Phase transitions in the Falicov–Kimball model away from half-filling

The canonical Monte Carlo method is used to study the order-disorder phase transition of the Falicov–Kimball model away from half-filling. It is shown that the transition from various inhomogeneous ground-state phases to the disordered phase can be either direct or indirect. The indirect transition means that the ground-state phase first, at critical temperature τ _c , changes to a different ordered phase and at the temperature, that can be several times higher than τ _c , finally changes to the disordered phase. It is shown that the Falicov–Kimball model, depending on the ground state phase, undergoes first order or second order phase transition or can even undergo both for the same parameters and different temperatures if the transition from the ground-state phase to the disordered phase is indirect.     

Phase diagram of muonium hydride: the significant effect of dimensionality

We carry out a theoretical investigation of the low-temperature phase diagram of muonium hydride in two dimensions, using numerical simulations. It is shown that the phase diagram of this substance is qualitatively different in two and three dimensions. Specifically, while in three dimensions it is essentially identical to that of parahydrogen, i.e., only displaying a single（crystalline） phase. In two dimensions it is very similar to that of ⁴He, with an equilibrium liquid phase that turns superfluid at a temperature as high as ～2.2 K, and crystallizes under applied pressure. This is a well-described case of a continuous-space condensed matter system whose ground state equilibrium phase is qualitatively altered by dimensional reduction.     

Simulation of adsorbate-induced faceting on curved surfaces

A simple solid-on-solid model, proposed earlier to describe overlayer-induced faceting of bcc(1 1 1) surface, is applied to faceting of curved surfaces covered by an adsorbate monolayer. Surfaces studied in this paper are formed by a part of sphere around the [1 1 1] pole. Results of Monte Carlo simulations show that the morphology of a faceted surface depends on the annealing temperature. At an initial stage the surface around the [1 1 1] pole consists of 3-sided pyramids and step-like facets, then step-like facets dominate and their number decreases with temperature, finally a single big pyramid is formed. It is shown that there is a reversible phase transition at which a faceted surface transforms to an almost spherical one. It is found that the temperature of this phase transition is an increasing function of the surface curvature. Simulation results show that measurements of high temperature properties performed directly and after fast cooling down to a low temperature lead to different results.     

‘Pseudo-proper’ ferroelectric phase transitions in oxyfluoride K3WO3F3

Based on the structural data on the phases of cryolite (ordered perovskite) K₃WO₃F₃, we develop a statistical model, which allows to describe the sequence of phase transitions observed in this compound using a unified approach. According to the model, the crystal possesses two structural subsystems: the K cations located in the octahedral positions and the WO₃F₃ octahedra in positions alternating with K cations. In the symmetric (cubic) phase, each subsystem can be found in one of the eight states. At decreasing temperature, an orientational phase transition in the subsystem of octahedra occurs first, followed by a phase transition to the low-temperature phase, caused by the loss of stability with respect to the ordering in the K cation subsystem. We find that the electric polarization occurs as pseudoproper and discuss the mechanisms of formation of the phase states.     

Growth of Ordered Domains in a Computer Model Alloy with Lattice Misfit

We study via Monte Carlo simulations the influence of elastic interactions on the ordering and decomposition of a two-dimensional model binary alloy with antiferromagnetic nearest and ferromagnetic next nearest neighbor type interactions following a quench into the coexistence region. The elastic interaction leads to the development of a platelet morphology for the segregated ordered and disordered regions. A length scale characterizing the coarsening process follows a law of the type R=a+bt ^1/3 with the growth b decreasing with the amount of ordered phase; this appears to be due to the presence of anti-phase boundaries between neighboring domains ordered on different sublattices which are difficult to eliminate. The application of uniaxial external stress results in rafting of the domains. Many of the simulation results are in agreement with experimentally observed effects in nickel-base superalloys.     

Micromemory effects in shape memory alloys

Summary Thermoelastic martensitic transformations (TMT) have been on the stage for several years in connection with shape memory alloys. Interest has recently grown in partial cycling or involving the incomplete reverse transformation in a pre-programmable way. Attention is here focussed on the hysteresis cycle of several TMT (NiTi, NiTiFe, AgCd) related either to a complete transformation or to incomplete cycling as required to activate stimulated stepwise martensite-to-austenite reversible transformation (SMART). The modifications of the hysteresis cycles are discussed in the light both of built-in kinetics barriers and of the hierarchy of symmetries between parent and product phase. In honour of Prof. Fausto Fumi on the occasion of his retirement from teaching.     

Study of the adsorption of interacting dimers on square lattices by using a cluster-exact approximation

A theoretical approach, based on exact calculation of the partition function on finite rectangular clusters, is introduced to study the adsorption of interacting homonuclear dimers on square lattices. An efficient algorithm allows us to calculate the detailed structure of the configuration space for m=(k×l) clusters with m varying from 8 to 48. The adsorption process has been monitored by following thermodynamic properties such as coverage versus chemical potential, internal energy and specific heat of the adlayer, etc. The analytical results are compared with those in [Surf. Sci. 411 (1998) 294], which were obtained by using Monte Carlo simulation and finite-size scaling techniques. The theoretical adsorption isotherms and phase diagrams (critical temperature versus coverage) for both attractive and repulsive lateral interactions are in good qualitative agreement with the computational data. This agreement between simulated and theoretical results supports the validity of the cluster-exact approximation proposed in this paper.     

小体积比两相分离早期过程的三维格子气模型研究

采用Monte Carlo模拟方法研究了小体积比三维格子气模型中的相分离过程, 比较了sc, fcc, bcc 等3种晶格类型和T/T_c=0.45–0.85等 5个相对温度的系统演化行为, 计算了系统的结构因子函数, 发现结构因子在相分离早期不满足标度关系, 生长指数等于1/6, 小于经典Lifshitz-Slyozov理论的值.关键词：格子气模型相分离过程Monte Carlo模拟生长律     

利用外延生长中动力学各向异性制备有序取向周期转变超晶格的方法

文章报道了关于外延生长中动力学各向异性导致有序取向转变的理论研究结果.以CoPt合金薄膜在Pt缓冲层上的外延生长为例,在生长温度适宜时,研究发现,随生长速率的加快,体系有序结构会逐渐从L10 ［001］有序过渡到L10 ［100］有序再发展到无序结构.基于这些机制,文章作者提出了一个制备有序取向周期变化超晶格的方法.     

The possibility of detection of finite temperature stripe ordering in 2D spinless Falicov–Kimball model

This paper announces a possibility of detection of finite-temperature stripe ordering in the two-dimensional Falicov–Kimball model at half-filling by extensive Monte Carlo simulations. Moreover, the tools to study orderings and to detect phase transition temperatures are presented. The use of Binder's cumulant is supplemented by finite-size magnetization profiles not analyzed previously in this context. Continuous character of phase transitions is announced. Analyses proving the existence of more complicated phases of finite-temperature stripe ordering than the checkerboard one conclude the paper.     

Polymer mixtures in confined geometries: Model systems to explore phase transitions

While binary (A,B) symmetric polymer mixtures ind = 3 dimensions have an unmixing critical point that belongs to the 3d Ising universality class and crosses over to mean field behavior for very long chains, the critical behavior of mixtures confined into thin film geometry falls in the 2d Ising class irrespective of chain length. The critical temperature always scales linearly with chain length, except for strictly two-dimensional chains confined to a plane, for whichT _c ∝N ^5/8 (this unusual exponent describes the fractal contact line between segregated chains in dense melts in two spatial dimensions,d = 2). When the walls of the thin film are not neutral, but preferentially attract one species, complex phase diagrams occur due to the interplay between capillary condensation and wetting phenomena. For ‘competing walls’ (one wall prefers A, the other prefers B) particularly interesting interface localization-delocalization transitions occur, while analogous phenomena in wedges are related to the ‘filling transition’.