Controlling Grain Boundaries by Magnetic Fields.

The ability to use external magnetic fields to influence the microstructure in polycrystalline materials has potential applications in microstructural engineering. To explore this potential and to understand the complex interactions between electromagnetic fields and solid-state matter transport we consider a phase-field-crystal model. Together with efficient and scalable numerical algorithms this allows the examination of the role that external magnetic fields play on the evolution of defect structures and grain boundaries, on diffusive timescales. Examples for planar and circular grain boundaries explain the essential atomistic processes and large scale simulations in 2D are used to obtain statistical data on grain growth under the influence of external fields.

Two-component structural phase-field crystal models for graphene symmetries.

We extend the three-point XPFC model of Seymour & Provatas (Seymour & Provatas 2016 Phys. Rev. B93, 035447 (doi:10.1103/PhysRevB.93.035447)) to two components to capture chemical vapour deposition-grown graphene, and adapt a previous two-point XPFC model of Greenwood et al. (Greenwood et al. 2011 Phys. Rev. B84, 064104 (doi:10.1103/PhysRevB.84.064104)) into a simple model of two-component graphene. The equilibrium properties of these models are examined and the two models are compared and contrasted. The first model is used to study the possible roles of hydrogen in graphene grain boundaries. The second model is used to study the role of hydrogen in the dendritic growth morphologies of graphene. The latter results are compared with new experiments.This article is part of the theme issue 'From atomistic interfaces to dendritic patterns'.

Grain Boundary Structures and Collective Dynamics of Inversion Domains in Binary Two-Dimensional Materials.

Understanding and controlling the properties and dynamics of topological defects is a lasting challenge in the study of two-dimensional materials, and is crucial to achieve high-quality films required for technological applications. Here grain boundary structures, energies, and dynamics of binary two-dimensional materials are investigated through the development of a phase field crystal model that is parametrized to match the ordering, symmetry, energy, and length scales of hexagonal boron nitride. Our studies reveal some new dislocation core structures for various symmetrically and asymmetrically tilted grain boundaries, in addition to those obtained in previous experiments and first-principles calculations. We also identify a defect-mediated growth dynamics for inversion domains governed by the collective atomic migration and defect core transformation at grain boundaries and junctions, a process that is related to inversion symmetry breaking in binary lattice.

Emergence of Chirality from Isotropic Interactions of Three Length Scales.

Chirality is known to play a pivotal role in determining material properties and functionalities. However, it remains a great challenge to understand and control the emergence of chirality and the related enantioselective process particularly when the building components of the system are achiral. Here we explore the generic mechanisms driving the formation of two-dimensional chiral structures in systems characterized by isotropic interactions and three competing length scales. We demonstrate that starting from isotropic and rotationally invariant interactions, a variety of chiral ordered patterns and superlattices with anisotropic but achiral units can self-assemble. The mechanisms for selecting specific states are related to the length-scale coupling and the selection of resonant density wave vectors. Sample phase diagrams and chiral elastic properties are identified. These findings provide a viable route for predicting chiral phases and selecting the desired handedness.

Honeycomb and triangular domain wall networks in heteroepitaxial systems.

A comprehensive study is presented for the influence of misfit strain, adhesion strength, and lattice symmetry on the complex Moiré patterns that form in ultrathin films of honeycomb symmetry adsorbed on compact triangular or honeycomb substrates. The method used is based on a complex Ginzburg-Landau model of the film that incorporates elastic strain energy and dislocations. The results indicate that different symmetries of the heteroepitaxial systems lead to distinct types of domain wall networks and phase transitions among various surface Moiré patterns and superstructures. More specifically, the results show a dramatic difference between the phase diagrams that emerge when a honeycomb film is adsorbed on substrates of honeycomb versus triangular symmetry. It is also shown that in the small deformation limit, the complex Ginzburg-Landau model reduces to a two-dimensional sine-Gordon free energy form. This free energy can be solved exactly for one dimensional patterns and reveals the role of domains walls and their crossings in determining the nature of the phase diagrams.

Exploring the complex world of two-dimensional ordering with three modes.

The world of two-dimensional crystals is of great significance for the design and study of structural and functional materials with novel properties. Here we examine the mechanisms governing the formation and dynamics of these crystalline or polycrystalline states and their elastic and plastic properties by constructing a generic multimode phase field crystal model. Our results demonstrate that a system with three competing length scales can order into all five Bravais lattices, and other more complex structures including honeycomb, kagome, and other hybrid phases. In addition, nonequilibrium phase transitions are examined to illustrate the complex phase behavior described by the model. This model provides a systematic path to predict the influence of lattice symmetry on both the structure and dynamics of crystalline and defected systems.

Anomalous fast dynamics of adsorbate overlayers near an incommensurate structural transition.

We investigate the dynamics of a compressively strained adsorbed layer on a periodic substrate via a simple two-dimensional model that admits striped and hexagonal incommensurate phases. We show that the mass transport is superfast near the striped-hexagonal phase boundary and in the hexagonal phase. For an initial step profile separating a bare substrate region (or "hole") from the rest of a striped incommensurate phase, the superfast domain wall dynamics leads to a bifurcation of the initial step profile into two interfaces or profiles propagating in opposite directions with a hexagonal phase in between. This yields a theoretical understanding of the recent experiments for the Pb/Si(111) system.

Patterning of heteroepitaxial overlayers from nano to micron scales.

Thin heteroepitaxial overlayers have been proposed as templates to generate stable, self-organized nanostructures at large length scales, with a variety of important technological applications. However, modeling strain-driven self-organization is a formidable challenge due to different length scales involved. In this Letter, we present a method for predicting the patterning of ultrathin films on micron length scales with atomic resolution. We make quantitative predictions for the type of superstructures (stripes, honeycomb, triangular) and length scale of pattern formation of two metal-metal systems, Cu on Ru(0001) and Cu on Pd(111). Our findings are in excellent agreement with previous experiments and call for future experimental investigations of such systems.

Corrigenda to "The Oldham Notebooks: An analysis of the development of IVF 1969-1978. II. The treatment cycles and their outcomes" [Reprod. Biomed. Soc. Online 1/1 (2015) 9-18].

[This corrects the article DOI: 10.1016/j.rbms.2015.04.003.].

Novel mechanical properties in lamellar phases of liquid-crystalline diblock copolymers.

Structural properties of flexible nematic diblock copolymers in the lamellar phase are investigated using a mean-field model. We address two complementary questions on the mechanics of the system: 1) How does the nematic order affect the elasticity of the one-dimensional solid? 2) What effect does the block copolymer microstructure has on the orientation of the nematic director? In the limit when the microstructure does not influence the nematic director orientation we predict a soft lamellar compression mode. When the microstructure does influence the nematic director orientation, small compressions lead to conventional elasticity, until a critical strain is reached, where there is then a transition to a softer response. On the other hand, we show that an identifiable lamellar symmetry provides a direction along which the nematic director prefers to align. Our model provides avenues to explore nonlinear properties of flexible diblock copolymers in which the monomers on both sides have mesogenic side groups.

Traveling waves of the solidification and melting of cubic crystal lattices.

Using the phase field crystal model (PFC model), an analysis of slow and fast dynamics of solid-liquid interfaces in solidification and melting processes is presented. Dynamical regimes for cubic lattices invading metastable liquids (solidification) and liquids propagating into metastable crystals (melting) are described in terms of the evolving amplitudes of the density field. Dynamical equations are obtained for body-centered cubic (bcc) and face-centered cubic (fcc) crystal lattices in one- and two-mode approximations. A universal form of the amplitude equations is obtained for the three-dimensional dynamics for different crystal lattices and crystallographic directions. Dynamics of the amplitude's propagation for different lattices and PFC mode's approximations is qualitatively compared. The traveling-wave velocity is quantitatively compared with data of molecular dynamics simulation previously obtained by Mendelev et al. [Modell. Simul. Mater. Sci. Eng. 18, 074002 (2010)MSMEEU0965-039310.1088/0965-0393/18/7/074002] for solidification and melting of the aluminum fcc lattice.

Nonlinear driven response of a phase-field crystal in a periodic pinning potential.

We study numerically the phase diagram and the response under a driving force of the phase field crystal model for pinned lattice systems introduced recently for both one- and two-dimensional systems. The model describes the lattice system as a continuous density field in the presence of a periodic pinning potential, allowing for both elastic and plastic deformations of the lattice. We first present results for phase diagrams of the model in the absence of a driving force. The nonlinear response to a driving force on an initially pinned commensurate phase is then studied via overdamped dynamic equations of motion for different values of mismatch and pinning strengths. For large pinning strength the driven depinning transitions are continuous, and the sliding velocity varies with the force from the threshold with power-law exponents in agreement with analytical predictions. Transverse depinning transitions in the moving state are also found in two dimensions. Surprisingly, for sufficiently weak pinning potential we find a discontinuous depinning transition with hysteresis even in one dimension under overdamped dynamics. We also characterize structural changes of the system in some detail close to the depinning transition.

Simulation of an atomistic dynamic field theory for monatomic liquids: freezing and glass formation.

We examine a phase field crystal model for simple liquid-solid systems consisting of a free energy functional related to the Ramakrishnan-Yussouff free energy of classical density functional theory and an equation of motion capable of describing long-time-scale behavior in the deeply supercooled regime. The thermodynamics and dynamics of freezing and glass formation in this model system are studied through large-scale three-dimensional Langevin simulations. At low cooling rates bcc crystals are formed by nucleation and growth from the melt. At large cooling rates no clear glass transition is observed, but a kinetically driven first-order transition from supercooled liquid to a disordered glasslike solid does occur. Despite the peculiarities of the transition, the structure and properties of the resulting disordered solid are shown to strongly resemble those of a typical glass. Consequences of pseudocritical behavior and heterogeneity near the liquid spinodal are also discussed.

Thermal fluctuations and phase diagrams of the phase-field crystal model with pinning.

We study the influence of thermal fluctuations in the phase diagram of a recently introduced two-dimensional phase field crystal model with an external pinning potential. The model provides a continuum description of pinned lattice systems allowing for both elastic deformations and topological defects. We introduce a nonconserved version of the model and determine the ground-state phase diagram as a function of lattice mismatch and strength of the pinning potential. Monte Carlo simulations are used to determine the phase diagram as a function of temperature near commensurate phases. The results show a rich phase diagram with commensurate, incommensurate, and liquidlike phases with a topology strongly dependent on the type of ordered structure. A finite-size scaling analysis of the melting transition for the c(2x2) commensurate phase shows that the thermal correlation length exponent nu and specific heat behavior are consistent with the Ising universality class as expected from analytical arguments.

Psychiatric illness delays diagnosis of esophageal cancer.

Evidence suggests that patients with psychiatric illnesses may be more likely to experience a delay in diagnosis of coexisting cancer. The association between psychiatric illness and timely diagnosis and survival in patients with esophageal cancer has not been studied. The specific aim of this retrospective cohort study was to determine the impact of coexisting psychiatric illness on time to diagnosis, disease stage and survival in patients with esophageal cancer. All patients with a diagnosis of esophageal cancer between 1989 and 2003 at the Portland Veteran's Administration hospital were identified by ICD-9 code. One hundred and sixty patients were identified: 52 patients had one or more DSM-IV diagnoses, and 108 patients had no DSM-IV diagnosis. Electronic charts were reviewed beginning from the first recorded encounter for all patients and clinical and demographic data were collected. The association between psychiatric illness and time to diagnosis of esophageal cancer and survival was studied using Cox proportional hazard models. Groups were similar in age, ethnicity, body mass index, and history of tobacco and alcohol use. Psychiatric illness was associated with delayed diagnosis (median time from alarm symptoms to diagnosis 90 days vs. 35 days in patients with and without psychiatric illness, respectively, P < 0.001) and the presence of advanced disease at the time of diagnosis (37% vs. 18% of patients with and without psychiatric illness, respectively, P= 0.009). In multivariate analysis, psychiatric illness and depression were independent predictors for delayed diagnosis (hazard ratios 0.605 and 0.622, respectively, hazard ratio < 1 indicating longer time to diagnosis). Dementia was an independent risk factor for worse survival (hazard ratio 2.984). Finally, psychiatric illness was associated with a decreased likelihood of receiving surgical therapy. Psychiatric illness is a risk factor for delayed diagnosis, a diagnosis of advanced cancer, and a lower likelihood of receiving surgical therapy in patients with esophageal cancer. Dementia is associated with worse survival in these patients. These findings emphasize the importance of prompt evaluation of foregut symptoms in patients with psychiatric illness.

Phase-field-crystal model for ordered crystals.

We describe a general method to model multicomponent ordered crystals using the phase-field-crystal (PFC) formalism. As a test case, a generic B2 compound is investigated. We are able to produce a line of either first-order or second-order order-disorder phase transitions, features that have not been incorporated in existing PFC approaches. Further, it is found that the only elastic constant for B2 that depends on ordering is C_{11}. This B2 model is then used to study antiphase boundaries (APBs). The APBs are shown to reproduce classical mean-field results. Dynamical simulations of ordering across small-angle grain boundaries predict that dislocation cores pin the evolution of APBs.

Diffusive atomistic dynamics of edge dislocations in two dimensions.

The fundamental dislocation processes of glide, climb, and annihilation are studied on diffusive time scales within the framework of a continuum field theory, the phase field crystal model. Glide and climb are examined for single edge dislocations subjected to shear and compressive strain, respectively, in a two-dimensional hexagonal lattice. It is shown that the natural features of these processes are reproduced without any explicit consideration of elasticity theory or ad hoc construction of microscopic Peierls potentials. Particular attention is paid to the Peierls barrier for dislocation glide or climb and the ensuing dynamic behavior as functions of strain rate, temperature, and dislocation density. It is shown that the dynamics are accurately described by simple viscous motion equations for an overdamped point mass, where the dislocation mobility is the only adjustable parameter. The critical distance for the annihilation of two edge dislocations as a function of separation angle is also presented.

Phase diagram and commensurate-incommensurate transitions in the phase field crystal model with an external pinning potential.

We study the phase diagram and the commensurate-incommensurate transitions in a phase field model of a two-dimensional crystal lattice in the presence of an external pinning potential. The model allows for both elastic and plastic deformations and provides a continuum description of lattice systems, such as for adsorbed atomic layers or two-dimensional vortex lattices. Analytically, a mode expansion analysis is used to determine the ground states and the commensurate-incommensurate transitions in the model as a function of the strength of the pinning potential and the lattice mismatch parameter. Numerical minimization of the corresponding free energy shows reasonable agreement with the analytical predictions and provides details on the topological defects in the transition region. We find that for small mismatch the transition is of first order, and it remains so for the largest values of mismatch studied here. Our results are consistent with results of simulations for atomistic models of adsorbed overlayers.

Cancer mortality in relatives of retinoblastoma patients.

The risk of other cancers in relatives of retinoblastoma (RTB) patients was determined by a survey of the mortality experience of siblings, parents, parental siblings, and grandparents of all U.S. or Canadian RTB patients referred to The University of Texas M.D. Anderson Hospital and Tumor Institute between 1944 and 1980. Expected mortality was ascertained by the application of age-, sex-, race-, and calendar year-specific U.S. mortality rates to the observed person-years. Among 607 relatives of 33 unilateral-sporadic RTB probands, no excess in cancer deaths was observed (observed/expected = 18/22). Among 733 relatives of 47 bilateral-familial RTB probands, a slight excess in cancer deaths was observed (41/31). A significant excess in cancer deaths was occurred in relatives under age 55 years (18/9) and in fathers (7/1) of the bilateral RTB probands. To determine whether the cancer excess was related to some unique allele associated with second tumors in RTB survivors, the cancer mortality of 203 relatives of the 14 RTB patients with second tumors was examined, and no excess was observed (11/11). To determine whether the excess might be attributable to an unexpressed RTB gene or precursor, the mortality experience was examined in 6 kindreds in which parents, unaffected by RTB, had more than 1 child with RTB. Among these 72 relatives a significant excess in cancer deaths was observed (8/2). The findings demonstrate a modest overall cancer excess in relatives of hereditary RTB patients and suggest it may be attributable to an unexpressed RTB gene or precursor in a small number of kindreds. Mechanisms for an apparent "precursor" might involve a delayed mutation, genetic mosaicism, or a submicroscopic balanced chromosomal translocation.