Short-range Effects Research Articles

Mechanisms of Radiation Damage and Properties of Nuclear Materials

AbstractRadiation damage effects in ceramics, e.g., nuclear waste forms, transmutation targets, and inert matrix fuels, may have important implications for the physical and chemical stability of these materials as the cumulative radiation dose increases over time. A key aspect of scientific research in this area is the ability to understand the fundamental damage mechanisms through the combination of experimental and atomistic modelling techniques. In this paper, we review some of the lessons learned from the significant body of data now available for pyrochlore-defect fluorite based materials, followed by an illustration of the advantages of working on simple compounds with well established interatomic potentials. We conclude the paper with a description of radiation damage processes in the LaxSr1-1.5xTiO3 defect perovskites, a system that includes phase transformations, short-range order effects, and complex defect behavior.

Nonuniform Electro-osmotic Flow on Charged Strips and Its Use in Particle Trapping

In this article, we investigate theoretically electro-osmotic flow set up by charged strips on an otherwise uncharged surface. Starting with a single-strip problem we demonstrate that for simple polynomial surface charge distributions several basic solutions can be derived in closed forms constituted by the analogous idea-flow solutions, which provide a more lucid way of revealing the flow features. These solutions reveal two types of flow topology: simple draining-in/pumping-out streaming and a pair of microvortices for symmetric and antisymmetric surface charge distributions, respectively. For an arbitrary surface charge distribution, more complicated flow structures can be found by the superposition of these basic solutions. We further extend the analysis to two uniformly charged strips and show how the flow characteristics vary with the strips' dimensions and surface zeta potentials. The far-field velocity behavior is also asymptotically identified and indicates that the hydrodynamic nature of the flow is typically long-range. An application to particle trapping with electro-osmotic vortices is also investigated theoretically for the first time. We show that in collaboration with short-range attraction effects the trapping can be facilitated by symmetric vortices with a converging stagnation point, but not by asymmetric vortices.

Self-consistent many-body perturbation theory in range-separated density-functional theory: A one-electron reduced-density-matrix-based formulation

In many cases, density-functional theory (DFT) with current standard approximate functionals offers a relatively accurate and computationally cheap description of the short-range dynamic electron correlation effects. However, in general, standard DFT does not treat the dispersion interaction effects adequately which, on the other hand, can be described by many-body perturbation theory (MBPT). It is therefore of interest to develop a hybrid model which combines the best of both the MBPT and DFT approaches. This can be achieved by splitting the two-electron interaction into long-range and short-range parts; the long-range part is then treated by MBPT and the short-range part by DFT. This work deals with the formulation of a general MBPT-DFT model (i.e., valid for any type of zeroth-order Hamiltonian) based on such a range separation. Applying the Rayleigh-Schr\"odinger formalism in this context, one finds that the generalized Bloch equation becomes self-consistent at each order of perturbation. This complication arises because the short-range part of the energy is a functional of the exact electron density, which is expanded in a perturbation series. In order to address this ``self-consistency problem'' and provide computable orbital-based expressions for any order of perturbation, a general one-electron reduced-density-matrix-based formalism is proposed. Two applications of our general formalism are presented: The derivation of a hybrid second-order M\o{}ller-Plesset-DFT model and the formulation of a range-separated optimized effective potential method based on a hybrid G\"orling-Levy-type MBPT-DFT model.

The effect of MM polarization on the QM/MM transition state stabilization: application to chorismate mutase

Hybrid quantum mechanics/molecular mechanics (QM/MM) calculations provide a mechanism for studying enzyme catalysed reactions at the molecular level. Here, through applications on the chorismate to prephenate rearrangement within the enzyme chorismate mutase, the feasibility of including MM polarization into these calculations has been demonstrated using the method of induced charges. MM polarization is shown to be a short-range effect, such that more than 75% of the energy of MM polarization occurs within a 5 Å residue-based cut-off of the substrate. MM polarization was shown to have a greater magnitude within the enzyme catalysed reaction than in the aqueous reaction, indicating that MM polarization may in principle have a significant effect on enzyme rate enhancement and mechanism. In both the enzyme and the aqueous case, the percentage contribution of MM polarization to the total stabilization energy was towards the upper end of the expected value. For the specific structures studied here, MM polarization lowered the energy barrier for the aqueous reaction, but the calculated contribution of MM polarization to both the reactant and transition structure stability were similar.

New flexible boundary hybrid solvation models for efficient biomolecular simulations

Two simple models are developed to accurately account for short-range solvation effects in molecular dynamics simulations of biomolecular systems in water. The methods limit the amount of solvent that must be treated explicitly to two or three solvation shells, thereby eliminating the need for periodic boundary conditions, which significantly reduces the computational complexity. In the dynamic boundary model, a confining potential is imposed on the solvent that responds dynamically to fluctuations in the solvent distribution and conformations of the biomolecular solute. In the exchange boundary solvation model, the molecules at the surface of the explicit solvation shell are allowed to undergo pairwise exchanges with the bulk to maintain a uniform hydration of the solute. The models are used to study the solvation thermodynamics of a 11-residue polyalanine helix in water. A broad range of structural and dynamic properties characterizing both the protein and the surrounding solvent are computed. Comparison with the results obtained from conventional periodic boundary condition simulations shows excellent agreement, and the new solvation models are found to improve computational efficiency by up two orders of magnitude.

Renormalization of the dephasing by zero point fluctuations

We study the role of zero point fluctuations (ZPF) in dephasing at low temperatures. Unlike the Caldeira\char21{}Leggett model, wherein the interaction is with a homogeneous fluctuating field of force, here we consider the effect of short range scattering by localized bath modes. We find that in the presence of ZPF, the inelastic cross section gets renormalized. Thus, ZPF might indirectly contribute to the dephasing at low temperatures.

Upward motion of earthquake hypocenters in the Wadati-Benioff seismic zones

Vertical migration of hypocenters of earthquakes (EQ) in the Wadati-Benioff focal zones is proved by special data processing. Migration is related to the ascent of earthquake hypocenters from the depths of the upper mantle (600‐700 km) to the Earth’s crust level. The total time of fluid ascent in the Tonga‐Bougainville and Java‐Sumatra zones is equal to 5.5‐6 and 10‐11 yr, respectively. This fact is important for explaining EQ triggering in the subduction regions and rift zones. In the last 10‐25 years, a wide range of investigations was performed to analyze short-range effects accompanying EQs, including electromagnetic, hydrogeochemical, and seismoacoustic phenomena in the Earth, and radiophysical and wave phenomena in the atmosphere and ionosphere. Important scientific projects were carried out in Japan, Russia (Kamchatka region), France, Italy, Greece, Indonesia, and a number of other countries [1]. The experimental results obtained in these works cannot be explained completely from the standpoint of the known models of EQ preparation for the following reasons: the last stage of the source preparation takes a short period of time (from a few days to a few weeks); the recorded effects are manifested sporadically in space and time; some effects are generated in the atmosphere, which gives grounds to suppose the effective transport of perturbations through the Earth’s surface; and electromagnetic effects are obviously related to fluid filtration through cracks and fractures in the crust. These peculiarities of the preparation process of the seismic source suggest the existence of local agents of geodynamic perturbation in a large volume of the Earth’s crust. New seismological data also indicate that the geological‐geophysical conditions are inhomogeneous in the preparation source. This is also confirmed by the instability of the solutions of the focal mechanism of the background EQ at the foreshock stage [2], the phenomenon of vertical migration of their hypocenters, small stresses in the Earth’s crust, and the longterm unstable (critical) state of the medium. Therefore, regardless of the large amount of collected empirical data, it was not possible to obtain an adequate scientific interpretation of the data from the standpoint of the existing models of EQ preparation. At present, the known general model of seismogenesis has been supplemented with a model in which the appearance and fluctuations of heat flux in the interior are the initial factors. They lead to a spatially scattered vertical migration of fluid formations, which form the conditions for rapid preparation of the seismic source owing to a strong reduction of stress in rock massifs under conditions of decreased strength of the medium and the existence of differently oriented active fractures and fissure zones in the Earth’s interior. In a short period of time, the variations in the volume and directions of near-surface migration in a nonuniform geothermal field lead to the main seismic shock and beginning of the aftershock sequence. The model suggested here includes the critical state of the medium as a necessary condition for migration without strong accumulation of stress at the last stage of the source preparation.

Short-ranged potential effects on the recurrence spectra of lithium M = 1 atoms in parallel electric and magnetic fields

This paper presents recurrence spectra of highly excited lithium atoms with M = 1 state in parallel electric and magnetic fields at a fixed scaled energy ∊ = -0.03. Short-ranged potentials including ionic core potential and centrifugal barrier are taken into account. Their effects on the states and photo-absorption spectrum are analysed in detail. This demonstrates that the geometric features of classical orbits are of special importance for modulations of the spectral pattern. Thus the weak polarization as well as the reduction of correlation of electrons induced by short-ranged potentials give rise to the recurrence spectra of lithium M = 1 atoms more compact than that of the M = 0 one, which is in good agreement with the experimental prediction.

Temperature dependence of effective g factor in diluted magnetic semiconductor (Ga,Mn)As

Time resolved magneto-optic Kerr rotation measurements of optically induced spin quantum beats are performed on heavily doped bulk (Ga,Mn)As diluted magnetic semiconductors (DMS). An effective g-factor of about 0.2–0.3 over a wide range of temperature for both as-grown and annealed (Ga,Mn)As samples is obtained. A larger effective g-factor at lower temperature and an increase of the spin relaxation with increasing in-plane magnetic field are observed and attributed to the stronger p-d exchange interaction between holes and the localized magnetic ion spins, leading to a larger Zeeman splitting and heavy-hole–light-hole mixing. An abnormal dip structure of the g-factor in the vicinity of the Curie temperature suggests that the mean-field model is insufficient to describe the interactions and dynamics of spins in DMS because it neglects the short-range spin correlation effect.

Scale patterns of diel distribution of the copepod Cyclops abyssorum Sars in a regulated lake: the relative importance of physical and biological factors

The relative importance of hydrological and biological factors in driving the diel distribution of the copepod Cyclops abyssorum from small to large scale is evaluated here in a lake where a complex hydrodynamics is at work and a clear environmental gradient is present. A set of statistical tools (which include the proposed partial variance) are used for characterizing the scale patterns of distribution and their daily trend. A day-night aggregation/dispersion process is observed at any of the spatial scales and directions considered. The value of the fractal dimension of the copepod distribution (D-horizontal = 1.89) suggests that short-range effects prevail over long-range ones in affecting the overall complexity of distribution. The geostatistical analysis shows that individuals form isotropic swarm-like assemblages of a few metre diameter during daytime which relax during night. The cohesion among population members of different age and sex also shows a daily fluctuation, with separation distance increasing vertically during daytime and horizontally during night. The present study shows how visual predation affects the whole structure of patchiness and explains the diel spacing among population fractions, whereas food availability prevails over water transport in driving the copepod distribution at a large scale.

Simple Formulas for Improved Point-Charge Electrostatics in Classical Force Fields and Hybrid Quantum Mechanical/Molecular Mechanical Embedding.

We present a simple damping scheme for point-charge electrostatics that could be used directly in classical force fields. The approach acts at the charge (or monopole) level only and allows the inclusion of short-range electrostatic penetration effects at a very low cost. Results are compared with density functional theory Coulomb intermolecular interaction energies and with several other methods such as distributed multipoles, damped distributed multipoles, and transferable Hermite-Gaussian densities. Realistic trends in the interactions are observed for atom-centered Mertz-Kollman corrected point-charge distributions. The approach allows increasing the selectivity of parameters in the case of metal complexes. In addition, two QM/MM calculations are presented where the damping function is employed to include the MM atoms located at the QM/MM boundary. The first calculation corresponds to the gas-phase proton transfer of aspartic acid through water and the second is the first step of the reaction catalyzed by the 4-oxalocrotonate tautomerase (4OT) enzyme. First, improved agreement is observed when using the damping approach compared with the conventional excluded charge method or when including all charges in the calculation. Second, in the case of 4OT, the damped charge approach is in agreement with previous calculations, whereas including all charges gives a significantly higher energy barrier. In both cases, no reparameterization of the van der Waals part of the force field was performed.

Anharmonic interactions in the Raman spectrum of ZnGa2Se4 and MnGa2Se4 ordered vacancy compounds

We present a study of anharmonic and local order effects in the Raman spectrum of the ordered vacancy semiconductors ZnGa2Se4 and MnGa2Se4. In MnGa2Se4 the asymmetry of the Se-vacancy breathing like mode and its temperature dependence are satisfactorily explained by a model based on the anharmonic coupling of the phonon with second order phonon excitations. In ZnGa2Se4, however, asymmetry is not fully accounted for with this model. We attribute the discrepancy to short-range order effects in the, on the average, cation disordered structure of ZnGa2Se4. These effects are taken into account by means of the spatial correlation model.

Investigations of the magnetic properties of the granular system Co0.6(Al2On)0.4 possessing isotropic positive magnetoresistance

Magnetostatic, magnetoresonance (FMR), and magnetotransport measurements are performed in the temperature range 100–360K in the granular ferromagnetic film Co0.59(Al2On)0.41 possessing isotropic (independent of the angle between the direction of the current and magnetization in the plane) positive magnetoresistance in magnetic fields ±500Oe are performed. It is established that this anomaly of the magnetoresistance is due to short-range order effects in the ordering imposed on the magnetic moments of granules in the absence of an external magnetic field by their dipole–dipole interaction. It is established that the blocking temperature of thermally activated re-orientations of the magnetic moments of the granules differs substantially with respect to the measurement time and the methods used for this investigation. It is about 200K for magnetization measurements and exceeds 470K for FMR and measurements of the magnetoresistance. It is shown that ferromagnetic resonance in the present system changes from the regime “independent resonance of individual clusters” to the regime “collective resonance of a system of magnetic granules in a film” when the direction of the magnetic field changes from in-plane to normal to the film.

Anisotropic, Polarizable Molecular Mechanics Studies of Inter- and Intramolecular Interactions and Ligand-Macromolecule Complexes. A Bottom-Up Strategy.

We present an overview of the SIBFA polarizable molecular mechanics procedure, which is formulated and calibrated on the basis of quantum chemistry (QC). It embodies nonclassical effects such as electrostatic penetration, exchange-polarization, and charge transfer. We address the issues of anisotropy, nonadditivity, and transferability by performing parallel QC computations on multimolecular complexes. These encompass multiply H-bonded complexes and polycoordinated complexes of divalent cations. Recent applications to the docking of inhibitors to Zn-metalloproteins are presented next, namely metallo-beta-lactamase, phosphomannoisomerase, and the nucleocapsid of the HIV-1 retrovirus. Finally, toward third-generation intermolecular potentials based on density fitting, we present the development of a novel methodology, the Gaussian electrostatic model (GEM), which relies on ab initio-derived fragment electron densities to compute the components of the total interaction energy. As GEM offers the possibility of a continuous electrostatic model going from distributed multipoles to densities, it allows an inclusion of short-range quantum effects in the molecular mechanics energies. The perspectives of an integrated SIBFA/GEM/QM procedure are discussed.

Loop corrections and naturalness in a chiral effective field theory

The loop expansion is applied to a chiral effective hadronic Lagrangian; with the techniques of Infrared Regularization, it is possible to separate out the short-range contributions and to write them as local products of fields that are already present in our Lagrangian. (The appropriate field variables must be re-defined at each order in loops.) The corresponding parameters implicitly include short-range effects to all orders in the interaction, so these effects need not be calculated explicitly. The remaining (long-range) contributions that must be calculated are nonlocal and resemble those in conventional nuclear-structure calculations. Nonlinear isoscalar scalar ( σ ) and vector ( ω ) meson interactions are included, which incorporate many-nucleon forces and nucleon substructure. Calculations are carried out at the two-loop level to illustrate these techniques at finite nuclear densities and to verify that the coupling parameters remain natural when fitted to the empirical properties of equilibrium nuclear matter. Contributions from the ωN tensor coupling are also discussed.

Specific Ion Adsorption and Short-Range Interactions at the Air Aqueous Solution Interface

We have investigated the surface composition of alkali-halide aqueous solutions using grazing incidence x-ray fluorescence. Using mixtures of salts as a means to enhance the short-range effects, small differences in concentration over a few angstrom could be resolved, with, for example I- or Br- > Cl-. In order to explain our data, we need to include an effective potential accounting for the short-range (A) solvent mediated couplings, responsible for specific effects together with dispersion forces. This attractive potential (few k{B}T for halides) leads to concentration profiles which are in good agreement with recent numerical simulations.

Two-loop corrections for nuclear matter in a covariant effective field theory

Although one-loop calculations provide a realistic description of bulk and single-particle nuclear properties, it is necessary to examine loop corrections to develop a systematic finite-density power-counting scheme for the nuclear many-body problem when loops are included. Moreover, it is imperative to study exchange and correlation corrections systematically to make reliable predictions for other nuclear observables. One must also verify that the natural sizes of the one-loop parameters are not destroyed by explicit inclusion of many-body corrections. The loop expansion is applied to a chiral effective hadronic Lagrangian; with the techniques of Infrared Regularization, it is possible to separate out the short-range contributions and to write them as local products of fields that are already present in our Lagrangian. (The appropriate field variables must be re-defined at each order in loops.) The corresponding parameters implicitly include short-range effects to all orders in the interaction, so these effects need not be calculated explicitly. The remaining (long-range) contributions that must be calculated are nonlocal and resemble those in conventional nuclear-structure calculations. Calculations at the two-loop level are carried out to illustrate these techniques at finite densities and to verify that the coupling parameters remain natural when fitted to the empirical properties of equilibrium nuclear matter.

Luminescence properties of europium (III) cryptates trapped in sol–gel glass

The Lanthanide complexes Eu-2.2.1 and Eu-2.2.2 were synthesized and then incorporated into silica based transparent organic–inorganic hybrid material by the sol–gel method as an example of how doped xerogel materials are emerging as an important means of producing new materials. The produced gels were diagnosed to monitor emission spectra of the luminescent trivalent europium (Eu 3+) complexes; emissions were compared to those for gels containing uncomplexed Eu 3+. Results of the experiments (emission and lifetimes) concerning the coordination sphere composition showed that a cryptand ligand with aromatic groups (short range effect) and the hydrophobic gel host (long range effect) settle efficient action in the antenna effect and isolate the central ion from efficient quenchers, as e.g. water molecules. Each ligand imposed a distinct splitting pattern on the europium emission bands that helps identify them.

Role of the image charge and electronic corrugation on charge-fraction azimuthal variations for keV Li and Na ions interacting with Cu(110) and Ag(110) under grazing incidence

The positive charge fraction resulting from charge exchange during the interaction of ${\mathrm{Li}}^{+}$ and ${\mathrm{Na}}^{+}$ ions on the Ag(110) and Cu(110) metal surfaces has been measured for different energies and incident azimuthal angles of the projectiles. Surprisingly, although metallic Cu and Ag are well described by the free-electron model, the positive charge fraction has been observed to vary significantly according to the azimuthal angle of incidence, for projectile energies lower than $4\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$ and incidence angle below 5\ifmmode^\circ\else\textdegree\fi{} with respect to the surface (grazing scattering conditions), and specifically around the low-index [001] and $[1\overline{1}0]$ directions. A detailed analysis of trajectory calculations for the $\mathrm{Li}∕\mathrm{Cu}(110)$ system allows one to distinguish between different types of trajectories (on tops, zigzags, and in rows), the relative importance of which varies rapidly with azimuthal angle. We obtain a good correlation between the observed azimuthal variation of the charge fraction and the mean value of the electronic density encountered by the projectile in the vicinity of the first atomic layers. We show that this short-range effect can be reflected on the observed variation of the charge fraction through the charge image effect on the trajectories. This effect is shown to be very sensitive to the electronic corrugation of the surface which is known to be low for Cu and Ag. Azimuthal experimental measurements thus appear to be a tool to characterize such low corrugations.

All orders proton breakup from exotic nuclei

We present a semiclassical method to treat the proton breakup from a weakly bound state in an exotic nucleus. The Coulomb interactions among the proton, the core, and the target are treated to all orders, including the full-multipole expansion of the Coulomb potential. The nuclear proton-target interaction is also treated to all orders. The core-target interaction is included as an absorption. The method is semianalytical, thus allowing for a detailed understanding of the short-range and long-range effects of the interactions in the reaction dynamics. It explains also the origin of possible asymmetries in the core parallel momentum distributions when the full-multipole expansion of the Coulomb potential is used. Calculations are compared to results of other, fully numerical, methods and to experimental data to establish the accuracy and reliability of the method.