Half-Metallicity and High-<Emphasis Type="Italic">T</Emphasis><Subscript>c</Subscript> Ferromagnetism in Si-containing Half-Heusler Alloys

A possibility of half-metallic ferromagnetism for the homogeneous phase in the half-Heusler alloy-based DMS CoTi_1−x Fe _x Sb is predicted. A phase diagram of the spinodal and binodal decomposition is constructed through the evaluation of the mixing free energies. By applying the Monte Carlo simulation method to the Ising model with realistic chemical pair interactions between Fe magnetic impurities, we simulate the spinodal decomposition and nanoscale separation in this alloy.     

First Principle Study of Spinodal Decomposition Thermodynamics in Half-Heusler Alloy CoTi<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Fe<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Sb

A possibility of half-metallic ferromagnetism for the homogeneous phase in the half-Heusler alloy-based DMS CoTi_1?x Fe _x Sb is predicted. A phase diagram of the spinodal and binodal decomposition is constructed through the evaluation of the mixing free energies. By applying the Monte Carlo simulation method to the Ising model with realistic chemical pair interactions between Fe magnetic impurities, we simulate the spinodal decomposition and nanoscale separation in this alloy.     

First-principles derived complexion diagrams for phase boundaries in doped cemented carbides

This article reviews a method for calculating an equilibrium interfacial phase diagram depicting regions of stability for different interface structures as function of temperature and chemical potentials. Density functional theory (DFT) is used for interfacial energies, Monte Carlo simulations together with cluster expansions based on DFT results for obtaining configurational free energies, and CALPHAD-type modeling for describing the thermodynamic properties of the adjoining bulk phases. An explicit case, vanadium doped cemented carbides, is chosen to illustrate the progress in the research field and the interfacial diagram, a complexion diagram, for the phase boundary WC(0 0 0 1)/Co is constructed as function of temperature and chemical potentials.     

Spin Compensation Temperatures in the Monte Carlo Study of a Mixed Spin-3/2 and Spin-1/2 Ising Ferrimagnetic System

The magnetic properties of mixed-spin S ^′ = 3/2 and S = 1/2 ferrimagnetic system have been studied by Monte Carlo simulations. The ground-state phase diagrams of mixed spin-3/2 and spin-1/2 Ising system are given. The critical and compensation temperatures have been found with different values of reduced exchange interactions. The variation of total magnetization with reduced exchange interactions of mixed spins is given with different temperatures and crystal fields. We have also given the variation of total magnetization with crystal field for different reduced exchange interactions and different temperatures. The magnetic hysteresis cycle is found to have different values of reduced exchange interactions, temperatures, and crystal fields. The multiple hysteresis and the superparamagnetic phase are established.     

Cold Rydberg Atoms for Quantum Simulation of Exotic Condensed Matter Interactions

Quantum simulators could provide an alternative to numerical simulations for understanding minimal models of condensed matter systems in a controlled way. Typically, cold atom systems are used to simulate e.g., Hubbard models. In this paper, we discuss a range of exotic interactions that can be formed when cold Rydberg atoms are loaded into optical lattices with unconventional geometries, such as long-range electron–phonon interactions and extended Coulomb-like interactions. We show how these can lead to proposals for quantum simulators for complex condensed matter systems such as superconductors. Continuous time quantum Monte Carlo is used to compare the proposed schemes with the physics found in traditional condensed matter Hamiltonians for systems such as high temperature superconductors.     

Finding new phases for precipitate-hardening in platinum and palladium alloys

Precipitate hardening (via ordered phases rather than phase separation) of platinum and palladium can be effective even with a small volume-fraction of the ordered phase [M. Carelse, C.I. Lang, Scripta Materialia 54 (7) (2006) 1311]. The approach is particularly well suited to jewelry alloys which must be 95 wt.% pure and where ordered phases of 7:1 or 8:1 stoichiometries can be formed. We examined eight systems where this approach may lead to new applications: Pt-Al, Pd-Al, Pd-Cu, Pd-Mg, Pd-Nb, Pt-Mo, Pt-V, and Pd-V. In each system, using first-principles-based cluster expansion modeling, we have identified high stoichiometric-ratio phases that are stable. Furthermore, using Monte Carlo simulations, we have estimated the order-disorder transition temperatures to identify experimentally feasible phases. In three cases, the computational results are verified by experiment, suggesting that the remaining predictions are likely to be useful as well.     

Phase Diagram of Antiferromagnet Film Sandwiches Between Ferromagnetic Surfaces

The Monte Carlo (MC) simulation is used to study the magnetic properties of a spin-1/2 Ising antiferromagnetic film sandwiches between two ferromagnetic surfaces. The influence of different exchange surface interactions on the phase diagram of such a film is studied. In the ordered phase, it is found that the antiferromagnetic layer of the film can change in ferromagnetic or ferrimagnetic phases, depending on the values of the surface, the perpendicular and the bulk magnetic couplings. Moreover, the sublattice magnetizations are computed as a function of temperature for a film of thickness n=10 and fixed values of magnetic couplings.     

From the interface energy to the solubility limit of aluminium in nickel from first-principles and Kinetic Monte Carlo calculations

We developed a way to predict the solubility limit of solute atoms in a binary alloy using a Kinetic Monte Carlo algorithm. The idea is to use the interface energies calculated by first-principles calculations to parameterize the pair interaction energies used in the Kinetic Monte Carlo algorithm. In order to validate this method, it was tested on a very well known case: the Ni-Al alloy. We found that the calculations are in very good agreement with the previously calculated phase diagrams.     

Temperature dependence of the Henry's law constant for hydrogen storage in NaA zeolites: a Monte Carlo simulation study

Grand canonical Monte Carlo simulations of hydrogen adsorption in zeolites NaA were carried out for a wide range of temperatures between 77 and 300 K and pressures up to 180 MPa. A potential model was used that comprised of three main interactions: van der Waals, coulombic and induced polarization by the electric field in the system. The computed average number of adsorbed molecules per unit cell was compared with available results and found to be in agreement in the regime of moderate to high pressures. The particle insertion method was used to calculate the Henry coefficient for this model and its dependence on temperature.     

Critical Properties of Two-dimensional Anisotropic Ising Model on a Square Lattice

We study the critical behavior of two-dimensional anisotropic Ising model on a square lattice by the finite cluster approximation based on a single-site cluster theory and by Monte Carlo techniques. We analize the influence of the report of the interactions J _h /J v on the critical temperature of the system, and we show that the critical exponents depend only on the very general properties of the systems and not on the detail of the interactions. Finally, we do a comparison between the finite cluster approximation, the mean-field approximation, and the Monte Carlo method on the level of the phase diagram in the plan \(\left (J_{h}/T_{c}\text { },\text { } J_{v}/T_{c}\right ).\)     

Cluster variation investigation of phase equilibria in Fe–Co system using simulated annealing approach

The cluster variation method (CVM) coupled with the constrained simulated annealing (CSA) algorithm for global free energy minimization is proposed as an efficient approach for the study of alloy phase diagram. The chemical order–disorder transitions in Fe–Co alloy system between 300 and 1100 K are investigated using the proposed CSA–CVM approach using the irregular tetrahedron approximation in a bcc lattice. The calculations have been carried out for a hypothetical paramagnetic Fe–Co alloy, i.e. with chemical interactions only, as well as for Fe–Co alloy with both magnetic and chemical interactions acting simultaneously, i.e., considering the quaternary Fe^↑Fe^↓Co^↑Co^↓ system. The calculated A2/B2 boundary is found to match well with that determined experimentally. The incorporation of magnetic interaction is also found to have a large influence on the order–disorder transitions. The main advantages of the CSA–CVM approach is that it works efficiently with no initial value dependence on optimization parameters, and in contrast to some of the previously used minimization techniques, it is always found to converge to the global minimum. Another new aspect of the present investigation is that the phase diagram is obtained here by plotting long range order versus composition for the most ordered phase (B2 in this study) and not the commonly used grand potential versus chemical potential curves for various phases, thereby saving a lot of labor. Precise predictions for the variation of heat capacity with temperature have also been made.     

Atomistic simulations of Ga atom ordering in Pu 5?at. % Ga alloys

We employ molecular dynamics and Monte Carlo (MC) methods to simulate the rearrangement of Ga atoms from a randomly distributed Pu-Ga alloy and study the resultant effect on thermodynamic properties. The results show that all of the first neighbor Ga-Ga bonds are removed at all temperatures considered (200, 400 and 600 K) while the number of 2NN and 3NN bonds increase, and the number of 4NN bonds decreases. These results imply that Ga atoms develop strong short range ordering in the solid solution. The ordering causes an enthalpy decrease about ~3–4 meV/atom for different temperatures in the 5 at. % Ga alloy. This energy change is clearly important in the calculation of the Pu-Ga phase diagram. In addition, MC calculations at 200 K show pronounced Ga segregation.     

A Monte Carlo calculation of lateral interactions in silver monolayers on W(110)

The lateral interactions in silver submonolayers on W(110) are estimated by comparing phase diagrams calculated by means of Monte Carlo techniques with an experimental phase diagram inferred from work function and thermal desorption spectroscopy measurements. Pairwise interactions alone, at least within the ranges considered in this paper, do not seem to be able to reproduce part of the experimental phase diagram. The additional use of three-body interactions, however, introduces a desired asymmetry into the calculated phase diagrams, resulting in a reasonably close fit with the experimental phase diagram. Best-fit estimates for the lateral interactions were obtained by assuming attractive first-nearest-neighbour and repulsive second- and third-nearest-neighbour interactions, together with two different types of attractive three-body interaction. Ground state phase diagrams for adsorbates with pairwise interactions extending up to the fifth nearest neighbour and two different types of three-body interaction are discussed in detail for attractive first-nearest-neighbour interactions and b.c.c. (110) substrates.     

Estimation of flux distributions with Monte Carlo functional expansion tallies

Monte Carlo methods provide a powerful technique for estimating the average radiation flux in a volume (or across a surface) in cases where analytical solutions may not be possible. Unfortunately, Monte Carlo simulations typically provide only integral results and do not offer any further details about the distribution of the flux with respect to space, angle, time or energy. In the functional expansion tally (FET) a Monte Carlo simulation is used to estimate the functional expansion coefficients for flux distributions with respect to an orthogonal set of basis functions. The expansion coefficients are then used in post-processing to reconstruct a series approximation to the true distribution. Discrete event FET estimators are derived and their application in estimating radiation flux or current distributions is demonstrated. Sources of uncertainty in the FET are quantified and estimators for the statistical and truncation errors are derived. Numerical results are presented to support the theoretical development.     

Antiferromagnetism and superconductivity transitions with SO(5) symmetry

Using Monte Carlo simulation we have computed the phase diagram of a superspin model of the SO(5) symmetry between antiferromagnetism (AF) and d-wave superconductivity (SC). A bicritical point is observed at a finite temperature where the second-order AF and SC transition lines merge tangentially into a first-order line in the temperature vs. doping rate phase diagram. In the regime of the doping rate where the SC is achieved at low temperatures, maximal AF correlation length is observed at a temperature above the SC transition point T _c when AF coupling is much stronger than SC coupling. The relationship between this behavior and the spin-gap phenomenon is addressed.     

Atomistic simulations of Ga atom ordering in Pu 5 at. % Ga alloys

We employ molecular dynamics and Monte Carlo (MC) methods to simulate the rearrangement of Ga atoms from a randomly distributed Pu-Ga alloy and study the resultant effect on thermodynamic properties. The results show that all of the first neighbor Ga-Ga bonds are removed at all temperatures considered (200, 400 and 600 K) while the number of 2NN and 3NN bonds increase, and the number of 4NN bonds decreases. These results imply that Ga atoms develop strong short range ordering in the solid solution. The ordering causes an enthalpy decrease about ~3–4 meV/atom for different temperatures in the 5 at. % Ga alloy. This energy change is clearly important in the calculation of the Pu-Ga phase diagram. In addition, MC calculations at 200 K show pronounced Ga segregation.     

On the stability of intermetallic phases

A general methodology using atomic clusters is applied to three problems connected to the study of alloy phase stability. The cluster method proposed by Allen and Cahn is applied to non-ideal hcp structures under tetrahedral approximation using multiatom interactions. The possible ground-state structures which are stable at absolute zero temperature are obtained. A geometrical representation in 4D parameter space of the possible strengths of multiatom interactions permitted for these structures is illustrated in terms of a 2D analogue. Extending these ideas, the cluster variation method (CVM) proposed by Kikuchi is applied to fcc structures under tetrahedral approximation to find the effect of multiatom interactions on the topology of the coherent phase diagrams in which all the phases present are derivable by mere rearrangement of atoms on the parent disordered structure. In addition, the possible invariant reactions are identified in such coherent phase diagrams. Finally the CVM is applied for calculating a model incoherent phase diagram, that of Ti-Zr system, where disordered hcp and bcc phases are present. The free energies of hcp and bcc phases are formulated using CVM procedures respectively under tetrahedral-octahedral and tetrahedral approximations. The CVM is shown to be in better agreement with the thermodynamic data and to be able to reproduce the correct value of measured enthalpy of transformation compared to that given by the regular solution model, which significantly overestimates the same.     

Shielding of proton accelerators

This paper discusses some of the methods that can be employed for calculating shielding of proton accelerators, showing that a simple analytical model is often useful for a first estimate before going into complex Monte Carlo simulations. In particular what we call the Monte Carlo 'hybrid' approach, which employs source terms and attenuation length data calculated by Monte Carlo simulations under generic geometrical conditions, with a point-source line-of-sight model is discussed. Examples are given of the application of this method to the shielding calculations of two versions of the CERN SPL (2- and 3.5-GeV energy), comparing its results with Monte Carlo simulations of the full geometry.     

Determination of the phase diagram from interatomic potentials: The iron–chromium case

Prior to applying any interatomic potential, it is important to know the stability of the different phases it describes. In the literature many methods to determine the phase diagram from an interatomic potential are described. Although for pure elements the procedure to obtain the thermodynamic functions is well established, for alloys it is not. In this work a method is developed to determine the phase diagram, i.e., solubility limits and spinodal gap, for the case of miscibility gaps. The method combines Monte Carlo simulations in the isobaric semi-grand canonical ensemble, full thermodynamic integration and Redlich–Kister expansions to parameterize the Gibbs free energy. Besides numerical inaccuracies, this method does not rely on any physical approximations to determine the phase diagram of a given interatomic potential. The method is applied to two different Fe–Cr potentials that are widely used in the literature. The resulting phase diagrams are discussed by comparing them to the experimental one and ones obtained in other works from the same potentials.     

电子和原子层次材料行为的计算机模拟

先进的理论和计算技术以及结合计算机的威力，提供在电子和原子层次上了解及其演化过程细节的可能性。具有无先例的准确性。使材料设计和性能预测成为可能。