Short-range Model Research Articles

Study of zone center phonons in wolframite ZnWO_4

A short-range force constant model was applied to investigate the phonons in wolframite ZnWO_4 crystals in their monoclinic phase having space group P2/c and symmetry C_{2h}. The normal symmetry coordinates for the monoclinic wolframite were computed. The zone center phonons were calculated by using 8 stretching and 6 bending force constants. The calculated results are in very good agreement with the observed ones. The infrared frequencies were assigned for the first time. The potential energy distribution was also investigated for determining the significance of the contribution from each force constant toward the Raman and infrared wavenumbers.

Air–Sea Transport, Dispersion, and Fate Modeling in the Vicinity of the Fukushima Nuclear Power Plant: A Special Conference Session Summary

The March 2011 tragedy at the coastal Fukushima Daiichi nuclear plant created a contaminant release that was transported and dispersed in both the air and the sea. Two months after the event, the authors began planning for a special July 2011 conference session (“Fukushima crisis: Air and sea transport modeling” at the 15th Annual Conference on Atmospheric Transport and Dispersion Modeling; see http://camp.cos.gmu.edu/Agenda-fifteenth-GMU-Conference.pdf) to draw together experts who were directly involved in the scientific modeling and decision making. The focus was on presentations describing the releases to the atmosphere and ocean, how the models performed, and how predictive modeling can effectively inform crisis decision making. This paper provides an overview of the short-range (within and near Japan) modeling conducted for the crisis and identifies some key steps that might improve the modeling response to incidents occurring in a complex coastal zone. Those steps include operational protocols to deal with source-term uncertainty, scientific approaches to addressing the linkage of air–sea source terms and their subsequent transformations, and improvements in coupled air–sea models for better prediction in coastal regions. The conference included anecdotes from those deployed in Japan and tasked to deliver and interpret plume model products. The conference presentations provide insights that could benefit emergency preparedness, response, and recovery for a similar disaster in the future. There is a need for a well-rehearsed approach to sharing plume model information among agencies and among international partners, and for scientific expertise to help interpret the operational relevance of models lacking source-term certitude.

LATTICE DYNAMICAL INVESTIGATIONS FOR RAMAN AND THE INFRARED FREQUENCIES OF Cs2HgCl4

A short-range force constant model has been applied in investigating the Raman and infrared modes in insulating Cs2HgCl4 in orthorhombic phase having space group Pnam using normal coordinate analysis. Calculations on zone centre phonons have been made with twelve stretching and three bending force constants. The calculated Raman and infrared wave numbers are in good agreement with the available experimental results. The potential energy distribution has also been investigated for determining the significance of contribution from each force constant towards the Raman and the infrared wave numbers.

Correspondence between long-range and short-range spin glasses

We compare the critical behavior of the short-range Ising spin glass with a spin glass with long-range interactions which fall off as a power sigma of the distance. We show that there is a value of sigma of the long-range model for which the critical behavior is very similar to that of the short-range model in four dimensions. We also study a value of sigma for which we find the critical behavior to be compatible with that of the three dimensional model, though we have much less precision than in the four-dimensional case.

Competing ordered states in bilayer graphene

We use a perturbative renormalization group approach with short-range continuum model interactions to analyze the competition between isotropic gapped and anisotropic gapless ordered states in bilayer graphene, commenting specifically on the role of exchange and on the importance of spin and valley flavor degeneracy. By comparing the divergences of the corresponding susceptibilities, we conclude that this approach predicts gapped states for flavor numbers $N=1,2,4$. We comment briefly on other approaches which can be used to shed light on the bilayer ground state and on the related ground state of chiral (ABC) trilayers.

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.

Microcanonical Phase Diagram of the BEG and Ising Models

The density of states of long-range Blume—Emery—Griffiths (BEG) and short-range Ising models are obtained by using Wang—Landau sampling with adaptive windows in energy and magnetization space. With accurate density of states, we are able to calculate the microcanonical specific heat of fixed magnetization introduced by Kastner et al. in the regions of positive and negative temperature. The microcanonical phase diagram of the Ising model shows a continuous phase transition at a negative temperature in energy and magnetization plane. However the phase diagram of the long-range model constructed by peaks of the microcanonical specific heat looks obviously different from the Ising chart.

One-dimensional infinite-component vector spin glass with long-range interactions

We investigate zero and finite temperature properties of the one-dimensional spin-glass model for vector spins in the limit of an infinite number m of spin components where the interactions decay with a power, \sigma, of the distance. A diluted version of this model is also studied, but found to deviate significantly from the fully connected model. At zero temperature, defect energies are determined from the difference in ground-state energies between systems with periodic and antiperiodic boundary conditions to determine the dependence of the defect-energy exponent \theta on \sigma. A good fit to this dependence is \theta =3/4-\sigma. This implies that the upper critical value of \sigma is 3/4, corresponding to the lower critical dimension in the d-dimensional short-range version of the model. For finite temperatures the large m saddle-point equations are solved self-consistently which gives access to the correlation function, the order parameter and the spin-glass susceptibility. Special attention is paid to the different forms of finite-size scaling effects below and above the lower critical value, \sigma =5/8, which corresponds to the upper critical dimension 8 of the hypercubic short-range model.

Functional Importance of Short-Range Binding and Long-Range Solvent Interactions in Helical Antifreeze Peptides

Short-range ice binding and long-range solvent perturbation both have been implicated in the activity of antifreeze proteins and antifreeze glycoproteins. We study these two mechanisms for activity of winter flounder antifreeze peptide. Four mutants are characterized by freezing point hysteresis (activity), circular dichroism (secondary structure), Förster resonance energy transfer (end-to-end rigidity), molecular dynamics simulation (structure), and terahertz spectroscopy (long-range solvent perturbation). Our results show that the short-range model is sufficient to explain the activity of our mutants, but the long-range model provides a necessary condition for activity: the most active peptides in our data set all have an extended dynamical hydration shell. It appears that antifreeze proteins and antifreeze glycoproteins have reached different evolutionary solutions to the antifreeze problem, utilizing either a few precisely positioned OH groups or a large quantity of OH groups for ice binding, assisted by long-range solvent perturbation.

Superlattice formed by quantum-dot sheets: Density of states and infrared absorption

Low-energy continuous states of electron in a heterosrtucture with periodically placed quantum-dot sheets are studied theoretically. The Green's function of an electron is governed by the Dyson equation with the self-energy function which is determined the boundary conditions at quantum-dot sheets with weak damping in the low-energy region. The parameters of a superlattice formed by quantum-dot sheets are determined using the short-range model of quantum dot. The density of states and spectral dependencies of the anisotropic absorption coefficient under midinfrared transitions from doped quantum dots into miniband states of a superlattice strongly depend on dot concentration and on the period of sheets. These dependencies can be used for the characterization of the multilayer structure and they determine the parameters of different optoelectronic devices exploiting the vertical transport of carriers through quantum-dot sheets.

Excluded volume versus hydrogen bonding: complementary or incompatible concepts?

An attempt has been made to assess individual effects of the repulsive and attractive interactions on the structure of associating fluids using short-range models descending from parent realistic models. It is shown that neither the excluded volume (repulsive interactions) nor the hydrogen bonding (attractive interactions) considered separately are able to produce the correct structure. However, when both these types of interactions are considered together, they faithfully reproduce the structure of water and other associating fluids. Consequently, although some properties of aqueous systems can be explained qualitatively/intuitively by hydrogen bonding only, disregarding simultaneously the excluded volume effects may be misleading.

Interacting resonant-level model with long-range interactions: Fast screening and suppression of the zero-bias conductance

The effects of long-range interactions in quantum transport are still largely unexplored, mainly due to the difficulty of devising efficient embedding schemes. In this work we present a substantial progress in the interacting resonant level model by reducing the problem to the solution of Kadanoff-Baym-like equations with a correlated embedding self-energy. The method allows us to deal with short- and long-range interactions and is applicable from the transient to the steady-state regime. Furthermore, memory effects are consistently incorporated and the results are not plagued by negative densities or nonconservation of the electric charge. We employ the method to calculate densities and currents with long-range interactions appropriate to low-dimensional leads, and show the occurrence of a jamming effect, which drastically reduces the screening time and suppresses the zero-bias conductance. None of these effects are captured by short-range model interactions.

An intercomparison of models used to simulate the short-range atmospheric dispersion of agricultural ammonia emissions

Ammonia emitted into the atmosphere from agricultural sources can have an impact on nearby sensitive ecosystems, either through elevated ambient concentrations or dry/wet deposition to vegetation and soil surfaces. Short-range atmospheric dispersion models are often used to assess these potential impacts on semi-natural ecosystems and a range of different models are used for these assessments. However, until now there has not been an intercomparison of the different models for the case of ammonia dispersion from agricultural sources and therefore it cannot be assumed that assessments are consistent. This paper presents an intercomparison of atmospheric concentration predictions made by a set of models commonly used for this type of assessment (ADMS; AERMOD; LADD and OPS-st). This intercomparison shows that there are differences between the concentration predictions of the models and some of these differences appear to be consistent and independent of the scenario modelled. The best model agreement was found for simple scenarios with area and volume sources, whereas the model agreement was worst for a scenario with elevated sources with exit velocities, for which ADMS predicted significantly smaller concentrations than the other models. The concentration predictions for the latter scenario depend strongly on the ability of the models to simulate the necessary processes, as well as the interaction of these processes with pre-processor calculations of meteorological data. When applied to two case study farms in Denmark and the USA, the performance of all of the models is judged to be ‘acceptable’ according to a set of objective criteria, although the LADD model version used is currently not suitable for simulations with elevated sources with exit velocities.

Spin-Density-Wave Gap with Dirac Nodes and Two-Magnon Raman Scattering in BaFe2As2

Raman selection rules for electronic and magnetic excitations in BaFe 2 As 2 were theoretically investigated and applied them to the separate detection of the nodal and anti-nodal gap excitations at the spin density wave (SDW) transition and the separate detection of the nearest and the next nearest neighbor exchange interaction energies. Raman spectra are composed of magnetic excitations with gradually decreasing intensity toward far above the SDW transition temperature ( T SDW ) and electronic excitations induced by the Brillouin zone folding below T SDW . The SDW gap has Dirac nodes, because many orbitals participate in the electronic states near the Fermi energy. Using a two-orbital band model the electronic excitations near the Dirac node and the anti-node are found to have different symmetries. Applying the symmetry difference to Raman scattering the nodal and anti-nodal electronic excitations are separately obtained. The low-energy spectra from the anti-nodal region have critical fluctuation just above T SDW and change into the gap structure by the first order transition at T SDW , while those from the nodal region gradually change into the SDW state. Magnetic excitations are observed as a very broad peak in all polarization configurations. The selection rule for two-magnon scattering from the stripe spin structure was obtained. Applying it to the two-magnon Raman spectra it is found that the magnetic exchange interaction energies are not presented by the short-range superexchange model, but the second derivative of the total energy of the stripe spin structure with respect to the moment directions. The selection rule and the peak energy are expressed by the two-magnon scattering process in an insulator, but the large spectral weight above twice the maximum spin wave energy is difficult to explain by the decayed spin wave. It may be explained by the electronic scattering of itinerant carriers with the magnetic self-energy in the localized spin picture or the particle–hole excitation model in the itinerant spin picture. The magnetic scattering spectra are compared to the insulating and metallic cuprate superconductors whose spins are believed to be localized.

Assessing the Impact of the Covariance Localization Radius when Assimilating Infrared Brightness Temperature Observations Using an Ensemble Kalman Filter

A regional-scale Observing System Simulation Experiment is used to examine how changes in the horizontal covariance localization radius employed during the assimilation of infrared brightness temperature observations in an ensemble Kalman filter assimilation system impacts the accuracy of atmospheric analyses and short-range model forecasts. The case study tracks the evolution of several extratropical weather systems that occurred across the contiguous United States during 7–8 January 2008. Overall, the results indicate that assimilating 8.5-μm brightness temperatures improves the cloud analysis and forecast accuracy, but has the tendency to degrade the water vapor mixing ratio and thermodynamic fields unless a small localization radius is used. Vertical cross sections showed that varying the localization radius had a minimal impact on the shape of the analysis increments; however, their magnitude consistently increased with increasing localization radius. By the end of the assimilation period, the moisture, temperature, cloud, and wind errors generally decreased with decreasing localization radius and became similar to the Control case in which only conventional observations were assimilated if the shortest localization radius was used. Short-range ensemble forecasts showed that the large positive impact of the infrared observations on the final cloud analysis diminished rapidly during the forecast period, which indicates that it is difficult to maintain beneficial changes to the cloud analysis if the moisture and thermodynamic forcing controlling the cloud evolution are not simultaneously improved. These results show that although assimilation of infrared observations consistently improves the cloud field regardless of the length of the localization radius, it may be necessary to use a smaller radius to also improve the accuracy of the moisture and thermodynamic fields.

The Weather Research and Forecasting Model's Community Variational/Ensemble Data Assimilation System: WRFDA

Data assimilation is the process by which observations are combined with short-range NWP model output to produce an analysis of the state of the atmosphere at a specified time. Since its inception in the late 1990s, the multiagency Weather Research and Forecasting (WRF) model effort has had a strong data assimilation component, dedicating two working groups to the subject. This article documents the history of the WRF data assimilation effort, and discusses the challenges associated with balancing academic, research, and operational data assimilation requirements in the context of the WRF effort to date. The WRF Model's Community Variational/Ensemble Data Assimilation System (WRFDA) has evolved over the past 10 years, and has resulted in over 30 refereed publications to date, as well as implementation in a wide range of real-time and operational NWP systems. This paper provides an overview of the scientific capabilities of WRFDA, and together with results from sample operation implementations at the U.S. ...

Extreme Value Statistics Distributions in Spin Glasses

We study the probability distribution of the pseudocritical temperature in a mean-field and in a short-range spin-glass model: the Sherrington-Kirkpatrick and the Edwards-Anderson (EA) model. In both cases, we put in evidence the underlying connection between the fluctuations of the pseudocritical point and the extreme value statistics of random variables. For the Sherrington-Kirkpatrick model, both with Gaussian and binary couplings, the distribution of the pseudocritical temperature is found to be the Tracy-Widom distribution. For the EA model, the distribution is found to be the Gumbel distribution. Being the EA model representative of uniaxial magnetic materials with quenched disorder like Fe(0.5)Mn)0.5)TiO(3) or Eu(0.5)Ba(0.5)MnO(3), its pseudocritical point distribution should be a priori experimentally accessible.

A lattice dynamical investigation for the Raman and the infrared frequencies of Bi 2W 2O 9

Studies on infrared and Raman-active phonons in the Aurivillius ferroelectric Bi 2W 2O 9 were performed theoretically. The Raman and infrared phonons were calculated by applying a short-range force constant model using normal coordinates having space group Pna2 1. Sixteen stretching, two repulsive and seventeen bending force constants were used for the calculations of the zone center phonons. The calculated Raman and infrared wavenumbers are in very good agreement with the experimental ones in the literature. Potential energy distribution for this compound has also been investigated for the contribution of each force constant towards the Raman and the infrared wavenumbers.

Implementing molecular dynamics on hybrid high performance computers – Particle–particle particle-mesh

The use of accelerators such as graphics processing units (GPUs) has become popular in scientific computing applications due to their low cost, impressive floating-point capabilities, high memory bandwidth, and low electrical power requirements. Hybrid high-performance computers, machines with nodes containing more than one type of floating-point processor (e.g. CPU and GPU), are now becoming more prevalent due to these advantages. In this paper, we present a continuation of previous work implementing algorithms for using accelerators into the LAMMPS molecular dynamics software for distributed memory parallel hybrid machines. In our previous work, we focused on acceleration for short-range models with an approach intended to harness the processing power of both the accelerator and (multi-core) CPUs. To augment the existing implementations, we present an efficient implementation of long-range electrostatic force calculation for molecular dynamics. Specifically, we present an implementation of the particle–particle particle-mesh method based on the work by Harvey and De Fabritiis. We present benchmark results on the Keeneland InfiniBand GPU cluster. We provide a performance comparison of the same kernels compiled with both CUDA and OpenCL. We discuss limitations to parallel efficiency and future directions for improving performance on hybrid or heterogeneous computers.

Geometrical and Anderson transitions in harmonic chains with constrained long-range couplings

Low-dimensional systems with long-range couplings usually present phase transitions which are absent in the short-ranged counterpart model. In this work, we show that a harmonic chain with long-range couplings restricted by a cost function proportional to the chain length N exhibits two distinct phase transitions. In the present model, two sites at a distance r>1 are connected by a spring with probability 1/r(α) with the constraint that the total length of the non-nearest-neighbor couplings is limited to λN, where λ is a cost parameter. A geometrical phase transition is found at α=1.5 between a phase with a finite number of long-range couplings and a phase on which the number of long-range couplings is proportional to the system size. Further, the normal vibrational modes of this chain display a phase transition from delocalized to localized modes at a smaller value of α. Maximum effective disorder is reached at α=2 for which the frequency of the lowest vibrational mode exhibits a pronounced peak.