Long-range Coulomb Forces Research Articles

Atomistic Modeling of Thermal Conductivity of Epoxy Nanotube Composites

The Green–Kubo method was used to investigate the thermal conductivity as a function of temperature for epoxy/single wall carbon nanotube (SWNT) nanocomposites. An epoxy network of DGEBA-DDS was built using the ‘dendrimer’ growth approach, and conductivity was computed by taking into account long-range Coulombic forces via a k-space approach. Thermal conductivity was calculated in the direction perpendicular to, and along the SWNT axis for functionalized and pristine SWNT/epoxy nanocomposites. Inefficient phonon transport at the ends of nanotubes is an important factor in the thermal conductivity of the nanocomposites, and for this reason discontinuous nanotubes were modeled in addition to long nanotubes. The thermal conductivity of the long, pristine SWNT/epoxy system is equivalent to that of an isolated SWNT along its axis, but there was a 27% reduction perpendicular to the nanotube axis. The functionalized, long SWNT/epoxy system had a very large increase in thermal conductivity along the nanotube axis (~700%), as well as the directions perpendicular to the nanotube (64%). The discontinuous nanotubes displayed an increased thermal conductivity along the SWNT axis compared to neat epoxy (103–115% for the pristine SWNT/epoxy, and 91–103% for functionalized SWNT/epoxy system). The functionalized system also showed a 42% improvement perpendicular to the nanotube, while the pristine SWNT/epoxy system had no improvement over epoxy. The thermal conductivity tensor is averaged over all possible orientations to see the effects of randomly orientated nanotubes, and allow for experimental comparison. Excellent agreement is seen for the discontinuous, pristine SWNT/epoxy nanocomposite. These simulations demonstrate there exists a threshold of the SWNT length where the best improvement for a composite system with randomly oriented nanotubes would transition from pristine SWNTs to functionalized SWNTs.

An anion effect on the separation of AgI-containing melts using sound waves

Sound velocities in molten ((LiF+AgI)) and ((LiBr+AgI)) mixtures have been measured to investigate the relationship between the sound velocity and the temperature and the role of the anion in the (liquid+liquid) phase transition. Our results show that the ((LiBr+AgI)) system is biphasic between the melting point and T=984K and becomes monophasic above this temperature. We show that the upper consolute critical temperature for the AgI-containing melts increases with decreasing anion size in the series F−>Cl−>Br−. The ((LiF+AgI)) melt remains biphasic at all temperatures investigated up to T=1218K. The temperature coefficients for the sound velocities in the upper and lower phases of the ((LiBr+AgI)) system have opposite signs because of the superposition of the temperature and composition factors. The difference between the magnitudes of the velocities for the coexisting phases decreases exponentially with increasing temperature and is described by a critical exponent of 0.85 for the ((LiBr+AgI)) melt near the critical temperature. This value is 15% less than that found for alkali halide melts, in which long-range Coulomb forces between ions prevail. This difference may result from the fact that silver halides are intermediate between the typical ionic salts and the fully covalently bonded ones.

Sound Velocities for Dissolving AgI + LiCl Melts

The ultrasonic velocities of a molten stratified mixture at a molar ratio of 0.3AgI and 0.7LiCl (the composition corresponding to the top of the miscibility gap) were measured along the saturation line over a wide temperature range using the pulse method to establish the characteristics of mixing salts comprising different chemical bonds. It is shown that the coefficients of the temperature dependences for the sound velocities in the coexisting phases have opposite signs as a result of the superposition of the temperature and concentration factors. It is found that the difference, Δu, between the magnitudes of the sound velocities for the coexisting phases decreases with increasing temperature and becomes zero at 1250 K. This temperature corresponds to the critical phase transition point, Tc. The temperature dependence of the sound velocity difference, Δu, for the system studied is described by the equation Δu ≈ (Tc – T)β, where β = 0.900, which is close to the value 0.896 obtained for the previously investigated AgI+NaCl melt but is less than the value found for alkali halide melts (β = 1.02), in which long-range Coulomb forces between ions prevail. The results are discussed in terms of the peculiarity of the chemical bond in silver iodide.

Giant dielectric permittivity and magneto-capacitance effects in low doped manganites

The effect of giant dielectric permittivity due to phase separation accompanied by charged inhomogeneities in low-doped manganites is discussed. The effect appears in the vicinity of the second-order magnetic phase transition and is caused by long-range Coulomb forces. The long-range Coulomb interaction is responsible for the formation of inhomogeneous charged states and determines their characteristic length scales. We derive the phase diagram of the inhomogeneous charged states in the framework of the phenomenological theory of phase transitions. The large value of the static dielectric function reduces the characteristic value of the Coulomb energy of the inhomogeneous state and makes the appearance of the magnetoelectric effect possible. We discuss the formation of a state with giant dielectric permittivity and magneto-capacitance effects in that case.

Nuclear Structure of the Heaviest Elements – Investigated at SHIP-GSI

The quest for the heaviest nuclei that can exist is a basic topic in natural science as their stability is characterized by a delicate interplay of short range nuclear forces acting between the nucleons (protons and neutrons) and long-range Coulomb forces acting solely between charged particles, i.e. the protons. As the stability of a nucleus is strongly correlated to its structure, understanding the nuclear structure of heaviest nuclei is presently a main challenge of experimental and theoretical investigations concerning the field of Superheavy Elements. At the velocity filter SHIP at GSI Darmstadt an extensive program on nuclear structure investigations has been started about a decade ago. The project covered both as well systematic investigations of single particle levels in odd-mass isotopes populated by α-decay as investigation of two- or fourquasi-particle states forming K isomers and was supplemented by direct mass measurements at SHIPTRAP and investigation of spontaneous fission properties. Recent experimental studies allowed to extend the systematics of low lying levels in N = 151 and N = 153 up to 255Rf and 259Sg, investigation of possible relations between nuclear structure and fission properties of odd-mass nuclei and investigation of shell strengths at N = 152 and towards N = 162.

“Magic” Surface Clustering of Borazines Driven by Repulsive Intermolecular Forces

It's a kind of magic: Hydroxy pentaaryl borazine molecules self-assemble into small clusters (see structure) on Cu(111) surfaces, whereas with symmetric hexaaryl borazine molecules large islands are obtained. Simulations indicate that the observed "magic" cluster sizes result from long-range repulsive Coulomb forces arising from the deprotonation of the B-OH groups of the hydroxy pentaaryl borazine.

Calculation of proton-3He elastic scattering between 7 and 35 MeV

Background: Theoretical calculations of the four-particle scattering above the four-cluster breakup threshold are technically very difficult due to nontrivial singularities or boundary conditions. Further complications arise when the long-range Coulomb force is present.Purpose: We aim at calculating proton-${}^{3}$He elastic scattering observables above three- and four-cluster breakup threshold.Methods: We employ Alt, Grassberger, and Sandhas (AGS) equations for the four-nucleon transition operators and solve them in the momentum-space framework using the complex-energy method whose accuracy and practical applicability is improved by a special integration method.Results: Using realistic nuclear interaction models we obtain fully converged results for the proton-${}^{3}$He elastic scattering. The differential cross section, proton and ${}^{3}$He analyzing powers, spin correlation and spin transfer coefficients are calculated at proton energies ranging from 7 to 35 MeV. Effective three- and four-nucleon forces are included via the explicit excitation of a nucleon to a $\ensuremath{\Delta}$ isobar.Conclusions: Realistic proton-${}^{3}$He scattering calculations above the four-nucleon breakup threshold are feasible. There is quite good agreement between the theoretical predictions and experimental data for the proton-${}^{3}$He scattering in the considered energy regime. The most remarkable disagreements are the peak of the proton analyzing power at lower energies and the minimum of the differential cross section at higher energies. Inclusion of the $\ensuremath{\Delta}$ isobar reduces the latter discrepancy.

Ultrafast inter-ionic charge transfer of transition-metal complexes mapped by femtosecond X-ray powder diffraction.

The transient electronic and molecular structure arising from photoinduced charge transfer in transition metal complexes is studied by X-ray powder diffraction with a 100 fs temporal and atomic spatial resolution. Crystals containing a dense array of Fe(II)-tris(bipyridine) ([Fe(bpy)3](2 +)) complexes and their [Formula: see text] counterions display pronounced changes of electron density that occur within the first 100 fs after two-photon excitation of a small fraction of the [Fe(bpy)3](2 +) complexes. Transient electron density maps derived from the diffraction data reveal a transfer of electronic charge from the Fe atoms and-so far unknown-from the [Formula: see text] counterions to the bipyridine units. Such charge transfer (CT) is connected with changes of the inter-ionic and the Fe-bipyridine distances. An analysis of the electron density maps demonstrates the many-body character of charge transfer which affects approximately 30 complexes around a directly photoexcited one. The many-body behavior is governed by the long-range Coulomb forces in the ionic crystals and described by the concept of electronic polarons.

Densities of a dissolving mixture of molten (AgI + NaCl)

The densities of a molten mixture of (0.5AgI+0.5NaCl) were measured along the saturation line over a wide temperature range by the hydrostatic weight method to establish the peculiarities of the mixing of salts with different chemical bonds. We showed that the difference between the magnitudes of the densities for the coexisting phases decreases with increasing temperature and becomes equal to zero at 1063.5K. This temperature corresponds to the critical phase transition point, Tc. The temperature dependence of the difference in densities, Δρ, is described by equation Δρ/ρc≈Tc-TTcβ, where ρc is the density at Tc. The index β=0.476 occurs at a lower value than that found for alkali halide melts (β=0.52) where long-range Coulombic forces between ions prevail.

Ultrasonic velocity for an equimolar mixture of molten AgI and NaCl in the biphasic region

The ultrasonic velocities of a molten stratifying mixture composed of 0.5 AgI and 0.5 NaCl (the composition corresponding to the top of the miscibility gap) were measured along the saturation line for a wide temperature range via the pulse method to establish the characteristics of mixing salts with different chemical bonds. We show that the difference, Δu, between the magnitudes of the sound velocities for the coexisting phases decreases with increasing temperature and becomes zero at 1064K. This temperature corresponds to the critical phase transition point, Tc. The temperature dependence of the sound velocity difference, Δu, is described by the equation Δu≈(Tc−T)Θ, where Θ=0.896, which is less than that found for alkali halide melts (Θ=1.02), in which long-range Coulomb forces between ions prevail. The results are discussed in terms of the peculiarity of the chemical bond in silver iodide.

Long-range transfer of electron–phonon coupling in oxide superlattices

The electron-phonon interaction is of central importance for the electrical and thermal properties of solids, and its influence on superconductivity, colossal magnetoresistance and other many-body phenomena in correlated-electron materials is the subject of intense research at present. However, the non-local nature of the interactions between valence electrons and lattice ions, often compounded by a plethora of vibrational modes, presents formidable challenges for attempts to experimentally control and theoretically describe the physical properties of complex materials. Here we report a Raman scattering study of the lattice dynamics in superlattices of the high-temperature superconductor YBa(2)Cu(3)O(7) (YBCO) and the colossal-magnetoresistance compound La(2/3)Ca(1/3)MnO(3) that suggests a new approach to this problem. We find that a rotational mode of the MnO(6) octahedra in La(2/3)Ca(1/3)MnO(3) experiences pronounced superconductivity-induced line-shape anomalies, which scale linearly with the thickness of the YBCO layers over a remarkably long range of several tens of nanometres. The transfer of the electron-phonon coupling between superlattice layers can be understood as a consequence of long-range Coulomb forces in conjunction with an orbital reconstruction at the interface. The superlattice geometry thus provides new opportunities for controlled modification of the electron-phonon interaction in complex materials.

An electromechanical atomic-scale finite element method for simulating evolutions of ferroelectric nanodomains

In this paper, a novel atomic-level computational method of perovskite ferroelectrics is established by combining the shell model and atomic-scale finite element method (AFEM). Its applicability is carefully testified for both bulk and nanoscale ferroelectrics, by comparing the calculated structural parameters and polarizations with the molecular dynamics (MD) simulations, first-principles calculations and experiment results. A comparison of the CPU time demonstrates that the developed method has a computational speed about 10 times over that of shell model MD method and its advantage becomes more evident as the computational scale becomes larger. Moreover, two effective calculation skills of long-range Coulomb force are introduced which can further enhance the computational efficiency by about 10 times. Using the developed atomic-level method, we investigate the various patterns of nanoscale domain structures in BaTiO3 and their evolutions under electrical loadings. A domain structure with coexistence of vortex and streamline polarization patterns is revealed. Furthermore, the simulations of domain evolutions not only reproduce well the two-step 90° domain switching process observed in experiments on single domain under an anti-parallel electric field, but also provide a full evolution diagram among different domain patterns under various electric fields. A quantitative analysis indicates that the direction-dependent coercive field of multi-domain structure can be well described by that of single domain based on a simple analytical model. This study on domain patterns and evolutions may help us understand the behaviors of ferroelectrics from the atomic level.

Thermoelastic parameter αKT of sodium chloride at high pressure and high temperature

Two different potential models to the molecular dynamics (MD) simulations have been applied to investigate the thermoelastic parameter αKT of sodium chloride (NaCl) under high pressure and high temperature. The first one is the shell model (SM) potential that due to the short-range interaction when pairs of ions are moved together as is the case in that polarization of a crystal due to the motion of the positive and negative ions, and the second one is the two-body rigid-ion Born–Mayer–Huggins–Fumi–Tosi (BMHFT) potential with full treatment of long-range Coulomb forces. Particular attention is paid to the comparison of the SM- and BMHFT-MD simulations with the Debye model for the first time, and this model combines with ab initio calculations within local density approximation (LDA) and generalized gradient approximation (GGA) using ultrasoft pseudopotentials and a plane-wave basis in the framework of density functional theory (DFT), and it takes into account the phononic effects within the quasi-harmonic approximation. Note that the MD calculated volumes using SM model is somewhat larger than both the DFT and experimental volumes despite not considering the temperature effect. Compared with SM potential, the MD simulated 300K isotherm of NaCl with BMHFT potential is very successful in reproducing accurately the measured volumes and the GGA calculated volumes. Generally, it is found that there exist minor differences between the LDA and GGA computed the thermoelastic parameter αKT of NaCl, with both average results giving good agreement with SM-MD simulations. At an extended pressure and temperature ranges, the variation of thermoelastic parameter αKT which play a central role in the formulation of approximate equations of state has also been predicted. The properties of NaCl are summarized in the pressure range of 0–300kbar and the temperature up to 2000K.

Protein interactions with nanoporous sol–gel derived bioactive glasses

Sol–gel derived bioactive glasses are excellent candidates for bone regenerative implant materials as they bond with bone, stimulate bone growth and degrade in the body. Their interactions with proteins are critical to understanding their performance after implantation. This study focuses on the interactions between fibrinogen and sol–gel glass particles of the 70S30C (70 mol.% SiO 2, 30 mol.% CaO composition). Sol–gel silica and melt-derived Bioglass® were also used for comparison. Fibrinogen penetration into the nanoporous glasses was observed by live tracking the fluorescent-labelled fibrinogen with confocal microscopy. The effect of pore size on protein penetration was investigated. Nanoporous networks with modal pore diameters larger than 6 nm were accessible to fibrinogen. When the modal nanopore diameter was decreased to 2 nm or less, the penetration of fibrinogen was inhibited. The surface properties of the glasses, which can be modulated by media pH, glass composition and final stabilisation temperature in the sol–gel process, have effects on fibrinogen adsorption via long-range Coulombic forces before the adsorption and via short-range interactions such as hydrogen bonding after the adsorption.

Towards a petascale tree code: Scaling and efficiency of the PEPC library

The highly scalable parallel tree code PEPC for rapid computation of long-range (1/ r) Coulomb forces is presented. It can be used as a library for applications involving electrostatics or Newtonian gravity in 3D. The code is based on the hashed oct-tree algorithm, in which particle coordinates are projected onto a space-filling curve prior to sorting and construction of multipole moments. However, standard particle sorting techniques can ultimately limit the scalability of such algorithms for thousands of cores, a bottleneck which can be alleviated by a recursive sort scheme specially adapted to the Morton curve. More serious limitations of the original locally essential tree concept of Salmon and Warren, which ultimately lead to a failure in memory scaling, are identified and analyzed rigorously. Benchmarks for the code on the IBM Blue Gene/P Jugene are presented which demonstrate scaling for multi-million particle systems on up to 8192 cores.

Two-proton radioactivity and three-body decay. V. Improved momentum distributions

Nowadays quantum-mechanical theory allows one to reliably calculate the processes of $2p$ radioactivity (true three-body decays) and the corresponding energy and angular correlations up to distances of the order of ${10}^{3}$ fm. However, the precision of modern experiments has now become sufficient to indicate some deficiency of the predicted theoretical distributions. In this paper we discuss extrapolation along the classical trajectories as a method to improve the convergence of the theoretical energy and angular correlations at very large distances (of the order of atomic distances), where only long-range Coulomb forces are still operating. The precision of this approach is demonstrated using the ``exactly'' solvable semianalytical models with simplified three-body Hamiltonians. It is also demonstrated that for heavy $2p$ emitters, the $2p$ decay momentum distributions can be sensitive to the effect of screening by atomic electrons. We compare theoretical results with available experimental data.

Three-nucleon force effects in proton-deuteron scattering

Possible three-nucleon forces that may be considered in addition to a widely accepted two-pion exchange three-nucleon force are studied to improve the fit to experimental data of proton-deuteron scattering. Calculations are performed with including effects of long-range Coulomb force by a recently developed method to solve the Faddeev equation in coordinate space. Two different kinds of three-nucleon potentials: a phenomenological potential and one due to the exchange of pion and sigma bosons, are presented and compared with experimental data of elastic differential cross sections, a tensor analyzing power, and a nucleon-to-nucleon polarization transfer coefficient.

Exploring the charge-ordering transition in [formula omitted] via thermal expansion measurements

We report results of high-resolution measurements of the c * ‐axis expansivity ( α c * ) at the charge-ordering (CO) transition for the quasi-1D (TMTTF) 2 X compounds with X = SbF 6 and Br and make a comparison with previous results for the X = PF 6 and AsF 6 salts. For X = SbF 6 , due to the screening of the long-range Coulomb forces, a sharp λ -type anomaly is observed at T CO , which contrasts with the step-like mean-field anomaly at T CO for PF 6 and AsF 6 , where CO occurs in the Mott–Hubbard charge-localized regime. For the latter two salts, a negative contribution to α c * is observed above T CO . This feature is assigned to the anions’ rigid-unit modes, which become inactive for T < T CO . Our α c * results for the X = Br salt, where such rigid-unit modes are absent, reveal no traces of such negative contribution, confirming the model based on the anions’ rigid-unit modes for the X = PF 6 and AsF 6 salts.

Mode coupling and evolution in broken-symmetry plasmas

The control of nonlinear processes and possible transitions to chaos in systems of interacting particles is a fundamental physical problem. We propose a new nonuniform solid-state plasma system, produced by the optical injection of current in two-dimensional semiconductor structures, where this control can be achieved. Due to an injected current, the system symmetry is initially broken. The subsequent nonequilibrium dynamics is governed by the spatially varying long-range Coulomb forces and electron-hole collisions. As a result, inhomogeneities in the charge and velocity distributions should develop rapidly, and lead to previously unexpected experimental consequences. We suggest that the system eventually evolves into a behavior similar to chaos.

Precise set of tensor analyzing power T20 data for the deuteron-proton breakup at 130 MeV

High-precision tensor analyzing power T-20 data of the H-1(d, pp)n reaction at 130 MeV beam energy have been determined for 81 kinematical configurations. They are compared to theoretical predictions based on various approaches to describe the dynamics of the three-nucleon (3N) system. The calculations are performed using modern realistic nucleon-nucleon potentials combined with three-nucleon force (3NF) models or with an effective 3NF resulting from the explicit treatment of the Delta-isobar in coupled-channels (CC) calculations. Alternatively, the framework of chiral perturbation theory is used to generate consistent two-nucleon and three-nucleon potentials at the currently numerically attainable order. Results of the CC calculations with the Delta degrees of freedom and including long-range Coulomb force are also shown. In general all predictions are consistent with each other and describe the experimental T-20 results quite well. In a few configurations small inconsistencies between the data and the results of all approaches are observed. Predicted effects of the 3NF are not big and in most cases do not lead to an improved description of the data. The Coulomb force effects are also small in size and often opposite to the effects of TM99 3NF.