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Online 1. Developing angular intensity correlations of Xray photons as a tool for studying structures of proteins in noncrystalline solutions [2019]
 Qiao, Shenglan, author.
 [Stanford, California] : [Stanford University], 2019.
 Description
 Book — 1 online resource.
 Summary

This dissertation offers lessons learned and tools developed as we attempt to apply correlated Xray scattering (CXS) to noncrystalline proteins in solution. It builds on our previous work of extracting correlation signals from scattering intensities of ensembles of mental nanoparticles, which has led to threedimensional (3D) structural insights not reflected in azimuthally averaged measurements. In his 1977 paper, Zvi Kam proposed the idea of correlating Xray photons scattered by an ensemble of randomly oriented particles suspended in solution. He found that if the exposure time is much shorter than the diffusion timescale of Brownian motion, correlations between photons scattered into different angles encode 3D structural information of the particles not accessible via conventional small or wideangle Xray scattering. The advent of the Xray free electron laser (XFEL) renders Kam's idea feasible for noncrystalline solutions of proteins. With femtosecond pulses and extremely high fluences, the XFEL is not only capable of probing ensembles of molecules essentially frozen in time but also delivering a large number of photons per pulse, a capability critical for enhancing angular intensity correlation signals. Meanwhile, probing proteins in solution removes the need for crystallization, allows measurements of mixtures of conformational states under physiological conditions, and broadens opportunities for timeresolved experiments. The body of work in this dissertation draws from scattering data collected with samples containing the Gprotein Gi alpha subunit during two separate beam times conducted at the Linac Coherent Light Source. The Gi alpha subunit was chosen for these proofofprinciple experiments because of its important role in the Gprotein coupled receptor signaling pathway. This dissertation has taken the first steps in developing and validating CXS as a tool for probing ensembles of biomolecules in solution. These first steps as well as ideas described in this dissertation to improve CXS towards a mature pipeline that yields reliable and detailed structural insights aim to inspire others in the solution scattering community to engage with the unique challenges and rewards of this technique.
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Online 2. From timeresolved to frequencyresolved xray scattering [2019]
 Ware, Matthew Robert, author.
 [Stanford, California] : [Stanford University], 2019.
 Description
 Book — 1 online resource.
 Summary

Gasphase timeresolved xray scattering (TRXS) measures internuclear separations in a molecule following laserinduced photoexcitation. TRXS constitutes an indirect measurement of the molecular motion because it captures information in reciprocalspace and realtime, which then must be inverted to recover the charge density as it changes in time. The spatial resolution of the recovered charge density is fundamentally restricted by the xray wavelength used in the experiment. There is no corresponding technical restriction on the ability to scan the delay between the pumplaser pulse and the xrayprobe pulses, and thus no lower limit on the ability to resolve beat frequencies from TRXS measurements. This observation motivates transforming the measured TRXS in reciprocalspace and realtime into its reciprocalspace and reciprocaltime representation through a temporal Fourier transform. This representation is called frequencyresolved xray scattering (FRXS). The novel aspect of this approach is that an interpretable and compact representation of the experimental measurement may be obtained in reciprocalspace and reciprocaltime without the difficulty of inverting the measurement to the traditional realspace and time representation, and thus FRXS presents an alternative to traditional analyses of TRXS. The traditional approach based on pair correlation functions is limited by the range of momentum transfer, Q, that is accessible at xray free electron lasers (FELs). FRXS does not suffer this limitation, and in fact, FRXS leverages the strengths of FELs, namely fine time resolution and (relatively) fast data accumulation. This enables a long range of pumpprobe delays to be measured in an experiment, thus improving the frequency resolution of an experiment, while maintaining sufficient temporal resolution to measure high beat frequencies. These advantages have been used to obtain compact representations of bound states and dissociations along lines in reciprocalspace and reciprocaltime, demonstrating an alternative to traditional analyses of timeresolved xray scattering for gasphase photochemistry.
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 Chen, Ruizhu, author.
 [Stanford, California] : [Stanford University], 2019.
 Description
 Book — 1 online resource.
 Summary

In this thesis, we use helioseismic methods to study two separate topics, the Sun's meridional circulation and sunquake events. The Sun's meridional circulation is a key component of solar dynamo and interior dynamics, playing an important role in transporting magnetic flux and redistributing angular momentum. A profile of the meridional circulation has long been sought, but results from previous studies were not fully consistent, due to a systematic center tolimb (CtoL) effect in helioseismic measurements that complicates the inference of meridional circulation in the deep interior. In the first part of this thesis, we measure the Sun's meridional circulation and its temporal evolution using 8 years of SDO/HMI Dopplervelocity observations, with a new CtoLeffectremoval method that we have developed in timedistance helioseismology. The longtimeaveraged meridional circulation profile is found to have a threelayer flow structure: an equatorward flow is sandwiched between two poleward flow zones above and beneath it, indicating a doublecell circulation in each hemisphere. Moreover, the 3layer flow pattern is more significant when the Sun's magnetic activity level is low, while significant changes are found in the flow structure during the active phase of the solar cycle. Besides, we also study the observational properties of the CtoL effect in the measured travel time of helioseismic waves. The CtoL effect is found isotropic relative to the azimuthal angle around the solar disk center. It also exhibits a significant frequency dependence  it reverses sign at a frequency around 5.4 mHz, and is strongest at around 4.0 mHz. The tendency of frequency dependence varies with diskcentric distance but not with the waves' travel distance. In the second part this thesis, we focus on sunquakes. Sunquakes are helioseismic power enhancements initiated by solar flares, but not all flares generate sunquakes. It is curious why some flares cause sunquakes while others do not. Here we propose a hypothesis to explain the disproportionate occurrence of sunquakes: during a flare's impulsive phase when the flare's impact acts upon the photosphere, a sunquake tends to occur if the background oscillation at the flare's footpoint happens to oscillate downward, in the same direction with the impulse from above. To verify this hypothesis, we survey 60 strong flares in Solar Cycle 24 to search for sunquakes, by reconstructing the oscillatory velocity in the flare sites using a helioseismic holography method. A total of 24 flares are found to be sunquake active, giving a total of 41 sunquakes. It is found that in 3 − 5 mHz frequency band, 25 out of 31 (81%) sun quakes show net downward oscillatory velocities during the flares' impulsive phases, and in 5 − 7 mHz frequency band, 33 out of 38 (87%) sunquakes show net downward velocities. These results support our hypothesis that a sunquake more likely occurs when a flare impacts a photospheric area that happens to have a downward background oscillation.
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Online 4. Impurity quantum phase transitions in quantum dot nanostructures [2019]
 Peeters, Lucas Bernd Marie, author.
 [Stanford, California] : [Stanford University], 2019.
 Description
 Book — 1 online resource.
 Summary

Electronic systems subject to competing interactions can end up in different phases as the balance between these interactions shifts. When a quantum critical point separates these phases, exotic electronic behavior often marks the vicinity of the transition. In this work, we construct nanopatterned devices to probe such critical phenomena. The basic element of our devices is the GaAs/AlGaAs quantum dot, an isolated region of electronic charge which is coupled to a twodimensional gas of weakly interacting electrons. We use different designs of quantum dots to realize different models. The first device studied in this work realizes the spin twochannel Kondo ('spin 2CK') model. In this model, a single impurity (i.e. a single spindegenerate dot) is coupled to two separate reservoirs. When the couplings to both reservoirs are unequal, the more strongly coupled reservoir screens the impurity spin degeneracy and forms a manybody singlet; this is known as the Kondo effect. When both reservoirs are coupled equally strongly, a nonFermi liquid ground state arises as a result of the overscreening by both reservoirs. We probe the anomalous scaling properties of this state, and show how it transitions into a more conventional Fermi liquid under the influence of various perturbations. The second device is first operated as a single metallic quantum dot in the quantum Hall regime. Spin degeneracy is broken, but the device can be tuned such that there is now a charge degeneracy which can then be screened by coupling to a reservoir. We tune to and away from equal couplings to see the effect of the twochannel Kondo state. Finally, we operate the second device in its full form as a doubledot device, to explore the competition between dotlead and interdot interactions.
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Online 5. The LZ dark matter search and development of a new gas phase technique to characterize low level electron emission from electrode grids [2019]
 Ji, Wei, 1990 author.
 [Stanford, California] : [Stanford University], 2019.
 Description
 Book — 1 online resource.
 Summary

Dark Matter is needed to explain many cosmological observations and therefore has been proposed for many decades, but it awaits direct detection. One of the most popular classes of dark matter candidates is Weakly Interacting Massive Particles (WIMPs), which have masses in the order of 100 GeV and couple to ordinary matter at weak scale. In WIMP direct detection experiments, we look for WIMPs being scattered by nuclei, a process which produces low energy (smaller than 100 keV) recoiling nuclei that can be observed. We are building LZ, a detector looking for WIMPs using liquid xenon in a dualphase time projection chamber (TPC), at 4850 feet underground at the Sanford Underground Research Facility (SURF) in Lead, South Dakota, USA. LZ aims to achieve the world's highest sensitivity to find WIMPs via WIMPnucleon interactions. After a brief discussion of dark matter and the LZ experiment, this dissertation presents the details of my study to solve the electron emission problem. The LZ TPC will consist of electrode grids and other metallic surfaces that can emit electrons when operated under voltage. Because the charge measurement in the LZ detector is sensitive to single electrons, electrons from the grids can be both a significant nuisance for data collection and a source of background at lowenergies, limiting the sensitivity of the experiment for lowmass WIMPs. This has motivated us to develop a test detector to study how to reduce this background. The test detector consists of a pair of grids biased to high voltage and operated in xenon gas. The electric field between the grid causes the electrons to produce electroluminescence scintillation light that is measured by PMTs. This new technique is sensitive to single electrons emitted by the grids, allowing a measurement of emission currents as low as attoamperes. We used this detector to study the properties of different grids and to determine what treatments can be done to reduce their electron emission. We found that passivation with citric acid reduces electron emission from stainless steel surfaces. This work was supervised by Professor Thomas Shutt and was completed in collaboration with members of the LZ collaboration and the SLAC LZ group.
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Online 6. Measurement and control of nonadiabatic dynamics a study in the acetylene dication [2019]
 LiekhusSchmaltz, Chelsea Elizabeth, author.
 [Stanford, California] : [Stanford University], 2019.
 Description
 Book — 1 online resource.
 Summary

Nonadiabatic structural dynamics occur in lighter molecules and are often described using Conical Intersections (CI) in molecules with at least three atoms. Light that is resonant near regions of strong nonadiabatic coupling can play an important role in controlling and understanding these dynamics. This thesis presents both simple model simulations and experimental data in the acetylene dication to analyze how light interacts with nonadiabatic dynamics. A key parameter that relates how the nuclear energy changes in time compared to the relative electronic energy is used to explain and predict the role that dipole coupling has in controlling nonadiabatic dynamics.
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Online 7. Measurements with optically levitated microspheres [2019]
 Rider, Alexander David, author.
 [Stanford, California] : [Stanford University], 2019.
 Description
 Book — 1 online resource.
 Summary

I discuss the development of optically levitated microspheres as a tool for precision measurements and tests of fundamental physics. Micronscale dielectric spheres are trapped by the radiation pressure at the focus of a Gaussian laser beam, where the optical suspension enables thermal, electrical, and mechanical isolation from the surrounding environment at high vacuum. Forces and torques can be measured from changes in the angle and polarization of light both transmitted through and reflected by the trapped particle. Additionally, the charge of the particle can be controlled with single electron precision. We have used these methods for the following three purposes: to search for fractionally charged particles and dark energy, to develop measurement techniques for surface potentials, and to construct an electrically driven microgyroscope.
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Online 8. Search of the Higgs boson produced through vector boson fusion decaying to a pair of bb with the ATLAS detector [2019]
 Jiang, Zihao, author.
 [Stanford, California] : [Stanford University], 2019.
 Description
 Book — 1 online resource.
 Summary

This thesis presents the search of the Higgs boson produced by Vector Boson Fusion (VBF) and decaying to bottom quarks. A search using the ATLAS detector was performed with 2016 protonproton collision data. The multivariate analysis measured the signal strengths of both the inclusive Higgs production and the vectorboson fusion production relative to the Standard Model prediction. This analysis led to the observation of Higgs coupling to bquarks in the summer of 2018. Furthermore, potential improvements of the analysis techniques using complex neural networks are investigated. In order to understand better the Quantum Chromodynamics (QCD) backgrounds of the Higgs search, the characteristic variables of the gluon splitting vertex are measured.
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Online 9. Angular momentum conservation law in lightfront quantum field theory and extended conformal symmetry of Abelian gauge theory in D ≠ 4 [2018]
 Chiu, YuJu, author.
 [Stanford, California] : [Stanford University], 2018.
 Description
 Book — 1 online resource.
 Summary

This thesis investigates two independent aspects of spacetime symmetries. The first part of my thesis is about the angular momentum conservation law in lightfront quantum field theory. We prove the lightfront Poincare invariance of the angular momentum conservation law and the helicity sum rule for relativistic composite systems. We show that the lightfront wavefunction (LFWF), which describes the internal structure of a bound state, is in fact frame independent, in contrast to instant form wavefunctions. In particular, we demonstrate that j3, the intrinsic angular momentum projected onto the lightfront direction, is independent of the bound state's 4momentum and the observer's Lorentz frame. The frame independence of j3 is a feature unique to the front form. The angular momentum conservation law leads directly to a nonperturbative proof of the constraint A(0)=1 and the vanishing of the anomalous gravitomagetic moment B(0)=0. Based on the conservation of angular momentum, we derive a selection rule for orbital angular momentum which can be used to eliminate certain interaction vertices in QED and QCD. We also generalize the selection rule to any renormalizable theory and show that there exists an upper bound on the change of orbital angular momentum in scattering processes at any fixed order in perturbation theory. The second part of my thesis investigates an extended conformal symmetry for Abelian gauge theory in general dimensions. Maxwell theory in d \neq 4 spacetime dimensions is an example of a scaleinvariant theory which does not possess conformal symmetry  the special conformal transformation (SCT) explicitly breaks the gauge invariance of the theory. We construct a nonlocal gaugeinvariant extension of the SCT, which is compatible with the BRST formalism and defines a new symmetry of the physical Hilbert space of the Maxwell theory for any dimension d \geqslant 3. We prove the invariance of Maxwell theory in d \geqslant 3 by explicitly showing that the gaugeinvariant twopoint correlation functions, the action, and the classical equation of motion are unchanged under such a transformation.
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Special Collections
Special Collections  Status 

University Archives  
3781 2018 C  Unavailable In process 
 Benjamin, Nathan, author.
 [Stanford, California] : [Stanford University], 2018.
 Description
 Book — 1 online resource.
 Summary

In this thesis, we study aspects of threedimensional gravity in Anti de Sitter (AdS) space by studying the holographically dual twodimensional conformal field theory (CFT). We begin by describing general constraints on the elliptic genus of a twodimensional supersymmetric conformal field theory which has a gravity dual with large radius in Planck units. We discuss the distinction between theories with supergravity duals and those whose duals have strings at the scale set by the AdS curvature, using symmetric orbifolds as a case study. We then move on to study extremal theories, conjectured to be dual to "pure" threedimensional gravity. We first provide an example of an extremal chiral N=2 superconformal field theory at c=24. We then consider extremal CFTs at large central charge, and consider the quantum corrections to the classical spectrum. Our conjecture passes various consistency checks, especially when generalized to include theories with supersymmetry. Finally, we examine a specific topdown construction of AdS3/CFT2 from string theory, called the D1/D5 system. We examine the lowlying quarter BPS spectrum of the K3 symmetric orbifold CFT at various points in moduli space, and look at a more refined count than the ordinary elliptic genus. We do a decomposition of the spectra into N=4 characters, and show that at large N the character decomposition satisfies an unusual property, in which the degeneracy only depends on a certain linear combination of left and rightmoving quantum numbers, suggesting deeper symmetry structure.
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Special Collections
Special Collections  Status 

University Archives  
3781 2018 B  Unavailable In process 