DRUM Collection: Physics Theses and Dissertations
http://hdl.handle.net/1903/2800
2014-10-25T08:44:10ZDuality methods in networks, computer science models, and disordered condensed matter systems
http://hdl.handle.net/1903/15829
Title: Duality methods in networks, computer science models, and disordered condensed matter systems
Authors: Mitchell, Joe Dan
Abstract: In this thesis, I explore lattice independent duality and systems to which it can be applied. I first demonstrate classical duality on models in an external field, including the Ising, Potts, and x-y models, showing in particular how this modifies duality to be lattice independent and applicable to networks. I then present a novel application of duality on the boolean satsifiability problem, one of the most important problems in computational complexity, through mapping to a low temperature Ising model. This establishes the equivalence between boolean satisfiability and a problem of enumerating the positive solutions to a Diophantine system of equations. I continue by combining duality with a prominent tool for models on networks, belief propagation, deriving a new message passing procedure, dual belief propagation. In the final part of my thesis, I shift to propose and examine a semiclassical model, the two-component Coulomb glass model, which can explain the giant magnetoresistance peak present in disordered films near a superconductor-insulator transition as the effect of competition between single particle and localized pair transport. I numerically analyze the density of states and transport properties of this model.2014-01-01T00:00:00ZCharacterization and applications of FeGa/PZT multiferroic cantilevers
http://hdl.handle.net/1903/15807
Title: Characterization and applications of FeGa/PZT multiferroic cantilevers
Authors: Wang, Yi
Abstract: Multiferroic materials and structures, which possess two or more ferroic properties, have been widely investigated because of their ability to transfer one different form of signals. The magnetoelectric (ME) effect, which results in induced voltage under applied magnetic field, makes multiferroic materials promising in applications for new types of transducers, sensors, and information storage devices. The laminated bulk composite multiferric devices had attracted a lot of attention because of their high ME coefficients, which define the strength of ME coupling.
We fabricated mechanically-resonant ME devices by depositing magnetostrictive FeGa and piezoelectric PZT thin films on Si cantilevers. Various sized cantilevers were found to exhibit different behaviors. With a 1 Oe AC magnetic driving field HAC, the small cantilever (0.95 mm × 0.2 mm × 5 μm) shows a high ME coefficient (33 V/(cm×Oe)) with a bias DC magnetic field of 66.1 Oe at the resonant frequency fr of 3833 Hz in vacuum. We found that the fr of the small cantilever continuously shifts with the bias magnetic field. A magnetic cantilever theory was used to explain this shift. In addition, we are able to demonstrate application of magnetic cantilevers in AC magnetic energy harvesters with an efficiency of 0.7 mW/cm<super>3</super>.
By driving the cantilever into the nonlinear regime with an AC magnetic field larger than 3 Oe or AC electric field larger than 5 mV, we are able to demonstrate its application in a robust multi-mode memory device based on bistable solutions of the Duffing oscillation. We can use the driving frequency, the driving amplitude, DC magnetic field, or DC electric field as the input, and use bistable vibration amplitudes of the device as the output.
We also show that parametric amplification can be used to substantially increase the ME coefficient by adding a pump voltage on the PZT layer. The parametric gain is sensitive to both the phase of pumping signal and the phase of the driving signal. The gain diverges as the pump voltage approaches the threshold. With parametric amplification, the ME coefficient can be boosted to a value as large as 2×10<super>6</super> V/(cm×Oe) from 33 V/(cm×Oe).2014-01-01T00:00:00ZInspiral-merger-ringdown models for spinning black-hole binaries at the interface between analytical and numerical relativity
http://hdl.handle.net/1903/15795
Title: Inspiral-merger-ringdown models for spinning black-hole binaries at the interface between analytical and numerical relativity
Authors: Taracchini, Andrea
Abstract: The long-sought direct detection of gravitational waves may only be a few years away, as a new generation of interferometric experiments of unprecedented sensitivity will start operating in 2015. These experiments will look for gravitational waves with frequencies from 10 to about 1000 Hz, thus targeting astrophysical sources such as coalescing binaries of compact objects, core collapse supernovae, and spinning neutron stars, among others. The search strategy for gravitational waves emitted by compact-object binaries consists in filtering the output of the detectors with template waveforms that describe plausible signals, as predicted by general relativity, in order to increase the signal-to-noise ratio.
In this work, we modeled these systems through the effective-one-body approach to the general-relativistic 2-body problem. This formalism rests on the idea that binary coalescence is universal across different mass ratios, from the test-particle limit to the equal-mass regime. It bridges the gap between post-Newtonian theory (valid in the slow-motion, weak-field limit) and black-hole perturbation theory (valid in the small mass-ratio limit, but not limited to slow motion). The project unfolded along two main avenues of inquiry, with the goal of developing faithful inspiral-merger-ringdown waveforms for generic spinning, stellar-mass black-hole binaries. On the one hand, we studied the motion and gravitational radiation of test masses orbiting Kerr black holes in perturbation theory, with the goal of extracting strong-field information that can be incorporated into effective-one-body models. On the other hand, we worked at the interface between analytical and numerical relativity by calibrating effective-one-body models against numerical solutions of Einstein's equations, and testing their accuracy when extrapolated to different regions of the parameter space. In the course of this project, we also studied conservative effects of the 2-body dynamics, namely the periastron advance, and devised algorithms for generating realistic initial conditions for spinning, precessing black-hole binaries.
The waveform models developed in this project will be employed in data-analysis pipelines and gravitational-wave searches of advanced LIGO and Virgo. In the near future, natural extensions of this work will be the inclusion of tidal effects in the comparable-mass regime (relevant for neutron-star/black-hole binaries), and spin precession in the test-particle limit.2014-01-01T00:00:00ZDirected Cell Migration: From Single Cells to Collectively Moving Cell Groups
http://hdl.handle.net/1903/15767
Title: Directed Cell Migration: From Single Cells to Collectively Moving Cell Groups
Authors: Guven, Can
Abstract: Unlike molecules, which are driven thermally by Brownian motion, eukaryotic cells move in a particular direction to accomplish designated tasks that are involved in diverse biological processes such as organ development and tumor progression. In this dissertation, I present experiments, analysis, and modeling of directed individual and collective cell migration. At subcellular scale, the migration of cells can be guided via the interaction of the cell cytoskeleton with the surrounding nanotopographic elements. I show that mechanical waves of actin polymerization are involved in this guidance–known as contact guidance–as dynamic sensors of surface nanotopography. The dynamics of guided actin waves were measured to build and test predictive models of contact guidance. The distributions of actin-wave propagation speed and direction were obtained from experimental observations of cell migration on nanotopographic surfaces as a function of the spacing between adjacent features (varying between 0.8 and 5 microns). I show that actin polymerization is preferentially localized to nanoscale features for a range of spacings. Additionally, the velocity of actin polymerization waves moving parallel to the direction of nanoridges depends on the nanoridge spacing. A model of actin polymerization dynamics in which nanoridges modify the distribution of the nucleation promoting factors captures these key observations. For individual cells, the question is how the intracellular processes result in directed migration of cells. I introduce a coarse-grained model for cell migration to connect contact guidance to intrinsic cellular oscillations.
The guidance of collective cell migration can be dictated via intercellular communication, which is facilitated by biochemical signals. I present a coarse-grained stochastic model for the influence of signal relay on the collective behavior of migrating <italic>Dictyostelium discoideum</italic> cells. In the experiment cells display a range of collective migration patterns including uncorrelated motion, formation of partially localized streams, and clumping, depending on the type of cell and the strength of the external concentration gradient of the signaling molecule cyclic adenosine monophosphate (cAMP). The collective migration model shows that the pattern of migration can be quantitatively described by considering the competition of two processes, the secretion of cAMP by the cells and the degradation of cAMP in the gradient chamber. With degradation, the model secreting cells form streams and efficiently traverse the gradient, but without degradation the model secreting cells form clumps without streaming. This observation indicates that streaming requires not only signal relay but also degradation of the signal. In addition, I show how this model can be extended to other eukaryotic systems that exhibit more complex cell-cell communication, in which the impact on collective migration is more subtle.2014-01-01T00:00:00Z