Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2003. / Includes bibliographical references (p. 111-117). / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Transition metal oxides with low dimensional geometry have displayed fascinating new phenomena such as high temperature superconductivity and unconventional magnetism. The first part of this thesis is related to this rich and diverse subject, where TiOCl is studied as an example of an S = 1/2 layered Mott insulator. Earlier experiments on this material indicating two-dimensional spin-liquid behavior are reviewed critically and are compared to new susceptibility data. The latter suggest a new picture, where band structure effects produce quasi-one-dimensional spin chains formed by t2g orbitals. Based on these findings, TiOC1 is proposed to be a new example of Heisenberg-chains which undergo a spin-Peierls transition. Within this picture, the effect of doping with non-magnetic Sc impurities can be explained in good agreement with the experiment. The magnetic energy scale of J - 660K and the frustration of the interchain geometry render TiOC1 unique among materials with a spin-Peierls transition. This unusual geometry is interpreted as the main reason for the failure of conventional mean-field theory to describe the details of the transition such as its first order character. It will be shown that a simple Ginzburg-Landau theory which takes proper account of interchain-frustration is capable of explaining this unconventional behavior. / (cont.) In the second part of the thesis, the problem of a doped dimerized spin chain is studied in the context of the tJJ'-model one dimension. The focus is on the regime J'/J - .5 where a spin gap is present at small doping and the undoped spin chain is strongly dimerized, and on the limit of small hole doping x as well as small J/t, J'/t. In this regime, earlier numerical calculations have not been able to yield conclusive results. Using a perturbative approach and Luttinger liquid arguments, it will be demonstrated for this non-integrable class of models that the charge degrees of freedom behave as non-interacting spinless solitons in the dilute hole limit. These results are verified up to third order in perturbation theory. The same approach is also used to evaluate the energy and mass renormalization of a single hole, where non-analytic corrections in powers of [the square root of] J/t are obtained. At J'/J = .5 a variational spin-polaron wave function for the hole is constructed and good agreement with the perturbative results is found. / by Alexander Seidel. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2007. / Includes bibliographical references (p. 169-182). / We investigate theoretical expectations for B-meson decay rates in the Standard Model. Strong-interaction effects described by quantum chromodynamics (QCD) make this a challenging endeavor. Exact solutions to QCD are not known, but an arsenal of approximation techniques have been developed. We apply effective field theory methods, in particular the sophisticated machinery of the soft-collinear effective theory (SCET), to B decays with energetic hadrons in the final state. SCET separates perturbative interactions at the scales mb and ... from hadronic physics at AQCD by expanding in ratios of these scales. After a review of SCET, we construct a complete reparametrization-invariant basis for heavy-to-light currents in SCET at next-to-next-to-leading order in the power-counting expansion. Next we classify AQCD/mb corrections to non-leptonic B - M1 M2 decays, where M1,2 are charmless mesons (flavor singlets excluded). The leading contributions to annihilation amplitudes as well as the leading "chirally enhanced" contributions are calculated and depend on twist-2 two-parton and twist-3 three-parton distributions. We demonstrate that non-perturbative strong phases in annihilation are suppressed. Using simple models, we find that the three-parton and two-parton terms have comparable magnitude, both consistent with the expected size of power corrections. Finally, we present a method for determining Vub from B --> [pi] data that is competitive with inclusive methods. At large q2, the form factor is taken from unquenched lattice QCD. At q2 = 0, we impose a model-independent constraint obtained from B --> [pi][pi] using SCET, and the form factor shape is constrained using QCD dispersion relations. Theory error is dominated by the input points, with negligible uncertainty from the dispersion relations. / by Christian Arnesen. / Ph.D.

Thesis (B.S.)--Massachusetts Institute of Technology, Dept. of Physics, 1939. / MIT copy bound with: Intensity scale sensitometer / Richard A. Bloomberg. 1939. / by Monarch L. Cutler. / B.S.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 1997. / Includes bibliographical references (p. 75-77). / by Ken D. Olumn. / Ph.D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2014. / Cataloged from PDF version of thesis. / Includes bibliographical references. / Theory predicts that the approach of catastrophic thresholds in natural systems may result in an increasingly slow recovery from small perturbations, a phenomenon called critical slowing down. In this thesis, we used replicate laboratory populations of the budding yeast Saccharomyces cerevisiae for direct observation of critical slowing down in spatio-temporal dynamics before population collapse. In the first project, we mapped the bifurcation diagram experimentally and found that the populations became more vulnerable to disturbance closer to the tipping point. Fluctuations of population density increased in size and timescale near the tipping point, in agreement with the theory. In the second project, we used spatially extended yeast populations to evaluate early warning signals based on spatio-temporal fluctuations. We found that indicators based on fluctuations increased before collapse of connected populations; however, the magnitude of increase was smaller than that observed in isolated populations, as local variation is reduced by dispersal. Furthermore, we propose a generic indicator based on deterministic spatial patterns, recovery length. In our experiments, recovery length increased substantially before population collapse, suggesting that the spatial scale of recovery can provide a warning signal before tipping points in spatially extended systems. In the third project, we characterized how different environmental drivers influence the dynamics of yeast populations. We compared the performance of early warning signals across multiple deteriorating environments. We found that the varying performance is determined by how a system responds to changes in a specific driver, which can be captured by a relation between stability and resilience. Furthermore, we demonstrated that the positive correlation between stability and resilience, as the essential assumption of indicators based on critical slowing down, can break down when multiple environmental drivers are changed simultaneously. / by Lei Dai. / Ph. D.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Physics, 2004. / Includes bibliographical references (p. 51-53). / We present results for optical measurements on charged CdSe nanocrystals. The injection of electrons into quantum confined states is confirmed by monitoring changes in the visible and infrared absorption spectra. Interestingly, the response is observed only in the nanocrystals that have been chemically treated with sodium biphenyl. Bleaching of the visible interband transition is observed upon electron injection into the nanocrystal films. It is expect that, with injected electrons in the 1S[e] state, more electrons are available to undergo the intraband transition and absorb more light in infrared region. However, the opposite effect is observed. The energy and cross section of the infrared absorption are consistent with a one-electron transition between the 1S[e] and 1P[e] states confined in the quantum dots. / by Fumiaki Toyama. / S.M.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 1989. / Vita. / Includes bibliographical references (leaves 228-235). / by John Frederick Daniel Marko. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 1997. / On t.p. ["delta"] appears as the lower case Greek letter. / Includes bibliographical references (157-163). / by Pamela Lené Washington Blakeslee. / Ph.D.

Thesis (S.B.)--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 (p. 83-84). / The differential cross section for the quasielastic d(n,np) neutron-induced deuteron breakup reaction was measured as a function of incident and scattered particle energies and angles, using a liquid deuterium target at the Weapons Neutron Research (WNR) spallation neutron beam at the Los Alamos Neutron Science Center at the Los Alamos National Laboratory. Cross sections were computed for the incident neutron energy ranges of 10010, 20010, and 30010 MeV. We used proton detectors covering a range of = 17 - 25 in the lab frame, and neutron detectors covering a range of = 55 - 76 in the lab frame. Experiment setup and data analysis are discussed, and preliminary results are presented for the cross section. / by Sho Uemura. / S.B.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Physics, 1999. / In title on t.p., all double-underscored letters appear as subscript and "[right arrow]" is the symbol above the preceding "e". / Includes bibliographical references (p. 199-203). / by Jiang Chen. / Ph.D.