Coupled electron-ion monte carlo calculations of dense metallic hydrogen

We present an efficient new Monte Carlo method which couples path integrals for finite temperature protons with quantum Monte Carlo calculations for ground state electrons, and we apply it to metallic hydrogen for pressures beyond molecular dissociation. We report data for the equation of state for temperatures across the melting of the proton crystal. Our data exhibit more structure and higher melting temperatures of the proton crystal than do Car-Parrinello molecular dynamics results. This method fills the gap between high temperature electron-proton path integral and ground state diffusion Monte Carlo methods and should have wide applicability.     

Diffusion correlation effects of molybdenum and silicon in molybdenum disilicide

Diffusion data for both principal directions of silicon and molybdenum as well as germanium are briefly summarised. Analysis is performed of the defect formation energies (available from previous ab initio calculations and experimental measurements) for diffusion mechanisms via home and foreign sublattices. The home sublattice mechanism is shown to be the preferred one for both silicon and molybdenum. Tracer correlation factors for silicon and molybdenum diffusion via sublattice vacancies in the respective sublattices of the tetragonal C11_b structure of molybdenum disilicide are calculated by a direct Monte Carlo simulation technique. Correlation factors for Si diffusion on its sublattice are compared with literature values that were calculated using a more complicated Monte Carlo method based on the matrix approach. It is shown that there is no need for this complicated approach and that the direct Monte Carlo simulation technique gives highly accurate correlation factors. Correlation factors and anisotropy ratios of vacancy-mediated diffusion in both sublattices are deduced and compared with experimental data. Tracer correlation in the tetragonal direction is shown to contribute 0.40?eV (i.e. over 55%) of the migration energy of the corresponding Si diffusivity. Two possible jump rates for Si diffusion are separately estimated. Mo diffusion correlation factors are calculated using the direct Monte Carlo technique. A comparison with experiment is made and the ratio of two possible jump rates is also estimated.     

Simulation of small molecule diffusion using continuous space disordered networks

Disordered networks were created and kinetic Monte Carlo simulations were conducted in order to assess the effects of jump network connectivity on the diffusion coefficient. Off-lattice jump networks were created using reverse Monte Carlo, with an objective function defined by agreement to specified inter-site connectivity, inter-site (jump path) distance and consecutive jump angle distributions. Both Gaussian and Poisson distributions of connectivity were applied, with average connectivities spanning a range appropriate for small molecules within a variety of polymers. Distance and angle distributions were taken from earlier work on jump networks in polypropylene. At short times, anomalous diffusion with a range of exponents n ≥ 0.4 was found over the domain of average connectivities. At longer times, diffusion was normal for connectivities above the percolation threshold, while particles were trapped for lower connectivities. The percolation threshold was slightly higher for Gaussian distributions of connectivity than for Poisson distributions. The diffusion coefficient increased linearly for connectivities well above the threshold, with slightly faster diffusion occurring for Gaussian distributions of connectivity.     

Tracer diffusion in concentrated lattice gas models. Rectangular lattices with anisotropic jump rates

An approximate theory is developed for tracer diffusion in rectangular lattice gas models with anisotropic jump rates to neighboring unoccupied sites in different directions. Comparison with Monte Carlo simulations on quadratic lattices with several ratios for the jump rates in orthogonal directions shows a satisfactory agreement in all cases investigated.     

Structure and proton conduction in CsDSO4

Caesium hydrogen sulphate is one of the most extensively studied superprotonic conductors with hydrogen bonds. A first order phase transition, from the low conductivity to the high conductivity phase, takes place at 414 K. The crystallographic structure of both phases has been established using high-resolution neutron powder diffraction. Quasielastic neutron scattering has provided information about the spatial and temporal characteristics of proton transport. In the present paper we report the results of reverse Monte Carlo modelling of neutron diffraction (total scattering) data which makes it possible to obtain the diffusion pathways for protons. The results are in good agreement with the qualitative model for proton transport proposed previously; in particular we show clearly that the proton motion is highly correlated with the rotation of the sulphate groups. In the low temperature phase we identify a weak correlation between protons and oxygens on the next-nearest-neighbour sulphate group which increases with temperature and probably drives the phase transition.     

Kinetic Monte Carlo simulations of Pd deposition and island growth on MgO(1 0 0)

The deposition and ripening of Pd atoms on the MgO(1 0 0) surface are modeled using kinetic Monte Carlo simulations. The density of Pd islands is obtained by simulating the deposition of 0.1 ML in 3 min. Two sets of kinetic parameters are tested and compared with experiment over a 200-800 K temperature range. One model is based upon parameters obtained by fitting rate equations to experimental data and assuming the Pd monomer is the only diffusing species. The other is based upon transition rates obtained from density functional theory calculations which show that small Pd clusters are also mobile. In both models, oxygen vacancy defects on the MgO surface provide strong traps for Pd monomers and serve as nucleation sites for islands. Kinetic Monte Carlo simulations show that both models reproduce the experimentally observed island density versus temperature, despite large differences in the energetics and different diffusion mechanisms. The low temperature Pd island formation at defects is attributed to fast monomer diffusion to defects in the rate-equation-based model, whereas in the DFT-based model, small clusters form already on terraces and diffuse to defects. In the DFT-based model, the strong dimer and trimer binding energies at charged oxygen vacancy defects prevent island ripening below the experimentally observed onset temperature of 600 K.     

Long jumps in the surface diffusion of large molecules

We have studied the diffusion of the two organic molecules DC and HtBDC on the Cu(110) surface by scanning tunneling microscopy. Surprisingly, we find that long jumps, spanning multiple lattice spacings, play a dominating role in the diffusion of these molecules--the root-mean-square jump lengths are as large as 3.9 and 6.8 lattice spacings, respectively. The presence of long jumps is revealed by a new and simple method of analysis, which is tested by kinetic Monte Carlo simulations.     

Surface diffusion of Cr adatoms on Au(111) by quantum tunneling

The low-temperature surface diffusion of isolated Cr adatoms on Au(111) has been determined using nonperturbing x rays. Changes in the x-ray magnetic circular dichroism spectral line shape together with Monte Carlo calculations demonstrate that adatom nucleation proceeds via quantum tunneling diffusion rather than over-barrier hopping for temperatures <40K. The jump rates are shown to be as much as 35 orders of magnitude higher than that expected for thermal over-barrier hopping at 10 K.     

VUV spectroscopy of magnetically trapped atomic hydrogen

We discuss the experimental and theoretical aspects of absorption spectroscopy of cold atomic hydrogen gas in a magnetostatic trap using a pulsed narrow-band source (bandwidth 100 MHz) at the Lyman- wavelength (121.6 nm). A careful analysis of the measured absorption spectra enables us to determine non-destructively the temperature and the density of the trapped gas. The development of this diagnostic technique is important for future attempts to reach Bose-Einstein condensation in trapped atomic hydrogen.     

Interstitial solute diffusion in metals

The diffusion of interstitial solute atoms, which were subject to nearest neighbour solute-solute interactions, was simulated by means of a modified Monte Carlo method. Solute correlation factors, nearest neighbour vacancy availability factors and effective jump frequency factors were calculated as functions of composition and temperature. Compared with non-interacting atoms attraction between solute atoms always retarded diffusion whereas repulsion always enhanced diffusion except within the ordered region.     

Fluorescence quenching kinetics in short polymer chains: Dependence on chain length

Monte Carlo simulations of chain conformations and the diffusion equation were used to analyze the fluorescence kinetics of short polymer chains labeled with a probe and a quencher at opposite ends. In simulations, three chain models were considered: an ideal chain (without volume interactions); a self-avoiding chain taking into account the exclusive volume effect; and a self-avoiding chain with limited flexibility between nearest segments. For each model, end-to-end distance distribution functions were obtained, which were different from Gaussian ones. The distribution functions were used in a diffusion equation to simulate the fluorescence kinetics of the probe affected by intramolecular end-to-end collisions of short chains. The kinetics has been numerically calculated for a representative experimental system in a nonviscous solution. The simulated time-resolved fluorescence decays were monoexponential except at very short times (<2 ns). Diffusion coefficients were calculated for different chain models and different chain lengths. The experimental data could be reproduced by assuming systematically smaller end-to-end diffusion coefficients for the shorter chains.     

Spin correlations and magnetic order in nonsuperconducting Nd(2-x)Ce(x)CuO(4+/-delta)

We report quantitative neutron scattering measurements of the evolution with doping of the Néel temperature, the antiferromagnetic correlations, and the ordered moment of as-grown, nonsuperconducting Nd(2-x)Ce(x)CuO(4+/-delta) (0相似文献   

Ep≤63 GeV 质子入射铅、铋引起的中子产生双微分截面的模拟(英文)

加速器驱动次临界系统(ADS) 液态Pb-Bi 散裂靶的设计中,需要可靠的理论计算工具精确地预言几个GeV 能量范围的质子引起的散裂反应产生的各种粒子和核素。利用蒙特卡罗模拟软件包Geant4 计算研究了800 MeV至3 GeV 质子入射铅、铋材料引起的中子产生双微分截面。比较了Geant4 不同物理模型得到的模拟结果与现有的实验数据。其中,Geant4 的QGSP BERT和QGSP INCL ABLA 物理模型模拟结果很好地再现了实验数据。本工作证实了Geant4 蒙特卡罗模拟软件包适合用于能量高达3 GeV 的质子入射铅、铋引起的中子产生双微分截面的模拟计算。A detailed design of the liquid Pb-Bi spallation target of the Accelerator Driven Systems (ADS) requires powerful and reliable computational tools that can accurately predict particles and nuclides production by the proton induced spallation reactions in the energy range of a few GeV. In this paper, the neutron production double-differential cross sections for Pb and Bi target materials at incident proton kinetic energies between 800 MeV and 3 GeV are studied by calculations with Monte Carlo simulation package Geant4. The simulated results of Geant4 with several physics models are compared with available experimental data. The simulated results generated by QGSP BERT and QGSP INCL ABLA physics models of Geant4 well reproduce the available experimental data. The present results validated that Geant4 Monte Carlo simulation package is suitable for simulations of neutron production double-differential cross sections of proton induced reaction on Pb and Bi targets in the incident energy range up to 3 GeV.     

Monte Carlo Hamiltonian: Linear Potentials

We further study the validity of the Monte Carlo Hamiltonian method. The advantage of the method,in comparison with the standard Monte Carlo Lagrangian approach, is its capability to study the excited states. Weconsider two quantum mechanical models: a symmetric one V(x) = |x|/2; and an asymmetric one V(x) = ∞, forx ＜ 0 and V(x) = x, for x ≥ 0. The results for the spectrum, wave functions and thermodynamical observables are inagreement with the analytical or Runge-Kutta calculations.     

Phenomenological coefficients in a concentrated alloy for the dumbbell mechanism

We present an adaptation of the self-consistent mean field (SCMF) theory to calculate the transport coefficients in a concentrated alloy for diffusion by the dumbbell mechanism. In this theory, kinetic correlations are accounted for through a set of effective interactions within a non-equilibrium distribution function of the system. Transport coefficients are calculated for the FCC and BCC multicomponent concentrated alloys for simple sets of jump frequencies, including different stabilities of the different defects. A first approximation leads to an analytical expression of the Onsager coefficients in a binary alloy, and a second approximation provides a more accurate prediction. The results of the SCMF theory are compared with existing models and available Monte Carlo simulations, and an interpretation of the set of effective interactions in terms of a competition between jump frequencies is proposed.     

Vacancy-assisted diffusion in silicon: a three-temperature-regime model

In this Letter we report kinetic lattice Monte Carlo simulations of vacancy-assisted diffusion in silicon. We show that the observed temperature dependence for vacancy migration energy is explained by the existence of three diffusion regimes for divacancies. This characteristic has been rationalized with an analytical model. In the intermediate temperature regime the divacancy dissociation plays a key role and an effective migration energy E{v}{m} approximately 2 eV is predicted, computed from either full ab initio values or mixed with experimental ones. The exact position of this temperature regime strongly depends on vacancy concentration. Previous contradictory experimental results are revisited using this viewpoint.     

Generalized pricing formulas for stochastic volatility jump diffusion models applied to the exponential Vasicek model

Path integral techniques for the pricing of financial options are mostly based on models that can be recast in terms of a Fokker-Planck differential equation and that, consequently, neglect jumps and only describe drift and diffusion. We present a method to adapt formulas for both the path-integral propagators and the option prices themselves, so that jump processes are taken into account in conjunction with the usual drift and diffusion terms. In particular, we focus on stochastic volatility models, such as the exponential Vasicek model, and extend the pricing formulas and propagator of this model to incorporate jump diffusion with a given jump size distribution. This model is of importance to include non-Gaussian fluctuations beyond the Black-Scholes model, and moreover yields a lognormal distribution of the volatilities, in agreement with results from superstatistical analysis. The results obtained in the present formalism are checked with Monte Carlo simulations.     

A hybrid Monte Carlo/fluid model of RF plasmas in aSiH4/H2 mixture

A fluid model for simulating the capacitively coupled steady RF plasmas in a monosilane-hydrogen mixture under plasma-processing conditions is developed. This model includes Monte Carlo treatment for electron transport properties in the mixture. The Monte Carlo simulation shows that the electron transport phenomena in the nonuniform RF field of 10 MHz are not in equilibrium with the local electric field. The nonequilibrium transport properties are incorporated in the fluid model (hybrid model) by modifying the equilibrium values of the swarm parameters using data from the Monte Carlo simulation. Using this model, the spatiotemporal variations of the charged species and the electric field in the sheath region of the RF plasma are calculated. For obtaining the steady RF plasma structure, ion-induced slow processes such as recombination and diffusion of ions are calculated by combining the hybrid model with rate equations for ions. Using the calculated steady RF plasma structure, a preliminary calculation of the silyl (SiH ₃) and hydrogen (H) radical distributions caused by the generation, diffusion, and reaction of the radicals are carried out. The effect of sticking on the profile of the radical distribution is presented     

Lattice-assisted proton hopping in oxides at low temperatures

Stimulated diffusion of protons in oxides such as ABO₃ crystals and rutile TiO₂ is discussed in the context of quantum Brownian motion. A self-consistent lattice-assisted proton hopping (LAPH) model is developed by going from white noise (characteristic of the standard stochastic theory of superionic conduction) to colored noise in the Markovian limit. This model differs from the commonly used ion jump models in that the hydrogen diffusion rate prefactor is identified as a quantity proportional to the frequency of phonon assistance. Application of the quantum fluctuation–dissipation theorem suggests that the dynamic activation energy for diffusion is a function of a bath-mode frequency. The LAPH model can predict enhanced rates of barrier jumping at room temperature compared to thermally activated proton diffusion. This indicates that low-temperature solid oxide devices are potential candidates for use in hydrogen energy research. The LAPH model offers a valid explanation for the mechanism of high protonic mobility recently observed for TiO₂ in a picosecond transient pump-probe experiment. This unexpected dominant lattice relaxation channel must be considered as a new classical-like (but low-temperature) proton transfer mechanism. For vibration-assisted protonic jumps to occur at low temperature, the phonon assistance must be classified as a low-frequency vibration specific to each lattice.     

Interaction-induced partitioning and magnetization jumps in the mixed-spin oxide FeTiO3-Fe2O3

In this study we report on jumps in the magnetic moment of the hemo-ilmenite solid solution (x)FeTiO(3)-(1-x)Fe(2)O(3) above Fe(III) percolation at low temperature (T<3 K). The first jumps appear at 2.5 K, one at each side of the magnetization loop, and their number increases with decreasing temperature and reaches 5 at T=0.5 K. The jumps occur after field reversal from a saturated state and are symmetrical in the trigger field and intensity with respect to the field axis. Moreover, an increase of the sample temperature by 2.8% at T=2.0 K indicates the energy released after the ignition of the magnetization jump, as the spin-currents generated by the event are dissipated in the lattice. The magnetization jumps are further investigated by Monte Carlo simulations, which show that these effects are a result of magnetic interaction-induced partitioning on a sublattice level.