Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2004. / Includes bibliographical references (p. 109-116). / We pose two key questions about color superconductivity: What are the effects of the large strange quark mass, and what are the observable consequences of color superconductivity? Motivated by the first question, we study crystalline color superconductivity. We adapt the Nambu-Gor'kov formalism to study this phase, and go on to examine distinctions between crystalline color superconductivity induced by quark mass differences and by quark chemical potential differences. Turning to the second question, we study neutrino scattering in proto-neutron stars cooling through the critical temperature for color-flavor locked quark matter. We include neutrino interactions with the fermionic excitations, important above and just below the critical temperature, and with the massless collective excitation, important just and well below the critical temperature. / by Joydip Kundu. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2006. / Includes bibliographical references (p. 133-156). / A large number of properties of solid state materials can now be predicted with standard first-principles methods such as the Local Density (LDA) or Generalized Gradient Approximation (GGA). However, known problems exist when using these methods for predicting the electronic structure and total energy of transition metal oxides. We demonstrate that the LDA+U method, in which localized d electrons are treated with a Hubbard-like model, gives considerably better predictions in band gap, magnetization, lattice constants, redox potential, mixing energy and other properties of lithium transition-metal oxides. The systematic improvement of both electronic structure and total energy properties suggests that the more accurate treatment of electron correlation on the transition metal sites will greatly enhance the predictive character of ab-initio methods for these materials. In a even more dramatic example, the more accurate electronic and total energy descriptions with LDA+U have enabled us to study the finite temperature phase diagram of LixFePO4. It is found that an unexpected solid solution phase are mainly stabilized by the entropy contribution of localized d electrons. This brings the less visited problem of the effects of electronic correlation on thermodynamics into our attention. / (cont.) The second part of this thesis is focused on application of methods used in solid state materials to proteins. Protein systems are well known for having extremely large conformation space, which makes their energy evaluation expensive even with empirical energy models. We are implementing a coarse graining scheme, called the cluster expansion, to help alleviate this problem. Traditionally, the cluster expansion is a powerful tool to model the energetics of solid-state materials with configurational disorder. We have developed a generalized cluster expansion framework suitable for protein studies, in which properties such as the energy can be unambiguously expanded in the amino acid sequence space. The cluster expansion coarse-grains over non-sequence degrees of freedom (e.g. sidechain configurations) and thereby simplifies the problem of designing proteins, or predicting the compatibility of a sequence with a given structure, by many orders of magnitude. With the help of this method, we have been able to perform a full-sequence design for a small peptide within a relatively short period of time. / by Fei Zhou. / Ph.D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2015. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 149-159). / In this thesis, we study charge transport in nanopatterned arrays of PbS colloidal quantum dots using conventional two-probe measurements and an integrated charge sensor. PbS dots are synthesized in solution with an organic ligand or cap that serves to passivate the surface of the dot, provide a tunnel barrier as well as colloidal stability. These dots can self assemble into an array as the solvent evaporates from a drop of solution on a surface. The self-assembled arrays can be thought of as tunable artificial solids, where the coupling between the dots can be tuned by changing the ligand. Using electron beam lithography followed by a lift-off process, we develop a novel technique to nanopattern these arrays and present the first colloidal quantum dot arrays of nanoscale dimensions. Nanopatterning makes it possible to study the electrical properties intrinsic to the dots unimpeded by macroscopic defects, such as cracking and clustering that typically exist in larger-scale arrays. We find that the electrical conductivity of the nanoscale films is higher than that of drop-cast, microscopic films made of the same type of dot. We achieve unprecedented versatility in integrating the patterned films into device structures, which will be valuable both for studying the intrinsic electrical properties of the dots and for nanoscale optoelectronic applications. From two-probe measurements on the nanopatterned arrays that are approximately 15 dots wide, we observe large noise in the current as a function of time. The noise is proportional to the current when the latter is varied by applying source-drain or gate voltage in a field-effect structure or when changing temperature. Owing to the small number of current paths in the system, we often observe telegraph switching, and find that the off times follow non-poissonian statistics. We show that the results can be understood in terms of a model in which a quasi-one-dimensional percolation path is turned on and off, by charging of a dot along the path. Long organic ligands lead to highly resistive colloidal quantum dot arrays, making the low bias regime inaccessible with conventional two-probe measurements. We use an integrated charge sensor to study transport in the low bias regime as a function of the coupling between the dots. We present transport measurements on butylamine and oleic acid capped PbS dots. The resistances measured are the highest measured for colloidal quantum dots. For the native oleic acid ligand, and weak coupling between the dots, the conduction mechanism is nearest neighbor hopping, and the conductance is simply activated. At low source-drain bias voltages, the activation energy is given by the energy required to release a carrier from a trap state plus the activation over barriers resulting from site disorder. The barriers from site disorder are eliminated with a sufficiently high source-drain bias. For the shorter ligand, which gives stronger coupling, the data are consistent with Mott's variable range hopping as the conduction mechanism. / by Nirat Ray. / Ph. D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2010. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 259-269). / Experiment E08-007 measured the proton elastic form factor ratio [mu]GE/GM in the range of Q2 = 0.3-0.7(GeV/c)2 by recoil polarimetry. Data were taken in 2008 at the Thomas Jefferson National Accelerator Facility in Virginia, USA. A 1.2 GeV polarized electron beam was scattered off a cryogenic hydrogen target. The recoil proton was detected in the left HRS in coincidence with the elastically scattered electrons tagged by the BigBite spectrometer. The proton polarization was measured by the focal plane polarimeter (FPP). In this low Q2 region, previous measurement from Jefferson Lab Hall A (LEDEX) along with various fits and calculations indicate substantial deviations of the ratio from unity. For this new measurement, the proposed statistical uncertainty (< 1%) was achieved. These new results are a few percent lower than expected from previous world data and fits, which indicate a smaller GEp at this region. Beyond the intrinsic interest in nucleon structure, the new results also have implications in determining the proton Zemach radius and the strangeness form factors from parity violation experiments. / by Xiaohui Zhan. / Ph.D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2015. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 213-224). / In this thesis, I study a class of exotic quantum matter named Symmetry-Protected Topological (SPT) phases. These are short-range-entangled quantum phases hosting non-trivial states on their boundaries. In the free-fermion limit, they are famously known as Topological Insulators (TI). Huge progress has been made recently in understanding SPT phases beyond free fermions. Here I will discuss three aspects of SPT phases in interacting systems, mostly in three dimensions: (1) Novel SPT phases could emerge in strongly correlated systems, with no non-interacting counterpart. In particular, I will discuss interaction-enabled electron topological insulators, including their classification, construction, characterization and realization. (2) When strong interactions are present, the surface of many SPT phases (including the familiar free fermion topological insulator) can be gapped without breaking any symmetry, at the expense of having intrinsic topological order on the surface. (3) Some topological phases that are non-trivial in the free fermion theory become trivial once strong interactions are introduced. The material of this thesis closely parallels that of Refs. [1, 2, 3, 4, 5, 6]. / by Chong Wang. / Ph. D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2018. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 187-194). / In this thesis we present several surveys of heavy element absorber characteristics at high redshift, gauging properties of diffuse intra- and intergalactic gas in the first several Gyr of the Universe. At z ~ 3.5, we study chemical abundances of Lyman limit systems (LLSs) and evaluate their potential to represent expected reservoirs of cold, low-metallicity gas whose accretion onto galaxies is necessary to maintain star formation. In an initial survey focused only on LLSs identified as potential lowmetallicity absorbers, based on the absence of metal lines in low-resolution spectra, we indeed found the selected systems have low abundances, with a median of [M/H]~ -2.2 and several systems at [M/H]</~ -3, comparable to the intergalactic medium. While this result is tempting to interpret as evidence that a sizable fraction of LLSs are candidates of low-metallicity gas accreting onto galaxies, in a follow-up study of all LLSs at z ~ 3.5 we found that LLS abundances can be reasonably described by a unimodal distribution with [M/H]~ -2.5. Additional diagnostics are hence needed to evaluate if (and how many) low abundance LLSs arise from accretion flows, since the overall LLS abundance distribution overlaps heavily with the IGM at this epoch. In a seperate work that constitutes the largest study of multiple ions in individual metalline absorption systems at z >/~ 6, we find that the bulk of high-redshift absorption systems are low-ionization and low-metallicity. Since H i absorption cannot be seen at z >/~ 6, we further argue from incidence rates and absorber characteristics that these absorbers are analogous to strong neutral hydrogen systems seen at lower redshift. We conclude that the non-detection of weaker H i absorption systems (such as LLSs) is consistent with lower metallicities and lower ionization states in the circumgalactic medium than is seen at later times. / by Thomas Cooper. / Ph. D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2012. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 255-264). / In this thesis, we search for the production of the Standard Model Higgs boson at the Large Hadron Collider, through its decay mode to two W bosons, which each in turn decay into a charged lepton and a neutrino. The Higgs boson is the only elementary particle predicted by the Standard Model of particle physics which has not yet been experimentally observed. The question of its existence has been the subject of a wide body of experimental and theoretical work for the past half-century. Using 4.9 fb-1 of integrated luminosity, this search is expected, on average, to exclude the Standard Model predicted Higgs boson production cross section for masses between 126 GeV/c 2 and 260 GeV/c 2 under the background-only hypothesis. We observe no statistically significant excesses in the data, and we exclude the Standard Model Higgs boson production for masses between 129 GeV/c 2 and 260 GeV/c 2 . This result represents a significant reduction of the mass region in which the Standard Model Higgs boson is allowed to exist. / by Si Xie. / Ph.D.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Physics, 2007. / Includes bibliographical references (leaves 55-59). / In this thesis, I carried out a simulation study to characterize the measurement of bottom quark flow in relativistic Pb+Pb collisions using the Compact Muon Solenoid experiment. The Hydjet event generator is used to produce sample collision events at four centrality values between 0 and 12 fm. These events are processed using the simulation, digitization, and reconstruction modules of the CMSSW software framework. Studies of these data produce information necessary to create a flow signal simulator, and attempts are made at suggesting values for experimental cuts in PT and DCA to reduce backgrounds. Using the signal simulator, it is determined that the muon elliptic flow coefficient, v [mu]/2, varies linearly with that of open beauty, v B/2, with the same constant of proportionality applicable in all background conditions. The expected statistical uncertainty of the flow measurement, RMSv2, is shown to vary linearly with background level. Finally, RMSv2 is shown to vary with event sample size as predicted by background-to-signal ratio counting statistics. / by Arthur James Franke. / S.B.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2017. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 123-138). / In this thesis we discuss recent results on the medium/probe interaction in the strongly coupled plasma produced in heavy-ion collisions. Such processes involve physics at multiple energy scales making a consistent theoretical description challenging. We show how insights from various regimes can be combined to extend our understanding of the underlying physics. As a first example, we start with the novel contributions to the drag force acting on a heavy quark moving through the strongly coupled holographic plasma. The new terms are proportional to the coefficient of the axial anomaly and to the chiral asymmetry in the medium. These chiral contributions to the drag force act either parallel to or antiparallel to an external magnetic field or to the vorticity of the fluid. We show that the chiral drag force vanishes for heavy quarks that are at rest in a suitably chosen frame in the weak field limit. In this frame, the heavy quark at rest sees counter propagating momentum and charge currents, both proportional to the axial anomaly coefficient, but feels no drag force. This provides strong concrete evidence for the absence of dissipation in chiral transport. Then, we extend this result introducing a concept of an "anomalous wind" felt by probes at rest. This phenomenon modifies previous results on the medium/probe interaction. We demonstrate that by deriving the anomalous correction to the screening length of the heavy quarkonium color potential. Finally, we discuss the strong magnetic field limit to the leading order in the weak medium/probe coupling. It is shown that the drag force is suppressed along the magnetic field and in the exact chiral limit tends to zero. This anisotropy in the drag force is present in the system even at zero chiral asymmetry and may lead to strong observable effects. Then, we turn to the main available probes of the plasma produced in experiments - jets. We supplement the holographic description corresponding to the infinite medium/probe coupling limit with an initial parameter distribution for an ensemble of jets motivated by the weakly coupled dynamics and similar to the case of proton-proton collisions. The model constructed in this way, combining insights from both weakly and strongly coupled regimes, is used to analyze the evolution of an ensemble, as it propagates through an expanding cooling droplet of strongly coupled plasma as in heavy ion collisions. Each jet in the ensemble is represented holographically by a string in the dual 4 + 1-dimensional gravitational theory. Firstly, the full string dynamics is approximated by assuming that the string moves at the speed of light. We study the evolution of the jet opening angle distribution upon propagating the droplet and study the medium effect on the mean opening angle within a simple two parametric phenomenological model. Then, we extend this result analyzing the full string dynamics for a range of possible initial conditions. That gives access to the dynamics of holographic jets just after their creation. We analyze the full jet shape modification and find the result that the jet shape scales with the opening angle at any particular energy. Further, we construct an ensemble of dijets with energies and energy asymmetry distributions taken from events in proton-proton collisions and jet shape taken from proton-proton collisions and scaled according to our result. We study how these observables are modified after the ensemble of dijets is propagated through the strongly-coupled plasma. The results of this simple model is in qualitative agreement with the experimental data. / by Andrey V. Sadofyev. / Ph. D.

<p> Particle Induced X-ray emission (PIXE) technique was employed for analysis of major and trace elements in two biomedical materials and one petrochemical material. The PIXE experiments were carried out using a 1.7 MeV Pelletron Accelerator and an MeV ion microprobe at the Louisiana Accelerator Center, University of Louisiana at Lafayette, USA. Data was acquired using an Oxford triplet conguration with OMDAQ2007 software and analyzed offline using the Dynamic Analysis technique on the GeoPIXE software. A proton beam of 2 MeV energy and a Si X-ray detector were used for PIXE measurements. Relative concentrations and elemental maps in the irradiated targets were subsequently determined. This thesis reports work on the overview of the micro-PIXE technique, its applications and investigation of dental amalgams, a petrochemical catalyst, and snail shells.</p><p>