bluuebunny/crossref_metadata_2025 huggingface dataset:
Deterministic folding: The role of entropic forces and steric specificities
Authors: da Silva Roosevelt A. , da Silva M. A. A. , Caliri A.
Published: July 2002

The inverse folding problem of proteinlike macromolecules is studied by using a lattice Monte Carlo (MC) model in which steric specificities (nearest-neighbors constraints) are included and the hydrophobic effect is treated explicitly by considering interactions between the chain and solvent molecules. Chemical attributes and steric peculiarities of the residues are encoded in a 10-letter alphabet and a correspondent “syntax” is provided in order to write suitable sequences for the specified target structures; twenty-four target configurations, chosen in order to cover all possible values of the average contact order χ (0.2381⩽χ⩽0.4947 for this system), were encoded and analyzed. The results, obtained by MC simulations, are strongly influenced by geometrical properties of the native configuration, namely χ and the relative number φ of crankshafts-type structures: For χ<0.35 the folding is deterministic, that is, the syntax is able to encode successful sequences: The system presents larger encodability, minimum sequence-target degeneracies and smaller characteristic folding time τf. For χ⩾0.35 the above results are not reproduced any more: The folding success is severely reduced, showing strong correlation with φ. Additionally, the existence of distinct characteristic folding times suggests that different mechanisms are acting at the same time in the folding process. The results (all obtained from the same single model, under the same “physiological conditions”) resemble some general features of the folding problem, supporting the premise that the steric specificities, in association with the entropic forces (hydrophobic effect), are basic ingredients in the protein folding process.
Renormalization of the indirect correlation function to extract the bridge function of simple fluids
Authors: Bomont J. M. , Bretonnet J. L.
Published: July 2002

The standard integral equation approach is used to extract the bridge function and other correlation functions of simple fluids. To do this, we firstly use the hybridized mean spherical approximation (HMSA), which involves a renormalization of the indirect correlation function. Secondly, we propose a new division scheme for the pair potential founded on the merging process of a test particle into the system. Thirdly, we take advantage of the thermodynamic consistent condition put forward in the HMSA to optimize the depth of the long-range part of the pair potential inside the core, making the procedure autonomous and state dependent. The method is tested on the pure Lennard-Jones fluid in supercritical conditions. Very good agreement is obtained for structural quantities and thermodynamic properties as compared to the Monte Carlo (MC) results for the bridge function and to the molecular dynamics (MD) results for the thermodynamic properties.
Dynamic displacement of N2 from Ru(0001) by incident D and H atoms
Authors: Diekhöner L. , Mortensen H. , Åkerlund C. , Baurichter A. , Luntz A. C.
Published: July 2002

Exposing a N2 covered Ru(0001) surface to a D or H atom beam leads to desorption of the N2 molecules. This displacement is kinetically prompt at all N2 coverages and the process is identified as dynamic displacement. By showing that the cross section for displacement by D atoms is roughly twice that for H atoms, we suggest that the mechanism for this dynamic displacement is some phonon mediated process rather than an electronically nonadiabatic one suggested earlier. As a contrast, the displacement of Xe adsorbed on Pt(111) induced by CO adsorption has also been measured. In this case, the displacement is not prompt and there is a total coverage on the surface that is necessary to induce desorption of Xe. This seems well described by a thermodynamic displacement mechanism.
Role of parallel flow curvature on the mitigation of Rayleigh–Taylor instability
Authors: Sarmah D. , Sen S. , Cairns R. A.
Published: July 2002

The effect of a radially varying parallel equilibrium flow on the stability of the Rayleigh–Taylor (RT) mode is studied analytically in the presence of a sheared magnetic field. It is shown that the parallel flow curvature can completely stabilize the RT mode. The flow curvature also has a robust effect on the radial structure of the mode. Possible implications of these theoretical findings to recent experiments are also discussed.
Bimolecular kinetics according to a stochastic analysis of reactant dynamics
Authors: Moro Giorgio J. , Severin Maria Gabriella
Published: July 2002

A stochastic representation of the reversible bimolecular process A+B⇌AB is introduced on the basis of the nearest-neighbor distribution. It leads to a description of the reactant pair dynamics under the action of its interaction potential, without introducing any boundary condition or sink function. In this way it becomes evident that reaction processes are particular manifestations of the molecular dynamics. The analysis of the eigenvalues of the time evolution operator allows one to identify the conditions for a well-defined time scale separation between the slow kinetic processes and the fast equilibration of the unbound pair. Correspondingly the rate equations for the reversible bimolecular kinetics are recovered from the long time behavior of the nearest-neighbor distribution. By means of asymptotic methods, analytical approximations are derived for the rate coefficients and their concentration dependence. This allows a simple rationalization of the effects of the interaction potential between the reagents.
Living poly(α-methylstyrene) near the polymerization line: VIII. Mass density, viscosity, and surface tension in tetrahydrofuran
Authors: Pendyala Krishna , Gu Xiangyun , Andrews Kevin P. , Gruner Karen , Jacobs D. T. , Greer S. C.
Published: July 2002

We consider the polymerization of α-methylstyrene, initiated by sodium naphthalide in the solvent tetrahydrofuran on time scales that permit full thermodynamic equilibrium between the monomer and the polymer. We present new measurements as a function of temperature of the mass density, the shear viscosity, and the liquid–vapor surface tension, and we compare the data to theoretical expectations when the polymerization is viewed as a phase transition. The mass density is well described by either mean field or nonmean field theories. The shear viscosity increases as the average degree of polymerization (DP) increases, but the exponent 3.4 is not reached, presumably because the DP is too small. The surface tension increases as the DP increases, indicating depletion of the polymer from the surface.
Electron localization function studies of the nature of binding in neutral rare-gas containing hydrides: HKrCN, HKrNC, HXeCN, HXeNC, HXeOH, and HXeSH
Authors: Berski Sławomir , Latajka Zdzisław , Silvi Bernard , Lundell Jan
Published: July 2002

Neutral rare-gas containing molecules HRgCN (Rg=Kr, Xe), HXeSH and HXeOH are investigated by means of topological analysis of the electron localization function (ELF). This analysis explains the type of bonding and delocalization of electron density in chemical systems based on the indirect probability of finding two electrons with the opposite spins. The calculations reveals that all studied species are charge-transfer systems with the approximate formulas: [HKr]+0.65[CN]−0.65, [HXe]+0.66[CN]−0.66, [HXe]+0.45[SH]−0.45, and [HXe]+0.57[OH]−0.57. The isomerization process from HRgCN to HRgNC increases the charge separation to 0.72e for Kr and 0.74e for Xe containing molecules. It is shown that the Rg–C, Rg–N, Xe–S, and Xe–O bonds belong to the unshared electron type and are mainly of the electrostatic origin. The minimum of ELF in the Kr–C and Kr–N linkage, studied at the B3LYP/6-311++G(2d,2p) computational level, yields relatively high values of about 0.4 and 0.3, respectively. There is a correlation between larger stability of HRgCN isomers and an increased exchange of the electron density between the lone electron pair of carbon and the nonbonding electron density of xenon: V(C)↔V(Xe) as compared to smaller V(N)↔V(Xe) delocalization in less stable HRgNC isomers. The analysis of the CN group reveals the covalent character of the carbon–nitrogen bond, which is confirmed on the basis of presence of the disynaptic valence V(C,N) attractor positioned near to nitrogen. The HRgCN→HRgNC isomerization results in depletion of the carbon–nitrogen bond V(C,N) and a large saturation of the valence nitrogen basin V(N).
Single-pass ion cyclotron resonance absorption
Authors: Breizman Boris N. , Arefiev Alexey V.
Published: July 2002

The ion response to the rf-field during single-pass ion-cyclotron resonance heating (ICRH) can be essentially nonlinear. This paper presents a self-consistent theory of the rf-wave propagation and ion motion through the resonance. An important ingredient of the problem is the ion flow along the magnetic field. The flow velocity limits the time the ions spend at the resonance, which in turn limits the ion energy gain. A feature that makes the problem nonlinear is that the flow accelerates under the effect of the ∇B force and rf-pressure. This acceleration can produce a steep decrease in the plasma density at the resonance, resulting in partial reflection of the incident wave.
Electron acceleration by static and oscillating electric fields produced in the interaction between femtosecond laser pulses and solid targets
Authors: Dong Q. L. , Zhang J.
Published: July 2002

The interaction of modest, femtosecond (fs) laser pulses with solid targets is studied with particle-in-cell (PIC) simulations. A bi-temperature distribution of hot electrons is found. The first hot electron temperature can be attributed to the resonance absorption of the laser field, whereas the second hot electron temperature is identified to be due to the combined acceleration by the static electric field in front of the target and by the laser induced oscillating electric field in the thin plasma layer between the vacuum and the target.
Optoelectronic properties of ZnSe/ZnMgSSe multiple quantum wells
Authors: Chang C. W. , Yang H. C. , Chen C. H. , Chang H. J. , Chen Y. F.
Published: July 2002

Optoelectronic properties of undoped ZnSe/ZnMgSSe multiple quantum wells (MQWs) grown by molecular beam epitaxy on (001) GaAs substrates have been investigated by photoluminescence, photoconductivity, and persistent photoconductivity (PPC) measurements. The features related to the band-to-band excitonic and impurity-related transitions of ZnSe/ZnMgSSe MQWs are observed and discussed. In addition, the parameters that describe the temperature dependence of the interband transition energy and broadening function of the excitonic feature are evaluated. PPC has been observed in undoped ZnSe/ZnMgSSe MQWs. The decay kinetics of the PPC effect can be described by a stretched-exponential function, Ippc(t)=Ippc(0)exp[−(t/τ)β], (0<β<1). Through the study of the PPC effect under various conditions, such as different temperature, different photon energy of photoexcitation, and different ZnSe well width, we identify that the carrier excitation from the defect level in ZnMgSSe barrier layer into the ZnSe well layer is the origin of the PPC effect in ZnSe/ZnMgSSe MQWs.
Structure and magnetism in Co/Pt multilayers
Authors: Huang J. C. A. , Lee C. H. , Yu K. L.
Published: July 2002

The structure of [Co(tCo)/Pt(10 Å)]30 multilayers has been studied by x-ray absorption spectroscopy and x-ray diffraction using in-plane and out- of-plane polarized synchrotron radiation. The x-ray absorption near-edge spectroscopy shows that the Co layer is like a fcc pseudomorphic structure for tCo less than 4 Å. For tCo=10 Å, the spectra in both directions are separated and look closer to the bulk hcp Co. The extended x-ray absorption fine structure studies reveal that the Co–Pt interface of the multilayers fitted with a sharp boundary better than an interdiffusion model. The in-plane x-ray diffraction shows that the Pt layer in the multilayer possesses a 2%–3.5% compressible strain. Although the saturation magnetization Ms does not depend on tCo or interfacial roughness in any simple form, the Ms values scale quite linearly with the in-plane Pt strain. We conclude that the interfacial strain is important for the perpendicular magnetization in the Co/Pt multilayers.
Spin transfer into an inhomogeneous magnetization distribution
Authors: Miltat J. , Albuquerque G. , Thiaville A. , Vouille C.
Published: July 2002

Based on specific examples, we examine the consequence of spin-polarized current injection into confined model micromagnetic configurations, namely a high remanence state known as the S state and a low, though nonzero, remanence state called the Leaf state. Magnetization dynamics is solved in the space and time domain owing to the Landau–Lifshitz–Gilbert equation. It is shown that, within model bounds, the S state is not propitious to fast switching under the sole influence of a polarized current, even if disregarding the current induced field, whereas Leaf state switching characteristics become extremely complex as soon as due account is made for the latter.
Micromagnetic predictions of bit decay caused by thermal fluctuations over long time scales
Authors: Xue Jianhua , Victora R. H.
Published: July 2002

Thermal decay in magnetic recording media increases with time, consequently decreasing the signal-to-noise ratio. A recently developed scaling technique, based on the equivalence of time and temperature within micromagnetics, has been used to predict bit decay over long time scales. An ideal square-wave field is used to record bits in thin film magnetic recording media. Comparison of bit thermal stability for longitudinal and perpendicular thin film media with similar KuV/kT is carried out at recording densities from 50 to 400 Gbit/in.2. Bit decay in a Co/Pd superlattice film at ∼100 Gbit/in.2 is also studied.
Imaging magnetic domain structure in sub-500 nm thin film elements
Authors: Kirk K. J. , McVitie S. , Chapman J. N. , Wilkinson C. D. W.
Published: July 2002

Magnetic imaging in the transmission electron microscope (TEM) has been used to examine submicron elements with the aim of discovering down to what element size complex domain patterns can form. The elements were squares, circles, triangles, and pentagons in the size range 100–500 nm and were made from 36 nm Co films or 8 nm Ni80Fe20 (NiFe) with in-plane magnetization. The magnetic domain structures in these elements were imaged at high resolution using the differential phase contrast imaging mode in a TEM. Nonuniform magnetization structures were seen in the images. Vortices were present at remanence in all shapes of 36-nm-thick Co elements down to 100 nm size and in circular NiFe elements down to 116 nm diameter. Triangular NiFe elements did not have a vortex state at remanence, instead the magnetization curved round within the element but did not achieve complete flux closure. In simulations of square and circular NiFe elements, it was found that defects at the edges of the elements encouraged reversal by a vortex mechanism, whereas for simulated elements with no defects, reversal was by rotation and occurred at much lower fields.
Far infrared spectroscopy on the three-dimensional dilute antiferromagnet FexZn1−xF2
Authors: Satooka J. , Katsumata K. , Belanger D. P.
Published: July 2002

Fourier-transform infrared spectroscopy measurements have been performed on the three-dimensional dilute antiferromagnet FexZn1−xF2 with x=0.99–0.58 in the far infrared (FIR) region. The FIR spectra are analyzed taking into account the ligand field and the local exchange interaction probability with J1–J3; |J1|, |J3|≪|J2|, where J1, J2, and J3 are the nearest neighbor, second nearest neighbor, and third nearest neighbor exchange interaction constants, respectively. The concentration dependence of the FIR spectra at low temperature is qualitatively well reproduced by our analysis, though some detailed structure remains unexplained.
Magnetic field dependence of spin-glass freezing temperature in a diluted antiferromagnet Mn0.55Mg0.45TiO3
Authors: Fukaya A. , Satooka J. , Ito A. , Nakajima K. , Kakurai K. , Nishiyama K. , Nagamine K.
Published: July 2002

In order to examine magnetic properties of a spin-glass Mn0.55Mg0.45TiO3 under magnetic fields, dc magnetization, ac susceptibility, muon spin relaxation (μSR), and neutron scattering measurements have been performed. A freezing temperature observed by the dc magnetization and the ac susceptibility strongly depends on magnetic field, H. But for the latter, the H dependence becomes weak, as the measuring frequency increases. In the μSR and the neutron scattering measurements, no significant field dependence is observed. These results demonstrate that the field dependence of the freezing temperature becomes weaker, as the time scale of measurements shortens.
The α-relaxation process in simple glass forming liquid m-toluidine. I. The temperature dependence of the dielectric response
Authors: Cutroni Maria , Mandanici Andrea , Spanoudaki Anna , Pelster Rolf
Published: July 2002

Wide band dielectric spectroscopy has been used to study the bulk dynamics of the simple supercooled liquid m-toluidine. Following the temperature and frequency dependence of the dielectric response we find a stretched relaxation process, the stretching parameter of which is temperature dependent. The observed decoupling between rotational and translational diffusion parameters at low temperatures may be the result of heterogeneous dynamics.
Magnetization and susceptibility of ultrathin Fe films on Cu(100)
Authors: Camley R. E.
Published: July 2002

Face-centered Fe on Cu(100) remains a challenging and rich magnetic film system due to its structural and magnetic instabilities. One magnetic phase has a spin structure where the first two layers are coupled ferromagnetically and the remaining layers are all antiferromagnetically coupled. We use a self-consistent local mean-field method, to calculate the magnetic structure of this phase for any temperature and applied field. We obtain theoretical results for both parallel and transverse susceptibility measurements and indicate how such measurements may be used to determine the anisotropy in the ferromagnet and antiferromagnet portions of the Fe film.
Effects of reduced dimensionality on the magnetic properties of ultrathin (Co/Cu)[111] films
Authors: Camarero J. , de Miguel J. J. , Hernando A. , Miranda R.
Published: July 2002

Due to the collective nature of the electronic interactions responsible for the appearance of ferromagnetic ordering, a strong influence of reduced dimensionality is to be expected in ultrathin magnetic films and heterostructures. In this article we review a number of experimental results covering several of those effects. For instance, the coercivity of ultrathin Co/Cu(111) films increases continuously up to ∼6 monolayer Co thickness. We show that this behavior is related to the thickness dependence of the Curie temperature of these low-dimensional samples. Moreover, magnetic anisotropy and interlayer exchange coupling are also strongly affected by the structural features. Because of their smaller interfacial roughness, films grown with the aid of a surfactant Pb layer exhibit perpendicular magnetization up to larger thicknesses than those grown without Pb. With respect to the exchange coupling, the first ones show oscillatory magnetic coupling and complete antiferromagnetic coupling for both perpendicular and parallel magnetization between Co layers, while for films grown without Pb there is always some amount of ferromagnetic coupling between Co layers.
Growth and stress relaxation of Co/NiO bilayers on MgO(001)
Authors: Warot B. , Snoeck E. , Baulès P. , Ousset J.-C. , Casanove M.-J. , Dubourg S. , Bobo J. F.
Published: July 2002

Co/NiO bilayers have been grown on a MgO(001) surface in an ultrahigh vacuum sputtering chamber under various deposition conditions. Investigations by x-ray diffraction, atomic force microscopy, and high resolution transmission electron microscopy give evidence for a close relationship between structure and deposition conditions for the two different layers. The role of the deposition rate on the oxide layer surface roughness is particularly emphasized. At high deposition rates, surface roughness increases with temperature, whereas the surface remains flat for low deposition rates. Whatever the growth conditions, the NiO layer grows epitaxially on the MgO substrate with the orientation relationship NiO(001)[100]//MgO(001)[100]. The misfit strain (about 1%), elastically accommodated in the thinner layers (3 nm), is fully relaxed in 50-nm-thick layers. A temperature dependence of the cobalt layer structure is observed: at room temperature it grows in its high temperature face-centered cubic structure, whereas it grows in the hexagonal close-packed structure at slightly higher temperatures. The influence of oxygen deficiency in NiO on the growth mode of the cobalt layer is also discussed.
Relaxor behavior in manganites (invited)
Authors: Kimura T. , Tokura Y. , Kumai R. , Okimoto Y. , Tomioka Y.
Published: July 2002

The impurity (Cr3+)-doping effect on the stability of charge and orbital ordering has been systematically investigated for Nd1/2Ca1/2Mn1−yCryO3 crystals by measurements of magnetotransport and x-ray diffraction. The random field in terms of eg orbital deficiencies on the Cr sites drives the charge and orbital correlations to dynamical and short range, which is most relevant to the high-resistive state exhibiting colossal magnetoresistance. In the Cr-doped manganite, we can observe the coexistence of ferromagnetic–metallic and charge–orbital ordered phases, their spatial distributions, diffuse x-ray scattering, magnetic-field annealing, and the aging effect on the magnetic and electric properties, etc. These phenomena are reminiscent of those of relaxor ferroelectrics composed of ferroelectric clusters embedded in a paraelectric matrix. We propose that the mixed-valent manganite can be viewed as a “magneto- and electrorelaxor.”
Extension of Gaussian-3 theory to molecules containing third-row atoms K, Ca, Ga–Kr
Authors: Curtiss Larry A. , Redfern Paul C. , Rassolov Vitaly , Kedziora Gary , Pople John A.
Published: July 2002

Gaussian-3 (G3) theory is extended to molecules containing the third-row nontransition elements K, Ca, Ga–Kr. Basis sets compatible with those used in G3 theory for molecules containing first- and second-row atoms have been derived. The G3 average absolute deviation from experiment for a set of 47 test reactions containing these elements is 0.94 kcal/mol. This is a substantial improvement over Gaussian-2 theory, which has an average absolute deviation of 1.43 kcal/mol for the same set. Variations of G3 theory are also presented that are based on reduced orders of perturbation theory. These variations also show similar improvement over the corresponding G2 methods. The use of scaling parameters in G3 theory for the third row was investigated and found to perform nearly as well as use of the higher level correction. In addition, these methods are evaluated on a set of molecules containing K and Ca for which the experimental data are not accurate enough for them to be included in the test set. Results for this set indicate that G3 theory performs significantly better than G2 for molecules containing Ca. When the 47 third-row systems are added to the G3/99 database the complete G3 average absolute deviation becomes 1.06 kcal/mol for 423 energies.
McMillan–Mayer theory for solvent effects in inhomogeneous systems: Calculation of interaction pressure in aqueous electrical double layers
Authors: Kjellander Roland , Lyubartsev Alexander P. , Marčelja Stjepan
Published: July 2002

We demonstrate how to use the McMillan–Mayer theory to include solvent effects in effective solute–solute interactions for inhomogeneous systems, extending a recent derivation [S. Marčelja, Langmuir 16, 6081 (2000)] for symmetric planar double layers to the general case. In the exact treatment, the many-body potential of mean force between the solute molecules can be evaluated for an inhomogeneous reference system in equilibrium with pure bulk solvent. The reference system contains only solvent and a finite number, n, of fixed solute molecules and it has an external potential that in some cases is different from that of the original system. It is discussed how the n-body potential of mean force between the ions for the relevant cases of large n values can be approximated by a sum of effective singlet and pair interactions evaluated in the presence of, on average, all n ions, i.e., at finite concentration. In examples considered in this work we use effective interionic pair potentials evaluated from bulk electrolyte calculations at finite electrolyte concentrations. We calculate the contribution to the double layer interaction pressure arising from the interaction between ions dissolved in aqueous electrolyte. In cases of moderate or high surface charge, calculations show several new effects. At small surface separations one finds attractive and then strongly repulsive contributions. For surface charge density around one negative charge per 70 Å2 the full results for pressures resemble “secondary hydration force” measured in classical experiments in 1980s. When there is a tendency for ions to adsorb at the surfaces there is a marked change in behavior. The force is then oscillatory, reminiscent of results obtained with the surface force apparatus at low electrolyte concentration.
High-resolution electron spin resonance spectroscopy of ethyl radicals in solid parahydrogen
Authors: Kumada Takayuki , Kumagai Jun , Miyazaki Tetsuo
Published: July 2002

High-resolution electron spin resonance (ESR) spectrum of ethyl radicals isolated in solid parahydrogen (>99.5 mol %) has been measured in the temperature range between 3.1 and 6.7 K. This study was carried out by UV photolysis of ethyl iodide doped in solid parahydrogen. Although ESR linewidth of the spectrum is broadened due to anisotropic terms of hyperfine interactions, the linewidth measured in parahydrogen remains comparable to that of the isotropic spectrum measured in liquid ethane [R. W. Fessenden, J. Chem. Phys. 37, 747 (1962)]. Small splittings were found in the resolved ESR signals and assigned as ethyl radicals with A(A′,A″) and E(E′,E″) symmetries, respectively. The ratio in the ESR intensities is not proportional to that of the degeneracy of each symmetry but varies as a function of temperature. We measured the signal intensities as a function of temperature and determined the rotational energy level splitting of methyl groups between the lowest with the A(A′,A″) symmetry and the second-lowest with the E(E′,E″) symmetry in solid parahydrogen to be 5.3±0.7 K. The level splitting is close to that measured by infrared-absorption study in gas phase and theoretical calculation, whereas the splitting measured in conventional matrices have been much smaller. Intermolecular interaction between guest radicals and surroundings in solid parahydrogen are found to be much smaller than those in other conventional matrices giving less shifted spectral parameters.
Field emission property of highly ordered monodispersed carbon nanotube arrays
Authors: Yuan Z. H. , Huang H. , Dang H. Y. , Cao J. E. , Hu B. H. , Fan S. S.
Published: July 2002

A template-synthesis technique based on self-ordered hexagonal nanopore alumina templates was applied to synthesize highly ordered carbon nanotube arrays via thermal decomposition of ethylene with iron as the catalyst electrodeposited into the bottom of the pores. The resultant carbon nanotubes are monodispersed and have open ends with uniform sizes. It was found that the insulating oxide layer lying between the carbon nanotubes and the underlying aluminum greatly obstructs the electron emission from the carbon nanotube arrays. After the oxide layer was removed, the arrays exhibited excellent field emission properties with a threshold electric field of ∼2.8 V/μm and emission current density of 0.08 mA/cm2 at 3.6 V/μm.
An integral equation study of percolation in systems of flexible and rigid macromolecules
Authors: Wang Xiaoling , Chatterjee Avik P.
Published: July 2002

An earlier integral equation approach to percolation in macromolecular fluids is extended to include attractive interactions between the sites representing flexible polymers. An analytic treatment based on the threadlike chain model shows that the percolation threshold near the theta temperature is closely related to the usual semidilute overlap concentration, and is significantly below the value found under athermal conditions. These results are consistent with the findings of numerical calculations on tangent-bead chain models. The location of the static percolation threshold in the density–concentration plane with relation to the liquid–vapor spinodal boundary is investigated. Numerically obtained results for the percolation thresholds for both flexible and rigid athermal, finite thickness, macromolecules are presented, which are consistent with prior findings based on the analytically tractable threadlike chain model.
Nuclear quadrupole moments for Al27 and Ga69 derived from four-component molecular coupled cluster calculations
Authors: Pernpointner Markus , Visscher Lucas
Published: July 2002

In this work we investigate different approaches for calculating electric field gradients in order to provide accurate theoretical values for the nuclear quadrupole moments (NQM) for aluminum and gallium. Electron correlation is included in a fully four-component framework at the CCSD(T) level. The resulting NQM for Al27 (146.0±0.4 mb) is in good agreement with earlier work, while the value for Ga69 (171±2 mb) is higher than suggested on basis of previous molecular calculations.
Superstrings and topological strings at large N
Authors: Vafa Cumrun
Published: July 2002

We embed the large N Chern–Simons/topological string duality in ordinary superstrings. This corresponds to a large N duality between generalized gauge systems with N=1 supersymmetry in four dimensions and superstrings propagating on noncompact Calabi–Yau manifolds with certain fluxes turned on. We also show that in a particular limit of the N=1 gauge theory system, certain superpotential terms in the N=1 system (including deformations if spacetime is noncommutative) are captured to all orders in 1/N by the amplitudes of noncritical bosonic strings propagating on a circle with self-dual radius. We also consider D-brane/anti-D-brane system wrapped over vanishing cycles of compact Calabi–Yau manifolds and argue that at large N they induce a shift in the background to a topologically distinct Calabi–Yau, which we identify as the ground state system of the brane/anti-brane system.
Direct observation of power-law behavior in the asymptotic relaxation to equilibrium of a reversible bimolecular reaction
Authors: Pines Dina , Pines Ehud
Published: July 2002

We report the first direct observation of power-law relaxation to equilibrium of the diffusion influenced reversible reaction, AB⇆A+B (B≫A). Our experimental findings confirm the predicted t−3/2 decay law relaxation of AB population a long time after the photoinitiation of the reaction. This t−3/2 relaxation of the excess-over-equilibrium population is similar to that found in diffusion influenced geminate recombination reactions.
Confined thin film diblock copolymer in the presence of an electric field
Authors: Ashok B. , Muthukumar M. , Russell T. P.
Published: July 2002

We have studied theoretically the effect of an external electric field on the equilibrium properties of lamellar and cylindrical assembly in confined diblock copolymers. The critical electric field at which a vertical morphology of lamellae is preferred over a horizontal configuration is found to have only a weak explicit dependence on the plate separation D when D is larger than a few bulk lamellar periods. In the case of the cylindrical morphology, the situation is found to be more complex, with the dimensions of the plates also playing a crucial factor. In both morphologies, the difference between the values of the polymer-plate interfacial tension for each of the two components is found to be a determining factor for the critical field.
Changes of the local pore space structure quantified in heterogeneous porous media by H1 magnetic resonance relaxation tomography
Authors: Borgia G. C. , Bortolotti V. , Fantazzini P.
Published: July 2002

Magnetic resonance imaging and relaxation analysis are combined in a spatially resolved technique (relaxation tomography), which is able to quantify the parameters connected to the local structure in the internal regions of a porous material saturated by water, giving information on the pore space structure beyond the nominal instrumental resolution. Voxel-by-voxel longitudinal (T1) and transverse (T2) relaxation curves are acquired in order to obtain T1, T2 and S(0) maps, where S(0) is the extrapolation to zero time of the total equilibrium magnetization corrected for T2 decay. The proposed method permits evaluation of the porosity (ratio of pore space to total volume), at different length scales, from the sample to the voxel, not all achievable by traditional methods. More striking is its ability to describe how porosity is shared among different classes of surface-to-volume ratios of diffusion cells (the regions that the individual water molecules, starting at their particular positions, can experience by diffusion before relaxing). This is a consequence of the fact that relaxation times of water confined in a porous material can, under favorable circumstances, distinguish regions with the same local porosity but with different pore sizes and connections. So, parameters can be introduced, such as the microporosity fraction, defined as the fraction of the “micropore” volume with respect to the total pore volume, and several voxel average porosities, defined as the average porosities of the voxels characterized by particular classes of diffusion cells. Moreover, the imaging methods enable us to get all this information in a user-defined region of interest. The method has been applied to quantify changes in the structure of carbonate cores with wide distributions of pore sizes induced by repeated cycles of freezing and heating of the sample. With freezing, the microporosity fraction decreases significantly; the voxel average porosity of voxels with T1 shorter than for free water tend to decrease; and the distributions of porosity as functions of T1 show a trend, with much more signal with the T1 of free water, in accordance with the picture suggesting large vugs breaking, with fractures contributing to homogenizing the structure of the pore space and favoring coupling between neighboring pores.
The multi-coefficient correlated quantum mechanical calculations for structures, energies, and harmonic frequencies of HF and H2O dimers
Authors: Park Chea-Yong , Kim Yangsoo , Kim Yongho
Published: July 2002

The accurate determination of interaction energies and structures of hydrogen-bonded complexes has been an important issue of ab initio theory for a long time. Extensive theoretical studies have been performed to correct electronic correlation and the basis set truncation error (BSTE) that is a consequence of the incompleteness of the one-electron basis set. We have used recently developed multilevel methods to calculate the structures, harmonic frequencies and the dissociation energies of the HF and water dimers. The seven multilevel methods, namely SAC-MP2/cc-pVDZ, SAC-MP4SDQ/cc-pVDZ, MC-QCISD, MCCM-CO-MP2, MCCM-UT-MP4SDQ, MCCM-UT-CCSD, and MCG3, have been tested. The MC-QCISD, MCCM-UT-MP4SDQ, MCCM-UT-CCSD, and MCG3 method predict the structures and harmonic frequencies of HF and H2O dimers reasonably well compared with experiments and high level ab initio results. Particularly, the MCCM-UT-MP4SDQ and MCCM-UT-CCSD methods show very good agreement of both the interfragment distances and the dissociation energies with experiments using significantly less cost than the high-level ab initio calculations.
The 3ν1 overtone band of trans-nitrous acid: Rotational and perturbation analysis and absolute intensity
Authors: Witonsky Scott K. , Canagaratna Manjula R. , Coy Stephen L. , Steinfeld Jeffrey I. , Field Robert W. , Kachanov Alexandre A.
Published: July 2002

High-resolution (0.013 cm−1) spectra of the second overtone of the OH stretch of trans-nitrous acid were recorded between 10 230 and 10 350 cm−1. The spectra were analyzed to yield a complete set of rotational parameters, including the A, B, and C rotational constants and the 5 quartic distortion constants. Two groups of anharmonic perturbations were observed and analyzed, and for one of these groups, the spectra were fit with a second model to extract an improved set of parameters that took into account these interactions. Much less information could be obtained from our spectra about the second group of perturbations, except that the same dark vibrational state was probably not responsible for both groups of perturbations. The anharmonic coupling matrix element between the 3ν1 “bright” state and one “dark” vibrational state was found to be 0.0338(31) cm−1, and possible assignments for the “dark” level are discussed. In addition, we report the first measurement of the absolute intensity of the 3ν1 overtone band, which was found to be 2.1±0.6×10−21 cm2 molecule−1 cm−1. Our findings are discussed in the context of the contribution of overtone-mediated processes to OH radical production in the upper atmosphere.
Selected error sources in resistance measurements on superconductors
Authors: Garcı́a-Vázquez Valentı́n , Pérez-Amaro Neftalı́ , Canizo-Cabrera A. , Cumplido-Espı́ndola B. , Martı́nez-Hernández R. , Abarca-Ramı́rez M. A.
Published: July 2002

In order to investigate the causes that produce some of the unwanted effects observed in the resistance versus temperature profiles, a variety of sources of error for resistance measurements in superconductors, using a standard four-probe configuration, have been studied. A piece of superconducting Y1Ba2Cu3O7−x ceramic material has been used as the test sample, and the resulting effects in both accuracy and precision in its temperature dependent resistance are reported here. Studied measurement error sources include thermal emf’s, temperature sweep rates, Faraday currents, electrical-contact failures at the sample’s surface, thermal contractions at mechanically attached instrumental wires, external electromagnetic fields, and slow sampling rates during data acquisition. Details of the experimental setup and its measurement error function are also given.
Three-phase fractionation of polydisperse fluids
Authors: Bellier-Castella L. , Baus M. , Xu H.
Published: July 2002

It is shown that the van der Waals free energy of polydisperse fluids, as introduced previously [L. Bellier-Castella, H. Xu, and M. Baus, J. Chem. Phys. 113, 8337 (2000)], predicts that for certain thermodynamic states (e.g., low temperatures and large polydispersities) the ordinary two-phase coexistences become metastable relative to a fractionation of the system into three phases, reducing thereby the polydispersity of each of the coexisting phases.
The relation between dynamics of wetting and nonwetting liquids
Authors: Neogi P.
Published: July 2002

The dynamics of wetting liquids has been related to the dynamics of nonwetting liquids. It appears that the response of wetting liquids form an envelop to the family of responses for liquids with nonzero contact angles. These are singular solutions, and the mathematics of singular solutions has been used to obtain results for the generally less accessible case of wetting liquids from the more accessible nonwetting liquids. Three cases have been analyzed. In the first, the well known Voinov–Hoffman–Tanner rule is derived for a spreading drop of a wetting liquid starting with a known expression for the nonwetting case. In the second, the theoretical results for the blow-off experiments for the nonwetting case have been used to obtain the results for the wetting case, allowing a successful comparison between the theory and experiments. In the third, the solution for drop spreading by Hocking and Rivers [J. Fluid Mech. 121, 425 (1982)] for nonwetting liquids where the lubrication theory approximation is not valid is used to derive the solution for the wetting. The present work appears to be the first in determining the connection between the dynamics of wetting and nonwetting liquids of any kind.
Micromechanics simulation of spontaneous polarization in ferroelectric crystals
Authors: Li Wenfang , Weng G. J.
Published: July 2002

Based on a modified equivalent inclusion method and a thermodynamic theory, a micromechanics model is developed to predict the evolution of internal stress and the effects of microstructural features (i.e., new domain concentration and porosity) and the external hydrostatic pressure on the transition temperature for single crystal ferroelectric ceramics upon cooling. During the cubic-to-tetragonal transition the material is a composite, with an internal stress due to phase transformation. Consideration of the thermodynamic driving force and resistance force leads to a kinetic equation that provides the volume concentration of new domains at a given level of temperature and applied stress. This new domain concentration results in an increase of overall electric polarization. Numerical calculations using the properties of BaTiO3 indicate that internal stress in the parent cubic phase continues to develop during spontaneous polarization, and that increased porosity greatly facilitates the transition process. Hydrostatic pressure provides an additional contribution to the Gibbs free energy of the system and also can increase the transition temperature. Its effect generally follows the Clausius-Clapeyron equation, but weak nonlinearity is found at very high pressure. This departure is attributed to the change in heat capacity during the cubic–tetragonal transition. A comparison with some observed data for the evolution of overall polarization of a BaTiO3 crystal serves to support the rationality of the model.
Optoelectronic and structural properties of Er-doped sputter-deposited gallium–arsenic–nitrogen films
Authors: Zanatta A. R. , Ribeiro C. T. M. , Freire F. L.
Published: July 2002

Erbium-doped gallium–arsenic–nitrogen thin films were prepared by cosputtering a crystalline GaAs target partially covered with small pieces of metallic erbium in an Ar+N2 atmosphere. The films were deposited near room temperature and under increasing partial pressures of nitrogen P(N2). The investigation of the films included ion beam analysis, optical spectroscopy in the infrared–visible–ultraviolet energy ranges, Raman scattering, and photoluminescence measurements. According to the experimental results, all films present an amorphous structure and nitrogen contents that scale with P(N2). Increasing amounts of nitrogen induce the widening of the optical band gap and a systematic redshift of the Raman signal of the films considered. The intensity of the Er-related light emission at ∼0.8 eV also increases at higher nitrogen contents. A detailed study of the absorption bands in the infrared region allowed the identification of different vibration modes involving gallium, arsenic, and nitrogen atoms. The combined analysis of the compositional data and of the infrared absorption bands provided a constant of proportionality between the nitrogen content in the films and the integrated absorption due to Ga–N bonds. Where applicable, comparisons between the optoelectronic and structural characteristics of amorphous and crystalline gallium–arsenic–nitrogen compounds were made.
Ion source improvement by electron injection from a ferroelectric cathode
Authors: Boscolo I. , Cialdi S. , Valentini M. , Gammino S. , Ciavola G. , Celona L. , Marletta S. , Riege H. , Handerek J.
Published: July 2002

With increasing rf power, the electron concentration in the plasma of electron cyclotron resonance (ECR) ion sources is decreasing in comparison to the ion concentration, so that the plasma is charging up positively. When reviewing the basic performance requirements of ECR sources it becomes evident that the direct injection of electrons into the ECR plasma is increasing the electron charge density and the ion current yield. Ferroelectric ceramics was used as very robust, electron emitting cathode material under the heavy-duty conditions inside the plasma chamber of an ECR ion source. The electron emission from the ferroelectric cathode is turned on by a high repetition-rate bipolar pulse of ±1.2 to 1.6 kV amplitude to the electrodes deposited on both sides of the disk-shaped cathode. Lead–barium–zirconium-titanate (30/70/30) cathodes doped with 2 mol % Bi2O3 were installed and tested in the Ar–ion plasma of the ECR ion source CAESAR at INFN-LNS, Catania. The aim was to visibly increase the yield of the ion output current by electron injection. The plasma was heated by rf in continuous mode, while the ferroelectric cathode was pulsed at a repetition rate of 50 to 250 Hz. Pulsed electron injection at 250 Hz led to a noticeable increase of 30% of the Ar8+ output current at a rf power level of 400 W. In addition, magnetohydrodynamic instabilities were damped during and after electron injection. These results strongly encourage further research and development on the application of ferroelectric cathodes for increasing the ion current yield from hot plasma ion sources.
Time-resolved two-photon photoemission spectroscopy of image potential states: A phenomenological approach
Authors: Jørgensen Solvejg , Ratner Mark A. , Mikkelsen Kurt V.
Published: July 2002

The dynamics of the image potential states n=3 and n=4 at the Cu(100) surface are studied. We solve the Liouville–von Neumann equation for an open system and treat the dissipation within the Lindblad formalism. We compute the two-photon photoemission spectrum (2PPE) using first order perturbation theory and study the populations of the image potential states and the two-photon photoemission spectrum as functions of the lifetimes and dephasings of the image potential states. The computed two-photon photoemission spectrum is compared with the measured spectrum obtained by Höfer et al. [Appl. Phys. B 68, 383 (1999); Science 277, 1480 (1997)].
Measuring the predissociation and rotational autoionization of the vibrationless Rydberg series in ammonia
Authors: Warntjes J. B. M. , Noordam L. D.
Published: July 2002

A novel method is presented to retrieve the average predissociation rate and the average rotational autoionization rate of a molecule by measuring the field ionization spectrum only. Both an intuitive picture and a quantitative description are derived for the competition between the three decay channels, field ionization, rotational autoionization, and predissociation. Field ionization spectra of various rotational states of ammonia are presented and fitted. From the fits we found that the average predissociation rate of the Rydberg series converging to the vibrationless N+=3 ionic state is about equal to the average rotational autoionization rate to the Rydberg series converging to the vibrationless N+=2 ionic state, namely 1×1015/n3 s−1.