Crystalline misfit-angle implications for solid sliding

For the contact of two finite portions of interacting rigid crystalline surfaces, we compute the dependence of the pinning energy barrier on the misfit angle and contact area. The resulting data are used to investigate the distribution of static frictional thresholds for a contact of polycrystal surfaces, as occurs at the touching points of dry or even lubricated friction. The simplicity of the model allows us to investigate a broad contact-size and angular range, thus obtaining the statistical properties of the energy barriers opposing sliding for a single asperity. These statistical properties are used as the input of a master-equation model to predict the sliding properties of two macroscopic surfaces in contact. The model is consistent with the well-established result that low temperature should generally favor stick-slip motion, while at high temperature sliding should be smooth.Comment: 11 pages, including 7 figure

Social Security Reform: Legal Analysis of Social Security Benefit Entitlement Issues

[Excerpt] Calculations indicating that the current Social Security program will not be financially sustainable in the long run under the present statutory scheme have fueled the current debate regarding Social Security reform. This report addresses selected legal issues that may be raised regarding entitlement to Social Security benefits as Congress considers possible changes to the Social Security program in view of projected long-range shortfalls in the Social Security Trust Funds. Social Security benefits are administered pursuant to Title II of the Social Security Act, known as the Old Age, Survivors and Disability Insurance (OASDI) program. Title II is part of a larger social insurance program in which Congress uses its power to tax and spend for the general welfare to promote the social goals of aiding the aged, survivors of workers, disabled persons, and persons of limited means. Beneficiaries under Title II have a legal entitlement to receive Social Security benefits as set forth by the Social Security Act and as administered by the Social Security Administration (SSA), an independent agency in the executive branch. An individual’s right to Social Security benefits is in a sense “earned,” since there is a general relationship between OASDI benefits and wages earned and the tax paid thereon. However, benefits are not directly measured by the amount of payments made through the years into the system. Thus, the fact that Social Security benefits are financed by taxes on an employee’s wages does not provide a limit on Congress’s power to fix the levels of benefits under the Social Security Act, or the conditions upon which they may be paid

On the relationship between topological and geometric defects

The study of topology in solids is undergoing a renaissance following renewed interest in the properties of ferroic domain walls as well as recent discoveries regarding topological insulators and skyrmionic lattices. Each of these systems possess a property that is `protected' in a symmetry sense, and is defined rigorously using a branch of mathematics known as topology. In this article we review the formal definition of topological defects as they are classified in terms of homotopy theory, and discuss the precise symmetry-breaking conditions that lead to their formation. We distinguish topological defects from geometric defects, which arise from the details of the stacking or structure of the material but are not protected by symmetry. We provide simple material examples of both topological and geometric defect types, and discuss the implications of the classification on the resulting material properties