Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 1998. / Includes bibliographical references (leaves 234-249). / by Christopher Michael Becker. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2007. / MIT Science Library Copy: printed in pages. / Also issued printed in pages. / Includes bibliographical references (leaves 177-183). / The ground state for a one dimensional spin 1/2 Heisenberg chain coupled to phonons is a dimerized singlet state known as a "spin-Peierls" state. Currently, the spin-Peierls state is realized in only a handful of known compounds. Even after decades of scientific scrutiny, there is an absence of direct measurements of the lattice dynamics associated with the transition. In this work we present an extensive study of a new one dimensional spin-Peierls compound, TiOC1. The magnetic susceptibility strongly indicates a singlet ground state, with two apparent anomalies observed at T, ,=65 K and Tc2=92 K. Specific heat measurements have been performed and the associated entropy changes quantified. The 65 K transition exhibits a thermal hysteresis, indicative of a first order phase transition. A detailed synchrotron x-ray study of the structure reveals the appearance of superlattice peaks at ( ... ) below 65 K. The intensity of the peaks drop very sharply above T, and a thermal hysteresis is observed which is consistent with a first order phase transition at 65 K. We find that the temperature region between 65 K and 92 K is characterized by a novel incommensurate state. The incommensurate reflections appear at ( ... ). The temperature dependence of the intensity of the incommensurate peaks shows a more gradual onset, with no thermal hysteresis. The incommensurate wavevectors change continuously as a function of temperature and can be analyzed in terms of a mean field theory of phase shifted discommensurations. The observation of the third harmonics enabled a careful characterization of the underlying real space superstructure. We find that all of the observed scattering can be reproduced by a one dimensional long-wavelength modulation of a locally dimerized structure. / (cont.) The lattice dynamics above T 2were characterized by inelastic x-ray scattering measurements. By analyzing the data in terms of a damped harmonic oscillator response function, we are able to extract the phonon frequency and damping for all observed modes. We find a longitudinal acoustic phonon branch whose damping increases for q-vectors close to the zone boundary, which is also associated with an apparent softening of the frequency. Both of these anharmonic effects increase as T2 is approached, and are consistent with a soft phonon description of the dimerization. The anomalous phonon damping and softening are then analyzed using the Cross & Fisher theory of spin-phonon interaction leading to a spin-Peierls transition. We find that the theory succeeds in describing the data for a narrow temperature range about Tc2, for q near the zone boundary. It does not account for the anharmonic effects observed at high temperatures. Our experimental analysis represents one of the most in-depth quantitative tests of the Cross & Fisher theory to date. In addition our results suggest that TiOCl is a particularly ideal realization of a spin-Peierls system. / by Eric T. Abel. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2010. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 293-329). / We present theoretical and numerical methods for studying Casimir forces and nonlinear frequency conversion in nanophotonic media consisting of arbitrary geometries and materials. The first section of the thesis focuses on the study of various geometry-enabled resonant effects leading to strong nonlinear interactions. The starting point of this work is a coupled-mode theory framework for modeling a wide range of resonant nonlinear frequency-conversion processes in general geometries, ameliorating the need for repeated and expensive finite-difference time-domain simulations. We examine the predictions of the theory for two particular nonlinear processes: harmonic generation and difference-frequency generation. Our results demonstrate strong enhancement of nonlinear interactions at a "critical" input power leading to 100% frequency conversion, among many other interesting dynamical effects. Using a quantum-mechanical description of light, based on cavity quantum electrodynamics, similar enhancement effects are demonstrated at the single-photon level, leading to the possibility of achieving all-optical switching of a single signal photon by a single gating photon in a waveguide-cavity geometry consisting of pumped four-level atoms embedded in a cavity. Finally, we describe how one may tailor the geometry of certain materials to enhance their nonlinear susceptibilities by exploiting a consequence of the Purcell effect. The second section of the thesis, the main contribution of this work, presents a new formulation for studying Casimir forces in arbitrary geometries and materials that directly exploits efficient and well-developed techniques in computational electromagnetism. To begin with, we present the step-by-step conceptual development of our computational method, based on a well-known stress tensor formalism for computing Casimir forces. A proof-of- concept finite-difference frequency-domain implementation of the stress-tensor method is described and checked against known results in simple geometries. Building on this work, we then describe the basic theoretical ingredients of a new technique for determining Casimir forces via antenna measurements in tabletop experiments. This technique is based on a (derived) correspondence between the complex-frequency deformation of the Casimir frequency-integrand for any given geometry and the real-frequency classical electromagnetic response of the same geometry, but with dissipation added everywhere. This correspondence forms the starting point of a numerical Casimir solver based on the finite-difference time-domain method, which we describe and then implement via an off-the-shelf time-domain solver, requiring no modifications. These numerical methods are then used to explore a wide range of geometries and materials, of various levels of complexity: First, a four-body piston-like geometry consisting of two cylinders next to adjacent walls, which exhibits a non-monotonic lateral Casimir force (explained via ray optics and the method of images); Second, a zipper-like, glide-symmetric structure that leads to a net repulsive force arising from a competition between attractive interactions. Finally, we examine a number of geometries consisting of fluid-separated objects and find a number of interesting results. These include: stable levitation and suspension of compact objects, dispersion-induced orientation transitions, and strong non-zero temperature Casimir effects. / by Alejandro Rodriguez-Wong. / Ph.D.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Physics, 2005. / Includes bibliographical references (p. 49-50). / I have investigated electrical properties of arrays of lead selenide (PbSe) nanocrystals (NC's) of approximately 6nm diameter. The films become substantially more conducting when annealed at 400K, although no chemical changes of the capping layer occur at this low temperature. There is no evidence based on TEM images of annealed and unannealed films that the interparticle spacing changes at 400K. The dependence of the conductance on the voltage applied to a gate separated from the sample by 350nm of SiO2 was also measured. At 77K and 150K, a U-shaped curve is observed with a minimum in conductance near zero gate voltage, indicating that both electrons and holes are injected. At 294K, the conductance falls monotonically with increasing gate voltage, indicating injection of holes only. I calculate the electron and hole mobilities, [mu]e and [mu]h at 77K and 150K and find effective mobilities 10 orders of magnitude smaller than those of bulk PbSe at 77K. / by Maria C. Schriver. / S.B.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Physics, 2010. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 93-95). / In this thesis, I designed and built laser systems to cool, trap and image lithium-6 and potassium-40 atoms. I also constructed the vacuum system for the experiment and experimentally tested a new method to coat the chamber with a Titanium-Zirconium- Vanadium alloy that acts as a pump. The final apparatus will use a 2D Magneto- Optical Trap (MOT) as a source of cool potassium and a Zeeman slower as a source of cool lithium. The atoms will then be trapped and cooled together in a double-species 3D MOT. In the 3D MOT, we will perform photoassociation spectroscopy on the atoms to determine the Li-K molecular energies and collisional properties. Using this information, we can transfer weakly-bound Feshbach LiK molecules into their ground state. LiK has an electric dipole moment and will open the door to the study of novel materials with very long-range interactions. This new material might form a crystal, a superfluid with anisotropic order parameter or a supersolid. / by Sara L. Campbell. / S.B.

Thesis (B.S.)--Massachusetts Institute of Technology, Dept. of Physics, 1918. / by Donald Browne. / B.S.

Includes bibliographical references (p. 155-162). / Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2007. / by Boris Kozinsky. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2009. / Cataloged from PDF version of thesis. / Includes bibliographical references. / This Thesis considers the scenario in which an extra-solar planet (exoplanet) passes in front of its star relative to our observing perspective. In this event, the light curve measured for the host star features a systematic drop in flux occurring once every orbital period as the exoplanet covers a portion of the stellar disk. This exoplanetary transit light curve provides a wealth of information about both the planet and star. In this Thesis we consider the transit light curve as a tool for characterizing the exoplanet. The Thesis can divided into two parts. In the first part, comprised of the second and third chapters, I assess what observables describing the exoplanet (and host) may be measured, how well they can be measured, and what effect systematics in the light curve can have on our estimation of these parameters. In particular, we utilize a simplified transit light curve model to produce simple, analytic estimates of parameter values and uncertainties. Later, we suggest a transit parameter estimation technique that properly treats temporally correlated stochastic noise when determining a posteriori parameter distributions. In the second part, comprised of the fourth and fifth chapters, I direct my attention to real exoplanetary transit light curves, primarily for two exoplanets: HD 149026b and HD 189733b. We analyze four transits of the ultra-dense HD 149026b, as measured by an instrument on the Hubble Space Telescope, in an effort to properly constrain the stellar and exoplanetary radius. In addition, we assess a detection of strong, wavelength dependent absorption, possibly due to an unusual atmospheric composition. For HD 189733b, we utilize seven ultra-precise Spitzer Space Telescope transit light curves in an effort to make the first empirical measurement of asphericity in an exoplanet shape. In particular, we constrain the parameters describing an oblate spheriod shape for HD 189733b and, attributing oblateness to rigid-body rotation, we place lower bounds on the rotation period of the exoplanet. / by Joshua Adam Carter. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2012. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 99-112). / DNA conformation within cells has many important biological implications, but there are challenges both in modeling DNA due to the need for specialized techniques, and experimentally since tracing out in vivo conformations is currently impossible. This thesis contributes two computational projects to these efforts. The first project is a set of online and offline calculators of conformational statistics using a variety of published and unpublished methods, addressing the current lack of DNA model-building tools intended for general use. The second project is a reconstructive analysis that could enable in vivo mapping of DNA conformation at high resolution with current experimental technology. / by Brian Christopher Ross. / Ph.D.

Thesis (Ph.D.) Massachusetts Institute of Technology. Dept. of Physics, 1956. / Vita. / Bibliography: leaves 84-86. / by Jay Taylor Last. / Ph.D.