Thesis (Ph.D.) Massachusetts Institute of Technology. Dept. of Physics, 1950. / Vita. / Bibliography: leaves 115-119. / by Karl Uno Ingard. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2003. / In title on t.p. "[delta]" appears subscript as the lower-case Greek letter. / Includes bibliographical references (p. 154-157). / Neutron scattering, magnetization and transport measurements were performed on single crystals of Bi2Sr2CoO6+[delta] to study the evolution of the magnetic properties as a function of the oxygen content . The oxygen content was varied by annealing single crystals in either a reducing or oxidizing environment to obtain an experimental range of 0.25 =/< [delta] =/< 0.5 and a corresponding average Co valence between +2.5 and +3. We show that the as-grown samples, which are oxygen rich ([delta approximately] 0.5) and therefore contain mostly Co3+ ions, enter an antiferromagnetic (AF) phase with a Neel temperature - 250 K. On the other hand, annealing as-grown crystals in vacuum to reach [delta approx.] 0.25 destroys the AF phase; these samples exhibit predominantly ferromagnetic (FM) behavior with Tc [approx.] 100 K. At intermediate doping, 0.25 < [delta] < 0.5, we find evidence for co-existence of FM and AF domains, which are characteristic of the [delta] = 0.25 and [delta] = 0.5 phases, respectively. The signature of the co-existence is the presence of simultaneous FM and AF magnetic Bragg peaks in the neutron diffraction pattern. Polarized neutron scattering measurements confirm that the FM and AF peaks do not arise from different components of a canted antiferromagnet. / (cont.) The FM regions give rise to a ferromagnetic-like peak in the susceptibility at the same temperature as the spins in the AF phase order. In addition, the FM regions exert a random field in the AF phase, above a critical field Hc. We explain the field dependence of the two-phase samples with a microscopic model. We propose that the FM clusters within the AF phase are the result of regions which are rich in Co2+. Furthermore, we suggest that oxygen facilitates the formation of electronically inhomogeneous regions. / by Kennedy Jessica Thomas. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 1994. / Includes bibliographical references (leaves 77-80). / by Oren Bergman. / Ph.D.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Physics, 1999. / Includes bibliographical references (leaves 96-99). / by Kieran Holland. / Ph.D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2015. / Cataloged from PDF version of thesis. / Includes bibliographical references. / The observation that microRNAs (miRNAs), through a titration mechanism can couple interactions of their common targets (competing endogenous RNAs or ceRNAs) has prompted a general "ceRNA hypothesis" that RNAs can regulate each other indirectly through global RNA-miRNA-RNA networks. These ceRNAs are said to "crosstalk" with each other by competing for common miRNAs. Although many individual ceRNAs have been found, fundamental questions about both the magnitude and generality of the crosstalk effect remain. In our study we combine RNA sequencing and single-molecule FISH (smFISH) approaches to both measure the magnitude of the crosstalk effect genome-wide by perturbing three known ceRNAs (Pten, Vapa, Cnot6l) and to identify mechanisms by which the crosstalk effect acts. We identify hundreds of putative ceRNAs and dissect the contributions of individual miRNAs in transmitting crosstalk. We demonstrate that while the crosstalk effect is pervasive, it nevertheless remains bounded by the size of the perturbation. Furthermore, we show that both the number and affinity of shared miRNA binding sites between targets is crucial in determining the magnitude of the crosstalk strength. Using the smFISH data, we examined the single-cell gene expression profiles of pairs of ceRNAs and found that ceRNA gene expression is correlated only in the presence of active miRNAs. Additionally, on inspecting the intra-cellular localization of RNA molecules, we found a miRNA-dependent colocalization of ceRNAs, suggesting a new signature of crosstalk between ceRNAs that extends and modifies the original hypothesis. / by Apratim Sahay. / Ph. D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2018. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 307-321). / We explore three incarnations of highly-entangled quantum matter: as descriptions of exotic, gapped phases in three spatial dimensions, as resources for fault-tolerant quantum computation, and as the by-product of the unitary evolution of a quantum state, on its approach to equilibrium. In Part 1, we study quantum information processing in platforms hosting Majorana zero modes. We demonstrate that certain highly-entangled states may be engineered in arrays of mesoscopic topological superconducting islands, and used for fault-tolerant quantum computation. We then discuss measurement-based protocols for braiding Majorana zero modes and detecting their non-Abelian statistics in on-going experiments on proximitized, semiconductor nanowires, before proposing new families of error-correcting codes for fermionic qubits, along with concrete realizations. In Part 11, we study gapped, three-dimensional phases of matter with sub-extensive topological degeneracy, and immobile point-like excitations - termed "fractons" - which cannot be moved without nucleating other excitations. We find two broad classes of fracton phases in which (i) composites of fractons form topological excitations with reduced mobility, or (ii) all topological excitations are strictly immobile. We demonstrate a duality between these phases and interacting systems with global symmetries along sub-systems, and use this to find new fracton phases, one of which may also be obtained by coupling an isotropic array of two-dimensional states with Z₂ topological order. We introduce a solvable model in which the fracton excitations are shown to carry a protected internal degeneracy, which provides a generalization of non-Abelian anyons in three spatial dimensions. In Part III, we investigate the dynamics of operator spreading and entanglement growth in quantum circuits composed of random, local unitary operators. We relate quantities averaged over realizations of the circuit, such as the purity of a sub-system and the out-of-time-ordered commutator of spatially-separated operators, to a fictitious, classical Markov process, which yields exact results for the evolution of these quantities in various spatial dimensions. Operator spreading is ballistic, with a front that broadens as a dimension-dependent power-law in time. In this setting, we also map the dynamics of entanglement growth in one dimension to the stochastic growth of an interface and to the Kardar-Parisi-Zhang equation, which leads to a description of entanglement dynamics in terms of an evolving "minimal cut" through the quantum circuit, and provides heuristics for entanglement growth in higher-dimensions. The material presented here is based on Ref. [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]. Ref. [11, 12] are not discussed in this thesis, but were completed during my time at MIT. / by Ksheerasagar Vijay. / Ph. D.

Thesis: S.B., Massachusetts Institute of Technology, Department of Physics, 2018. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 67-69). / Gravitational microlensing is a powerful modelling tool, that is essential in accurately understanding the lensing behavior of astronomically strong lensed objects. Using Joachim Wambsganss micro-magnification maps, we discuss the creation of a Monte-Carlo tool that can determine the likelihood for microlensing to account for flux ratio anomalies between macro-models and observations. We apply this tool to the study of iPTF16geu, a recently discovered type IA lensed supernova, and determine that it is unlikely for microlensing alone to account for flux ratio anomalies. We apply the tool, and an extension of the tool that allows us to predict the source's intrinsic magnitude, to Huchra's Lens. We study the light curves of Huchra's lens over time, and predict that image B is stuck in an uninteresting place in its micro-magnification map, causing its microlensing to consistently corrupt the source light curve over the past twenty years. Using Charles Keeton's lens model, a macro-modelling tool, we investigate the quadruply lensed system, DES J0408-5354. We present a new macro-model for the system, which predicts that image C, a perturbed saddle point, is outside the second perturbing galaxy relative to the primary lensing galaxy. This represents a new macro-model for the system, supported by recent unpublished Hubble observations. Finally, we present a method for investigating the quasar continuum emitting region size, and a way to test the point-like assumption at varied wavelengths. We discuss the framework and describe the process for how future work can provide essential constraints on the quasar continuum emitting region. / by Daniel Alexander Yahalomi. / S.B.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2004. / Includes bibliographical references (p. 187-197). / This thesis is a compilation of theoretical and computational work in condensed matter physics related to three topics in structure development. First, I study photonic crystals composed of polaritonic media, focusing on the unique features of the band structures and Bloch states in dispersive media with and without losses. I discuss three novel localization phenomena in these structures: node switching, flux expulsion, and negative effective permeability. Second, I examine the importance of surface interfaces to melting using density functional theory. I demonstrate that single-layer coatings of Gallium Arsenide on Germanium and vice versa have a huge impact on the substrate melting temperature, causing superheating and induced melting, respectively. Finally, I develop reaction-diffusion and stochastic models of the Min-protein oscillations in bacteria that reproduce all main experimental observations. These models explain the origin of instability that ultimately causes dynamic pattern formation and have successfully been used to predict nucleotide binding rates in E. coli. In round cells, I provide evidence that oscillations can be used as a general mechanism for protein targeting and detecting the cell's geometry. / by Kerwyn Casey Huang. / Ph.D.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Physics, 2005. / Includes bibliographical references (p. 37). / Since their discovery in mid 80's, a complete theory of high temperature superconductors is yet to take its final shape. Theory of fractionalization attempts to explain the phenomenon by assuming that the electron is split into two particles, chargon and spinon, carrying charge and spin respectively. Although capable of producing the qualitative features of the phase diagram, this theory is not been able to account for a number of experimental observations. A simple mean field model based on fractionalization ideas is proposed in this work which can possibly get around some of the drawbacks of the original fractionalization theory. Chapter one discusses various aspects of superconductivity along with BCS theory and chapter two talks about the motivation behind considering this model along with its basic features. / by Dinesh V. Raut. / S.M.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2015. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 103-109). / Atomic interferometers have a resolution limit set by the projection noise in measurements on ensembles of uncorrelated atoms. To overcome this classical limit and extend precision measurements into the quantum regime, we need to generate complex entangled states of large atomic ensembles and measure the atomic states with high-quality detection. This thesis describes two experiments in this context. The first experiment demonstrates single-atom resolution and detection sensitivity more than 20 dB below the projection noise limit for hyperfine-state-selective measurements on mesoscopic ensembles containing 100 or more atoms. The measurement detects the atom-induced shift of the resonance frequency of an optical cavity containing the ensemble. The second experiment generates entangled states of 3,000 atoms with non-Gaussian spin distributions. Atoms interact with a weak cavity field, and the heralded detection of a single photon with certain polarization prepares the entangled states. By measuring the non-Gaussian spin distributions using the atom-cavity interaction, we construct a negative Wigner function, manifestly demonstrating that the atoms are entangled. We also show that nearly all of 3000 atoms are involved in the entanglement using an entanglement measure known as the entanglement depth. / by Hao Zhang. / Ph. D.