Glassy dynamics of polymethylphenylsiloxane (PMPS) is studied by broadband dielectric spectroscopy in one-dimensional (1D) and two-dimensional (2D) nanometric confinement; the former is realized in thin polymer layers having thicknesses down to 5 nm, and the latter in unidirectional (thickness 50 µm) nanopores with diameters varying between 4 and 8 nm. Based on the dielectric measurements carried out in a broad spectral range at widely varying temperatures, glassy dynamics is analyzed in detail in 1D and in 2D confinements with the following results: (i) the segmental dynamics (dynamic glass transition) of PMPS in 1D confinement down to thicknesses of 5 nm is identical to the bulk in the mean relaxation rate and the width of the relaxation time distribution function; (ii) additionally a well separated surface induced relaxation is observed, being assigned to adsorption and desorption processes of polymer segments with the solid interface; (iii) in 2D confinement with native inner pore walls, the segmental dynamics shows a confinement effect, i.e., the smaller the pores are, the faster the segmental dynamics; on silanization, this dependence on the pore diameter vanishes, but the mean relaxation rate is still faster than in 1D confinement; (iv) in a 2D confinement, a pronounced surface induced relaxation process is found, the strength of which increases with the decreasing pore diameter; it can be fully removed by silanization of the inner pore walls; (v) the surface induced relaxation depends on its spectral position only negligibly on the pore diameter; (vi) comparing 1D and 2D confinements, the segmental dynamics in the latter is by about two orders of magnitude faster. All these findings can be comprehended by considering the density of the polymer; in 1D it is assumed to be the same as in the bulk, hence the dynamic glass transition is not altered; in 2D it is reduced due to a frustration of packaging resulting in a higher free volume, as proven by ortho-positronium annihilation lifetime spectroscopy.

No description available.

The spin-glass is a peculiar magnetic phase, exhibiting non-trivial dynamics at low temperatures, characterized by an continuously evolving state without long-range order. The behavior requires some degree of disorder to occur, often in the way of impurities or random exchange energy between the spins. However, recent research have found structurally ordered systems exhibiting glassy behaviour. This project aims to further investigate these self-induced spin-glasses. The report provides a short introduction to atomistic spin-dynamics and applies the theory to study self-induced spin-glasses in hexagonal systems with the help of simulations. A variation approach was applied by running simulation using a range of spin-exchange couplings in the Heisenberg Hamiltonian. These systems were then studied by the means of their autocorrelation function and compared to known glassy systems from the Edwards-Andersson model. The resulting behaviour is presented for three different hexagonal structures and glassy behaviour is indicated in stacked hexagonal systems. It is however argued that the autocorrelation function is not sufficient to classify these systems, instead further observables are needed. Nevertheless, the method of studying self-induced spin-glasses by varying couplings in the Heisenberg Hamiltonian is promising. As even with the few spin interactions used in this report we observe the slow relaxation time associated with spin-glasses. Given some extra considerations when choosing the exchange used for the simulation, a self-induced glassy state should be able to be recreated using the method described in this report. / Spinn-glas är en speciell magnetisk fas som uppvisar icke trivial dynamik vid låga temperaturer, en kontinuerlig utveckling samt en avsaknad av ordning på stora skalor. Detta beteende kräver en viss gradav oordning för att uppstå, ofta i form av föroreningar i materialet eller slumpmässiga interaktioner mellan olika spinn. Forskning har dock visat att även strukturellt ordnade system kan uppvisa spinn-glas beteende. Därmed är målet med detta projekt att fortsätta undersöka dessa själv-inducerade spinn-glas. Rapporten ger en kort introduktion till atomistisk spinn-dynamik och applicerar denna teori för att studera själv-inducerade spin-glas i hexagonala system. I projektet simulerades system med varierande spin-interaktioner i Heisenberg Hamiltonianen. Dynamiken undersöktes med hjälp av en korrelationsfunktion som jämfördes mot kända spinn-glas från Edwards-Andersson modellen. Resultat presenteras för tre hexagonala strukturer och spinn-glas-liknande beteende observeras i de tre-dimensionella systemen. Det kan dock argumenteras att korrelationsfunktionen inte är tillräcklig för att klassificera dessa system och att mer kvantitativa mått krävs. Trots detta anses metoden, att variera spinn-interaktioner i Heisenberg Hamiltonianen, vara lovande. Detta eftersom den långa avslappningstiden associerad med spinn-glas påträffades, trots de få interaktioner som användes i denna rapport. Ett själv-inducerat spin-glass borde därmed kunna skapas med de metoder som presenteras i rapporten, givet en mer systematisk metod vid val av interaktionsparametrar för simuleringen.

We numerically investigate the dynamical properties of driven magnetic flux vortices in disordered type-II superconductors for a variety of temperatures, types of disorder and sample thicknesses. We do so with the aid of Langevin molecular dynamics simulations of a coarsegrained elastic line model of flux vortices in the extreme London limit. Some original findings of this doctoral work include the discovery that flux vortices driven through random point disorder show simple aging following drive quenches from the moving lattice state to both the pinned glassy state (non-universal aging) and near the critical depinning region (universal aging); estimations of experimentally consistent critical scaling exponents for the continuous depinning phase transition of vortices in three dimensions; and an estimation of the boundary curve separating regions of linear and non-linear electrical transport for flux lines driven through planar defects via novel direct measurements of vortex excitations. / Ph. D. / The works contained in this dissertation were undertaken with the goal of better understanding the dynamics of driven magnetic flux lines in type-II superconductors under different conditions of temperature, material defects and sample thickness. The investigations were conducted with the aid of computer simulations of the flux lines which preserve physical aspects of the system relevant to long-time dynamics while discarding irrelevant microscopic details. As a result of this work, we found (among other things) that when driven by electric currents, flux lines display very different dynamics depending on the strength of the current. When the current is weak, the material defects strongly pin the flux lines leaving them in a disordered glassy state. Sufficiently high current overpowers the defect pinning and results in the flux lines forming into a highly ordered crystal-like structure. In the intermediate critical current regime, the competing forces become comparable resulting in very large fluctuations of the flux lines and a critical slowing down of the flux line dynamics.

We investigate the relaxation properties of two distinct systems: magnetic vortex lines in disordered type-II superconductors and charge carriers in the Coulomb glass in disordered semiconductors. We utilize an elastic line model to simulate magnetic flux lines in disordered type-II superconductors by performing Langevin molecular dynamics simulations. We study the non-equilibrium relaxation properties of flux lines in the presence of uncorrelated point-like disorder or extended linear defects analyzing the effects of rapid changes in the system's temperature or magnetic field on these properties. In a previously-equilibrated system, either the temperature is suddenly changed or the magnetic field is abruptly altered by adding or removing random flux lines to or from the system. One-time observables such as the radius of gyration are measured to characterize steady-state properties, and two-time correlation functions such as the vortex line height autocorrelations are computed to investigate the relaxation dynamics in the aging regime and therefore distinguish the complex relaxation features that result from the different types of disorder in the system. This study allows us to test the sensitivity of the system's non-equilibrium aging kinetics to the selection of initial states and to make closer contact to experimental setups. Furthermore, we employ Monte Carlo simulations to study the relaxation properties of the two-dimensional Coulomb glass in disordered semiconductors and the two-dimensional Bose glass in type-II superconductors in the presence of extended linear defects. We investigate the effects of adding non-zero random on-site energies from different distributions on the properties of the correlation-induced Coulomb gap in the density of states and on the non-equilibrium aging kinetics highlighted by the autocorrelation functions. We also probe the sensitivity of the system's equilibrium and non-equilibrium relaxation properties to instantaneous changes in the density of charge carriers in the Coulomb glass or flux lines in the Bose glass. / Ph. D.

No description available.

No description available.

No description available.

No description available.

During high strain rate deformations of metals and metallic alloys, narrow regions of intense plastic deformations have been observed experimentally. The phenomenon is termed strain localization and is usually a precursor to catastrophic failure of a structure. Similar phenomenon has been observed in glassy polymers deformed both at slow and high strain rates. Whereas strain localization is attributed to material softening due to thermal heating in metallic alloys, it is believed to be due to the reorganization of the molecular structure in polymers. Here we numerically study the strain localization in Tungsten Heavy Alloys (WHAs), and glassy polymers. WHAs are heterogeneous materials and thus inhomogeneities in deformations occur simultaneously at several places. Thus strains may localize into narrow bands at one or more places depending upon the microstructure of the alloy. We analyze the strain localization phenomenon during explosion and implosion of WHA hollow cylinders. We have developed a procedure to generate three-dimensional microstructures from planar images so that the two have the same 2-point correlation function. The WHA considered here is comprised of W particulates in a Nickel-Iron (NiFe) matrix, and each constituent is modeled as a heat conducting, strain hardening, strain-rate hardening and thermally softening elastic-plastic material. Furthermore, the porosity is taken to evolve in each constituent and the degradation of material properties due to porosity is incorporated into the problem formulation. It is found that the strain localization initiation in WHA hollow cylinders does not significantly depend on microstructural details during either explosive or implosive loading. However, the number of disconnected regions of localized deformations is influenced by the microstructure. We have generalized constitutive equations for high strain rate deformations of two glassy polymers, namely, Polycarbonate (PC) and poly (methyl methacrylate) (PMMA). These have been validated by comparing computed results with test findings in uniaxial compression at different axial strain rates, and subsequently used to study strain localization in a plate with a through-the-thickness elliptic hole at the centroid and pulled axially at a nominal strain rate of 5,000 /s. For the problems studied, the intensely deformed narrow regions have very high shear strains in WHAs, but large axial strains in glassy polymers. / Ph. D.