe the formation of volume holograms in a polymeric medium containing photopolymerizable acrylate monomers that undergo spatially modulated gelation as a result of exposure to a visible “write” beam. The model refines the simple diffusion model of Zhao and Mouroulis [J. Mod. Opt. 41, 1929 (1994)], by including cure dependence of both the photoreaction kinetics and the monomer diffusivity. These dependences are determined by experimental measurements, using near infrared spectroscopy to quantify the degree of cure and the time dependence of the hologram formation to infer the cure-dependent diffusivity. The cure-dependent diffusion coefficient can be fit by an expression from a free-volume theory, and the cure-dependent reaction rate coefficient is found to be proportional to the diffusivity, showing the reaction rate to be diffusion limited. With the model parameters determined experimentally, predictions are then made of the first, second, and third harmonics of the grating profile, and these are found to be in good agreement with the measured values. The results show the validity of the model and its usefulness in predicting the optimal exposure conditions and performance of a given holographic material.
Electronic structure of CMR manganites (invited)
Authors: Goodenough John B.
Published: July 2002

A “colossal” negative magnetoresistance (CMR) occurs in manganites at a first-order ferromagnetic transition. The Mn4+ and high-spin Mn3+ ions each contain localized t3 configurations; the t3–pπ–t3 superexchange interactions are antiferromagnetic. The orbital degeneracy of localized Mn3+:t3e1, Eg5 configurations is lifted by cooperative static or dynamic Jahn–Teller deformations. Strong e-electron coupling to oxygen displacements, static or dynamic, introduces ferromagnetic e1–pσ–e0 interactions either via superexchange or, for fast Mn3+ to Mn4+ electron transfer relative to the spin-relaxation time (τh<τs), via a stronger double exchange. At the first-order ferromagnetic transition, a change from τh≈ℏ/Wσ>ωR−1 to τh<ωR−1 occurs within mobile molecular units, where Wσ is the bandwidth for states of e-orbital parentage and ωR−1 is the period of the optical-mode lattice vibration that traps a mobile hole as a small-polaron Mn4+ ion. TC increases with the fraction of double-exchange couplings, and this fraction increases with Wσ and ωR at the transition from polaronic to itinerant-electron behavior below TC. The bandwidth Wσ∼εσλσ2 cos φ〈cos(θij/2)〉 depends on the covalent mixing parameter λσ, which increases with pressure, as well as on the Mn–O–Mn bond angle (180°−φ), which increases with the tolerance factor t that measures the equilibrium bond-length mismatch, and on the angle θij between neighboring spins so that Wσ increases with the spontaneous magnetization on cooling below TC. In the compositions Ln0.7A0.3MnO3 with A=Ca or Sr, TC increases with t over the range 0.96⩽t⩽0.98 where the transition at TC is first order. The CMR is greatest near t≈0.96; it reflects a trapping out of mobile holes with decreasing temperature in the paramagnetic phase and their progressive release with decreasing temperature in the ferromagnetic phase where spin entropy is exchanged for configurational entropy.
Colossal magnetoresistance and charge order in La1−xCaxMnO3 (invited)
Authors: Ramirez A. P. , Cheong S.-W. , Schiffer P.
Published: July 2002

Double-exchange ferromagnets with their associated large magnetoresistance have recently been considered as candidates in magnetic storage applications. We review materials aspects of one compound family, La1−xCaxMnO3. There exist two distinct low temperature regimes; (i) ferromagnetic and metallic (x<0.5) and (ii) charge-ordered and semiconducting (x>0.5). We describe transport, magnetic, thermal and acoustic response in each regime.
Spin-reorientation transition in nano-, micro- and single-crystalline Nd2Fe14B
Authors: Kou X. C. , Dahlgren M. , Grössinger R. , Wiesinger G.
Published: July 2002

Spin reorientation of Nd2Fe14B with different degree of grain sizes was detected by measuring the temperature dependence of the ac susceptibility. The studied materials include: (1) a spherical Nd2Fe14B single crystal that shows no coercivity. (2) Microcrystalline Nd2Fe14B obtained by induction-melting or by sintering: Due to the presence of random-oriented multi-domain grains (⩾5000 nm), the bulk ingot shows no coercivity, but sintered materials do. (3) Microcrystalline Nd2Fe14B produced by the hydrogenation-disproportionation-desorption-recombination (HDDR): The HDDR treated materials show a coercivity due to the presence of single-domain grains of a size of about 200–300 nm. (4) Nanocrystalline Nd2Fe14B prepared by mechanical alloying and by rapid quenching: Due to the presence of ultrafine Nd2Fe14B grains (<50 nm), these materials show a high coercivity and an enhanced remanence. For the first time, a decrease of the spin-reorientation temperature, an intrinsic magnetic property, with decreasing the Nd2Fe14B grain size is found. The decrease of the spin-reorientation temperature in nanocrystalline Nd2Fe14B can be ascribed to the strong inter-grain exchange change coupling among nanocrystalline Nd2Fe14B.
Low rare-earth Nd–Fe–B bonded magnets with improved irreversible flux loss
Authors: Nishio T. , Koyama S. , Kasai Y. , Panchanathan V.
Published: July 2002

The effects of Nb addition on magnetic properties and irreversible flux loss were studied on bonded magnets with compositions of Nd8Fe86−xB6Nbx, where x=0–3. An irreversible flux loss was measured at Pc=2 after exposure at 393 K for 3.6 ks in air. It was found that the Nb addition of 2 at. % or more decreased the irreversible flux loss. The value was less than 5%, similar to that obtained with MQP-B magnets. Br decreased with the increase in Nb content. The effect of Nd content on the magnetic properties and irreversible flux loss was also studied on the bonded magnets with compositions of NdxFe92−xB6Nb2, where x=6–8. The bonded magnet with the optimum composition of Nd6.5Fe85.5B6Nb2 had  Br similar to that of MQP-Q magnets that did not contain Nb, and the value of irreversible flux loss was comparable to that of MQP-B magnets. A multipole magnetizability test on the bonded magnets with the above optimum composition was carried out, and the total flux of the bonded magnets was about 10% higher than that of MQP-B magnets for the 23 mm outside diameter magnet with 48 poles.
Oxidation of crystalline Si in an O2 plasma: Growth kinetics and oxide characterization
Authors: Martinet C. , Devine R. A. B. , Brunel M.
Published: July 2002

The growth of anodic oxides of crystalline Si in an O2 plasma has been studied. The kinetics are found to follow those expected when neutral O atoms in the plasma capture an electron at the substrate surface and diffuse through the growing oxide under the influence of an applied positive electric field. The oxides have been characterized using infrared spectroscopy, glancing incidence x-ray reflectometry, and electric capacitance/voltage measurements. Shifts to lower wave numbers of the transverse and longitudinal optic infrared modes associated with the asymmetric stretch of the bridging O’s with respect to thermally grown oxide values are observed. We conclude that these shifts result from structural modifications induced by ultraviolet photons present in the O plasma. A simple model suggests that the plasma grown oxides have a density ∼3.8% larger than thermal SiO2 and an average Si–O–Si bridging bond angle ∼1.6° smaller. The plasma grown oxides are inhomogeneous in the volume, the bond angle shift increasing to ∼−2.3°, and the density decreasing to +2% with respect to thermal oxide near the Si substrate/SiO2 interface. X-ray reflectivity data, which are consistent with film inhomogeneity, enable us to determine the film thickness and surface (air/SiO2) roughness. The latter is always ⩽1 nm. Electrical measurements suggest that the oxides contain fixed oxide charges and interface states which can be reduced by low temperature annealing. The leakage currents remain high, however.
A mechanistic picture of the effects of rubbing on polyimide surfaces and liquid crystal pretilt angles
Authors: Paek S.-H. , Durning C. J. , Lee K.-W. , Lien A.
Published: July 2002

A mechanistic picture of the effects of rubbing on polyimide alignment layer (AL) surfaces and nematic liquid crystal (LC) pretilt angles (ΘP), leading to establishing a mechanism of determination of ΘP’s, was proposed on the basis of the study of the change in surface polarity of a polyimide AL with the rubbing force and density, and the subsequent changes in the ΘP and the azimuthal anchoring energy. In the regime of relatively weak rubbing where inhomogeneous, patchy surface modification (the microscopic reorientation of polymer functional groups and the orientation of polymer backbones) occurs, the surface polarity, the anchoring energy, and the ΘP all increase monotonically with rubbing strength. These increases correlate to an increase in the area fraction of the reoriented/oriented AL surface which generates in-plane orientation of the first LC monolayer at the surface and subsequently bulk alignment. Beyond this region in which the AL surface gets modified sufficiently to induce fully developed in-plane orientation of the LC monolayer, the surface chemistry and roughness have dominant effects on the ΘP and the anchoring energy. The increase in surface polarity and the generation of a larger reoriented surface area by high-force rubbing, which enhances the attractive interaction between the LC monolayer and the rubbed surface, contribute to increases in both the anchoring energy and the thermal stability of ΘP and to a decrease of ΘP. This decrease of ΘP with increasing AL surface polarity was verified by studying the relationship between ΘP and surface polarity under constant rubbing conditions by manipulating the surface polarity via the degree of imidization of a poly(amic acid) AL.
Analysis of the impedance response due to surface states at the semiconductor/solution interface
Authors: Hoffmann Peter M. , Oskam Gerko , Searson Peter C.
Published: July 2002

Electronic surface states at semiconductor/solution interfaces can mediate processes such as trapping and detrapping of majority and minority charge carriers, recombination, or charge transfer to or from the solution. We have calculated the complete impedance response due to these processes using a kinetic approach. Specific cases are discussed and diagnostic parameters for the capacitance and conductance are presented. Experimental results on n-Si(111) in fluoride solutions are used to illustrate the obtained expressions.
Structural, optical, and electrical properties of hydrogenated amorphous silicon germanium alloys
Authors: Chou Yu-Pin , Lee Si-Chen
Published: July 2002

Hydrogenated amorphous silicon germanium alloys (a-SiGe:H) have been prepared by rf glow discharge of silane, germane, and hydrogen gas mixture at substrate temperature of 200 and 250 °C. The structural properties of the films have been investigated by infrared, Raman, and secondary ion mass spectroscopy. It is found that there is a preferential incorporation of germanium into the film relative to silicon and the films with high germane gas phase composition Xg>0.4 tend to oxidize in atmosphere. Besides, polysilane is enhanced in the films with low germane gas phase composition. The electrical properties including dark, photo conductivities, and conduction activation energy are measured. As for the optical properties, optical transmission is adopted to determine the optical gap while photoluminescence spectra together with temperature variation are used to study the band tail states of the films. By applying Brodsky’s quantum well model, the various optical and electrical properties could be explained successfully.
Switching of deep levels in CuInSe2 due to electric field-induced Cu ion migration
Authors: Nádaždy V. , Yakushev M. , Djebbar E. H. , Hill A. E. , Tomlinson R. D.
Published: July 2002

To search for the origin of the 0.26 eV deep hole trap, which is well established in p-type CuInSe2 single crystals and thin films, we investigated the influence of electric field-induced Cu ion migration on this level. We show that this hole trap can be removed from the space charge region of a reverse-biased Schottky barrier in p-type CuInSe2 single crystals by annealing at 490 K for 10 min. Simultaneously, a new hole trap at 0.49 eV from the valence band is created in this region. Conversely, the application of a forward bias under the same annealing conditions causes a decrease in the concentration of the 0.49 eV hole trap and a recreation of the 0.26 eV hole trap to the original state. These two processes were found to be reversible. Our results are completely consistent with recent evidence of an electric field-induced Cu ion migration via a vacancy mechanism in CuInSe2. According to a recent theoretical paper on the defect physics of CuInSe2, a candidate for experimentally observed 0.26 eV deep hole trap seems to be the simple acceptor CuIn(−/0).
Giant sonic stop bands in two-dimensional periodic system of fluids
Authors: Kushwaha M. S. , Djafari-Rouhani B.
Published: July 2002

Periodic binary systems can give rise to genuine acoustic stop bands within which sound and vibrations remain forbidden. We compute extensive band structures for two-dimensional (2D) periodic arrays of air cylinders in water. Complete, multiple, huge stop bands are found for both square and hexagonal lattices. The lowest stop bands are largest for a range of filling fraction 10%⩽f⩽55%, with a gap/midgap ratio of 1.8. The most interesting finding of the present investigation is that the low-frequency, flat passbands for a perfectly periodic system correspond to the discrete modes of a single airy cylinder. This is attributed to the low filling fraction and huge density contrast in air and water. We stress that such a simple inhomogeneous system as made up of air and water exhibits the largest stop bands ever reported for 2D or 3D elastic as well as dielectric composites.
Influence of Zn/In codoping on the optical properties of lithium niobate
Authors: Kasemir K. , Betzler K. , Matzas B. , Tiegel B. , Wahlbrink T. , Wöhlecke M. , Gather B. , Rubinina N. , Volk T.
Published: July 2002

The influence of a combined doping with Zn and In on the optical properties of lithium niobate is studied. While each of these dopants is known to improve the material’s nonlinear optical properties, it is an open question whether further improvements, especially of the nonlinear optical features, can be achieved by a suitable combination of dopants. Our measurements show that lithium niobate imposes certain limits which cannot be overcome. This can be described by a simple model which is based on the defect structure of lithium niobate.
Photoemission spectroscopy of LiF coated Al and Pt electrodes
Authors: Schlaf R. , Parkinson B. A. , Lee P. A. , Nebesny K. W. , Jabbour G. , Kippelen B. , Peyghambarian N. , Armstrong N. R.
Published: July 2002

Thin lithium fluoride (LiF) interlayers between the low work function electrode and the electron transport layer in organic light emitting diodes (OLED) result in improved device performance. We investigated the electronic structure of LiF coated Al and Pt electrodes by x-ray photoemission spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS). Thin LiF films were grown in several steps onto Ar+ sputtered Al and Pt foils. After each growth step the surfaces were characterized in situ by XPS and UPS measurements. After evaluating band bending, work function and valence band offset for both samples, their band lineups were determined. Our measurements indicate that despite the insulating character of LiF in both samples, band bending is present in the LiF layer. The difference in band bending between the samples allows the conclusion that the driving force for the development of the band bending results from the contact potential between the metal and the LiF overlayer. The band bending is most likely caused by a redistribution of charged Frenkel or Schottky type defects within the LiF layer. The work function of both samples after LiF deposition was dramatically lowered compared to the values obtained on the clean sputtered metal surfaces.
A study on the capacitance–voltage characteristics of metal-Ta2O5-silicon capacitors for very large scale integration metal-oxide-semiconductor gate oxide applications
Authors: Chih-ming Lai Benjamin , Kung Nan-hui , Ya-min Lee Joseph
Published: July 2002

In this work, the capacitance–voltage characteristics of Au/Ta2O5/Si capacitors are studied. The Ta2O5 films are deposited by plasma-enhanced chemical vapor deposition system. Constant voltage stress is applied to the Ta2O5 capacitors. The flatband voltage VFB is measured before and after the constant voltage stress. The flatband voltage shift due to the stress is explained by the trapping of electrons and holes. An interface trapped charge density (Dit) about 2×1011 eV−1 cm−2 in the midgap of silicon is extracted by using the conductance method. The discharging transient current after a constant voltage stress is measured and correlated with the electron and hole trapping. The trapped electron and hole densities in Ta2O5 are calculated by using the tunneling front model. The VFB shift and the discharging transient currents are explained based on the energy band diagram of the metal/Ta2O5/silicon system.
Effect of oxygen content on the structural, transport, and magnetic properties of La1−δMn1−δO3 thin films
Authors: Zhao Y. G. , Rajeswari M. , Srivastava R. C. , Biswas A. , Ogale S. B. , Kang D. J. , Prellier W. , Chen Zhiyun , Greene R. L. , Venkatesan T.
Published: July 2002

(110) oriented La1−δMn1−δO3 thin films with different oxygen content were grown on (001) LaAlO3 substrates by pulsed laser deposition. Samples prepared in higher oxygen partial pressures show a ferromagnetic transition around 200 K. The transport is thermally activated with a change in slope at the ferromagnetic transition. Samples prepared and annealed in vacuum show signatures of mixed ferromagnetic and antiferromagnetic phases, and are insulators. The pure antiferromagnetic phase (as expected and observed in bulk materials with optimum oxygen stoichiometry) was not obtained in our experiments, even in the strongly reduced films.
Henkel plots of single-domain ferromagnetic particles
Authors: Garcı́a-Otero J. , Porto M. , Rivas J.
Published: July 2002

The use of Henkel plots as a tool to analyze the type and strength of interaction between particles in fine magnetic particle systems is wide spread. It is commonly accepted that noninteracting systems in general show linear Henkel plots, while interacting systems show curved plots. Using extensive Monte Carlo simulations Henkel plots for noninteracting and interacting systems of particles that show different anisotropies are studied. It is found that a direct relation between linearity and noninteraction exists only for systems of uniaxial particles at low temperatures, while particles with cubic anisotropy always show positive deviation of the Henkel plot in the whole range of temperatures. On the other hand, dipolar interaction always results in negative deviation. In the case of particles with cubic anisotropy and dipolar interaction, the deviation changes gradually from positive to negative with increasing strength of the interaction.
Amorphization and recrystallization of yttrium iron garnet under swift heavy ion beams
Authors: Costantini J.-M. , Desvignes J. M. , Toulemonde M.
Published: July 2002

The room-temperature dc conductivity is used to monitor the damage and structural modifications induced by swift heavy ion irradiations in yttrium iron garnet (Y3Fe5O12 or YIG) epitaxial layers doped with calcium (CaYIG) or silicon (SiYIG), with a variable conductivity due to a variable degree of compensation, and amorphous YIG layers. Irradiations are performed with heavy ions in the 0.8–6 MeV amu−1 energy range, in the electronic slowing down regime, with an electronic stopping power ranging between 7 and 41 MeV μm−1 above the amorphous track formation threshold (4.5 MeV μm−1) in this low-ion velocity range. A conductivity decrease versus ion fluence is found in the case of the high-conductivity uncompensated epilayers whereas an increase occurs for the low-conductivity compensated ones, either p-type (CaYIG) or n-type (SiYIG). These results are discussed by considering the competing effects of disorder on the carrier density and mobility in the case of compensated and uncompensated semiconductors. In both cases, the low-fluence data display a plateau at around the same conductivity value corresponding to the amorphous YIG above an amorphous fraction around 50% regardless of the ions. All the high-fluence data exhibit a power-law behavior without saturation, above a threshold fluence decreasing with increasing amorphization cross section (A). These results are interpreted by the formation of amorphous tracks and of a more conducting nanophase after recrystallization of the tracks under ion impacts. All the data are rescaled versus the product of A times fluence (φ) where amorphization dominates for Aφ⩽1, whereas recrystallization dominates for Aφ>10. However, significantly larger A values than the ones previously determined from the RBS-channeling data are derived from a mean-field analysis of the low-fluence conductivity data with a 2D Bruggeman model. These deviations are ascribed to a contribution of the crystalline track halos where internal stresses are accumulated due to the atomic density difference between the crystal and amorphous phase. A simple phenomenological approach of the amorphization and recrystallization processes is proposed on the basis of two kinetic rate equations with a recrystallization cross section (S) at least one order of magnitude smaller than A. These S values are in agreement with a thermal spike model assuming vaporization of the amorphous YIG phase along the ion path. At such high temperatures in the ion tracks, the garnet phase may decompose into a more conducting nanocrystalline phase. Finally, an exp(−T)−1/4 law for the thermal dependence of conductivity at low temperature is found in the nanophase like in the amorphous one, most probably because of the strong contribution of the disordered grain boundary cores in the conduction process.
Calculation of the magnetic properties of CoN clusters on Pd(111)
Authors: Félix-Medina R. , Guirado-López R. , Dorantes-Dávila J. , Pastor G. M.
Published: July 2002

The magnetic properties of CoN clusters (N⩽7) deposited on Pd(111) have been determined in the framework of a self-consistent d-band model Hamiltonian. Results are given for the spin and orbital magnetic moments and for the magnetic anisotropy energy for different structures of the CoN clusters. We find that the magnetic orbital contribution to the total magnetic moment can be of the order of 20%. The magnetization direction of CoN changes in most cases from in-plane to off-plane upon deposition. We observe that the main contribution to the magnetic anisotropy energy results from the cluster–substrate interface. Comparison of the magnetic properties between deposited and free clusters is also provided.
Two-component model of polaronic transport
Authors: Rubinstein Mark
Published: July 2002

A phenomenological two-component polaron model, originally formulated to describe the temperature-dependent transport properties of La2/3Ca1/3MnO3(LCMO), has been applied to treat the resistivity and Hall effect of EuO and ZrTe5.
Accurate localized and delocalized vibrational states of HCN/HNC
Authors: Bačić Z. , Light J. C.
Published: July 2002

Results of the first accurate quantum calculation of the delocalized, large amplitude motion vibrational (J=0) levels of HCN/HNC, lying above the isomerization barrier, are presented. The recently developed DVR-DGB quantum method [Z. Bačić and J. C. Light, J. Chem. Phys. 85, 4594 (1986)] is employed in this work. A model, empirical surface by Murrell et al. is used. All modes are included; the energy level calculation does not involve any approximations. Over a hundred vibrational levels are calculated accurately for this model surface. A number of them lie above the isomerization barrier; some are extensively delocalized over both HCN and HNC minima. Analysis shows that for HCN/HNC the threshold for significant delocalization is determined by the height of the vibrationally adiabatic bending barrier. In addition, the nearest neighbor level spacing distribution is obtained and compared to that of LiCN/LiNC. Various computational aspects of the DVR-DGB approach, which is applicable to any triatomic molecule, are also discussed. The method is very suitable for efficient, accurate treatment of floppy molecules and molecules which can isomerize. The DVR-DGB (i.e., ray eigenvector) basis provides a rapidly convergent expansion for the delocalized (and localized) states. Consequently, a single diagonalization of the DVR-ray eigenvector Hamiltonian matrix, whose size is modest relative to the number of accurately determined energy levels, yields the energies of both localized and delocalized states. Accurate evaluation of the two-dimensional integrals in the potential matrix elements requires only 3–4 Gauss–Hermite quadrature points per dimension.
The diffusion of small ions through phospholipid bilayers
Authors: Georgallas A. , MacArthur J. D. , Ma X.-P. , Nguyen C. V. , Palmer G. R. , Singer M. A. , Tse M. Y.
Published: July 2002

The passage of the alkali metals Li, Na, K, Rb, and Cs through saturated phosphatidylcholine membranes has been measured using particle induced gamma ray and x-ray emission to observe the ions. Simultaneous measurements of these five cations has not been possible with more traditional methods involving ion specific electrodes or radioactive tracers. To the authors’ knowledge this is the first time this technique has been used in lipid bilayer research. The ion leakage was determined for an incubation period of 30 min at several temperatures which spanned the bilayer melting temperature. The dependence of this permeability on temperature, acyl chain length, and ion size is described theoretically in terms of the density fluctuations in the bilayer. A leakage rate which depends on the mass of the ion was observed. This dependence shows that the heavier the ion the faster it diffuses out of the vesicle. The effect of this selectivity is shown to be most pronounced near the melting temperature of the lipid. Moreover, this size dependence suggests that the permeating entity is a hydrated ion, rather than a naked ion or a neutral, bound ion pair.
Atom scattering from surfaces with isolated impurities: Calculations for hard wall and soft potentials
Authors: Drolshagen Gerhard , Vollmer Rüdiger
Published: July 2002

The angular intensity distribution for atom scattering from surfaces with isolated defects is calculated. For a two-dimensional model time-dependent wave packet calculations are carried out for potentials with different steepness and variable defect size and height. The influence of the potential softness is studied by comparison with results from a corresponding hard wall model. Within the hard wall model the Eikonal approximation (EA) and an improved Eikonal approximation (IEA) which accounts for double collision effects are tested against numerically exact results. While, in general, the EA works well only in the vicinity of the specular peak the IEA is quite accurate over the entire angular range. Finally, wave packet calculations are carried out for a model of a flat surface with isolated steps, fitted to represent a stepped Pt(111) surface. The results are compared to recent measurements of the incoherent elastic diffraction for this system. Basic measured features like a rainbow maximum and large angle interference oscillations are well reproduced by the calculations.
Dielectric relaxation measurements of poly(vinyl acetate) in glassy state in the frequency range 10−6–106 Hz
Authors: Nozaki Ryusuke , Mashimo Satoru
Published: July 2002

Dielectric relaxation measurements covering a wide frequency range extending from 10−6 to 106 Hz were made on poly(vinyl acetate) with conventional glass transition temperature Tg of 31 °C at temperatures between 26.85 and 84.77 °C. It was found that temperature dependence of frequency of the maximum dielectric loss below Tg cannot be described by the Williams–Landel–Ferry equation which gives a complete explanation to the dependence in the temperature range sufficiently higher than Tg, but by a simple expression of Arrhenius type with activation energy 138 kcal/mol. It was also found that distribution of the relaxation times changes abruptly in the vicinity of Tg. Potential barrier height for the chain motion has a distribution to some extent below Tg.
On the vibrational T2 processes in partially ordered systems
Authors: Rothschild Walter G. , Perrot Michel , Guillaume François
Published: July 2002

The stretched (‘‘extended’’ or ‘‘fractional’’) exponential, exp{−(t/T)α}, where 0<α≤1 and T (ps) a time constant, is introduced as a modulation function for a vibrational T2 process in order to describe the non-Markovian dynamics of the bath of amorphous systems in the picosecond domain. The theory simulates well published dephasing data of probe oscillators in concentrated aqueous solutions of KSCN, molten mixtures of Li and Rb nitrates, and of polycrystalline and smectic phases of the liquid crystal 4-n-octyloxy-4′-cyanobiphenyl. The temperature and phase dependence of dispersion parameter α follows an accepted model of constrained-sequential relaxation with a scale-invariant (‘‘fractal’’) distribution of relaxation times.
Rotational predissociation of (rare gas atom)–(slow rotating diatomic molecule) complexes
Authors: Gianturco F. A. , Palma A. , Villarreal P. , Delgado-Barrio G. , Roncero O.
Published: July 2002

We present an adiabatic angular approximation, closely related to the infinite order sudden approximation, to treat rotational predissociation of triatomic van der Waals complexes formed by a rare gas atom and a diatomic molecule that is treated as a rigid rotor. The metastable states of the complex are obtained as discrete solutions of the Schrödinger equation while the corresponding continuum solutions allow us to estimate the rates for rotational predissociation, within the framework of the Golden Rule approximation, after the relevant discrete-continuum dynamical couplings are calculated. Applications to the He–CO, He–N2, and He–O2 systems are presented.
Imaginary time path integral Monte Carlo route to rate coefficients for nonadiabatic barrier crossing
Authors: Wolynes Peter G.
Published: July 2002

Nonadiabatic transitions such as some biological electron transfers are difficult to simulate on a computer because they seem to need long real time quantum dynamics simulations. In fact rates are often dominated by saddle point trajectories in imaginary time. This paper describes a way in which this observation can be exploited with path integral Monte Carlo methods.
Formation and dissociation of negative ion resonances in methanol and allylalcohol
Authors: Kühn Alexander , Fenzlaff Heinz-Peter , Illenberger Eugen
Published: July 2002

Dissociative electron attachment by methanol, different deuterated methanols, and allylalcohol is studied including an analysis of the translational excess energy release of the ionic fragments. Although the energetic threshold for negative ion formation (OH−, O−) is near 2 eV, these ions are generated in methanol only within a prominent resonance at 10.5 eV. OH− and OD− formation is characterized by hydrogen scrambling in the temporary parent ion while the formation of CH3O− (and the deuterated analog) proceeds directly (no hydrogen scrambling). Only O− is generated with considerable translational excess energy (1.0±0.2 eV). In contrast to methanol, allylalcohol captures electrons at 1.7 eV to form OH− and the enolate anion (M–H)−. This resonance is interpreted as a Π radical anion comparable to that known in ethylene.
Atom scattering from isolated adsorbates on surfaces: Rainbows, diffraction interferences, and trapping resonances
Authors: Yinnon A. T. , Kosloff R. , Gerber R. B.
Published: July 2002

The scattering of He atoms from a CO molecule adsorbed on a Pt surface is studied theoretically by methods that include: (1) Numerically exact solutions of the time-dependent Schrödinger equation for the scattered wavepacket; (2) The sudden approximation; (3) Classical trajectories. The methods are used to obtain detailed insight into the collision dynamics, and to predict and understand interesting features in the angular intensity distribution of the scattered atoms. The analysis and interpretation of the exact quantum results is facilitated by calculations of the probability current density of the scattered particles. Some of the main results are: (i) The angular intensity distribution exhibits nonspecular maxima of two types: Several of the peaks are rainbow effects induced by the adsorbate, while others (at angles nearer to the specular) are Fraunhofer diffraction interferences. Both types of peaks contain useful, largely complementary, information on adsorbate geometry and on the He/adsorbate interaction. (ii) The angular intensity distribution is quantitatively sensitive to the adsorbate distance from the surface, suggesting possible determination of that distance from experimental data. (iii) The corrugation due to the adsorbate leads to scattering resonances associated with temporary trapping of the scattered atom at the defect site. This is a new effect of potential importance for experimental studies of atom/defect interactions. The results obtained here suggest that He scattering from isolated adsorbates exhibits distinct, substantial effects, measurement of which should yield very useful data on the adsorbates and on their interactions with gas-phase atoms.
Mechanisms for O− electron stimulated desorption via dissociative attachment in condensed CO
Authors: Azria R. , Parenteau L. , Sanche L.
Published: July 2002

The formation of O− ions via dissociative attachment (DA) in electron stimulated desorption from condensed CO is reported. The 2Π states of CO− previously observed in the gas phase and CO− states with the forbidden symmetry Σ− are involved below and above 13 eV, respectively, in the DA processes. Measurements of the kinetic energy of O− ions indicate that they suffer post dissociation interactions with the neighboring CO molecules before leaving the solid.
Accurate integral equation theory for the central force model of liquid water and ionic solutions
Authors: Ichiye Toshiko , Haymet A. D. J.
Published: July 2002

The atom–atom pair correlation functions and thermodynamics of the central force model of water, introduced by Lemberg, Stillinger, and Rahman, have been calculated accurately by an integral equation method which incorporates two new developments. First, a rapid new scheme has been used to solve the Ornstein–Zernike equation. This scheme combines the renormalization methods of Allnatt, and Rossky and Friedman with an extension of the trigonometric basis-set solution of Labik and co-workers. Second, by adding approximate ‘‘bridge’’ functions to the hypernetted-chain (HNC) integral equation, we have obtained predictions for liquid water in which the hydrogen bond length and number are in good agreement with ‘‘exact’’ computer simulations of the same model force laws. In addition, for dilute ionic solutions, the ion–oxygen and ion–hydrogen coordination numbers display both the physically correct stoichiometry and good agreement with earlier simulations. These results represent a measurable improvement over both a previous HNC solution of the central force model and the ex-RISM integral equation solutions for the TIPS and other rigid molecule models of water.
Raman study of the solid-state polymerization of acetylene at high pressure
Authors: Aoki K. , Usuba S. , Yoshida M. , Kakudate Y. , Tanaka K. , Fujiwara S.
Published: July 2002

The mechanism and kinetics of the pressure-induced polymerization of acetylene were studied by Raman spectroscopy. The polymerization reaction occurred in the orthorhombic phase at room temperature and pressures above 3.5 GPa. Dominant formation of trans-polyacetylene suggested that the monomer underwent trans opening of the triple bond and polymerized along the diagonal of the bc plane of the unit cell. The reaction was described as an idealized one-step and one-dimensional growth process by an Avrami equation with an exponent 1.34.
Study of the diffraction mediated selective adsorption through the close-coupling and diabatic distorted wave formalisms. Application to the 4He–Cu(110) system
Authors: Hernández M. , Roncero O. , Miret-Artés S. , Villarreal P. , Delgado-Barrio G.
Published: July 2002

Two different formalisms, one based on close-coupling equations and other on the distorted wave approximation, have been applied to the low energy elastic scattering of 4He atoms by a corrugated copper surface, Cu(110). Diffraction intensities and energetic positions and half-widths (selective adsorption resonances) have been obtained and compared with experimental and previous theoretical results, respectively. For this trapping process, and within the close-coupling framework, a plot of the diffraction channels (open and closed) has turned out to be very useful and a good guide in order to know the disposition of them at each interesting angular region. Also, a very accurate method, already employed successfully to treat resonances in the vibrational and/or rotational predissociation of van der Waals molecules, has been found to be also adequate here.