Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Physics, 2013. / Cataloged from PDF version of thesis. / Includes bibliographical references (page 24). / In this research, the manipulation of colloidal systems composed of superparamagnetic particles in water is studied by a simulation method. In response to an external magnetic field, the dipoles drive the beads to self-assemble into chains, which rotate and consequently move across a nearby surface. Under strong surface-interaction, the dynamic and equilibrium structures are modeled using a Bell model and measured using Monte Carlo-type update steps. It is shown that the walking motion can be characterized as two different regimes corresponding to an increase of the rotating arm from half to all of the chain-length as the activation barrier of binding interaction increases with a constant overall increase in energy. When operating at rotational frequencies from 1 Hz to 9 Hz and applied field from I mT to 9 mT, the corresponding translational velocities of chain-like rotors can be approximated with a two-state model until the fragmentation transition of chain-like rotors takes place. The translational velocities of chain-like rotors scale linearly with respect to number of beads. / by Yi-Han Su. / S.B.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2006. / Includes bibliographical references (p. 121-126). / In this thesis, we study dynamical pairing of fermions caused by the time-dependent interaction. Fermionic pairing develops on time scales short compared to the quasiparticle relaxation time where existing approaches to the problem including the Boltzmann kinetic equation and time-dependent Ginzburg-Landau theory are not applicable. 'The nonequilibrium dynamics can be explored in cold Fermi gas at a Feshbach resonance, a system with magnetically tunable interaction. Motivated by recent experiments, in Chapters 2 and 3 we study the pairing of fermions when a sudden switch of interaction induces the Bardeen-Cooper-Schrieffer (BCS) instability of the system. In this case, the pairing amplitude is found to be an oscillatory function of time with predictable characteristics. Another dynamical regime corresponding to a linear magnetic field sweep from atomic to molecular side at a Feshbach resonance is explored in Chapter 4. We find that pairing correlations of fermions in the initial state are encoded in the momentum distribution of molecules after the sweep. Finally, in Chapter 5, we consider pair excitations caused by the harmonic modulation of the interaction, and compare our results to experimental observations. / by Roman A. Barankov. / Ph.D.

Thesis (B.S.) Massachusetts Institute of Technology. Dept. of Physics, 1952. / MIT copy bound with: Determination of the effect of the concentration of paramagnetic ions in aqueous solution on proton magnetic resonance line widths / George Windsor Masters, Jr. 1952. / by Burton Richter. / B.S.

Massachusetts Institute of Technology. Dept. of Physics. Thesis. 1966. Ph.D. / Bibliography: leaves 138-140. / Ph.D.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Physics, 2012. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 67-68). / Topological insulators are a new class of three-dimensional quantum materials whose interior or bulk is an insulator but whose surface is a conductor. Bi₂Se₃ is a prototypical topological insulator that physicists at MIT are manufacturing and studying. Various interesting properties of the topological insulator include the flow of pure spin currents and topological protection. Pure spin current, distinct from electric current, is a net flow of spin without a net flow of charge. Recent research at MIT has revealed that shining circularly polarized laser light on a topological insulator turns its surface's pure spin current into a spin-polarized electrical current. The band structure of the bulk of a topological insulator resembles that of an ordinary insulator; the conduction band and valence band are separated, with the Fermi level falling between them. However, for Bi₂Se₃, the conducting surface's dispersion relation can be modeled by a Dirac cone, which crosses the Fermi level. Electrons with opposing spins reside on opposite sides of the Dirac cone. Illuminating a topological insulator with either left or right circularly polarized light depopulates one side of the Dirac cone, leaving on the other side the desired spin-polarized electrical current. In the experiment performed for this thesis, thin films of Bi₂Se₃ were grown on substrates of sapphire via molecular beam epitaxy (MBE). Electrical devices on a micron scale were then fabricated on the thin film surface and used to measure surface currents. Steps of this experiment included characterizing the surface quality of a sapphire substrate using atomic force microscopy (AFM), making electrical devices with Bi₂Se₃ via the processes of optical lithography, ion milling, and electron beam metal deposition. Photocurrents across these electrical devices were induced by the manipulation of optics and lasers and measured using low noise electronics. Experimental results revealed that it was indeed possible to induce spin-polarized electrical currents on thin films of MBE grown Bi₂Se₃. The desired photocurrent was observed when the laser beam spot size was enlargened to illuminate the entirety of the Bi₂Se₃ device simultaneously. These results were not replicable when the laser was more tightly focused onto a smaller area. Scanning the focused laser beam across the Bi₂Se₃ confirmed that different photocurrents were being induced at different points; these results led us to the conclude that there was something inhomogenous about our device. The reason behind this device inhomogeneity is still under investigation. / by Claudia Lau. / S.B.

Massachusetts Institute of Technology. Dept. of Physics. Thesis. 1967. B.S. / MIT copy bound with: Flexible discussion under student control in the Eliza computer program : coordination of small discussion units illustrated in a tutorial discussion of the train paradox of special relativity / by Paul Raymond Hayward [1967] -- The De Hauss-Van Alphen effect in zinc / by Lynne Howard Holley [1967] -- Investigation of an experiment in the electroproduction of meson resonances at SLAC / by Frederick Harris Kuttner [1967] -- Theoretical spectra for Venus and Jupiter at wavelengths near 1 cm. / by Sara Elizabeth Law [1967] -- The design and operation of wire spark chambers / by Charles Henry MacFarland [1967] -- Tunneling studies in superconducting gallium single crystals / by Michael Edward Malinowski [1967] -- Energy loss and straggling of alpha particles / by Philip J. Manly [1967] -- Infrared and raman spectra of some ionic and molecular crystals / by Jesse Z. Mase [1967] -- A theoretical investgation of self trapping / by William E. Murray, Jr. [1967] / Bibliography: leaf 29. / by John Christie Hamilton. / B.S.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 1996. / Vita. / Includes bibliographical references (p. 71-76). / by Rodrigo Barbosa Capaz. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2002. / Includes bibliographical references (p. 92-95). / 16 years and counting ... In 1986 Bednorz and Muller discovered the layered perovskite structure (La - Ba)2Cu04 which showed the phenomenon of superconductivity at the unprecedented high temperature of Tc = 33 K. In the ensuing months and years it became more and more apparent that the cuprates, as the materials came to be known, show very peculiar correlations in their "normal" state at temperatures T > Tc. The majority of this thesis is concerned with this abnormal "normal" state, attempting to extract a coherent picture for the strange phenomenology. The underlying theoretical framework is a slave particle description of the tJ model proposed by Wen and Lee. The conceptual background is rooted in Anderson's proposal of spin charge separation as the key emergent phenomenon in cuprate physics. After a brief motivation we look at single particle tunneling into the cuprate's superconducting state from the perspective of both d-wave BCS and the SU(2) slave boson theory of Wen and Lee. Both approaches work well close to zero tunneling bias. The slave particle formulation however also naturally incorporates the particle/hole asymmetric background that is seen in experiments. / (cont.) The question of single particle correlations studied experimentally via angle resolved photo emission experiments motivates the analysis of the next chapter. The broad spectral line-shapes seen experimentally imply the absence of well defined quasi-particles in the Fermi liquid sense. We study how gauge fluctuations arising from our use of slave particle coordinates affect the physical hole spectral function. The primary influence of gauge fluctuations turns out to be through their confining tendency on the vertex rather than as a scattering field for the slave particles. The last chapter discusses the effect of gauge fluctuations on the spin susceptibility and shows that they play a vital role in restoring Neel correlations. This allows us to give a natural explanation for the spin related phenomenology of underdoped cuprates. / by Walter Rantner. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 1995. / Includes bibliographical references (p. 455-461). / by Joseph Noël Robin Côté. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2010. / 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. / Collagenolysis (collagen degradation) is a physiological process involved in normal tissue maintenance, but excessive collagenolysis has been associated with the progression of cancer metastasis, atherosclerosis, and other diseases. Despite considerable efforts to understand the steps involved, the exact mechanism of collagenolysis remains unknown. One proposed mechanism suggests that the enzymes that degrade collagen, collagenases, physically unwind the triple-helical structure of collagen to gain access to the peptide bond that is cleaved. This unwinding mechanism would in principle have large energetic requirements, but neither ATP nor other energy-rich molecules are necessary for collagenolysis. An alternative mechanism is that collagen exists in multiple states, some featuring structures that are unfolded in the vicinity of the collagenase cleavage site, and that collagenases preferentially bind to and stabilize these partially unfolded structures before degradation occurs. The focus of this work is to investigate this alternative mechanism, particularly as it pertains to the conformational ensemble of collagen, using both experimental and computational methods. In particular, this work concentrates on: (1) devising a reaction scheme for this mechanism and verifying that it can explain existing experimental observations; (2) generating a structural model of the type I collagen collagenase cleavage site using molecular dynamics simulations; (3) validating this structural model by performing degradation experiments and analyzing them in light of the reaction scheme described in (1); and (4) validating a similar structural model for type III collagen by designing and characterizing a self-assembling collagen-like model peptide that includes the sequence of the collagenase cleavage site. Together, these data present a detailed and comprehensive analysis of the conformational ensemble of collagen near the collagenase cleavage site and its role in the molecular mechanism of collagen degradation. / by Paul S. Nerenberg. / Ph.D.