Monte Carlo Study of Magnetic and Thermal Phase Transitions of a Diluted Magnetic Nanosystem

Monte Carlo (MC) simulation method with the Metropolis algorithm is used to study the magnetic and thermal phase transition properties of a spherical nanoparticle. The system consists of two concentric spheres of rays R _C and R _S, respectively (R _c < R _s). For r < R _c, the spin is σ = ±3 /2 and ±1 /2, and for R _C < r ≤ R _S, the spin is S = ±7 /2, + 5/2, ±3 /2, and ±1 /2 with antiferromagnetic interface coupling. Between R _C and R _S, the sites are populated with the probability (p). We present a detailed discussion on the magnetic and thermal phase transition characteristics of the system under consideration. Our investigations show that this system can be used as a magnetic nanostructure possessing potential applications in magnetism.     

Wetting and Layering Transitions in a Nano-Square Structure: Monte Carlo Study

In this work, we study the magnetic wetting and layering transitions in a nano-square structure. The system is consisting of particles with spins s= 1/2 and S=1 placed at the square sites. We study the effect of the exchange couplings between different spins, namely, s- s, S- s, and S-S. The wetting phenomena and layering transitions are found under the effect of the temperature and the external magnetic field, for fixed values of the exchange coupling parameters. When studying the temperature effect on the magnetizations, using Monte Carlo simulations, the ground-state phase diagrams confirm these results for very low temperatures. Over the 2 × 2 × 2 × 3 = 24 possible configurations, only 6 configurations are found to be stable in the ground-state phase diagrams. Though the model established equations are valid for any system sizes, we give results and numerical values for a fixed system size formed only by four square layers.     

Quantum Monte Carlo Study of Entanglement in Quantum Spin Systems

We perform an extensive quantum Monte Carlo investigation of entanglement properties in quantum spin systems close to or at a quantum critical point. Making use of the Stochastic Series Expansion method, we can systematically estimate the bipartite entanglement of the ground-state wavefunction in a large class of anisotropic spin models on unfrustrated lattices and in a uniform magnetic field. The behavior of the entanglement estimators as a function of the field shows remarkable universal features independent of the lattice dimensionality, marking both the occurrence of a field-induced quantum phase transition and of an exactly factorized state.PACS numbers: 03.67.Mn, 75.10.Jm, 73.43.Nq, 05.30.-d     

Path-Integral Monte Carlo Study of Phonons in the bcc Phase of 3He

Using Path Integral Monte Carlo and the Maximum Entropy method, we calculate the dynamic structure factor of solid ³He in the bcc phase at a finite temperature of T = 1.6 K and a molar volume of 21.5 cm³. From the single phonon dynamic structure factor, we obtain both the longitudinal and transverse phonon branches along the main crystalline directions, [001], [011] and [111]. Our results are compared with other theoretical predictions and available experimental data.     

Nanostructure of the interface modified by grafted polymers: a Monte Carlo simulation

The thermodynamic, conformational and orientational properties of polymer melts grafted on a solid substrate were obtained from a novel Monte Carlo (MC) simulation of coarse-grained model of polyethylene (PE). The interface between a non-interacting hard surface and a bulk PE melt, with all chains of which are grafted on the plane, has been studied. Different PE melts, of mean molecular length from C40 and C80, have been investigated, at grafting densities ranging from 0.92 to 1.85 nm. Profiles of monomer density and free end density, bond orientation, and average monomer position along a chain were studied. Quantitative measured in the simulations are derived from the analytical self-consistent field (SCF) theory and compared with the simulation data. The conformational and orientational properties can be quite accurately described by the theory, with some discrepancies observed near the wall and at the tail of the profile. Additional results concerning thermodynamic and surface energy of the brush are also presented.     

Monte Carlo Study of the Phase Diagrams of a Ferrimagnetic Nanowire with Alternate Layers

A Monte Carlo simulation based on Metropolis algorithm has been used to investigate the magnetic properties of an Ising ferrimagnetic nanowire on a hexagonal structure, which is made up with L alternate layers of spins σ = ±3/2, ±1/2 and S = ±1, 0. The critical behaviors are studied, in the absence as well as in the presence of crystal field interactions. The obtained results show the existence of some interesting phenomena, such as the appearance of different types of phase diagrams. Furthermore, tricritical point, isolated critical point, critical end point, and compensation behavior are found for appropriate values of the system parameters.     

Magnetism in Nanoislands: a Monte Carlo Study

The properties of nanoislands consisting of mixed spins-2 and -5/2 have been investigated using Monte Carlo simulations. Some interesting phenomena have been found in the thermal variations of the nanoisland system. The shapes of total magnetization and total magnetic susceptibility are greatly influenced by the exchange couplings. The reduced transition temperature of the nanoisland is deduced. The magnetic hysteresis cycles are obtained for different temperatures, crystal fields, and exchange interactions. Some results obtained in the nanoisland may have potential applications in different research fields, such as electronics, optics, mechanics, and even biomedicine and molecular devices.     

Study of Magnetic Phase Transition in Diluted Nanostructure Spinel Ferrites

The magnetic properties of CoFe_2−2x Cr_2x O₄ spinels are studied by different theoretical methods: the mean field theory, the high-temperature series expansions (MFT, HTSE) and the probability distribution law with the different size L (nm). The exchange interactions and the critical temperature T _c (K) are given for the nano-spinel ferrites. The magnetic phase diagram of CoFe_2−2x Cr_2x O₄ is deduced. The saturation magnetization, the coercivity field and magnetic energy are obtained.     

Monte Carlo Study of Magnetic Properties in Semiconducting MnTe

The magnetic properties of antiferromagnetic MnTe alloys have been studied by Monte Carlo simulations within Ising model framework. The considered Hamiltonian includes nearest-neighbor interactions, external magnetic field, and crystal field. Magnetizations and magnetic susceptibility versus temperature are computed for a fixed size. The blocking temperature is established, and the magnetic hysteresis cycle is deduced for different values of temperature. The remanent magnetization and the coercive field are obtained. In addition, the magnetization versus the first exchange interaction (J ₁) with a fixed value of the second exchange interaction (J ₂) and the third second interaction (J ₃) is estimated. The magnetization versus the crystal field is deduced.     

Monte Carlo Study of the Magnetic Properties of ErRh Layers

The magnetic properties of antiferromagnetic ErRh layers have been studied using Monte Carlo simulations within the framework of the Ising model. The Hamiltonian considered includes both nearest-neighbor interactions and external magnetic field. Magnetizations and magnetic susceptibility versus temperature are computed for a fixed size. In addition, blocking temperature versus both size and applied external magnetic field is estimated. Hysteresis cycle versus temperature, saturation magnetization, coercive field, and the remanent magnetization are reported as well.     

Nanographene Magnetic Properties: A Monte Carlo Study

The magnetic properties of an Ising antiferromagnetic model on a nanographene lattice which have spins that can take the values S=±3/2, ±1/2, are studied by Monte Carlo simulations. We only consider the nearest-neighbor interactions between the sites i and?j. The zero field cooled and field cooled magnetizations, magnetic susceptibilities and hysteresis cycle are obtained for a nanographene structure with the effect of exchange interaction between sites i and?j. The blocking temperature is also obtained for specific values of the external magnetic field.     

Monte Carlo simulations in zeolites

In this short review, the applications of Monte Carlo simulations to the study of the adsorption and diffusion of hydrocarbons in zeolites are discussed. We focus on those systems for which the conventional molecular simulation techniques, molecular dynamics and Monte Carlo, are not sufficiently efficient. In particular, to simulate the adsorption and diffusion of long-chain hydrocarbons, novel Monte Carlo techniques have been developed. Here we discuss configurational-bias Monte Carlo (CBMC) and kinetic Monte Carlo (KMC). CBMC was developed to compute the thermodynamic properties. KMC is applied to compute transport properties. The use of these methods is illustrated with examples of technological importance.     

Monte Carlo模拟薄膜生长的研究

阐述了Monte Carlo方法在薄膜生长中的应用和最新进展;简要论述了Monte Carlo算法的类型及各自的特点;结合MonteCarlo方法的特点,提出了模拟薄膜生长的模型以及处理方法.同时,归纳出MonteCarlo模拟薄膜生长需要解决的主要问题.     

Phase Transitions in Four-Fermion Models

The most important advances in the physics of four-fermion theories are illustrated using the Nambu–Jona-Lasinio and Gross–Neveu models. Magnetic-field-catalyzed dynamical chiral symmetry breaking is considered, and the magnetic-field effect on the effective fermion mass is assessed. The models in question are used to interpret the recently discoveredPparity breaking in high-T _c superconductors. High-T _c materials are considered in which the superconducting transition can be interpreted within the Gross-Neveu model as due to the formation of Berezinskii–Kosterlitz–Thouless vortex–antivortex pairs. The generalized Gross–Neveu model with superconducting states is described. Examples are presented of nonperturbative kink–antikink solutions in the Gross–Neveu model, and their role in the restoration of spontaneously broken symmetry is discussed. A number of Gross–Neveu models on a sphere are considered with application to C₆₀ fullerenes and related matrices with T _c = 117 K. It is shown that the curvature effect restores chiral symmetry according to the critical phase dependence r _c(T _c).     

Quantum Monte Carlo Study of Inhomogeneous Bond Dilution in the Two-Dimensional Heisenberg Antiferromagnet

The problem whether a two-dimensional Heisenberg antiferromagnet on a square lattice can be driven through a quantum phase transition by either bond or site dilution has attracted a lot of recent interest. Such a diluted system is of direct relevance for antiferromagnetic layered cuprates compounds, doped with nonmagnetic impurities. Both experimental results and numerical analysis now give evidence that such systems with homogeneous site and bond dilution are driven through a classical percolation transition instead of a quantum phase transition. In this paper we show that inhomogeneous bond dilution introduces the different scenario of a percolative quantum phase transition. The ground state of the bond-diluted system can be a spin liquid characterized by an infinite percolating network with vanishing antiferromagnetic order parameter. This quantum disordered phase appears as an intermediate regime between the geometrically disordered phase and the antiferromagnetic ordered phase. Here we investigate this phenomenon using Stochastic Series Expansion Quantum Monte Carlo simulations.PACS numbers: 75.10.Jm, 75.10.Nr, 75.40.Cx, 64.60.Ak     

蒙特卡罗方法模拟陶瓷晶粒生长研究进展

概述了蒙特卡罗模拟方法的基本原理和思想,介绍了陶瓷烧结过程中,蒙特卡罗分别模拟单相系统和二相系统中晶粒生长的模型,综述了国内外蒙特卡罗方法模拟陶瓷烧结过程中晶粒生长的研究进展,指出了当前该研究领域中存在的问题,提出了今后发展的主要方向:将模型与具体材料和实际工艺联系起来,模拟定量化,并向三维模拟发展,以解决实际问题.     

Solid-Fluid Phase Coexistence of Hard Heteronuclear Dumbbells via Cell Theory and Monte Carlo Simulation

We study the solid-fluid equilibria of hard heteronuclear dumbbells using cell theory and isobaric ensemble Monte Carlo simulations. Calculations for six cases of L* (bond length) and σ* (sphere diameter ratio) near the homonuclear limit are discussed with two base-centered monoclinic orientationally-ordered crystal structures which have been considered. The two crystal structures exhibit nearly identical properties at freezing within the accuracy of the calculations for the cases we present. The reduced pressure at coexistence increases with decreasing σ*.