Superconducting Transition-Edge Sensor Physics

Despite record-setting performance demonstrated by superconducting Transition Edge Sensors (TESs) and growing utilization of the technology, a theoretical model of the physics governing TES devices superconducting phase transition has proven elusive. Earlier attempts to describe TESs assumed them to be uniform superconductors. Sadleir et al. 2010 shows that TESs are weak links and that the superconducting order parameter strength has significant spatial variation. Measurements are presented of the temperature T and magnetic field B dependence of the critical current Ic measured over 7 orders of magnitude on square Mo/Au bilayers ranging in length from 8 to 290 microns. We find our measurements have a natural explanation in terms of a spatially varying order parameter that is enhanced in proximity to the higher transition temperature superconducting leads (the longitudinal proximity effect) and suppressed in proximity to the added normal metal structures (the lateral inverse proximity effect). These in-plane proximity effects and scaling relations are observed over unprecedentedly long lengths (in excess of 1000 times the mean free path) and explained in terms of a Ginzburg-Landau model. Our low temperature Ic(B) measurements are found to agree with a general derivation of a superconducting strip with an edge or geometric barrier to vortex entry and we also derive two conditions that lead to Ic rectification. At high temperatures the Ic(B) exhibits distinct Josephson effect behavior over long length scales and following functional dependences not previously reported. We also investigate how film stress changes the transition, explain some transition features in terms of a nonequilibrium superconductivity effect, and show that our measurements of the resistive transition are not consistent with a percolating resistor network model.

The effect of maternal employment on adolescents' transition to young adulthood

The purpose of this study was to examine relationships between multiple characteristics of maternal employment, parenting practices, and adolescents’ transition outcomes to young adulthood. The research addressed four main research questions. First, are the characteristics of maternal work (i.e., hours worked, multiple jobs held, work schedules, earnings, and occupation) related to adolescents’ enrollment in post-secondary education, employment, or involvement in neither of these types of activities as young adults? Second, are the work characteristics related to parental involvement and monitoring, and are the parenting practices related to adolescents’ transition outcomes? Third, do parental involvement and monitoring mediate any relationships between the characteristics of maternal employment and adolescents’ transition outcomes? Finally, do any associations between characteristics of maternal employment and parenting practices and adolescents’ transition outcomes vary by poverty status, race/ethnicity, or gender? To address these research questions, secondary data analysis was conducted, using data from the National Longitudinal Survey of Youth (NLSY) from 1998 through 2004. The study sample consisted of 849 youths who were 15 through 17 years of age in either 1998 or 2000, and were 19 through 21 years of age when their transition outcomes in young adulthood were measured four years later. Multinomial logistic and ordinary least squares regression models were estimated to answer the research questions. Study findings indicated that of the maternal work characteristics, mothers’ multiple jobs held, occupation, and work schedule were significantly related to the youths’ transition outcomes. When mothers held multiple jobs for 1 to 25 weeks per year, and when mothers held jobs involving lower levels of occupational complexity, their youths were more likely to experience employment rather than post-secondary education. Adolescents whose mothers worked a standard work schedule were less likely to experience other types of transitions than post-secondary education. With regard to the effects of maternal employment on parenting practices, none of the maternal work variables were related to parental involvement, and only one variable, mothers working less than 40 hours per week, was negatively related to parental monitoring. In addition, when parents were more involved with their youths’ education, the youths were less likely to transition into employment and other types of transitions rather than post-secondary education. The parenting practices did not mediate the relation between the significant work variables (holding multiple jobs, work schedule, and occupation) and youths’ transition outcomes. Finally, none of the interactions between maternal work characteristics and poverty status, race/ethnicity, and gender met the criteria for determining significance; but in a series of sub-group analyses, some differences according to poverty status and gender were found. Despite the lack of mediation and moderation, the findings of this study have important implications for social policy and social work intervention. Based on the findings, suggestions are made in these areas to improve working mothers’ lives and their adolescents’ development and successful transition to adulthood. Finally, directions for future research are discussed.

Shape and chemically anisotropic colloidal particles: A theoretical study of their structure, phase behavior, and slow dynamics

A detailed non-equilibrium state diagram of shape-anisotropic particle fluids is constructed. The effects of particle shape are explored using Naive Mode Coupling Theory (NMCT), and a single particle Non-linear Langevin Equation (NLE) theory. The dynamical behavior of non-ergodic fluids are discussed. We employ a rotationally frozen approach to NMCT in order to determine a transition to center of mass (translational) localization. Both ideal and kinetic glass transitions are found to be highly shape dependent, and uniformly increase with particle dimensionality. The glass transition volume fraction of quasi 1- and 2- dimensional particles fall monotonically with the number of sites (aspect ratio), while 3-dimensional particles display a non-monotonic dependence of glassy vitrification on the number of sites. Introducing interparticle attractions results in a far more complex state diagram. The ideal non-ergodic boundary shows a glass-fluid-gel re-entrance previously predicted for spherical particle fluids. The non-ergodic region of the state diagram presents qualitatively different dynamics in different regimes. They are qualified by the different behaviors of the NLE dynamic free energy. The caging dominated, repulsive glass regime is characterized by long localization lengths and barrier locations, dictated by repulsive hard core interactions, while the bonding dominated gel region has short localization lengths (commensurate with the attraction range), and barrier locations. There exists a small region of the state diagram which is qualified by both glassy and gel localization lengths in the dynamic free energy. A much larger (high volume fraction, and high attraction strength) region of phase space is characterized by short gel-like localization lengths, and long barrier locations. The region is called the attractive glass and represents a 2-step relaxation process whereby a particle first breaks attractive physical bonds, and then escapes its topological cage. The dynamic fragility of fluids are highly particle shape dependent. It increases with particle dimensionality and falls with aspect ratio for quasi 1- and 2- dimentional particles. An ultralocal limit analysis of the NLE theory predicts universalities in the behavior of relaxation times, and elastic moduli. The equlibrium phase diagram of chemically anisotropic Janus spheres and Janus rods are calculated employing a mean field Random Phase Approximation. The calculations for Janus rods are corroborated by the full liquid state Reference Interaction Site Model theory. The Janus particles consist of attractive and repulsive regions. Both rods and spheres display rich phase behavior. The phase diagrams of these systems display fluid, macrophase separated, attraction driven microphase separated, repulsion driven microphase separated and crystalline regimes. Macrophase separation is predicted in highly attractive low volume fraction systems. Attraction driven microphase separation is charaterized by long length scale divergences, where the ordering length scale determines the microphase ordered structures. The ordering length scale of repulsion driven microphase separation is determined by the repulsive range. At the high volume fractions, particles forgo the enthalpic considerations of attractions and repulsions to satisfy hard core constraints and maximize vibrational entropy. This results in site length scale ordering in rods, and the sphere length scale ordering in Janus spheres, i.e., crystallization. A change in the Janus balance of both rods and spheres results in quantitative changes in spinodal temperatures and the position of phase boundaries. However, a change in the block sequence of Janus rods causes qualitative changes in the type of microphase ordered state, and induces prominent features (such as the Lifshitz point) in the phase diagrams of these systems. A detailed study of the number of nearest neighbors in Janus rod systems reflect a deep connection between this local measure of structure, and the structure factor which represents the most global measure of order.