Designing electronic properties of two-dimensional crystals through optimization of deformations

One of the enticing features common to most of the two-dimensional electronic systems that are currently at the forefront of materials science research is the ability to easily introduce a combination of planar deformations and bending in the system. Since the electronic properties are ultimately determined by the details of atomic orbital overlap, such mechanical manipulations translate into modified electronic properties. Here, we present a general-purpose optimization framework for tailoring physical properties of two-dimensional electronic systems by manipulating the state of local strain, allowing a one-step route from their design to experimental implementation. A definite example, chosen for its relevance in light of current experiments in graphene nanostructures, is the optimization of the experimental parameters that generate a prescribed spatial profile of pseudomagnetic fields in graphene. But the method is general enough to accommodate a multitude of possible experimental parameters and conditions whereby deformations can be imparted to the graphene lattice, and complies, by design, with graphene's elastic equilibrium and elastic compatibility constraints. As a result, it efficiently answers the inverse problem of determining the optimal values of a set of external or control parameters that result in a graphene deformation whose associated pseudomagnetic field profile best matches a prescribed target. The ability to address this inverse problem in an expedited way is one key step for practical implementations of the concept of two-dimensional systems with electronic properties strain-engineered to order. The general-purpose nature of this calculation strategy means that it can be easily applied to the optimization of other relevant physical quantities which directly depend on the local strain field, not just in graphene but in other two-dimensional electronic membranes.Comment: 37 pages, 9 figures. This submission contains low-resolution bitmap images; high-resolution images can be found in version 1, which is ~13.5 M

Mapping unstructured mesh codes onto local memory parallel architectures

Initial work on mapping CFD codes onto parallel systems focused upon software which employed structured meshes. Increasingly, many large scale CFD codes are being based upon unstructured meshes. One of the key problems when implementing such large scale unstructured problems on a distributed memory machine is the question of how to partition the underlying computational domain efficiently. It is important that all processors are kept busy for as large a proportion of the time as possible and that the amount, level and frequency of communication should be kept to a minimum. Proposed techniques for solving the mapping problem have separated out the solution into two distinct phases. The first phase is to partition the computational domain into cohesive sub-regions. The second phase consists of embedding these sub-regions onto the processors. However, it has been shown that performing these two operations in isolation can lead to poor mappings and much less optimal communication time. In this thesis we develop a technique which simultaneously takes account of the processor topology whilst identifying the cohesive sub-regions. Our approach is based on an unstructured mesh decomposition method that was originally developed by Sadayappan et al [SER90] for a hypercube. This technique forms a basis for a method which enables a decomposition to an arbitrary number of processors on a specified processor network topology. Whilst partitioning the mesh, the optimisation method takes into account the processor topology by minimising the total interprocessor communication. The problem with this technique is that it is not suitable for dealing with very large meshes since the calculations often require prodigious amounts of computing processing power. The problem can be overcome by creating clusters of the original elements and using this to create a reduced network which is homomorphic to the original mesh. The technique can now be applied to the image network with comparative ease. The clusters are created using an efficient graph bisection method. The coarseness of the reduced mesh inevitably leads to a degradation of the solution. However, it is possible to refine the resultant partition to recapture some of the richness of the original mesh and hence achieve reasonable partitions. One of the issues to be addressed is the level of granuality to obtain the best balance between computational efficiency and optimality of the solution. Some progress has been made in trying to find an answer to this important issue. In this thesis, we show how the above technique can be effectively utilised in large scale computations. Results include testing the above technique on large scale meshes for complex flow domains

The Dog that Finally Barked:England as an Emerging Political Community

This report presents evidence which suggests the emergence of a new kind of Anglo-British identity in which the English component is increasingly the primary source of attachment for English people. It also suggests that English identity is becoming more politicised: that is, the more English a person feels, the more likely they are to believe that the current structure of the UK is unfair and to support a particularly English dimension to the governance of England

Burundi: A Critical Security Perspective

In the last few years Critical Security Studies (CSS) has emerged as a new approach to the academic study of security. This article argues that its genesis is best understood as a reaction to two developments, namely ‘real world’ changes after the end of the Cold War and the far-reaching philosophical debates that have recently been taking place within the social sciences. The authors argue for a conceptualisation of CSS based on an explicit commitment to human emancipation. They then illustrate their preferred understanding of security through a discussion of Burundi. This case study not only illustrates the theoretical claims of CSS but also serves as a contribution to a more comprehensive understanding of the security issues with which this country and its inhabitants are faced

Marxism in foreign policy

The problematic implications of the long absence of a dedicated encounter between Marxism and FPA (foreign policy analysis) are discussed. This absence has been marked by a series of different starting points and theoretical preferences between both intellectual projects. A paradigmatic turn for the incorporation of FPA and international politics into a revised Marxist research program is needed. Whereas FPA originated within a United States–centric Cold War context, growing out of the subfield of “comparative foreign policy,” which initially pursued a positivistic methodology, Marxism’s European theoretical legacy afforded neither international relations nor foreign policy analysis any systematic place since its inception in the 19th century. Recurring rapprochements were qualified successes due to Marxism’s tendency to relapse into structuralist versions of grand theorizing. While these could speak to general theories of international relations in the field of IR (international relations) from the late 20th century onward, FPA fell again and again through the cracks of this grand analytical register. Marxist FPA has only very recently been recognized as a serious research program, notably within the two traditions of neo-Gramscian international political economy (IPE) and Marxist historical sociology. With this move, Marxism has started to identify a problematique and produced a nascent literature that should bear fruit in the future

Unions of the mind:The UK as a subjective state

Those seeking to understand attitudes to decentralization focus on attitudes to constitutional change or to the ideal level of government to control particular areas of jurisdiction. Within this is a wider approach to understanding subjective dimensions of multi-level states and the different communities of interest or polities that exist within them. Drawing on data from successive rounds of the Future of England (including parallel surveys in Scotland and Wales), this article develops a conceptual framework through which to understand political unions as well as a multi-dimensional measure through which to evaluate the location of unions from a scale that runs from subjective unionism to subjective autonomism. It outlines the various unions of the mind, including an identity union, a union of economic solidarity, of social solidarity and of fairness (or legitimacy) and then proceeds to map these within the UK. It then evaluates what impact each of these has on attitudes to the wider state, including attitudes to its continued survival. The article draws primarily on individual-level survey data collected by the authors but refers also to campaigns for constitutional change in unions and relates this to what we know about how individuals conceive of the states in which they live

Optimal control of plate shape with incompatible strain fields

A flat plate can bend into a curved surface if it experiences an inhomogeneous growth field. In this article a method is described that numerically determines the optimal growth field giving rise to an arbitrary target shape, optimizing for closeness to the target shape and for growth field smoothness. Numerical solutions are presented, for the full non-symmetric case as well as for simplified one-dimensional and axisymmetric geometries. This system can also be solved semi-analytically by positing an ansatz for the deformation and growth fields in a circular disk with given thickness profile. Paraboloidal, cylindrical and saddle-shaped target shapes are presented as examples, of which the last two exemplify a soft mode arising from a non-axisymmetric deformation of a structure with axisymmetric material properties.Comment: 21 pages, 6 figure