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Dynamics of Classical Scattering in a Periodic Lattice
No full textThis project concerns the analysis of mathematical model of classical scattering. I consider a 2-dimensional model of a particle that is scattered through a symmetric lattice and interact through the Lennard-Jones potential. The model is investigated as a Hamiltonian system and simulations are done in the context of molecular dynamics. The analysis is done following the formalism of chaotic scattering. In particular, the fractal dimension d_c of the scattering function and the mean exit time tau from the scattering region are computed. By increasing the lattice size from 2-by-2 to 4-by-4, I find an increase in both tau and d_c. This is an indication of an increase in the positive Lyapunov exponent chi_1 with increasing lattice size.This project concerns the analysis of mathematical model of classical scattering. I consider a 2-dimensional model of a particle that is scattered through a symmetric lattice and interact through the Lennard-Jones potential. The model is investigated as a Hamiltonian system and simulations are done in the context of molecular dynamics. The analysis is done following the formalism of chaotic scattering. In particular, the fractal dimension d_c of the scattering function and the mean exit time tau from the scattering region are computed. By increasing the lattice size from 2-by-2 to 4-by-4, I find an increase in both tau and d_c. This is an indication of an increase in the positive Lyapunov exponent chi_1 with increasing lattice size
Structural Changes in Cylinder-Forming PS-b-PB Thin Films During Solvent Vapor Annealing
No full textThe orientational changes of cylinders in Polystyrene-b-Polybutadiene (PS-b-PB) diblock copolymer thin films during solvent vapor annealing (SVA) was investigated using White- Light Interferometry (WLI) and time-resolved grazing-incidence small-angle x-ray scattering (GISAXS). The SVA procedure was performed with three different solvents of varying selec- tivity to the two blocks: Ethyl Acetate (EAC), Toluene (TOL) and Cyclohexane (CHX). A change from standing to lying cylinders was found using all three solvents. The orientational change appears to depend on the minimal polymer volume fraction phi_min of the film, and different threshold values were found for each solvent: phi_min,EAC = 0.66, phi_min,TOL = 0.63, phi_min,CHX = 0.49. In one experiment with CHX, the switch from standing to lying cylin- ders happens through a disordered phase spanning 15 minutes. This, along with other circumstantial evidence, hints at the timescales of structural changes.The orientational changes of cylinders in Polystyrene-b-Polybutadiene (PS-b-PB) diblock copolymer thin films during solvent vapor annealing (SVA) was investigated using White- Light Interferometry (WLI) and time-resolved grazing-incidence small-angle x-ray scattering (GISAXS). The SVA procedure was performed with three different solvents of varying selec- tivity to the two blocks: Ethyl Acetate (EAC), Toluene (TOL) and Cyclohexane (CHX). A change from standing to lying cylinders was found using all three solvents. The orientational change appears to depend on the minimal polymer volume fraction phi_min of the film, and different threshold values were found for each solvent: phi_min,EAC = 0.66, phi_min,TOL = 0.63, phi_min,CHX = 0.49. In one experiment with CHX, the switch from standing to lying cylin- ders happens through a disordered phase spanning 15 minutes. This, along with other circumstantial evidence, hints at the timescales of structural changes
Oxygen Dynamics in the High-Temperature Superconductor LSCO+O
No full textUnderstanding the mechanisms of high-temperature superconductivity in the cuprates remains one of the great unsolved questions of condensed matter physics. Hole-doping the Mott-insulator La2CuO4 with various chemical species creates a superconductor with a transition temperature Tc that depends heavily on the amount and type of doped species. While the amount of hole-dopingtypically dictates the phase diagram, there are certain situations where superconductivity can be suppressed or enhanced due to structural modifications caused by the dopant species. In this thesis, I investigate these structural modifications and, in particular, related dynamics in order to better understand any possible relationship to superconductivity. A particular interesting sample in this context is La2−xSrxCuO4+δ (LSCO+O), where doping can be performed with two distinct chemical species, Sr and O. Doping with Sr is ‘quenched’, meaning that Sr has a fixed, random distribution on La sites. On the other hand, doping with O is performed after crystal growth using electrochemical methods. Oxygen dopants are ‘annealed’ in the sense that they sit on interstitial sites and are mobile at room temperature. In addition, doping with O creates a superconductor with slightly better Tc that is equipped with a number of unique phenomena such as electronic phase separation and complex superstructures. In thisthesis, various aspects of phonon dynamics in LSCO+O are investigated through a combination of theoretical and experimental methods. Density functional theory and molecular dynamics is used to numerically estimate the phonon band structure and density of states. Inelastic neutron scattering is used to probe phonon dynamics and the two methods are compared. Despite the fact that Density Functional Theory is known to struggle with the electronic structure of the cuprates, excellent agreement between phonon dynamical simulations and experiments are found. An analysis of this relationship reveals unique dynamics of octahedral tilt patterns due to interstitial oxygen. We speculate that these tilt patterns may assist in relieving the inherent frustration between magnetism and superconductivity in LSCO+O. Finally, anomalous features in the Cu-O bond stretching phonon of two different LSCO+O samples is observed with neutron scattering. This ‘phonon anomaly’ is suspected to be a signature of charge density fluctuations that might be important for superconductivity. While previously observed in Sr-doped La2−xSrxCuO4, this result rules out a phonon anomaly caused by structural mechanism related to the specific dopant type
Karakterisering af vesikler ved dynamisk lysspredning
No full textFormålet med projektet er at undersøge alkohols indvirkning på brudstyrken (lysis ten- sion) af DMPC-vesikler. Ved at ekstrudere multilamellare vesikler opnår vi en monodis- pers opløsning af unilamellere vesikler. Ved at variere mængden af alkohol i membranen forventer vi en ændring af brudstyrken. En model for ekstruderingsprocessen foreslår, at denne ændring vil give anledning til en ændret størrelse af de ekstruderede vesikler. Vi måler den hydrodynamiske radius af vesiklerne med dynamisk lysspredning. Da vi har haft eksperimentelle problemer med systematiske fejl og usikkerheder, er vi ud fra vores resultater ikke i stand til at sige noget generelt om alkohols betydning for brud- styrken. Vi er derfor ikke i stand til at eftervise den foreslåede model. Vi diskuterer, på baggrund af vores eksperimentelle vanskeligheder, fejlkilder og usikkerheder med det formål at videregive erfaringer om forsøg med dynamisk lysspredning.Formålet med projektet er at undersøge alkohols indvirkning på brudstyrken (lysis ten- sion) af DMPC-vesikler. Ved at ekstrudere multilamellare vesikler opnår vi en monodis- pers opløsning af unilamellere vesikler. Ved at variere mængden af alkohol i membranen forventer vi en ændring af brudstyrken. En model for ekstruderingsprocessen foreslår, at denne ændring vil give anledning til en ændret størrelse af de ekstruderede vesikler. Vi måler den hydrodynamiske radius af vesiklerne med dynamisk lysspredning. Da vi har haft eksperimentelle problemer med systematiske fejl og usikkerheder, er vi ud fra vores resultater ikke i stand til at sige noget generelt om alkohols betydning for brud- styrken. Vi er derfor ikke i stand til at eftervise den foreslåede model. Vi diskuterer, på baggrund af vores eksperimentelle vanskeligheder, fejlkilder og usikkerheder med det formål at videregive erfaringer om forsøg med dynamisk lysspredning
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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