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Open Access Publications from the University of California

Scholarly Works (175 results)

  • Thesis
  • Peer Reviewed
UCLA Electronic Theses and Dissertations (2020)

Two-dimensional materials provide a new platform for discovery of exotic physics and phenomena. They possess rich electronic properties, susceptibility to electric control, danglingbond-free interfaces, and readily controlled thicknesses. These properties make them accessible, engineerable, and integrable into emergent heterostructures for previously unachievable properties and applications like atomically-thin magnetoelectric devices for ultracompact spintronics. Although layered materials like graphene are not inherently spin-polarized, magnetic proximity effect-induced spin splitting has been identified as an effective way to realize spin transport in graphene. Except the magnetic proximity effect, recently discovered long-range intrinsic magnetic orders in the van der Waals materials rise fundamental research interests. This dissertation includes the experimental realization of monolayer graphene magnetized by an underlying antiferromagnet. By coupling graphene to an antiferromagnetic thin film, exchange splitting energy as large as 134 meV at 2 K. This exchange splitting energy can be modulated through field coolings, which is reflected through the shifted quantum Hall plateau and quantum oscillations in graphene. Further, magneto-optic Kerr measurement supports magnetism in the graphene at low temperatures. This work establishes a key functionality for future graphene-based spin logic and memory devices. Different from the induced magnetism into graphene, the interface-induced Dzyaloshinskii-Moriya interaction and N�eel-type skyrmion lattice in an intrinsic van derWaals ferromagnetic material Fe3GeTe2 have also been discussed. By coupling Fe3GeTe2 to 1T’-WTe2, a large interfacial Dzyaloshinskii-Moriya interaction can be induced at the interface probably due to the inversion symmetry breaking. Transport measurements have shown the topological Hall effect in this heterostructure for temperatures below 100 K. Furthermore, Lorentz transmission electron microscopy is used to directly image N�eel-type skyrmions along with aligned and stripe-like domain structures. This interfacial coupling induced Dzyaloshinskii-Moriya interaction is estimated to have a large energy of 1.0 mJ/m2, which is much larger than the critical value required to stabilize the N�eel-type skyrmions. This work paves a path towards the skyrmionic devices based on van der Waals layered heterostructure. Based on either induced or intrinsic magnetism in van der Waals materials, possible quantum anomalous Hall insulators and van der Waals heterostructureare listed and proposed.

    Cover page: Magnetism in van der Waals Heterostructures: Graphene/CrSe and WTe2/Fe3GeTe2
    • Article
    • Peer Reviewed
    UC Davis Previously Published Works (2015)

    Background

    A previous study identified a Yersinia enterocolitica transposon mutant, GY448, that was unable to export the flagellar type three secretion system (T3SS)-dependent phospholipase, YplA. This strain was also deficient for motility and unable to form colonies on Lauria-Bertani agar medium. Preliminary analysis suggested it carried a mutation in csrA. CsrA in Escherichia coli is an RNA-binding protein that is involved in specific post-transcriptional regulation of a myriad of physiological activities. This study investigated how CsrA affects expression of the flagellar regulatory cascade that controls YplA export and motility. It also explored the effect of csrA mutation on Y. enterocolitica in response to conditions that cue physiological changes important for growth in environments found both in nature and the laboratory.

    Results

    The precise location of the transposon insertion in GMY448 was mapped within csrA. Genetic complementation restored disruptions in motility and the YplA export phenotype (Yex), which confirmed this mutation disrupted CsrA function. Mutation of csrA affected expression of yplA and flagellar genes involved in flagellar T3SS dependent export and motility by altering expression of the master regulators flhDC. Mutation of csrA also resulted in increased sensitivity of Y. enterocolitica to various osmolytes, temperatures and antibiotics.

    Conclusions

    The results of this study reveal unique aspects of how CsrA functions in Y. enterocolitica to control its physiology. This provides perspective on how the Csr system is susceptible to adaptation to particular environments and bacterial lifestyles.
      Cover page: CsrA impacts survival of Yersinia enterocolitica by affecting a myriad of physiological activities
      • Article
      • Peer Reviewed
      UC San Diego Previously Published Works (2024)
      Algae biotechnology holds immense promise for revolutionizing the bioeconomy through the sustainable and scalable production of various bioproducts. However, their development has been hindered by the lack of advanced genetic tools. This study introduces a synthetic biology approach to develop such tools, focusing on the construction and testing of synthetic promoters. By analyzing conserved DNA motifs within the promoter regions of highly expressed genes across six different algal species, we identified cis-regulatory elements (CREs) associated with high transcriptional activity. Combining the algorithms POWRS, STREME, and PhyloGibbs, we predicted 1511 CREs and inserted them into a minimal synthetic promoter sequence in 1, 2, or 3 copies, resulting in 4533 distinct synthetic promoters. These promoters were evaluated in vivo for their capacity to drive the expression of a transgene in a high-throughput manner through next-generation sequencing post antibiotic selection and fluorescence-activated cell sorting. To validate our approach, we sequenced hundreds of transgenic lines showing high levels of GFP expression. Further, we individually tested 14 identified promoters, revealing substantial increases in GFP expression─up to nine times higher than the baseline synthetic promoter, with five matching or even surpassing the performance of the native AR1 promoter. As a result of this study, we identified a catalog of CREs that can now be used to build superior synthetic algal promoters. More importantly, here we present a validated pipeline to generate building blocks for innovative synthetic genetic tools applicable to any algal species with a sequenced genome and transcriptome data set.
        Cover page: Bioinformatic Prediction and High Throughput In Vivo Screening to Identify Cis-Regulatory Elements for the Development of Algal Synthetic Promoters.Creative Commons 'BY-NC-ND' version 4.0 license
        • Article
        • Peer Reviewed
        LBL Publications (2017)
        The observation of neutrinoless double-beta decay (0νββ) would show that lepton number is violated, reveal that neu-trinos are Majorana particles, and provide information on neutrino mass. A discovery-capable experiment covering the inverted ordering region, with effective Majorana neutrino masses of 15 - 50 meV, will require a tonne-scale experiment with excellent energy resolution and extremely low backgrounds, at the level of ∼0.1 count /(FWHM·t·yr) in the region of the signal. The current generation 76Ge experiments GERDA and the Majorana Demonstrator, utilizing high purity Germanium detectors with an intrinsic energy resolution of 0.12%, have achieved the lowest backgrounds by over an order of magnitude in the 0νββ signal region of all 0νββ experiments. Building on this success, the LEGEND collaboration has been formed to pursue a tonne-scale 76Ge experiment. The collaboration aims to develop a phased 0νββ experimental program with discovery potential at a half-life approaching or at 1028 years, using existing resources as appropriate to expedite physics results.
          Cover page: The large enriched germanium experiment for neutrinoless double beta decay (LEGEND)
          • Article
          • Peer Reviewed
          UCLA Previously Published Works (2016)
          We present high precision measurements of elliptic flow near midrapidity (|y|<1.0) for multistrange hadrons and ϕ meson as a function of centrality and transverse momentum in Au+Au collisions at center of mass energy √[sNN]=200  GeV. We observe that the transverse momentum dependence of ϕ and Ω v2 is similar to that of π and p, respectively, which may indicate that the heavier strange quark flows as strongly as the lighter up and down quarks. This observation constitutes a clear piece of evidence for the development of partonic collectivity in heavy-ion collisions at the top RHIC energy. Number of constituent quark scaling is found to hold within statistical uncertainty for both 0%-30% and 30%-80% collision centrality. There is an indication of the breakdown of previously observed mass ordering between ϕ and proton v2 at low transverse momentum in the 0%-30% centrality range, possibly indicating late hadronic interactions affecting the proton v2.
            • Article
            • Peer Reviewed
            UCLA Previously Published Works (2015)
            We report the observation of transverse polarization-dependent azimuthal correlations in charged pion pair production with the STAR experiment in p^{↑}+p collisions at RHIC. These correlations directly probe quark transversity distributions. We measure signals in excess of 5 standard deviations at high transverse momenta, at high pseudorapidities η>0.5, and for pair masses around the mass of the ρ meson. This is the first direct transversity measurement in p+p collisions.
              Creative Commons 'BY-NC-ND' version 4.0 license
              • Article
              • Peer Reviewed
              UCLA Previously Published Works (2015)
              Dihadron correlations are analyzed in √sNN = 200 GeV d + Au collisions classified by forward charged particle multiplicity and zero-degree neutral energy in the Au-beam direction. It is found that the jetlike correlated yield increases with the event multiplicity. After taking into account this dependence, the non-jet contribution on the away side is minimal, leaving little room for a back-to-back ridge in these collisions.
                Cover page: Effect of event selection on jetlike correlation measurement in d+Au collisions at sNN=200&nbsp;GeVCreative Commons 'BY-NC-ND' version 4.0 license
                • Article
                LBL Publications (2007)
                Measurements of two-particle correlations on transverse momentum pt for Au-Au collisions at sqrt sNN = 130 GeV are presented. Significant large-momentum-scale correlations are observed for charged primary hadrons with 0.15 <_ pt <_ 2 GeV/c and pseudorapidity lbar eta rbar <_ 1.3. Such correlations were not observed in a similar study at lower energy and are not predicted by theoretical collision models. Their direct relation to mean-pt fluctuations measured in the same angular acceptance is demonstrated. Positive correlations are observed for pairs of particles which have large pt values while negative correlations occur for pairs in which one particle has large pt and the other has much lower pt. The correlation amplitudes per final state particle increase with collision centrality. The 3 observed correlations are consistent with a scenario in which the transverse momentum of hadrons
                  Cover page: Two-Particle Correlations on Transverse Momentum and Momentum Dissipation in Au-Au Collisions at sqrt sNN = 130 GeV
                  • Article
                  • Peer Reviewed
                  UCLA Previously Published Works (2015)
                  A search for the quantum chromodynamics (QCD) critical point was performed by the STAR experiment at the BNL Relativistic Heavy Ion Collider, using dynamical fluctuations of unlike particle pairs. Heavy ion collisions were studied over a large range of collision energies with homogeneous acceptance and excellent particle identification, covering a significant range in the QCD phase diagram where a critical point may be located. Dynamical Kπ, pπ, and Kp fluctuations as measured by the STAR experiment in central 0-5% Au + Au collisions from center-of-mass collision energies sNN=7.7 to 200 GeV are presented. The observable νdyn was used to quantify the magnitude of the dynamical fluctuations in event-by-event measurements of the Kπ, pπ, and Kp pairs. The energy dependences of these fluctuations from central 0-5% Au + Au collisions all demonstrate a smooth evolution with collision energy.
                    Cover page: Energy dependence of Kπ, pπ, and Kp fluctuations in Au + Au collisions from sNN=7.7 to 200 GeVCreative Commons 'BY-NC-ND' version 4.0 license
                    • Article
                    • Peer Reviewed
                    UCLA Previously Published Works (2015)
                    The STAR Collaboration presents for the first time two-dimensional di-hadron correlations with identified leading hadrons in 200 GeV central Au. +. Au and minimum-bias d. +. Au collisions to explore hadronization mechanisms in the quark gluon plasma. The enhancement of the jet-like yield for leading pions in Au. +. Au data with respect to the d. +. Au reference and the absence of such an enhancement for leading non-pions (protons and kaons) are discussed within the context of a quark recombination scenario. The correlated yield at large angles, specifically in the ridge region, is found to be significantly higher for leading non-pions than pions. The consistencies of the constituent quark scaling, azimuthal harmonic model and a mini-jet modification model description of the data are tested, providing further constraints on hadronization.
                      Creative Commons 'BY-NC-ND' version 4.0 license
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