POPULATION ANNEALING: ANALYSIS, OPTIMIZATION AND APPLICATION TO GLASSY SYSTEMS

Glasses are physical systems that lack structural order and exhibit extremely slow dynamics, which makes them challenging to study. In this thesis we apply Monte Carlo methods to two distinct glassy systems: the 3D Edwards-Anderson spin glass and a binary hard sphere fluid. While significant progress has been made on theoretical and experimental fronts, much of our current understanding of glasses has come from numerical simulations. Standard Monte Carlo techniques cannot be used to perform equilibrium simulations due to slow dynamics in the glassy regime. As a result, several specialized techniques have been developed in order to simulate such systems, including the main topic of this thesis, population annealing Monte Carlo. Population annealing is a sequential Monte Carlo algorithm used to perform equilibrium simulations of systems with rough free energy landscapes, such as glasses. Unlike standard Monte Carlo algorithms, which are unable to overcome large free energy barriers, population annealing is able to sample disparate regions of configuration space in parallel. In this thesis, we discuss the optimization, analysis, and application of population annealing to model glassy systems. The 3D Edwards-Anderson spin glass has a long history of research, however, the nature of its low-temperature phase remains unclear. We use a carefully optimized canonical ensemble version of population annealing in order to obtain new benchmark values of observables that are important to understanding its low-temperature phase. We also derive and numerically test several useful metrics that provide accurate estimates of the systematic and statistical errors of a simulation. The binary hard sphere fluid is an example of a system that undergoes a dynamic transition from a fluid to a disordered glassy solid. Whether this system also undergoes a thermodynamic glass transition remains an open question. We use an NVT ensemble version of population annealing in order to simulate the binary fluid at high density and we present two new methods to measure the configurational entropy deep in the glass regime. Using our new numerical techniques, we are able to predict the location and existence of the thermodynamic glass transition

Climate change at the ecosystem scale: a 50-year record in New Hampshire

Observing the full range of climate change impacts at the local scale is difficult. Predicted rates of change are often small relative to interannual variability, and few locations have sufficiently comprehensive long-term records of environmental variables to enable researchers to observe the fine-scale patterns that may be important to understanding the influence of climate change on biological systems at the taxon, community, and ecosystem levels. We examined a 50-year meteorological and hydrological record from the Hubbard Brook Experimental Forest (HBEF) in New Hampshire, an intensively monitored Long-Term Ecological Research site. Of the examined climate metrics, trends in temperature were the most significant (ranging from 0.7 to 1.3 °C increase over 40–50 year records at 4 temperature stations), while analysis of precipitation and hydrologic data yielded mixed results. Regional records show generally similar trends over the same time period, though longer-term (70–102 year) trends are less dramatic. Taken together, the results from HBEF and the regional records indicate that the climate has warmed detectably over 50 years, with important consequences for hydrological processes. Understanding effects on ecosystems will require a diversity of metrics and concurrent ecological observations at a range of sites, as well as a recognition that ecosystems have existed in a directionally changing climate for decades, and are not necessarily in equilibrium with the current climate

Heap Reference Analysis Using Access Graphs

Despite significant progress in the theory and practice of program analysis, analysing properties of heap data has not reached the same level of maturity as the analysis of static and stack data. The spatial and temporal structure of stack and static data is well understood while that of heap data seems arbitrary and is unbounded. We devise bounded representations which summarize properties of the heap data. This summarization is based on the structure of the program which manipulates the heap. The resulting summary representations are certain kinds of graphs called access graphs. The boundedness of these representations and the monotonicity of the operations to manipulate them make it possible to compute them through data flow analysis. An important application which benefits from heap reference analysis is garbage collection, where currently liveness is conservatively approximated by reachability from program variables. As a consequence, current garbage collectors leave a lot of garbage uncollected, a fact which has been confirmed by several empirical studies. We propose the first ever end-to-end static analysis to distinguish live objects from reachable objects. We use this information to make dead objects unreachable by modifying the program. This application is interesting because it requires discovering data flow information representing complex semantics. In particular, we discover four properties of heap data: liveness, aliasing, availability, and anticipability. Together, they cover all combinations of directions of analysis (i.e. forward and backward) and confluence of information (i.e. union and intersection). Our analysis can also be used for plugging memory leaks in C/C++ languages.Comment: Accepted for printing by ACM TOPLAS. This version incorporates referees' comment

Longitudinal Eigenvibration of Multilayer Colloidal Crystals and the Effect of Nanoscale Contact Bridges

Longitudinal contact-based vibrations of colloidal crystals with a controlled layer thickness are studied. These crystals consist of 390 nm diameter polystyrene spheres arranged into close packed, ordered lattices with a thickness of one to twelve layers. Using laser ultrasonics, eigenmodes of the crystals that have out-of-plane motion are excited. The particle-substrate and effective interlayer contact stiffnesses in the colloidal crystals are extracted using a discrete, coupled oscillator model. Extracted stiffnesses are correlated with scanning electron microscope images of the contacts and atomic force microscope characterization of the substrate surface topography after removal of the spheres. Solid bridges of nanometric thickness are found to drastically alter the stiffness of the contacts, and their presence is found to be dependent on the self-assembly process. Measurements of the eigenmode quality factors suggest that energy leakage into the substrate plays a role for low frequency modes but is overcome by disorder- or material-induced losses at higher frequencies. These findings help further the understanding of the contact mechanics, and the effects of disorder in three-dimensional micro- and nano-particulate systems, and open new avenues to engineer new types of micro- and nanostructured materials with wave tailoring functionalities via control of the adhesive contact properties

A retrospective comparative study of multiple choice questions versus short answer questions as assessment tool in evaluating the performance of the students in medical pharmacology

Background: The aim was to assess the effectiveness of multiple choice versus short answer questions (SAQs) as assessment tools for evaluating performance of 2nd MBBS students.Methods: The study was observational, retrospective study of written pen and paper type assessment that utilized a sample of 100 2nd year medical students. Study consisted of two parts; part I was multiple-choice questions (MCQs) based on endocrine system where four options were given for a question and the single best answer was to be ticked. MCQ was timed at 20 mins for 30 questions with 1 mark each. There was no negative marking. Part II was SAQ on same system where 16 SAQ were given. SAQ was timed at 60 mins for 30 marks questions. After the test students had to reply on a feedback form.Result: There was a strong correlation between the marks scored in two formats of test and there was no statistical difference between the two set of marks.Conclusion: SAQs are as effective as MCQs in assessing the performance of the students in medical pharmacology

Étude expérimentale de la formulation de mortier a base de sable silteux du Togo et de liant de sachets plastiques type « voltic »

La présente étude est destinée à valoriser les déchets plastiques par la recherche d’une formule optimale de mortier à base de sable silteux du Togo et de sachets plastiques fondus du type « voltic » offrant des propriétés mécaniques optimales. Trente six (36) échantillons de mortier à base de mélange de sable silteux et de liant de sachets plastiques à des dosages variant de 7% à 28% ont été préparés et soumis aux essais physiques et mécaniques après imbibition ou non dans de l’eau. Il ressort des résultats que le dosage en liant de sachets plastiques offrant de résistance optimale en compression (σc), au fendage (τd), en traction par flexion (σf) et par fendage (σd) varient de 16% à 22% avec des résistances à la rupture respectives d’environ 10,3 MPa (à 22%), 2,2 MPa (à 16%), 5,3 MPa (à 18%) et 1,4 MPa (à 16%). Le matériau présente des modules d’élasticité en compression (Ec), en flexion (Ef) et au fendage (Gd) respectivement d’environ 3 800 MPa, 700 MPa et 60 MPa pour des taux de liants de 22%. Pour de faibles taux de liants (8 à 10%), la densité du mortier et son absorption d’eau (en masse) sont optimales (respectivement de 1,95% et 0,29%). Cette absorption d’eau est nulle pour les teneurs en liant supérieures à 18% montrant le comportement étanche des mortiers. Les mortiers peuvent donc être utilisés pour les travaux de pavages, d’étanchement des toitures et de murs pour des taux de liants compris entre 18 à 22%.Mots-clés : mortier, sachets plastiques, sable silteux, eau, propriétés physico-mécaniques