A comment on free-fermion conditions for lattice models in two and more dimensions

We analyze free-fermion conditions on vertex models. We show --by examining examples of vertex models on square, triangular, and cubic lattices-- how they amount to degeneration conditions for known symmetries of the Boltzmann weights, and propose a general scheme for such a process in two and more dimensions.Comment: 12 pages, plain Late

Control of thermal expansion, behavior on compression, and guest loading in framework materials

Each chapter in this thesis focuses on open framework systems found to display anomalous properties relating to their unique structures. The systems investigated were chosen for their open framework form, flexibility, and modifiability. These three key factors can allow for the structural control necessary for tunable thermal expansion and phase stability to be achieved in materials that inherently display large negative thermal expansion (NTE). Concepts necessary for the understanding of thermal expansion research will be covered in sufficient detail as well as examples from the different materials classes being investigated in this thesis. Definitions, mechanisms, applications, and real examples of thermal expansion, behavior upon compression, and composite research are reviewed in chapter 1. Chapters 2, 3 and 4 examine the control of thermal expansion in ReO3-type metal fluoride systems. ReO3-type materials exhibit the simplest crystalline structure found to exhibit strong NTE. Because of its inherent simplicity, modification and analysis of these structures is relatively straightforward. This being the case, the coefficients of thermal expansion (CTEs) of ReO3-type materials have been found to range from strongly positive to strongly negative. This fact reveals deeper complications involving the ionicity, bond strength, and general flexibility of these structures that lead to diverse thermodynamic behavior. In these chapters certain metal fluorides were chosen that have previously been found to exhibit both structural tunability and large NTE. The structural tunability allows for certain flexible structural components to be replaced by more rigid counterparts (systematically) to control the properties related directly to flexibility, such as thermal behavior. Specifically, Mg1-xZr1+xF6+2x and Sc1-xZrxF3+x solid solutions were found to display tunable thermal expansion and phase stability upon the introduction of edge-sharing polyhedra within their structures. The introduction of this specific defect is mediated by the metal ratio within these systems. The use of defects to control thermodynamic behavior is a common theme in this thesis, but the work in chapter 4 shows that the techniques involved in behavioral control must be modified specifically for each system they are being applied to. In chapter 4, Ca[Zr(IV)1-xNb(V)x]F6+x solid solutions are investigated using the same general technique applied successfully to the materials of chapters 1 and 2. Interestingly, it is shown that this specific variation of the technique does not appear to introduce the same defect that was found in the previous two systems. This new defect introduced leads to changes in the structural behavior that are non-ideal for application but demonstrates the importance of developing a diverse toolkit when approaching thermodynamic behavioral modification. Chapters 5 and 6 investigate an even broader range of thermal behavior control techniques, which span from structural scaffold modification to guest inclusion. A comparison of how effective these techniques are for a diverse set of metal organic frameworks (MOFs) is drawn and previously unexplored routes of structural control are introduced for these systems. While some techniques facilitated direct tunability of thermal expansion from positive to negative within a measured temperature range, others simply showed non-systematic changes in the thermal expansion or just the controlled inhibition of pronounced NTE. Insights into the origin of these differences in thermal behavior are obtained through an in-depth analysis of synchrotron‐radiation total scattering and diffraction experiments, as well as complementary molecular simulations performed by collaborators. The implications of these works on the prospects for MOFs as an emergent material class for NTE-related applications are also discussed. While several metal–organic frameworks are known to display negative thermal expansion, there have been no reports where the thermal expansion of a MOF has been tuned continuously from negative to positive through the formation of single-phase solid solutions. In the system Zn-DMOF-TMx, Zn2[(bdc)2–2x(TM-bdc)2x][dabco], the introduction of increasing amounts of TM-bdc, with four methyl groups decorating the benzene dicarboxylate linker, leads to a smooth transition from negative to positive thermal expansion in the a–b plane of this tetragonal material. The temperature at which zero thermal expansion occurs evolves from ∼186 K for the Zn-DMOF parent structure (x = 0) to ∼325 K for Zn-DMOF-TM (x = 1.0). The formation of mixed linker solid solutions is likely a general strategy for the control of thermal expansion in MOFs and its prospects are also discussed. Finally, Chapter 7 expands upon the recent discovery of a new class of hybrid ReO3-type fluoride perovskites that contain a neutral molecule (He) in the perovskite A-site. These experiments involve taking a prototypical NTE metal fluoride, ScF3, and exposing it to high pressure helium at high temperature. The pore aperture and activation barriers for diffusion are compared to that of the first known hybrid fluoride perovskite with the formula [He]CaZrF6. Evidence for the inclusion of He in ScF3 is reviewed and future directions for the project are discussed.Ph.D

Communicating new knowledge on previously reported genetic variants

Genetic tests often identify variants whose significance cannot be determined at the time they are reported. In many situations, it is critical that clinicians be informed when new information emerges on these variants. It is already extremely challenging for laboratories to provide these updates. These challenges will grow rapidly as an increasing number of clinical genetic tests are ordered and as the amount of patient DNA assayed per test expands; the challenges will need to be addressed before whole-genome sequencing is used on a widespread basis. Information technology infrastructure can be useful in this context. We have deployed an infrastructure enabling clinicians to receive knowledge updates when a laboratory changes the classification of a variant. We have gathered statistics from this deployment regarding the frequency of both variant classification changes and the effects of these classification changes on patients. We report on the system's functionality as well as the statistics derived from its use. Genet Med 2012:14(8):713–71

Racial Residential Segregation and Race Differences in Ideal Cardiovascular Health among Young Men

Background: Race disparities in cardiovascular disease (CVD) related morbidity and mortality are evident among men. While previous studies show health in young adulthood and racial residential segregation (RRS) are important factors for CVD risk, these factors have not been widely studied in male populations. We sought to examine race differences in ideal cardiovascular health (CVH) among young men (ages 24–34) and whether RRS influenced this association. Methods: We used cross-sectional data from young men who participated in Wave IV (2008) of the National Longitudinal Survey of Adolescent to Adult Health (N = 5080). The dichotomous outcome, achieving ideal CVH, was defined as having ≥4 of the American Heart Association’s Life’s Simple 7 targets. Race (Black/White) and RRS (proportion of White residents in census tract) were the independent variables. Descriptive and multivariate analyses were conducted. Results: Young Black men had lower odds of achieving ideal CVH (OR = 0.67, 95% CI = 0.49, 0.92) than young White men. However, RRS did not have a significant effect on race differences in ideal CVH until the proportion of White residents was ≥55%. Conclusions: Among young Black and White men, RRS is an important factor to consider when seeking to understand CVH and reduce future cardiovascular risk

Gas of self-avoiding loops on the brickwork lattice

An exact calculation of the phase diagram for a loop gas model on the brickwork lattice is presented. The model includes a bending energy. In the dense limit, where all the lattice sites are occupied, a phase transition occuring at an asymmetric Lifshitz tricritical point is observed as the temperature associated with the bending energy is varied. Various critical exponents are calculated. At lower densities, two lines of transitions (in the Ising universality class) are observed, terminated by a tricritical point, where there is a change in the modulation of the correlation function. To each tricritical point an associated disorder line is found.Comment: 19 pages, 6 figures. to appear in J. Phys. A : Math. & Ge

Learning from Monte Carlo Rollouts with Opponent Models for Playing Tron

This paper describes a novel reinforcement learning system for learning to play the game of Tron. The system combines Q-learning, multi-layer perceptrons, vision grids, opponent modelling, and Monte Carlo rollouts in a novel way. By learning an opponent model, Monte Carlo rollouts can be effectively applied to generate state trajectories for all possible actions from which improved action estimates can be computed. This allows to extend experience replay by making it possible to update the state-action values of all actions in a given game state simultaneously. The results show that the use of experience replay that updates the Q-values of all actions simultaneously strongly outperforms the conventional experience replay that only updates the Q-value of the performed action. The results also show that using short or long rollout horizons during training lead to similar good performances against two fixed opponents

Enhancing neural-network performance via assortativity

The performance of attractor neural networks has been shown to depend crucially on the heterogeneity of the underlying topology. We take this analysis a step further by examining the effect of degree-degree correlations -- or assortativity -- on neural-network behavior. We make use of a method recently put forward for studying correlated networks and dynamics thereon, both analytically and computationally, which is independent of how the topology may have evolved. We show how the robustness to noise is greatly enhanced in assortative (positively correlated) neural networks, especially if it is the hub neurons that store the information.Comment: 9 pages, 7 figure

Extended scaling relations for planar lattice models

It is widely believed that the critical properties of several planar lattice models, like the Eight Vertex or the Ashkin-Teller models, are well described by an effective Quantum Field Theory obtained as formal scaling limit. On the basis of this assumption several extended scaling relations among their indices were conjectured. We prove the validity of some of them, among which the ones by Kadanoff, [K], and by Luther and Peschel, [LP].Comment: 32 pages, 7 fi