Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2012. / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Cataloged from student-submitted PDF version of thesis. / Includes bibliographical references (p. 189-194). / The Murchison Widefield Array (MWA) is a new low-frequency radio array under construction in Western Australia with a primary goal of measuring the power spectrum of the 21-cm signal from neutral hydrogen during the Epoch of Reionization (EoR). In this thesis, we detail efforts to characterize the MWA system, and present scientific results from a 32-element prototype interferometer deployed at the MWA site. We develop simulations and perform anechoic chamber measurements to verify the performance of the MWA antenna tiles. We develop a calibration and imaging pipeline for the MWA which uses w-projection widefield imaging techniques and direction-dependent point spread functions. Using data from an MWA expedition in March 2010, we produce confusion-limited maps covering ~ 2700 square degrees in a region of sky with low galactic temperature. We develop a blind source detection and extraction algorithm, and use it to perform a blind survey in these maps, and detect 655 sources at high significance with an additional 871 candidates. We compare these sources with existing low-frequency radio surveys in order to assess the MWA-32T system performance, and to identify new candidates for ultra-steep spectrum radio sources. In order to constrain the EoR, we apply two power spectrum estimation techniques to this dataset: a Fast Fourier Transform in order to rapidly compute power spectra, and a quadratic estimation method which uses inverse covariance weighting to produce an optimal estimate. We use a principal component analysis to identify and remove the foreground contaminants. In the resulting two-dimensional power spectra, we find the predicted "wedge" feature due to the chromaticity of the instrumental response, and identify a sensitive region free of strong contaminants which can be used for characterizing the EoR signal. We then use these data to produce new limits on the EoR power spectrum at z = 9. / by Christopher Leigh Williams. / Ph.D.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Physics, 2002. / Also issued in leaves. / Includes bibliographical references (p. 113-121). / I analyze 68 oscillation trains detected in a search of 159 thermonuclear bursts from eight neutron star X-ray binaries observed with the Rossi X-ray Timing Explorer. I use all data that were public as of September 2001. The frequencies of the oscillations are uniformly distributed between 270-620 Hz, and are nearly constant for a given source. They typically have fractional rms amplitudes of 5%. During a burst, the frequencies of the oscillations generally drift upward by -4 Hz to stable limiting values. Neither the amplitudes of the oscillations nor the evolution of their frequencies are simply related to the time scales and energetics of the bursts. If the frequency drift is accounted for with smooth functions, the oscillations are coherent in 70% of the bursts, and their asymptotic frequencies are stable to a few parts in 1000. This suggests that the asymptotic frequencies are determined by the spin of the neutron star. However, residual dispersion in the frequencies from 4U 1636-536 is uncorrelated with the known orbit of the system, so some mechanism other than the orbital motion of the neutron star must cause them to vary. Models for the oscillations suggest that they originate from patterns in the stellar surface brightness that drift in the direction opposite the rotation of the neutron star, yielding oscillation frequencies lower than that of the spin. The patterns produce oscillations that are very sinusoidal - harmonic components have amplitudes that are less than 5-10% of those of the main signals. This suggests that the patterns are very symmetric. The types of bursts that exhibit oscillations are correlated with the pulsation frequency. / (cont.) In particular, oscillations between 500-650 Hz occur almost exclusively in bursts that exhibit photospheric radius expansion, while oscillations with frequencies of 250-400 Hz are slightly more likely to occur in bursts without radius expansion. This appears to be because (i) oscillations from all of the sources are observed only when the persistent accretion rate is relatively high (- 0.1lMEdd), while (ii) in - 300 Hz sources radius expansion occurs at lower M, yet in - 600 Hz sources it occurs at higher M. / by Michael Patrick Muno. / Ph.D.

Thesis: S.B., Massachusetts Institute of Technology, Department of Physics, 2016. / Cataloged from PDF version of thesis. / Includes bibliographical references (page 59). / This paper explores the impact of neutron backgrounds on the Ricochet experiment. The purpose of the Ricochet experiment is to determine the existence of sterile neutrinos via coherent neutrino-nucleus scattering. Coherent neutrino-nucleus scattering is an ideal detection method because it is a neutral current process. All active neutrino flavors will therefore be detected uniformly (no non-active, or sterile neutrinos will be detected because they do not couple to the Z boson). By varying the distance between the detector and the neutrino source, we consider deficits in neutrino flux to be evidence of oscillation to a fourth, non-active neutrino. Neutron backgrounds could interfere with neutrino detection. We therefore calibrate and employ Neutral Current Detectors (NCD's) for the purpose of neutron detection to measure the expected neutron spectrum incident on the detector. We furthermore design a Monte Carlo simulation to model the expected neutron capture rate of the neutrino detector in this setup. / by Elise Newman. / S.B.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Physics, 2006. / Includes bibliographical references (p. 45-47). / Magnetic holes are isolated intervals of depleted interplanetary magnetic field (IMF) strength on timescales of several seconds to several hours. These intervals have been seen as often as several times per day in the ecliptic and at high heliospheric latitudes from 1 AU to 5.4 AU. We present a scale-free statistical technique for identifying magnetic holes and evaluating their significance relative to turbulent fluctuations in the solar wind. We apply this technique in a comprehensive search for magnetic holes in the Wind and ACE magnetometer data streams through 2004. Using magnetic field and ion measurements on the Wind spacecraft, we present the first statistical study of magnetic hole plasma signatures on the kinetic scale and we present a comparison of magnetic holes as kinetic and microscale phenomena. Magnetic holes are shown to be pressure-balanced structures with similar properties on all scales. Temperature anisotropy measurements are combined with magnetic field measurements to give direct evidence that the mirror-mode instability in the solar wind is a likely source of magnetic holes. Two-satellite correlations between ACE and Wind indicate that magnetic holes are stationary with respect to the solar wind and elongated along the IMF. / by M.L. Stevens. / S.M.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2008. / Includes bibliographical references (p. 189-202). / The central claim of this thesis is that nanoscale devices offer a platform to study and demonstrate new forms of control over both quantum and classical degrees of freedom in solid-state systems. To support this claim, I present a series of theoretical discussions that demonstrate how static and/or time-varying fields can be used to control spin degrees of freedom in GaAs quantum dots. This work is motivated by recent experiments in single and double quantum dots that have demonstrated many interesting phenomena arising from the coupled dynamics of electron and nuclear spins. In addition, I will present some results on the control of superconducting flux qubits, obtained in collaboration with the Orlando group at MIT. The control techniques discussed in this thesis may help provide new directions for experimental research on nuclear spin dynamics in solids, and may be applied to help enable future spintronics or quantum information processing tasks. / by Mark S. Rudner. / Ph.D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2015. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 159-178). / Two-dimensional (2d) layered materials, such as graphene and hexagonal boron nitride (hBN), can be isolated separately and then stacked together to form heterostructures with crystalline interfaces between the layers. In this thesis, I present a series of experiments which explore the quantum transport of electrons in heterostructures made from graphene and hBN. Depending on the relative alignment, or "twist", between the layers, a crystal of hBN can be either a non-perturbing substrate for the graphene, or a method to induce a band gap and superlattice potential for the graphene electrons. In the case of two stacked graphene layers, a relative twist can electronically decouple the layers from each other, despite a tiny 0.34nm interlayer spacing. This twist-dependent physics can be used to realize new electronic states in graphene, especially in the presence of strong magnetic fields and electron-electron interactions. By applying a strong tilted magnetic field to graphene which is decoupled from its hBN substrate, we are able to realize a quantum spin Hall state and measure its electronic properties. An analogous bilayer quantum spin Hall state is also realized in twisted bilayer graphene, by taking advantage of the twist decoupling between the layers and the effects of electron-electron interactions. A different set of experiments explores the competition of a magnetic field with the effects of the superlattice potential which arises when a graphene sheet is nearly aligned to its hBN substrates. The large superlattice potential allows us to study graphene transport in Hofstadter's butterfly-the fractal spectrum for electrons under the simultaneous influence of a lattice and a magnetic field. / by Javier Daniel Sanchez-Yamagishi. / Ph. D.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Physics, 1997. / Includes bibliographical references (leaves 106-108). / by Bryndol Avery Sones. / M.S.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2012. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 131-133). / Topological Insulator is a newly found state of matter. Unlike phases described by the traditional Landau theory of symmetry breaking, the topological phases do not break symmetry, and it is not obvious in which measurable quantity will the topological index manifest itself. In this thesis, our main goal is to understand how topological classification produces measurable consequences in periodic insulators. We first warm up by investigating the charge conjugation invariant insulator in one spatial dimension. We show there are two topological distinct classes and derive an integral formula for the topological index that distinguishes between them. We then show that the topological index appear as a Berry's phase when one adiabatically turns on a electric field. We then study the effective theory induced by this Berry's phase and show that there are measurable consequences. We then generalize the discussion to three spatial dimensions. It is hard to capture the topological terms in the effective theory by conventional perturbation methods. We then introduce a new formalism to calculate properties produced by those topological terms such as the polarization and the magnetization, in a unified way. The formalism is based on a perturbative expansion of the Green's functions in powers of a uniform field strength, instead of the potential. In particular, this formalism allows us to capture the effective action describing the three dimensional topological insulator defined under time reversal symmetry, which previously can only be calculated via pumping. Finally, we discuss measurable consequences from the effective theory, in various different boundary settings. Among the properties we have calculated, we find we can identify part of them as of bulk nature, and some other part of them more as an effect associated with boundaries. For the part that are associated with boundaries, the Maxwell relation in the bulk can be violated. For example, the isotropic orbital magneto-polarizability and the orbital electric-susceptibility are different with periodic boundary conditions. However, they become identical whenever there is a boundary. / by Kuang-Ting Chen. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2001. / Includes bibliographical references (p. 145-155). / I have studied the intergalactic medium (IGM) located in galaxy clusters and groups via the so-called "X-ray Forest" the X-ray absorption lines produced by the hot IGM. I gave a semi-analytic calculation of the X-ray forest distribution based on the Press-Schechter formalism. The warm-hot intergalactic medium - the IGM between 10⁵ - 10⁷ K - was investigated via hydrodynamic simulation. I analyzed the simulated spectra which were synthesized in a way similar to simulating the Lya forest and studied column density and the Doppler b-parameter distributions of highly-ionized metals. Finally, we studied X-ray spectra of two high-redshift quasars, S5 0836+710 and PKS 2149-306, obtained with the Chandra High Energy Transmission Grating Spectrometer (HETGS). While no absorption features were detected, a method of constraining the IGM via the X-ray forest theory was proposed. I also discuss constraining the IGM via the X-ray Gunn-Peterson test. / by Taotao Fang. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2013. / Title as it appears in MIT degrees awarded booklet, September 2013: Mediating light and matter: excitons in organic and nanostructured materials. Vita. Cataloged from PDF version of thesis. / Includes bibliographical references (pages [151]-165). / Over the past 20 years a new classes of optically active materials have been developed that are composites of nano-engineered constituents such as molecules, polymers, and nanocrystals. These disordered materials have enabled devices such as organic light emitting diodes, color tunable lasers, and low-cost photovoltaics, and hold promise as a platform for all-optical computing. The defining optical and electronic characteristic of molecular and nanostructured materials is the exciton, a bound electron hole pair. Excitons, which can be generated optically or electrically, are the nanoscale carriers of energy, acting as intermediates between photons and electronic excitations. The goal of this thesis is to add to the present understanding of two fundamental aspects of excitons in molecular and nanostructured materials. First we focus on the spatial transport of excitons, which is central to the operation of photovoltaics, LEDs, and potential excitonic transistors. Despite its importance, the precise dynamics of exciton transport and how it relates to disorder, the defining characteristic of molecular and nanostructured materials, remains elusive. Here we develop a technique for direct visualization of exciton transport. We reveal unambiguously that transport occurs by random walk diffusion and that it transitions to sub diffusive as energetic disorder is increased. Furthermore, we harness exciton transport in J-aggregate materials to build a platform for the enhancement of absorption and fluorescence of organic molecules and quantum dots. Second we turn to the interaction of excitons with optical microcavities. Using the thermally stable excitons in molecular materials, it is possible to create strongly coupled states of excitons and photons, known as polaritons. A longstanding research goal has been creating polaritons at high densities in order to study condensation phenomena and as a route to low threshold organic lasers. In this thesis we elucidate that a key mechanism that prevents polariton condensation is exciton-exciton annihilation. In order to circumvent annihilation, we develop a new microcavity architecture with an intracavity excitation scheme and demonstrate room temperature lasing through a polariton mode. Finally, we show super radiant lasing from an organic microcavity, an alternative method over strong coupling that results in a substantially reduced lasing threshold. / by Gleb M. Akselrod. / Ph.D.