bluuebunny/crossref_metadata_2025 huggingface dataset:
Exact solution of the Schrodinger equation across an arbitrary one-dimensional piecewise-linear potential barrier
Authors: Lui Wayne W. , Fukuma Masao
Published: July 2002

A simple yet exact method of solving the Schrodinger equation across an arbitrary one-dimensional piecewise-linear potential is described. It is based on the analytical solution of the Schrodinger equation across a linear potential, where the solution can be expressed as a linear combination of the Airy functions. Proper boundary conditions are imposed at the interface between adjacent linear intervals of the potential, and a transfer-matrix procedure is utilized in the derivation. The quantum mechanical transmission coefficient across a potential barrier and eigenenergies of a potential well can also be easily calculated using this method.
Kinetics of dry oxidation of silicon. I. Space-charge-limited growth
Authors: Wolters D. R. , Zegers-van Duynhoven A. T. A.
Published: July 2002

Recent studies of the kinetics of dry oxidation of silicon have shown that the time dependence is more precisely described by a power-of-time law than by a linear-parabolic expression. This study shows the power law to be correct. This follows from the interdependence of the experimental linear and parabolic rate constants. The linear-parabolic expression appears to be equivalent to the first terms of a series expansion of the power-of-time law. The omission of higher-order terms gives systematic deviations known as the ‘‘anomalous’’ initial regime. An ionic space-charge-limited growth model is introduced, based on the classical oxidation theory of Wagner and accounting for the effect of internal fields on conduction. First, it is shown that both the magnitude of ionic and electronic conduction are sufficiently high, which is illustrated for an oxide layer of 300 Å growing at 870 °C. It is made plausible that the oxide-fixed charge density, Qf, is sufficiently large at high temperatures to cause large internal fields. An expression is derived which accounts for the mutual Coulomb repulsion of charges in very dense space-charge layers. The excellent fit of the derived expression and its application will be discussed in part II.
Thermal field emission observation of single-crystal LaB6
Authors: Nagata Haruto , Harada Ken , Shimizu Ryuichi
Published: July 2002

TFE (thermal field emission) properties of LaB6 〈100〉 and 〈310〉 single crystals were investigated by emission pattern observation. It was found that field evaporation with the tip temperature held at ∼1500 °C is very useful to get a clean pattern of fourfold symmetry. Each of four bright spots in the clean pattern was presumed to correspond to 〈310〉 emission. It is proposed, as the most appropriate operating condition, to use the 〈310〉LaB6 tip at a temperature ∼1000 °C in vacuum of 10−9 Torr region, promising a new TF emitter of high brightness and stability for practical use.
Investigation of bulk laser damage threshold of lithium niobate single crystals by Q-switched pulse laser
Authors: Furukawa Yasunori , Yokotani Atsushi , Sasaki Takatomo , Yoshida Hideji , Yoshida Kunio , Nitanda Fumio , Sato Masayoshi
Published: July 2002

Bulk laser damage threshold of 2-in.-diam. MgO-doped LiNbO3 crystals grown using the Czochralski technique has been measured using a Q-switched Nd3+:LiYF4 laser. The threshold showed dependence on the crystal quality. The highest bulk laser damage threshold (14 J/cm2 at 1.053 μm wavelength with 1-ns pulse width) was obtained in the LiNbO3 crystal doped with 1 mol % of MgO. The lowering of bulk laser damage threshold was observed in the crystals doped with MgO at more than 3 mol %. These crystals contained the local aggregations of MgO which increased the scattering centers and decreased the transparency.
Spontaneous emission from a dipole in a semiconductor microcavity
Authors: Deppe D. G. , Lei C.
Published: July 2002

The effect of a semiconductor microcavity on the radiative spontaneous recombination of an electron-hole pair strategically placed (by virtue of a quantum well) in the microcavity is considered. First-order perturbation theory is used in the quantum mechanical calculation of the spatially anisotropic radiation rate and shows a strong influence of the cavity, and dipole position in the cavity, on the spontaneous photon emission process. Calculations are compared with previous experiments [T. J. Rogers, D. G. Deppe, and B. G. Streetman, Appl. Phys. Lett. 57, 1858 (1990)].
Guided-wave optical properties of sol-gel ferroelectric films
Authors: Wood V. E. , Busch J. R. , Ramamurthi S. D. , Swartz S. L.
Published: July 2002

Sol-gel processing methods have been used to prepare thin films of lead titanate, Pb(Zr,Ti)O3, and Pb(Mg1/3Nb2/3)O3 on sapphire and single-crystal strontium titanate substrates. These films have been characterized using x-ray diffraction, optical waveguiding measurements at 633 nm wavelength, and optical transmission measurements. The films, in general, contain a mixture of perovskite and pyrochlore phases, with perovskite formation being favored by higher processing temperatures, by use of the SrTiO3 substrates, and by the addition of excess lead precursor compound to the starting solutions. The films have refractive indices close to, but lower than, those of the corresponding bulk materials. The reduction of the refractive indices from the bulk values is believed to result primarily from incomplete densification. Transmission measurements show that the optical dispersion in the PbTiO3 films is similar to that in the bulk crystal. Although all films showed scattering losses, waveguiding over distances up to 1 cm was observed in some films. For multimode waveguides, conformity of the mode indices to those expected for a uniform guide is suggested as one test of film uniformity as a function of depth.
Characterization of laser vaporization plasmas generated for the deposition of diamond-like carbon
Authors: Pappas David L. , Saenger Katherine L. , Cuomo Jerome J. , Dreyfus Russell W.
Published: July 2002

Pulsed laser vaporization of graphite is rapidly emerging as an effective technique for the preparation of high quality diamond-like carbon films. However, the dynamics of the process and mechanisms by which diamond-like properties are obtained have not been well understood. The characteristics of the vapor plume generated by 248 nm KrF excimer laser irradiation of a graphite target are investigated using laser induced fluorescence and a Langmuir probe. It is found that the kinetic energy of the C2 molecule increases with laser fluence, reaching a value in excess of 12 eV in the moderate fluence range (3–5 J/cm2) employed for deposition. The Cn+ ions are 5–10 times more energetic and comprise ∼10% of the vapor flux. A notable finding is that irradiation of the surface at an angle of 70° with respect to the target normal increases the ion velocity when compared with 0° laser incidence at the same surface fluence. Analysis of the films prepared under such conditions supports the theory that diamond-like film character is directly related to the kinetic energy of the depositing species.
Electrostatic forces on small particles in low-pressure discharges
Authors: Daugherty J. E. , Porteous R. K. , Graves D. B.
Published: July 2002

The electrostatic forces on an isolated particulate in a low-pressure discharge are calculated assuming a screened Coulomb potential profile for the particle sheath. In the absence of particle sheath distortion due to a flowing plasma and assuming that there is no charge ‘‘bound’’ to the particle in the form of ions in trapped orbits, the monopolar and dipolar forces on the particle are essentially identical to the corresponding forces on a charged sphere in vacuum.
Excited-state absorption in the infrared emission domain of Nd3+-doped Y3Al5O12, YLiF4, and LaMgAl11O19
Authors: Guyot Yannick , Moncorge Richard
Published: July 2002

Detailed excited-state excitation spectra are recorded in the infrared emission domains around 1.06 and 1.32 μm of the Nd3+-doped Y3Al5O12, YLiF4, and LaMgAl11O19 laser crystals. The positions of the observed lines are compared with that deduced from the energy levels of the Nd3+ ion in these materials.
Optical recombination mechanisms in Eu2+-doped CaS and SrS thin films
Authors: Swiatek K. , Godlewski M. , Niinistö L. , Leskelä M.
Published: July 2002

Excitation and deexcitation mechanisms of Eu2+ emission in CaS:Eu and SrS:Eu thin films have been studied. The photoluminescence spectra and the luminescence decay kinetics were examined as function of the temperature. It was found that both internal transitions within the Eu2+ ions and resonant energy transfer between different Eu centers are responsible for the Eu2+ luminescence excitation in low temperatures. Moreover, at temperatures T≳100 K, electron transfer between different europium ions over the conduction band (CB) of the host lattice influences efficiency of the emission. This is possible since the Eu ion can change its charge state from 3+ to 2+ by trapping a CB electron via the 4f65d1 Eu2+ excited state.
Infrared studies of C60 thin films
Authors: Song Zhizhong , Zhang Fangqing , Gong Jinlong , Zhang Guoquan , Chen Guanghua
Published: July 2002

Fourier-transformation infrared spectroscopy (FTIR) was used to study C60 thin films which have been annealed for various times. The results show that the intermolecular interactions between C60 molecules in the films affect the vibrational modes. The 1428 cm−1 mode is the most sensitive mode to the interaction. A vibrational mode at about 669 cm−1 appears in the IR spectra of the films and increases with the annealing time; this mode represents the intermolecular interactions between the C60 molecules in the films. The peak position shift and the relative height of 1428 cm−1 mode increase with the increase of the 669 cm−1 mode peak height. The mechanism of the changes of vibrational modes has been discussed.
The effect of electric field on the excitonic states in coupled quantum well structures
Authors: Takahashi Yutaka , Kato Yoshimine , Kano Satoru S. , Fukatsu Susumu , Shiraki Yasuhiro , Ito Ryoichi
Published: July 2002

We have calculated by a variational procedure the excitonic states in symmetric coupled quantum well structures of GaAs/AlGaAs in the presence of an external electric field perpendicular to the interfacial plane using a nonseparable, nonspherical hydrogenic trial wavefunction. The exciton binding energies have been calculated as a function of external electric field for various well and barrier widths. The nonspherical trial wavefunction leads to a better estimate of the exciton binding energy, especially in high electric field, than that calculated with a spherical one. The calculations show that the binding energy is strongly dependent on the external electric fields: the variation (increase or decrease) is related to the spatial distribution of the wavefunctions. The results of the calculation are consistent with our experimental results.
Heat capacity measurements of antiferromagnetic CoO/NiCoO superlattices (abstract)
Authors: Abarra E. N. , Takano K. , Hellman F. , Berkowitz A. E.
Published: July 2002

We present heat capacity and magnetic measurements of antiferromagnetic (AFM) CoO/Ni0.5Co0.5O superlattices grown by reactive sputtering. X-ray data verify the structure and the high quality of the superlattice. Neutron-diffraction studies of similar superlattices have shown AFM ordering through several bilayers despite the short-range nature of the spin interaction in the constituent materials. We have recently developed a unique thin film microcalorimeter capable of measuring thin films from 1.5 K to well above room temperature, permitting specific heat measurements on these superlattices for the first time. Magnetic measurements were made by coupling the superlattices to a 30 nm Ni81Fe19 overlayer and measuring the temperature dependence of the exchange anisotropy field. We examine the effect of exchange coupling at the interfaces by varying the thickness of the bilayers and their constituents. When the layers of the CoO/Ni0.5Co0.5O superlattice are thin, we observe a single broad heat capacity peak at a temperature between the Néel temperatures of bulk CoO and Ni0.5Co0.5O. This peak is at a temperature that corresponds to the superlattice magnetic blocking temperature, the temperature at which the exchange field goes to zero. For thicker layers, we observe the disappearance of the superlattice peak, and the emergence of two broad peaks close to the individual Néel temperatures of CoO and Ni0.5Co0.5O. We compare the temperature dependence of the specific heat of the superlattices to various models.
Anomalous shock compression behavior of yttria-doped tetragonal zirconia
Authors: Mashimo Tsutomu , Nakamura Akira , Nishida Minoru , Matsuzaki Susumu , Kusaba Keiji , Fukuoka Kiyoto , Syono Yasuhiko
Published: July 2002

Shock-wave measurement and shock-compression recovery experiments of yttria (Y2O3) -doped (3.0 mol %) tetragonal zirconia in the pressure range up to 125 GPa were performed to study the anomalous shock compression and release behaviors. The minute three-wave structure with the transition points of 15–17 and 33–35 GPa was observed also by the inclined-mirror method, which is consistent with the VISAR (velocity interferometer system for any reflector) data. The shock velocities of the second wave were so fast that the first transition could not be identified as a normal elastoplastic transition. The shock velocity Us versus particle velocity Up relation of the final wave was given by Us=2.71+2.39Up km/s. The equation of state of the final phase was analyzed by using a program based on the Debye model, the Mie–Grüneisen equation, and the Birch–Marnaghan equation. The volume change between the tetragonal phase and the final phase at zero pressure was estimated to be 18%, which was larger than that expected for the tetragonal-orthorhombic II transition. The bulk moduli of the final phase at zero pressure was estimated to be 550 GPa. An instability was observed at release in the stress history of 28 GPa in peak stress. A certain amount of the monoclinic phase was observed at fractured surfaces of the recovered specimens from the shock compressions of below and above the first transition point (15–17 GPa) by Raman spectrum and transmission electron microscope observations. This might be related to the observed anomalous shock compression and release behaviors.
Electroluminescence in thin solid films of closely packed CdS nanocrystals
Authors: Artemyev M. V. , Sperling V. , Woggon U.
Published: July 2002

Thin solid films of closely packed CdS nanocrystals with sizes between 1.5 and 1.8 nm have been prepared and sandwiched between indium–tin–oxide (ITO) and silver contacts. At room temperature, these CdS nanocrystals start to emit light visible to the unaided eye at a bias above 15 V. The spectral characteristics are voltage controlled and tunable between 1.8 eV and 2.8 eV with increasing bias from 20 to 40 V. The current–voltage dependence indicates hopping conductivity of the carriers injected. At high voltage the electroluminescence and photoluminescence spectra are similar showing a strong red shift of the luminescence maximum with respect to the absorption peak at 3.75 eV which we explain by an effective radiative recombination through deep traps in the interface region. Consequently, the tunability of electroluminescence is attributed to excitation of different deep traps by electron transport across neighbouring CdS nanocrystals.
Influence of chemical bond of carbon on Ni catalyzed graphitization
Authors: Yudasaka Masako , Tasaka Kohji , Kikuchi Rie , Ohki Yoshimasa , Yoshimura Susumu , Ota Etsuro
Published: July 2002

Carbon diffuses into metal and recrystallizes as graphite, which is termed graphitization of carbon by metal. This study has revealed that the temperature at which this graphitization occurs depends on the initial state of the carbon. The lowest temperatures at which graphitization by Ni occurs for diamond, diamondlike amorphous carbon, graphite, and graphitelike amorphous carbon were 700, 500, 900, and 700 °C, respectively. It is shown that the temperature ranges at which graphitization by Ni occurs are correlated to the temperature ranges at which graphite can be formed on Ni by chemical vapor deposition using organic substances as starting materials.
Diffuse x-ray scattering of misfit dislocations at Si1−xGex/Si interfaces by triple crystal diffractometry
Authors: Bhagavannarayana G. , Zaumseil P.
Published: July 2002

Heterostructure Si1−xGex layers on (001)Si substrates with different degrees of relaxation R, ranging from the nearly pseudomorphic state to the nearly relaxed state (R∼0.8), were characterized by diffuse x-ray scattering measurements. The lattice constants a⊥ and a∥, and from these the Ge content x, the relaxation R, and misfit dislocation density Dd, were determined by single crystal diffractometry. The thickness of the SiGe layers was measured in the as-grown state by double crystal diffractometry. A standard triple crystal x-ray diffractometer was used to analyze the diffuse x-ray scattering (DXS) intensity by rotation of the analyzer crystal (ΔΘ) at a fixed sample position (α). The intensities were measured around the 004 reciprocal lattice point of the Si substrate using a wide open counter and a wide range of rotation angle (ΔΘ) for the analyzer crystal. The diffuse scattering increases steeply with increasing dislocation density up to a critical value of Dd≈5×104 cm−1. Then it reaches saturation and decreases slowly as Dd increases. This behavior can be explained by the superposition of the strain fields of neighboring dislocations above a critical density, so that only the heavily distorted regions near the dislocation core participate in scattering. This is confirmed by the fact that the DXS measured for different α values follows the Stokes–Wilson scattering ( IDXS∝q−4; q being the scattering vector). We show that the diffuse scattering technique is a very sensitive tool to detect relaxation at relatively low levels of dislocation densities Dd less than 5×104 cm−1, where diffractometric techniques that measure the lattice constants are near their resolution limits.
On the initial oxidation of iron: Quantification of growth kinetics by the coupled-currents approach
Authors: Graat P. C. J. , Somers M. A. J. , Vredenberg A. M. , Mittemeijer E. J.
Published: July 2002

Polycrystalline iron was oxidized at pO2=10−4 Pa and at temperatures ranging from 300 to 500 K. Ellipsometry was used for monitoring the oxide-film thickness as a function of time. The oxidation kinetics were described quantitatively by application of the model due to Fromhold and Cook by considering coupled currents of cations and electrons. At the lower temperatures tunneling is the dominant electron transport mechanism and an excellent agreement of experimental and calculated oxidation kinetics was obtained by adopting a time-dependent difference of the work functions of the metal-oxide and the oxide-oxygen interfaces. At the higher temperatures the experimental kinetics can be described quantitatively for a film thickness up to about 3 nm. Above this thickness electron transport becomes dominated by thermal emission rather than by tunneling. To investigate the influence of the surface pretreatment on the oxidation kinetics a sample was oxidized at pO2=10−4 Pa at 330 K, both after sputter cleaning and after sputter cleaning plus subsequent annealing at 700 K. The difference in initial oxidation rate observed after the two pretreatments was discussed in terms of corresponding differences in the work functions.
Formation and microstructural analysis of co-sputtered thin films consisting of cobalt nanograins embedded in carbon
Authors: Delaunay J.-J. , Hayashi T. , Tomita M. , Hirono S.
Published: July 2002

Cobalt–carbon thin films were deposited with a carbon concentration ranging from 27 to 57 at.% at different substrate temperatures. The morphology and phases of as-deposited films were investigated by transmission electron microscopy and x-ray diffraction. The effect of annealing on the microstructure is reported. Under particular conditions of substrate temperature, carbon concentration, and subsequent annealing, a granular morphology consisting of nanocrystalline cobalt grains embedded in graphitelike carbon was obtained. The cobalt grains were uniform in size. The particle size could be controlled in the range from 4 to 7 nm by varying the carbon concentration. The cobalt phase was found to depend on the carbon concentration and substrate temperature. The hexagonal close-packed cobalt phase was observed only via the formation of the metastable carbide δ′-Co2C and its subsequent decomposition upon annealing. Otherwise the cobalt phase has a heavily faulted close-packed structure or a random stacking structure.
Mechanical properties of the 3D polymerized, sp2–sp3 amorphous, and diamond-plus-graphite nanocomposite carbon phases prepared from C60 under high pressure
Authors: Brazhkin V. V. , Lyapin A. G. , Popova S. V. , Klyuev Yu. A. , Naletov A. M.
Published: July 2002

Mechanical properties (Vicker’s hardness, Young’s modulus, and fracture toughness coefficient) have been studied for the three-dimensionally polymerized, amorphous, and nanocrystalline diamond-plus-graphite composite carbon phases prepared from fullerite C60 by temperature treatment under pressure. The hardness was found to increase gradually with the synthesis temperature. The experimental values of hardness are well correlated with the density of samples regardless of the phase structural nature, displaying the same dependence as amorphous carbon films. It has been shown that the hardness and Young’s modulus of both polymerized crystalline and disordered phases, though not as high as those of diamond, are comparable to the properties of cubic BN, while fracture toughness coefficient can be higher than that for diamond. An unusual combination of high hardness and high plasticity has been established for strongly polymerized C60.
Effect of deposition temperature and growth rate on the bond structure of hydrogen free carbon films
Authors: Koskinen J. , Hirvonen J.-P. , Keränen J.
Published: July 2002

The effect of deposition temperature and growth rate on the bond structure of hydrogen free carbon films has been investigated. A sharp transition temperature of the sp3 content of the carbon films as a function of deposition temperature has been reported several times. The value of this transition temperature has varied from 150 to about 300 °C depending on the growth mechanism. In this article, high values varying from 300 to 400 °C for the transition temperature are reported. High momentary growth rates of up to 2000 nm/s have been obtained by using pulsed vacuum arc deposition. The transition temperature has been observed to depend on the momentary growth rate of the carbon film. The transition has been explained to be a relaxation process which includes diffusion of carbon atoms at the near surface. The results have been analyzed by using a model which was originally developed for radiation enhanced diffusion. An activation energy of 0.65 eV was obtained. The model also predicts qualitatively the transition temperatures in the case of the lower deposition rates reported in literature. For the frequency factor of the diffusion coefficient a value of the order of 10−8 cm2/s was obtained.
Correlation between spin polarization and magnetic moment in ferromagnetic alloys
Authors: Choy Tat-Sang , Chen Jian , Hershfield Selman
Published: July 2002

The correlation between the spin polarization of the tunneling current in ferromagnet/Al2O3/Al tunnel junction experiments and the magnetic moment in ferromagnets has been a mystery [Phys. Rev. B 16, 4907 (1977) and Phys. Rep. 238, 173 (1994)]. In this study, an attempt is made to explain this correlation. By assuming that the s electrons are responsible for the tunneling, the tunneling currents are proportional to the s density of states at the Fermi level [Phys. Rev. B 8, 3252 (1973)]. A tight-binding coherent potential approximation model for itinerant magnetism is applied to calculate the band structure of Fe- and Ni-based alloys [Rev. Mod. Phys. 46, 465 (1974)]. The Slater–Pauling curve for the bulk magnetic moment is recovered, and the spin polarization of the s electrons at the Fermi level is found to correlate well with the magnetic moment. The calculation is carried out in a model where the approximate band structure of an alloy is calculated by using two extra tight-binding parameters besides those of the host. As the average number of electrons is changed by the impurities, the total Coulomb energy and hence the splitting between the up and down spin bands has to be modified self-consistently. The modified band structure is then used to obtain the magnetic moment and the spin polarization. The relation between the magnetic moment and spin polarization can be understood by noting that within approximately 1.5 eV of the Fermi level, the density of states of the s band for either spin is roughly an increasing function of energy and therefore of the number of electrons. For a fixed total number of electrons, the magnetic moment increases with the number of spin-up electrons, corresponding to a larger spin-up density of states at the Fermi level. Thus, the spin polarization increases as the magnetic moment increases.
Theoretical study of the formation of closed curved graphite-like structures during annealing of diamond surface
Authors: Kuznetsov V. L. , Zilberberg I. L. , Butenko Yu. V. , Chuvilin A. L. , Segall B.
Published: July 2002

In recent high resolution transmission electron microscopic studies we have found that high temperature vacuum annealing (1200–1800 K) of ultradispersed (2–5 nm) and micron size diamond produces fullerene-like graphitic species, namely, onion-like carbon and closed curved graphite structures (multilayer nanotubes and nanofolds), respectively. Here we undertake theoretical studies to help in the understanding of the experimental data for these systems. (1) Calculations of cluster models by a standard semiempirical method (MNDO a software package) are used to explain the preferential exfoliation of {111} planes over other low index diamond planes. (2) The same approach suggests the likelihood that the graphitization is initiated by a significant thermal displacement of a single carbon atom at temperatures close to the Debye temperature. (3) At the diamond–graphite interface we have observed the formation of two curved graphitic sheets from three diamond {111} planes. We suggest that the evolution of this interface proceeds by a “zipper”-like migration mechanism with the carbon atoms of the middle diamond layer being distributed equally between the two growing graphitic sheets. (4) The observed mosaic packaging of closed curved graphite structures during the diamond surface graphitization is suggested to be a self-assembling process. This process is explained in terms of the “stretching” of a bowed graphite hexagonal network. The stretch is due to the fact that, if relaxed, the network would be smaller than the initially transformed hexagonal diamond (111), and to the increased separation between the separated sheet and the surface. The initial phase of the process is studied quantitatively using a molecular mechanics simulation.
Tensile testing of AlCu thin films on polyimide foils
Authors: Macionczyk F. , Brückner W.
Published: July 2002

Mechanical properties of sputtered AlCu(0.5 wt %) thin films, 0.2–2.0 μm thick, were determined by tensile testing. For comparison, tensile tests were also performed on bulk samples of the same composition. The films were deposited on thin polyimide foils. They were characterized with respect to the surface, microstructure, residual stress, and concentration of copper and oxygen. Stress-strain curves of the films were obtained by separating the force working on the polyimide foil from that working on the metal-polyimide compound. Young’s modulus of the films almost corresponded to the bulk value. Films with a thickness >1.5 μm broke by formation of macrocracks while thinner films showed formation of microcracks. The Hall–Petch model, additional strengthening by small grain size, and the role of grain boundary sliding for crack formation are discussed.
Spin vortex states and hysteretic properties of submicron size NiFe elements
Authors: Pokhil Taras , Song Dian , Nowak Janusz
Published: July 2002

Micromagnetic structures in submicron size circular permalloy elements of various thickness (5–50 nm) have been experimentally studied using a magnetic force microscope (MFM). One and two vortex micromagnetic states were observed in the elements. It was found that one vortex state is more favorable under external field of low values, while two vortex state forms under high fields. Switching between one and two vortex states results in a hysteresis in the micromagnetic response of the elements. The transition field from one to two vortex state increases with the increase of element thickness and, therefore, with the increase of the demagnetizing effect of the element edges. The two vortex state was not observed in the elements with a thickness of 50 nm or greater. The micromagnetic behavior correlates with hysteresis loops of a free layer in a magnetic tunnel junction of size, shape, and free layer thickness similar to elements examined with the MFM.
Effect of van der Waals forces on axial buckling of a double-walled carbon nanotube
Authors: Ru C. Q.
Published: July 2002

An elastic model is presented to study infinitesimal buckling of a double-walled carbon nanotube under axial compression. A simple formula is derived for the critical axial strain, which clearly indicates the role of the van der Waals forces between the inner and outer tubes characterized by two parameters. In particular, the analysis shows that inserting an inner tube into a single-walled nanotube does not increase the critical axial strain as compared to the single-walled nanotube under otherwise identical conditions, despite the fact that the total critical axial force of the double-walled nanotube could be increased due to an increase in the cross-sectional area.
Nonequilibrium molecular dynamics study of shear flow in soft disks
Authors: Heyes D. M. , Morriss G. P. , Evans D. J.
Published: July 2002

Nonequilibrium molecular dynamics calculations of homogeneous shear flow in two dimensions have been performed on soft disks close to the freezing transition. Simulations at discrete shear rates, γ̇, reveal complex phase changes and dynamical behavior during extensive shear thinning and a difference in the values of the diagonal components of the pressure tensor. A shear thickening regime was also found near to γ̇=10 but it had disappeared at γ̇=50, in line with recent work by Woodcock. The calculations reveal that there is a change from an amorphous liquid structure at γ̇=1.0 to a fluid microscopically layered along the stream lines at γ̇=10 and higher shear rates. An intermediate two-phase region was observed at γ̇=5 in which both amorphous and ordered phases coexist within the MD cell, containing 896 disks. In the ordered phase the particle dynamics are dominated by the imposed shear flow and can be reproduced in large part by simple models which exclude thermal motion. The time correlation functions are highly oscillatory which reflect well defined motion on a local distance scale. The soft disks spend most of their time trapped between similar disks in adjacent layers, in between rapid periods of ‘‘free-flight’’. Instantaneous pictures of the high shear rate (γ̇≳10) structure consequently show the presence of oscillating transitory strings of highly overlapping soft disks formed roughly perpendicular to those stably formed along the streamlines.
Rotational state distributions of NO molecules after interaction with germanium surfaces
Authors: Mödl A. , Robota H. , Segner J. , Vielhaber W. , Lin M. C. , Ertl G.
Published: July 2002

The rotational state distributions of NO molecules scattered at clean and oxidized Ge surfaces were determined by means of laser induced fluorescence (LIF). For low incident kinetic energies (Ekin≲200 meV) scattering is dominated by trapping/desorption and the molecules coming off the surface are translationally accommodated to the surface temperature. The rotational state populations can in this case be approximated by a Boltzmann distribution. The rotational temperature Trot equals the surface temperature Ts, however only up to about 250 K. Further increase of Ts leads to a leveling off of Trot until a limiting value Trot,lim≈400 K is reached. (Rotational cooling in desorption.) At higher kinetic energies direct-inelastic scattering dominates, and for Ekin=820 meV a pronounced non-Boltzmann distribution (rotational rainbow) is observed. The observed effects are qualitatively interpreted in terms of current theoretical models.
Raman cross section measurements in the visible and ultraviolet using an integrating cavity: Application to benzene, cyclohexane, and cacodylate
Authors: Trulson Mark O. , Mathies Richard A.
Published: July 2002

A new technique has been developed for the measurement of absolute Raman scattering cross sections. The Raman scattering of a sample in an integrating cavity is compared to the elastic scattering from a reference scatterer of unit yield. The fundamental advantage of this approach is that it permits the measurement of absolute cross sections throughout the visible and UV regions with a simple experimental apparatus and procedure. The 992 cm−1 mode of benzene was measured in the visible region and compared satisfactorily to the values obtained by Kato and Takuma [J. Chem. Phys. 54, 5398 (1971)] and Abe et al. [J. Raman Spectrosc. 6, 38 (1977)]. The scattering cross section of the 608 cm−1 mode of sodium cacodylate, an aqueous intensity standard, was measured from 647 to 240 nm. Analysis within the A-term formalism provided a functional form for interpolation between measured points and revealed that the 608 cm−1 cacodylate mode derives intensity from states in the vicinity of 83 kK (kilokaiser). The 802 cm−1 skeletal mode of cyclohexane was measured over the same spectral region to provide a nonaqueous intensity standard. In this case the principal contribution to the preresonance scattering arises from electronic states near 115 kK and not from the lowest allowed σ → σ* (CC) states. To compare scattering mechanisms, the CH stretching modes of cyclohexane were referenced to the 802 cm−1 mode. The CH stretching modes of cyclohexane derive A-term intensity from the strongly allowed σ → σ*(CH) state near 85 kK, as expected from simple valence arguments.
The static structure factor for fully aligned charged rod-like macromolecules
Authors: Dhont Jan K. G.
Published: July 2002

A calculation of the static structure factor for a system of fully aligned rod-like charged macromolecules, based on the Ornstein–Zernike equation in MSA approximation is presented. The hard core of the molecules is modeled by an ellipsoid. For very long rods this should be a good model. To obtain explicit results, for the long-ranged interaction potential a superposition of screened Coulomb potentials is chosen, although the theory is applicable for any long-ranged interaction potential. The full Ornstein–Zernike equation is reduced to a set of one-dimensional Ornstein–Zernike equations, which are coupled through their closure relations. This feature makes it necessary to obtain the structure factor by a numerical procedure.
Solute partitioning into chain molecule interphases: Monolayers, bilayer membranes, and micelles
Authors: Marqusee J. A. , Dill Ken A.
Published: July 2002

A mean field lattice theory is presented for the mixing of small molecule solutes within interfacial phases of chain molecules, and for their partitioning from an external solvent. Interphases include monolayers, bilayers, vesicles, cylindrical and spherical micelles, and stationary phases of grafted chains used in reversed phase liquid chromatography. Transfer depends on the entropy of mixing; the configurations of the chains; the contact interactions among solute and chains, treated here in the regular solution approximation; and the volumetric expansion of curved interphases. The theory predicts stable gradients of solute concentration at equilibrium due to the variation of chain organization with depth. For example, alkyl solutes should prefer mid-bilayer, since the surfactant chains are most disordered there; this prediction agrees with recent small-angle neutron scattering experiments. Increasing the surface density of the interphase chains should lead to solute expulsion. Most solutes should prefer the outer regions of spherical micellar cores due to the greater volume available there, even though densities are predicted to be highest at the core center.
Dynamics of concentration fluctuation in a polymer blend on both sides of the phase boundary
Authors: Sato Takahiro , Han Charles C.
Published: July 2002

The dynamics of concentration fluctuation in a deuterated polystyrene/poly(vinylmethylether) blend system has been studied by the temperature jump and also by the reverse quench light scattering techniques. The growth or decay rate R(q) in Cahn–Hilliard–Cook’s equation is almost proportional to the square of scattering wave vector. This indicates that the interfacial free energy contribution to the concentration fluctuation dynamics in the wavelength range examined is very small. The interdiffusion coefficient D which has been obtained as a function of temperature, is continuous at the phase separation boundary. The mobility M, which can be extracted from D by combining the results of both statics and kinetics has an Arrhenius type of temperature dependence on both sides of the phase boundary.
The IR spectrum of ethylene in solid xenon: Host–guest mode coupling
Authors: Collins Susan T. , Casey Patricia A. , Pimentel George C.
Published: July 2002

Ethenes (C2H4, C2D4, cis-1,2-C2D2H2, and trans-1,2-C2D2H2) isolated in xenon matrices show dramatic and reversible infrared spectral changes between 12 and 50 K; most bands broaden with increasing temperature. In the xenon matrix, the C2H4 ν7 band is unusually broad, much broader than in the argon spectrum, while the corresponding C2D4 mode remains sharp. Though sharper than C2H4 ν7, many other bands are doublets in which one peak shows different temperature dependence than the other. The C2D4 ν7 band, however, is only a singlet. Among several possible explanations for the unusual breadth, the most likely is that C2H4 ν7 couples with the xenon lattice phonon modes through the librational movement of C2H4 within the cage. The doublets are attributed to two sites with which different modes and isotopes interact to varying degrees.
Solvation thermodynamics: An approach from analytic temperature derivatives
Authors: Yu Hsiang-Ai , Roux Benoit , Karplus Martin
Published: July 2002

A method is presented that uses integral equation theory to determine analytic temperature derivatives of the radial distribution functions. It is illustrated by studying the solvation thermodynamics of monatomic solutes in aqueous solution. The results agree well with the density derivative method developed previously [Yu and Karplus, J. Chem. Phys. 89, 2366 (1988)]. An expression for the solvation enthalpy is derived which allows direct comparison with experimental and isobaric–isothermal (NPT) ensemble simulation data. Satisfactory agreement with experiment is found for pure water and for the aqueous solvation of monovalent ions. Simple equations that exploit the site–site HNC closures are given for the decomposition of the potential of mean force into its enthalpic (or energetic) and entropic components. Since the extended RISM (HNC-RISM) theory yields an incorrect (trivial) value of the dielectric constant, two different ways to correct for the asymptotic behavior of the solute–solute potential of mean force are compared. They lead to similar results but the method in which the solvent dielectric constant is modified from the outset can be applied more generally.

The interactions between nonpolar and between polar solutes in water are decomposed into enthalpic and entropic contributions. This is difficult to do by computer simulations because of the lack of precision in such calculations. The association of nonpolar solutes in water is found to have comparable enthalpic and entropic contributions; this result disagrees with the usual description of an entropy-dominated hydrophobic interaction. For ions, the somewhat surprising result is that the association of like-charged species is enthalpy driven while for oppositely charged ions entropic effects are dominant. The process of bringing two like-charged ions together leads to higher local charge density; the more favorable solvation enthalpy arising from this increase in charge density (q2 dependence) more than compensates for the Coulombic repulsion. For oppositely charged ions, association leads to a partial charge neutralization in which the favorable Coulombic attraction is overwhelmed by the loss of stabilizing solvation enthalpy. The entropic increase is due to the greater freedom of the surrounding water molecules resulting from the partial charge neutralization.
Gaussian approximation and Brownian dynamics simulations for Rouse chains with hydrodynamic interaction undergoing simple shear flow
Authors: Zylka Waldemar
Published: July 2002

A bead-spring-chain model including hydrodynamic-interaction effects is used to calculate the rheological properties of dilute polymer solutions undergoing steady shear flow. The hydrodynamic fluctuations are explicitly taken into account within the context of the recently developed Gaussian approximation for the hydrodynamic interaction. In this Gaussian approach expressions for the non-Newtonian stress tensor at arbitrary shear rate are calculated. For short chains, the shear-rate–dependent predictions for the viscometric functions are presented. Also, the Brownian dynamics simulation technique is applied to determine exact rheological behavior of the model. Comparing the simulation results to the corresponding predictions of the Gaussian, consistent-averaging, and preaveraging approximations the validity and the superiority of the Gaussian method is clearly demonstrated.
Molecular dynamics simulation of methyl group relaxation in water
Authors: Widmalm Göran , Pastor Richard W. , Bull T. E.
Published: July 2002

Recent theoretical studies of stochastic classical and quantum dynamics suggest that quantum effects may be significant in the nuclear magnetic resonance (NMR) relaxation of a methyl group attached to intermediate and large sized molecules at normal temperatures. The magnitude of the effects depends on the reorientational correlation time of the molecule, τ0, the barrier to internal rotation, V, and the correlation time for the collision induced randomization of the internal angular momentum, τω. This note reports the results of molecular dynamics simulations used to estimate τω for a methyl group in water. The simulations indicate that extended diffusion, rather than Fokker–Planck–Langevin, dynamics are appropriate for this system. Using τω determined in the simulation and V=13 KJ/mol (3 Kcal/mol), predictions of the classical and quantum models differ by 40% for the 13C relaxation of a methyl on an intermediate sized molecule, such as a disaccharide. In larger molecules, the differences are smaller due to smaller contributions from the internal dynamics to the NMR relaxation process, unless τω and/or V are larger. Experimental evidence suggests that τω is an order of magnitude or more larger in a hydrocarbon environment than in water. With the larger τω, a plausible example is given where the models’ predictions differ by 80%. More work is needed, however, to better define τω in hydrophobic regions of macromolecules.
The time lag in nucleation theory
Authors: Wu David T.
Published: July 2002

The time lag in transient nucleation is solved exactly for both the simple monomer approximation and the more complicated multipath kinetics with detailed balancing. It is shown that only initial and steady-state quantities are required for its calculation. The time lag within the continuum approximation is also considered. By evaluating the exact expression approximately, a simple functional form is obtained. The controversy over the ‘‘prefactor’’ in previous approximate time lags is resolved. It is shown that in general none of the approximate time lags are correct. The ‘‘relative time lag’’ is introduced. It is useful when detailed balance is not obeyed and also when the lower cutoff cluster size is not known precisely.
Vanishing interfacial tension in an immiscible polymer blend
Authors: Shull Kenneth R. , Kellock Andrew J. , Deline Vaughn R. , MacDonald Scott A.
Published: July 2002

The properties of the interface between polystyrene (PS) and a random copolymer of polystyrene and poly(parahydroxystyrene) (PPHS) were modified by addition of a diblock copolymer of deuterated polystyrene (dPS) and poly(2-vinylpyridine) (PVP). Segregation of diblock copolymer chains to the interface between the immiscible blend components is driven by the compatibility of the dPS block with the PS phase and by a strong favorable interaction between the PVP block and the PPHS units of the random copolymer. Forward recoil spectrometry was used to measure the equilibrium excess of copolymer chains at the interface as a function of the copolymer volume fraction in the bulk PS phase. A dramatic increase in the interfacial adsorption is identified with the formation of emulsified ‘‘droplet’’ phases which appear when the interfacial free energy between the immiscible blends becomes vanishingly small. Dynamic secondary-ion mass spectrometry was used to characterize these emulsified phases, and to verify our picture of the copolymer adsorption process. Comparison to predictions from a detailed self-consistent-field theory gives an estimate for the magnitude of favorable interaction between PVP and the PS/PPHS copolymer (≂ 0.16kBT per repeat unit) and the magnitude of the interfacial tension between the two phases in the absence of a diblock copolymer (≂2 dyn/cm).
The number of contacts in a self-avoiding walk of variable radius of gyration in two and three dimensions
Authors: Victor J-M. , Imbert J-B. , Lhuillier D.
Published: July 2002

A simple scaling law is proposed for the dependence of the number of contacts on the radius of gyration of a self-avoiding walk (SAW). We test our proposal on SAWs generated by a Monte Carlo simulation on square and cubic lattices. The distribution of the number of contacts is then combined with the distribution of configurations previously derived to deduce the free energy of a polymer chain for low values of the interaction parameter χ. As compared to the free energy of Flory, the new expression takes a much better account of the spatial correlations between distant monomers of the chain.
Surface wave scattering at nonuniform fluid interfaces
Authors: Chou T. , Nelson David R.
Published: July 2002

Effects of spatially varying interfacial parameters on the propagation of surface waves are studied. These variations can arise from inhomogeneities in coverage of surface active substances such as amphiphillic molecules at the fluid/gas interface. Such variations often occur in phase coexistence regions of Langmuir monolayers. Wave scattering from these surface inhomogeneities are calculated in the limit of small variations in the surface parameters by using the asymptotic form of surface Green’s functions in the first-order Born approximation. When shear viscosity of the bulk fluid and variations in surface elastic moduli become important, modes other than transverse capillary waves can change the characteristics of propagation. Scattering among these modes provides a mechanism for surface wave attenuation in addition to viscous damping on a homogeneous surfactant covered interface. Experimental detection of waves attenuation and scattering is also discussed.