Short-range Interaction Model Research Articles

Area-Independence of the Biexciton Oscillator Strength in CdSe Colloidal Nanoplatelets.

Colloidal CdSe nanoplatelets (NPLs) are unique systems to study two-dimensional excitons and excitonic complexes. However, while absorption and emission of photons through exciton formation and recombination have been extensively quantified, few studies have addressed the exciton-biexciton transition. Here, we use cross-polarized pump-probe spectroscopy to measure the absorption coefficient spectrum of this transition and determine the biexciton oscillator strength (fBX). We show that fBX is independent of the NPL area and that the concomitant biexciton area (SBX) agrees with predictions of a short-range interaction model. Moreover, we show that fBX is comparable to the oscillator strength of forming localized excitons at room temperature while being unaffected itself by center-of-mass localization. These results confirm the relevance of biexcitons for light-matter interaction in NPLs. Moreover, the quantification of the exciton-biexciton transition introduced here will enable researchers to rank 2D materials by the strength of this transition and to compare experimental results with theoretical predictions.

On the many-body problem with short-range interaction

The classical problem of the interaction of charged particles is considered in the framework of the concept of short-range interaction. Difficulties in the mathematical description of short-range interaction are discussed, for which it is necessary to combine two models, a nonlinear dynamic system describing the motion of particles in a field, and a boundary value problem for a hyperbolic equation or Maxwells equations describing the field. Attention is paid to the averaging procedure, that is, the transition from the positions of particles and their velocities to the charge and current densities. The problem is shown to contain several parameters; when they tend to zero in a strictly defined order, the model turns into the classical many-body problem. According to the Galerkin method, the problem is reduced to a dynamic system in which the equations describing the dynamics of particles, are added to the equations describing the oscillations of a field in a box. This problem is a simplification, different from that leading to classical mechanics. It is proposed to be considered as the simplest mathematical model describing the many-body problem with short-range interaction. This model consists of the equations of motion for particles, supplemented with equations that describe the natural oscillations of the field in the box. The results of the first computer experiments with this short-range interaction model are presented. It is shown that this model is rich in conservation laws.

Dynamic response functions of two-dimensional Dirac fermions with screened Coulomb and short-range interactions

We consider a screened Coulomb interaction between electrons in graphene and determine their dynamic response functions, such as a longitudinal and a transverse electric conductivity and a polarization function and compare them to the corresponding quantities in the short-range interaction model. The calculations are performed to all orders for short-range interaction by taking into account the self-energy renormalization of the electron velocity and using a ladder approximation to account for the vertex corrections, ensuring that the Ward identity (charge conservation law) is satisfied. Our findings predict a resonant response of interacting electron-hole pairs at a particular frequency below the threshold $qv=\omega$ and further predict an instability for sufficiently strong interactions.

Interfacial premelting of ice in nano composite materials.

The interfacial premelting in ice/clay nano composites was studied by high energy X-ray diffraction. Below the melting point of bulk water, the formation of liquid water was observed for the ice/vermiculite and ice/kaolin systems. The liquid fraction is gradually increasing with temperature. For both minerals, similar effective premelting layer thicknesses of 2-3 nm are reached 3 K below the bulk melting point. For the quantitative description of the molten water fraction in wet clay minerals we developed a continuum model for short range interactions and arbitrary pore size distributions. This model quantitatively describes the experimental data over the entire temperature range. Model parameters were obtained by fitting using a maximum entropy (MaxEnt) approach. Pronounced differences in the deviation from Antonow's rule relating interfacial free energy between ice, water, and clay are observed for the charged vermiculite and uncharged kaolin minerals. The resultant parameters are discussed in terms of their ice nucleation efficiency. Using well defined and characterized ice/clay nano composite samples, this work bridges the gap between studies on single crystalline ice/solid model interfaces and naturally occurring soils and permafrost.

Adlayer structure and lattice size effects on catalytic rates predicted from KMC simulations: NO oxidation on Pt(111).

Repulsive and/or attractive interactions between surface adsorbates have an important effect on the structure of the adsorbate layer and consequently on the rate of heterogeneous catalytic reactions. Thus, developing reaction models that take into account adsorbate-adsorbate interactions is crucial for making accurate predictions of the catalytic rate and surface coverage during reaction. In the present work, we employ kinetic Monte Carlo simulation to model the catalytic NO oxidation on Pt (111), adopting a cluster expansion (CE) Hamiltonian approach for treating the aforementioned interactions. We investigate CEs of increasing complexity, ranging from pairwise 1st nearest neighbor to long-range and many-body terms. We show that energetic models incorporating solely short-range interactions result in ordered adlayer structures, which are disrupted by anti-phase boundaries and defective regions when the size of the periodic lattice is non-commensurate to the structure of the stable adlayer. We find that O2 dissociates on sites located in these defective regions, which are predominantly responsible for the activity, and the predicted catalytic rate is strongly depended on the lattice size. Such effects are absent when employing non-periodic lattices, whereon the catalytic activity appears more intense on edges/corner sites. Finally, inclusion of long-range interactions in the model Hamiltonian induces relative disorder in the adsorbate layer, which is ascribed to the "softening" of the repulsive interactions between adspecies. Under these circumstances, the distribution of activation energies for O2 dissociation is broader as compared to short-range interaction models and on this basis we explain the disparate catalytic rate predictions when using different CEs.

On the Inclusion of Short-distance Bystander Effects into a Logistic Tumor Control Probability Model.

Currently, interactions between voxels are neglected in the tumor control probability (TCP) models used in biologically-driven intensity-modulated radiotherapy treatment planning. However, experimental data suggests that this may not always be justified when bystander effects are important. We propose a model inspired by the Ising model, a short-range interaction model, to investigate if and when it is important to include voxel to voxel interactions in biologically-driven treatment planning. This Ising-like model for TCP is derived by first showing that the logistic model of tumor control is mathematically equivalent to a non-interacting Ising model. Using this correspondence, the parameters of the logistic model are mapped to the parameters of an Ising-like model and bystander interactions are introduced as a short-range interaction as is the case for the Ising model. As an example, we apply the model to study the effect of bystander interactions in the case of radiation therapy for prostate cancer. The model shows that it is adequate to neglect bystander interactions for dose distributions that completely cover the treatment target and yield TCP estimates that lie in the shoulder of the dose response curve. However, for dose distributions that yield TCP estimates that lie on the steep part of the dose response curve or for inhomogeneous dose distributions having significant hot and/or cold regions, bystander effects may be important. Furthermore, the proposed model highlights a previously unexplored and potentially fruitful connection between the fields of statistical mechanics and tumor control probability/normal tissue complication probability modeling.

Effect of long- and short-range interactions on the thermodynamics of dipolar spin ice

The thermodynamic properties of dipolar spin ice on square, honeycomb and shakti lattices in the long-range and short-range dipole interaction models are studied. Exact solutions for the density of states, temperature dependencies of heat capacity, and entropy are obtained for these lattices with a finite number of point dipoles by means of complete enumeration. The magnetic susceptibility and average size of the largest low-energy cluster are calculated for square spin ice by means of Wang–Landau and Metropolis methods. We show that the long-range interaction leads to a blurring of the energy spectrum for all considered lattices. The inclusion of the long-range interaction leads to a significant change in the thermodynamic behaviour. An additional peak of heat capacity appears in the case of the honeycomb lattice. The critical temperature shifts in the direction of low or high temperatures; the direction depends on the lattice geometry. The critical temperature of the phase transition of square spin ice in the long-range model with frustrated ground states is obtained with the Wang–Landau and Metropolis methods independently.

Resolved-particle simulation by the Physalis method: Enhancements and new capabilities

We present enhancements and new capabilities of the Physalis method for simulating disperse multiphase flows using particle-resolved simulation. The current work enhances the previous method by incorporating a new type of pressure-Poisson solver that couples with a new Physalis particle pressure boundary condition scheme and a new particle interior treatment to significantly improve overall numerical efficiency. Further, we implement a more efficient method of calculating the Physalis scalar products and incorporate short-range particle interaction models. We provide validation and benchmarking for the Physalis method against experiments of a sedimenting particle and of normal wall collisions. We conclude with an illustrative simulation of 2048 particles sedimenting in a duct. In the appendix, we present a complete and self-consistent description of the analytical development and numerical methods.

The Secondary Organic Aerosol Processor (SOAP v1.0) model: a unified model with different ranges of complexity based on the molecular surrogate approach

Abstract. In this paper the Secondary Organic Aerosol Processor (SOAP v1.0) model is presented. This model determines the partitioning of organic compounds between the gas and particle phases. It is designed to be modular with different user options depending on the computation time and the complexity required by the user. This model is based on the molecular surrogate approach, in which each surrogate compound is associated with a molecular structure to estimate some properties and parameters (hygroscopicity, absorption into the aqueous phase of particles, activity coefficients and phase separation). Each surrogate can be hydrophilic (condenses only into the aqueous phase of particles), hydrophobic (condenses only into the organic phases of particles) or both (condenses into both the aqueous and the organic phases of particles). Activity coefficients are computed with the UNIFAC (UNIversal Functional group Activity Coefficient; Fredenslund et al., 1975) thermodynamic model for short-range interactions and with the Aerosol Inorganic–Organic Mixtures Functional groups Activity Coefficients (AIOMFAC) parameterization for medium- and long-range interactions between electrolytes and organic compounds. Phase separation is determined by Gibbs energy minimization. The user can choose between an equilibrium representation and a dynamic representation of organic aerosols (OAs). In the equilibrium representation, compounds in the particle phase are assumed to be at equilibrium with the gas phase. However, recent studies show that the organic aerosol is not at equilibrium with the gas phase because the organic phases could be semi-solid (very viscous liquid phase). The condensation–evaporation of organic compounds could then be limited by the diffusion in the organic phases due to the high viscosity. An implicit dynamic representation of secondary organic aerosols (SOAs) is available in SOAP with OAs divided into layers, the first layer being at the center of the particle (slowly reaches equilibrium) and the final layer being near the interface with the gas phase (quickly reaches equilibrium). Although this dynamic implicit representation is a simplified approach to model condensation–evaporation with a low number of layers and short CPU (central processing unit) time, it shows good agreements with an explicit representation of condensation–evaporation (no significant differences after a few hours of condensation).

Heisenberg-like ferromagnetism in3d−4fintermetallicLa0.75Pr0.25Co2P2with localized Co moments

The critical behavior near the continuous paramagnetic to ferromagnetic transition in a single crystal of ${\mathrm{La}}_{0.75}{\mathrm{Pr}}_{0.25}{\mathrm{Co}}_{2}{\mathrm{P}}_{2}$ has been determined based on high-resolution bulk magnetization data near ${T}_{C}$ \ensuremath{\sim} 167 K, where long-range order is established in the Co sublattice. Scaling equation of state analysis and the Kouvel-Fisher method under a moderate applied magnetic field yielded critical exponents ($\ensuremath{\beta}=0.3685\ifmmode\pm\else\textpm\fi{}0.0017$, $\ensuremath{\gamma}=1.3361\ifmmode\pm\else\textpm\fi{}0.0083$), consistent with the $d$ = 3, $n$ = 3 Heisenberg model of short-range interactions. Calculation of the Rhodes-Wohlfarth ratio confirmed that a localized rather than itinerant description of the $3d$ Co moments is appropriate in the ferromagnetic region of the sample. The critical susceptibility exponent \ensuremath{\gamma} was found to decrease systematically from the Heisenberg model value toward the mean-field model value as the maximum applied magnetic field considered in the analysis was increased above 2 T. The phenomenon is discussed in terms of mixed exchange mechanisms due to the coexistence of $3d$ and $4f$ magnetic sublattices and ordered clusters in the paramagnetic region.

Spin waves on the surface of a semiconductor nanotube with a superlattice

Landau-Silin spin waves spectra on the surface of a cylindrical semiconductor nanotube with a superlattice were studied. The energy spectrum of the tube was derived within the framework of short-range electron interaction model in Hartree-Fock approximation. The components of the tensor of electron gas dynamic spin susceptibility were calculated using the random phase approximation. Spin waves spectra in the cases of low and high electron density were obtained. It was shown that the wave frequencies exhibit oscillations similar to de Haas-van Alphen oscillations upon variation of nanotube parameters and Aharonov-Bohm oscillations upon variation of the magnetic flux through its cross-section. The beating pattern in the spectrum of magnons was found.

Entanglement mean field theory: Lipkin–Meshkov–Glick Model

Entanglement mean field theory is an approximate method for dealing with many-body systems, especially for the prediction of the onset of phase transitions. While previous studies have concentrated mainly on applications of the theory on short-range interaction models, we show here that it can be efficiently applied also to systems with long-range interaction Hamiltonians. We consider the (quantum) Lipkin---Meshkov---Glick spin model, and derive the entanglement mean field theory reduced Hamiltonian. A similar recipe can be applied to obtain entanglement mean field theory reduced Hamiltonians corresponding to other long-range interaction systems. We show, in particular, that the zero temperature quantum phase transition present in the Lipkin---Meshkov---Glick model can be accurately predicted by the theory.

Soft Wall Ion Channel in Continuum Representation with Application to Modeling Ion Currents in α-Hemolysin

A soft repulsion (SR) model of short-range interactions between mobile ions and protein atoms is introduced in the framework of continuum representation of the protein and solvent. The Poisson-Nernst-Plank (PNP) theory of ion transport through biological channels is modified to incorporate this soft wall protein model. Two sets of SR parameters are introduced. The first is parametrized for all essential amino acid residues using all atom molecular dynamic simulations; the second is a truncated Lennard-Jones potential. We have further designed an energy-based algorithm for the determination of the ion accessible volume, which is appropriate for a particular system discretization. The effects of these models of short-range interactions were tested by computing current-voltage characteristics of the α-hemolysin channel. The introduced SR potentials significantly improve prediction of channel selectivity. In addition, we studied the effect of the choice of some space-dependent diffusion coefficient distributions on the predicted current-voltage properties. We conclude that the diffusion coefficient distributions largely affect total currents and have little effect on rectifications, selectivity, or reversal potential. The PNP-SR algorithm is implemented in a new efficient parallel Poisson, Poisson-Boltzmann, and PNP equation solver, also incorporated in a graphical molecular modeling package HARLEM.

Activity coefficients of concentrated strong and weak electrolytes by a hydration equilibrium and group contribution model

A new speciation-based group contribution model for activity coefficients is proposed to estimate the equilibrium properties of aqueous solutions containing electrolytes. The chemical part of the model accounts for the hydration equilibrium of water and ions with the formation of ion n-water complexes in a single stage process; the hydration number n and the hydration equilibrium constant K are the two independent parameters in this part. The physical part of the model is the UNIFAC group contribution model for short-range interactions. Each ion is considered as a group. Long-range interactions are accounted for by a Pitzer contribution (Debye–Hückel theory). The model is compared with experimental data at 25 °C including water activity, osmotic coefficients, activity coefficients, and pH of binary diluted and concentrated electrolyte solutions (up to 20 mol kg −1 for NaOH, 16 mol kg −1 for HCl, etc.).

SIMULATIONS OF COLLISION CASCADES IN Cu–Nb LAYERED COMPOSITES USING AN EAM INTERATOMIC POTENTIAL

The embedded atom method (EAM) is used to construct an interatomic potential for modelling interfaces in Cu – Nb nanocomposites. Implementation of the Ziegler–Biersack–Littmark (ZBL) model for short-range interatomic interactions enables studies of response to ion bombardment. Collision cascades are modelled in fcc Cu , bcc Nb , and in Cu – Nb layered composites in the experimentally-observed Kurdjumov–Sachs (KS) orientation relation. The interfaces in these composites reduce the number of vacancies and interstitials created per keV of the primary knock-on atom (PKA) by 50–70% compared to fcc Cu or bcc Nb .

A model for short-range interactions in fibre suspensions

This paper introduces a model for short-range fibre–fibre interaction and combines it with the Shaqfeh–Fredrickson result for long-range hydrodynamic interactions. The short-range interactions are assumed to be localised, such that the interaction forces may be taken to act at points on the fibre axes. Only interactions of viscous Newtonian character are explored in this work; the approach, however, may be used to treat more general nonlinear interactions, such as non-Newtonian vicous forces or friction. To simulate the orientation evolution, a convective discretisation technique combined with a modification of the Folgar–Tucker rotary diffusion model is proposed. The rotational parallel-plate geometry is analysed, and transient as well as steady state responses are computed. Experiments are performed on suspensions of polyamide fibres in silicon oil, using a rotational parallel-plate rheometer. It is demonstrated that the effect of fibre volume fraction and aspect ratio are well captured by this theory. Normal force as well as torque is adequately predicted. Other effects, however, such as shear thinning and effect of absolute particle size, are not predicted by the linear theory.

Critical behavior ofLa1−xSrxMnO3(0⩽x⩽0.35)by thermal diffusivity measurements

An ac photopyroelectric calorimeter has been used to measure the thermal diffusivity of the perovskite manganite family ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$ on a set of single crystals with doping range $0\ensuremath{\leqslant}x\ensuremath{\leqslant}0.35$. Taking into account that the inverse of the thermal diffusivity has the same critical behavior as the specific heat, the critical exponent $\ensuremath{\alpha}$ of the magnetic transitions has been obtained. The results point to short-range interaction models for pure magnetic transitions. In the pure and lightly doped samples $(xl0.10)$, where the transition is antiferromagnetic-paramagnetic, the critical exponent is consistent with the Heisenberg model $(\ensuremath{\alpha}=\ensuremath{-}0.11)$. For the highly doped samples $(xg0.28)$, where there is a pure ferromagnetic-paramagnetic transition, the critical exponent is exactly that of an Ising behavior $(\ensuremath{\alpha}=+0.11)$. For $0.10\ensuremath{\leqslant}xl0.28$ no universality class was found and this behavior has been discussed taking into account the complexity of the phase diagram in this concentration range.

Atomistic simulation of radiation effects in carbon-based materials and nitrides

Molecular dynamics computer simulations have proven to be an extremely useful method to examine the basic mechanisms of ion-induced damage production in materials. In the particular case of insulators with a high degree of ionic bonding, there are some complications in the simulations associated with how the charges on atoms should be handled. In this article I will review different approaches of handling the ionicity and discuss the limits of their applicability. I will also present some of our recent simulation results in GaN and hydrogenated amorphous carbon. Comparison of defect production in collision cascades in GaN simulated with two different models, one with explicit ionic interactions and another without, gives the surprising result that the primary damage production does not seem to be affected by explicit ionic interactions. This indicates that at least in GaN it is justified to use fast short-range interaction models in simulations of the primary states of radiation damage.

Non-Equilibrium Relaxation Study of Ferromagnetic Transition in Double-Exchange Systems

Ferromagnetic transition in double-exchange systems is studied by non-equilibrium relaxation technique combined with Monte Carlo calculations. Critical temperature and critical exponents are estimated from relaxation of the magnetic moment. The results are consistent with the previous Monte Carlo results in thermal equilibrium. The exponents estimated by these independent techniques suggest that the universality class of this transition is the same as that of short-range interaction models but is different from the mean-field one.

Continuous phase transition in a spin-glass model without time-reversal symmetry.

We investigate the phase transition in a strongly disordered short-range three-spin interaction model characterized by the absence of time-reversal symmetry in the Hamiltonian. In the mean-field limit the model is well described by the Adam-Gibbs-DiMarzio scenario for the glass transition; however, in the short-range case this picture turns out to be modified. The model presents a finite temperature continuous phase transition characterized by a divergent spin-glass susceptibility and a negative specific-heat exponent. We expect the nature of the transition in this three-spin model to be the same as the transition in the Edwards-Anderson model in a magnetic field, with the advantage that the strong crossover effects present in the latter case are absent.