Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2006. / Includes bibliographical references (p. 271-276). / We report on a search for an annual variation of a daily sidereal modulation of the frequency difference between co-located ¹²⁹Xe and ³He Zeeman masers that sets a stringent limit on boost-dependent Lorentz and CPT violation involving the neutron, consistent with no effect at the level of 150 nHz. In the framework of the general Standard-Model Extension, our result provides the first clean test for the fermion sector of the symmetry of spacetime under boost transformations at a level of 10-27 GeV [1]. We also report progress on the optimization and operation of the Harvard-Smithsonian ¹²⁹Xe/³He dual noble gas maser. The ¹²⁹Xe/³He maser is the first device to sustain simultaneous active maser oscillations on distinct transitions in two intermingled atomic species, and it allows sensitive differential measurement of the ¹²⁹Xe and ³He nuclear spin-1/2 Zeeman transition frequencies [2, 3, 4]. The optimized ¹²⁹Xe /³He maser will be used for highly sensitive tests of Lorentz and CPT symmetry. / by Federico Canè. / Ph.D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2015. / Cataloged from PDF version of thesis. / Includes bibliographical references. / MicroRNAs (miRNAs) are a class of small non-coding RNAs which play important roles in post transcriptional gene regulation. miRNAs regulate more than half of mammalian protein-coding genes. They have been found to participate in almost every cellular process and their dysregulation is associated with many diseases. miRNAs recognize their targets by base paring to miRNA response elements (MREs), which are predominantly located at 3' untranslated region (3'UTR) of mRNAs. This thesis focuses on a microRNA activity reporter system to investigate various aspects of miRNA regulation on its endogenous 3'UTR targets. Mutation of selected MREs on 3'UTRs (MutUTRs) was designed and validated as miRNA unregulated control. It does not require genetic modifications of cellular background and effectively abolishes the majority of miRNA regulation with minimum perturbation to the UTR sequences. MicroRNAs can induce target silencing via mRNA transcript degradation and translational inhibition. But the relative contributions from the two sources have been under debate. It is also unclear how miRNA regulation varies for different target expression. MicroRNA regulation at the transcriptional and translational levels was quantified at single cell resolution over a target expression range of more than 100 fold using our reporter system. The transcriptional regulation was found to be uniform throughout the range of measurement, whereas translational regulation decreases at high target expression. Our data also suggests that translational regulation increase initially at low target expression for certain targets. For all UTRs under study, miRNA regulation from the two sources were found to be on the same order. In addition to target repression, miRNAs also control target expression noise. MicroRNAs decrease protein expression noise for lowly expressed genes, but increase noise for highly expressed genes, and the noise regulation seems to happen at translational level. By linking reporter assays to transcriptome expression, our findings suggest that microRNAs confer precision to protein expression in vivo, and transcriptional regulation might dominate for endogenous targets. Finally we applied the reporter system as miRNA decoys to study miRNA-mediated- crosstalk. We also propose that the reporter systems could be used to study alternative polyadenylation, which is usually accompanied by consequential loss of MREs. / by Yannan Zheng. / Ph. D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2005. / Includes bibliographical references (p. 279-293). / The network of detectors comprising the Laser Interferometer Gravitational-wave Observatory (LIGO) are among a new generation of detectors that seek to make the first direct observation of gravitational waves. While providing strong support for the General Theory of Relativity, such observations will also permit new tests of physical theory in regions of strong space-time curvature and high matter-energy density. However, the observed signals are expected to occur near the limit of detector sensitivity. The problem of identifying such small signals is the primary focus of this work. This work presents a novel method for the identification of astrophysically unmodeled bursts of gravitational radiation in data from networks of interferometric detectors. The method is based on the Q transform, a multiresolution time-frequency transform that efficiently targets waveforms within a finite region of time, frequency, and Q space. The method is also based on a modification of linear prediction that greatly simplifies the resulting statistical analysis by whitening interferometric detector data prior to Q transform analysis. Together, these techniques form the basis of a complete analysis pipeline that is equivalent to a template-based matched filter search for minimum uncertainty waveforms in the whitened data stream. This method is then applied to search for gravitational-wave bursts with duration less than 1 second and frequency content between 64 and 1024 Hz in coincident data from two detectors during second LIGO science run. Although no gravitational-wave bursts are identified, / (cont.) upper bounds are reported for the rate of gravitational-wave bursts as a function of signal strength for isotropic and galactic populations of sources with both abstract and astrophysically motivated waveform. The results indicate a maximum of 0.09 events per day at the 90% confidence level for bursts with characteristic strain amplitude in excess of 10-20 to 10-19 strain Hz-l/2 depending on waveform. A comparison with previous searches demonstrates that this search is one of the most sensitive to date for gravitational-wave bursts of unknown waveform, and is inconsistent with recent indications for an statistical excess of events by the ROG collaboration at above the 99% confidence level. / by Shourov Keith Chatterji. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2011. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 85-90). / Graphene, a two-dimensional hexagonal lattice of carbon, has jumped to the forefront of condensed matter research in the past few years as a high quality two-dimensional electron system with intriguing scientific and practical applications. Both the monolayer and bilayer allotropes are of tremendous theoretical interest, each in its own specific ways. We will focus on the transport properties of graphene in various gated configurations and magnetic fields. We proceed by stating the motivations to study these unusual materials and follow up by deriving the machinery needed to model and understand the low energy behavior. We will see that graphene offers many very strange and unexpected phenomena. We will begin with monolayer in crossed electric and magnetic fields and use the Lorentz symmetry of the Dirac equation to solve for magnetoconductance. Next, we proceed to study monolayer quasiparticles in a deconfining potential and a magnetic field. The twodimensional nature of graphene allows us to study competition between the two external fields. Finally, a look at bilayer graphene in a p-n-p junction shows a case of confinement by chirality - where transitions between states at the same energy are forbidden by an emergent quantum property, chirality. The purpose of this thesis is to provide a taste of the bizarre possibilities that occur in graphene, and to convince the reader that graphene really is unique and worthy of detailed study. The general writing philosophy is to use toy models to provide a simple intuitive picture, followed by a more quantitative analysis. / by Nan Gu. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 1997. / Includes bibliographical references. / by Janet Ann Cutro. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 1993. / Includes bibliographical references (leaves 83-87). / by Ite Albert Yu. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 1995. / Includes bibliographical references (leaves 202-207). / by Kyungwon An. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 1998. / Vita. / Includes bibliographical references (p. 139-145). / by Neer Ruben Asherie. / Ph.D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2018. / 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 157-176). / Quantum materials are characterized by a macroscopic behavior that does not have a clear classical counterpart. Among the experimental probes capable of exploring the exotic properties of quantum materials, ultrafast laser pulses can offer a perspective that is not readily accessible with other conventional techniques. With extremely short pulses (10~100 fs), the strength of electric field is strong enough to cause nonlinear optical phenomena in which the frequency of response is different from that of the input. In this thesis, we discuss how second harmonic generation (SHG) and time and angle-resolved photoemission spectroscopy (tr-ARPES) based on high harmonic generation can offer a unique pathway toward studying quantum materials in the context of broken symmetries and nonequilibrium electronic structure, respectively. In particular, we measured the hidden magnetism buried at a topological insulator ferromagnet interface with SHG, and developed a tr-ARPES setup with 16 and 30 meV energy resolutions using 11 and tunable 24~33 eV photons, respectively. Such high photon energy laser pulses allow us to map the nonequilibrium band structure of a large number of quantum materials, as the setup is capable of probing a much wider momentum range compared to previous tr-ARPES experiments. The capability of our setup is demonstrated through the nonequilibrium band structure of ZrTe5, the quasiparticle recombination dynamics of Bi2Sr2CaCu2O8+[delta], and the dynamics of photoinduced replica bands in WSe2. / by Changmin Lee. / Ph. D.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Physics, 1985. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Bibliography: leaves 360-375. / by Donald Paul Monroe. / Ph.D.