Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Physics, 2008. / Includes bibliographical references (leaves 61-62). / Due to the unique electronic properties of bismuth, bismuth nanowires provide an attractive low-dimensional system for studying quantum confinement effects, and have generated much interest in both optical and thermoelectric applications. Two especially interesting features of bismuth nanowires are the non-parabolic nature of the electronic energy bands near the Fermi level and the large anisotropy of the carrier pockets. As a result of these features, the electronic properties of bismuth nanowires depend strongly on both crystalline orientation and nanowire diameter. In order to study the effect of crystalline orientation, we first derive a simple method to transform from hexagonal to Cartesian coordinates in the bismuth lattice. We then investigate an important indirect electronic transition in bismuth nanowires, and we use the Lax two-band model to develop a theoretical model for studying the diameter dependence of the energy of this transition for nanowires of any crystalline orientation. Our theoretical model shows good agreement with previous experimental results, and demonstrates that the parabolic approximation of the non-parabolic electronic energy bands is inappropriate. Finally, we perform room-temperature IR spectroscopy measurements, in both the reflection and transmission modes, on three sets of bismuth nanowire samples of different crystalline orientation, each fabricated by a different research group. Our results confirm that the notable differences in the measured electronic spectra of the three sets of samples are physical in nature, and are not due to differences in the experimental setups. / by Andrei J. Levin. / S.B.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Physics, 2008. / Includes bibliographical references (p. 43-45). / The transition to Advanced LIGO introduces new sensitivity requirements for the LIGO interferometers. When light scatters away from the main laser beam, then scatters off the beam tube and returns to the main beam, noise is introduced into the phase of the laser. The Auxiliary Optics Support subsystem uses baffles and beam dumps to control this scatter, but the baffle material and shape contribute some scatter as well. Careful selection of baffle material for Advanced LIGO is necessary in order to minimize baffle backscatter. Characterization of potential materials will also inform the geometry and placement of baffles. To this end, I developed a scatterometer experiment designed to measure the Bidirectional Reflectance Distribution Function (BRDF) of a material. The arrangement was used to measure the BRDF for black welder's glass, the prime candidate material for baffles in Advanced LIGO. I found that black glass has a BRDF on the order of 10-, putting it within sensitivity requirements. / by Cassandra R. Hunt. / S.B.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 1997. / Includes bibliographical references (p. [99]-101). / by Partha Saha. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2005. / Includes bibliographical references (p. 129-140). / High-temperature superconducting cuprates, which are the quintessential example of a strongly correlated system and the most extensively studied materials after semiconductors, spurred the development in the fields of material science and experimental and theoretical physics. As first noted by Anderson, these materials are doped Mott insulators and the novel phenomenology emerges in the regime intermediate to the Neel state and the Fermi liquid metal where electrons evolve from being local moments to itinerant entities. This thesis attempts to describe the evolution of the electronic properties between the above two limits as of interest to high-temperature superconductors. We use the tt't"J model to describe doped Mott insulators and resort to numerical methods and to both well known and new approximate analytical techniques to deal with the absence of on-site double electron occupancy. We first address the problem of a single hole in the tt't"J model in terms of the exact diagonalization and the self-consistent Born approximation methods. We show spins dress the doped hole in two distinct ways, namely with a staggered moment and a liquid spin configurations. The resulting two-band picture captures the momentum space anisotropy consistent with experimental observations in underdoped cuprates. Next we use the SU(2) slave boson mean-field theory of the tt'J model. The role of next-nearest-neighbor hopping t' on the phase diagram is studied. We find that when t' > 0.5J a Z2 state with true spin-charge separation exists, which can be a candidate for an eventual pseudogap phase in electron doped materials. / (cont.) We then derive a new formulation of the tt't"J model in terms of holes instead of electrons and introduce a mean-field description of doped Mott insulators that provides an effective two-fluid model of vacancies which captures the effect of staggered moment and d-wave singlet bond short-range spin correlations on the hole dynamics. It leads to a mean field phase diagram consistent with that of hole and electron doped cuprates. Moreover, it provides a framework to describe the observed evolution of the electron spectral function from the undoped insulator to the overdoped Fermi metal, as we conclude from extensively comparing the mean-field results to non-trivial angle-resolved photoemission spectroscopy and tunneling conductance data. / y Tiago Castro Ribeiro. / Ph.D.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Physics, 2012. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 73-75). / This thesis reports the achievement of a two-species apparatus for use in an upcoming experiment with fermionic ultracold atomic gases. First, we describe the construction of a laser system capable of cooling and trapping gaseous lithium-6 atoms in a 3D Magneto-Optical Trap. Second, we discuss the realization of a 2D Magneto-Optical Trap which, in our experiment, acts as a high-flux source of cold potassium-40 atoms. These two systems are critical first steps in cooling the lithium and potassium atoms to quantum degeneracy. / by Vinay Ramasesh. / S.B.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2017. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 186-201). / Dynamical tidal effects impact the orbital motion of extended bodies, imprinting themselves in several measurable ways. This thesis explores the saturation of weakly nonlinear dynamical tidal interactions within two very different systems: hot Jupiters orbiting main-sequence hosts with radiative cores and compact stellar remnants inspiraling due to gravitational radiation. In addition, it discusses general aspects of detecting Gravitational Waves with ground-based laser interferometers. Data quality and noise reduction along with source parameter estimation, with particular emphasis on localization, are discussed in great detail. Conclusions drawn from statistical ensembles of simulated signals are applied to the first three confirmed detections of Gravitational Waves, all from the coalescence of binary black hole systems. / by Reed Clasey Essick. / Ph. D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 1994. / Includes bibliographical references (p. 194-207). / by John Zhihao Yin. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 1996. / Includes bibliographical references (p. 161-166). / by Alexander Belopolsky. / Ph.D.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Physics, 1999. / Includes bibliographical references. / A visible continuum array diagnostic has been constructed to perform experiments on the Alcator C-Mod tokamak. It views plasma Bremsstrahlung in a narrow spectral region free from atomic lines. Signals from thirty chordal views on the midplane are digitized and then Abel inverted to obtain emissivity as a function of minor radius. A procedure based on Green's functions was developed to deal with the problem of noise inherent to Abel inversion. It has a demonstrated ability to pick out the original signal from among noise of equal magnitude. Bremsstrahlung intensity is proportional to the square of electron density times "Z effective", which is a measure of impurities. In conjunction with an independent density diagnostic, the continuum array gives time-resolved impurity density profiles. Alternatively, when it is known that Z~1, the array gives electron density profiles with excellent time and space resolution and coverage of the plasma. An eigenfunction expansion method was used to obtain highly accurate solutions to the transport equation, using posited values of diffusion and convection coefficients and matching the data at the initial time and at the edge of the analysis region. The method assumes the simplest model of constant diffusion and convection linear in r, both constant in time. Possible values of coefficients are systematically scanned to find the best fit to the data. The fits are excellent, which justifies the model. A formal error analysis is done. Impurity injections are analyzed. It is shown that the transport of light elements can be analyzed in the core without the need for diagnostic beams. The elemental composition need not be known. A new electron transport regime was investigated. It is sometimes entered into at the ends of shots when the current is being ramped down. It features a small core plasma and greatly enhanced inward transport, producing very high density. Its potential as a tokamak operational scenario is unclear. / by Cindy R. Christensen. / Ph.D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2017. / 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 (pages 135-141). / From its inception, statistical mechanics has aspired to become the link between biology and physics. But classical statistical mechanics dealt primarily with systems in thermal equilibrium, where detailed balance forbids the directed motion characteristic of living things. Formal variational principles have recently been discovered for nonequilibrium systems that characterize their steady-state properties in terms of generalized thermodynamic quantities. Concrete computations using these principles can usually only be carried out in certain limiting regimes, including the near-equilibrium regime of linear response theory. But the general results provide a solid starting point for defining these regimes, demarcating the extent to which system's behavior can be understood in thermodynamic terms. I use these new results to determine the range of validity of a variational procedure for predicting the properties of near-equilibrium steady states, illustrating my conclusions with a simulation of a sheared Brownian colloid. The variational principle provides a good prediction of the average shear stress at arbitrarily high shear rates, correctly capturing the phenomenon of shear thinning. I then present the findings of an experimental collaboration, involving a specific example of a nonequilibrium structure used by living cells in the process of endocytosis. I first describe the mathematical model I developed to infer concentrations of signaling molecules that control the state of this structure from existing microscopy data. Then I show how I performed the inference, with special attention to the quantification of uncertainty, accounting for the possibility of "sloppy modes" in the high-dimensional parameter space. In the final chapter I identify a trade-off between the strength of this kind of structure and its speed of recovery from perturbations, and show how nonequilibrium driving forces can accelerate the dynamics without sacrificing mechanical integrity. / by Robert Alvin Marsland, III. / Ph. D.