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Caltech Authors (167.915 recursos)

Repository of works by Caltech published authors.

Status = In Press

Mostrando recursos 1 - 20 de 2.126

  1. Charge carrier transport across grain boundaries in graphene

    Mendez, J. P.; Arca, F.; Ramos, J.; Ortiz, M.; Ariza, M. P.
    We evaluate the charge carrier transmission across asymmetric grain boundaries (GB) in a graphene lattice within the Landauer-Büttiker formalism. We employ a tight-binding model for C-based materials that accounts for lattice strain introduced by topological defects, such as grain boundaries. In particular, we investigate electronic transmission across grain boundaries found to be stable up to high temperatures. Our calculations suggest that the introduction of GBs generally preserves the zero-transport gap property of pristine graphene. However, only some specific asymmetric GBs open a moderate transport gap, which can be as high as ≈ 1.15 eV. We find that the GBs that...

  2. Eulerian-Lagrangian method for simulation of cloud cavitation

    Maeda, Kazuki; Colonius, Tim
    We present a coupled Eulerian-Lagrangian method to simulate cloud cavitation in a compressible liquid. The method is designed to capture the strong, volumetric oscillations of each bubble and the bubble-scattered acoustics. The dynamics of the bubbly mixture is formulated using volume-averaged equations of motion. The continuous phase is discretized on an Eulerian grid and integrated using a high-order, finite-volume weighted essentially non-oscillatory (WENO) scheme, while the gas phase is modeled as spherical, Lagrangian point-bubbles at the sub-grid scale, each of whose radial evolution is tracked by solving the Keller-Miksis equation. The volume of bubbles is mapped onto the Eulerian grid...

  3. The origin of ultra diffuse galaxies: stellar feedback and quenching

    Chan, T. K.; Kereš, D.; Wetzel, A.; Hopkins, P. F.; Faucher-Giguère, C.-A.; El-Badry, K.; Garrison-Kimmel, S.; Boylan-Kolchin, M.
    We test if the cosmological zoom-in simulations of isolated galaxies from the FIRE project reproduce the properties of ultra diffuse galaxies (UDGs). We show that outflows that dynamically heat galactic stars, together with a passively aging stellar population after imposed quenching, naturally reproduce the observed population of red UDGs, without the need for high spin halos, or dynamical influence from their host cluster. We reproduce the range of surface brightness, radius and absolute magnitude of the observed red UDGs by quenching simulated galaxies at a range of different times. They represent a mostly uniform population of dark matter-dominated dwarf galaxies...

  4. The Fate of Atomic Spin in Atomic Scattering Off Surfaces

    Kretchmer, Joshua S.; Chan, Garnet Kin-Lic
    We explore model electron dynamics of an atom scattering off a surface within the time-dependent complete active space self consistent field (TD-CASSCF) approximation. We focus especially on the scattering of a hydrogen atom and its resulting spin-dynamics starting from an initially spin-polarized state. Our results reveal competing electronic time-scales that are governed by the electronic structure of the surface as well as the character of the atom. The timescales and nonadiabaticity of the dynamics are reported on by the final spin-polarization of the scattered atom, which may be probed in future experiments.

  5. Enantioselective Synthesis of Vicinal All-Carbon Quaternary Centers via Iridium-Catalyzed Allylic Alkylation

    Hethcox, J. Caleb; Shockley, Samantha E.; Stoltz, Brian M.
    The development of the first enantioselective transition metal‐catalyzed allylic alkylation providing access to acyclic products bearing vicinal all‐carbon quaternary centers is disclosed. The iridium‐catalyzed allylic alkylation reaction proceeds with excellent yields and selectivities for a range of malononitrile‐derived nucleophiles and trisubstituted allylic electrophiles. The utility of these sterically congested products is explored through a series of diverse chemo‐ and diastereoselective product transformations to afford a number of highly valuable, densely functionalized building blocks, including those containing vicinal all‐carbon quaternary stereocenters.

  6. Broadly neutralizing antibodies from human survivors target a conserved site in the Ebola virus glycoprotein HR2–MPER region

    Flyak, Andrew I.; Kuzmina, Natalia; Murin, Charles D.; Bryan, Christopher; Davidson, Edgar; Gilchuk, Pavlo; Gulka, Christopher P.; Ilinykh, Philipp A.; Shen, Xiaoli; Huang, Kai; Ramanathan, Palaniappan; Turner, Hannah; Fusco, Marnie L.; Lampley, Rebecca; Kose, Nurgun; King, Hannah; Sapparapu, Gopal; Doranz, Benjamin J.; Ksiazek, Thomas G.; Wright, David W.; Saphire, Erica Ollmann; Ward, Andrew B.; Bukreyev, Alexander; Crowe, James E., Jr.
    Ebola virus (EBOV) in humans causes a severe illness with high mortality rates. Several strategies have been developed in the past to treat EBOV infection, including the antibody cocktail ZMapp, which has been shown to be effective in nonhuman primate models of infection and has been used under compassionate-treatment protocols in humans. ZMapp is a mixture of three chimerized murine monoclonal antibodies (mAbs) that target EBOV-specific epitopes on the surface glycoprotein. However, ZMapp mAbs do not neutralize other species from the genus Ebolavirus, such as Bundibugyo virus (BDBV), Reston virus (RESTV) or Sudan virus (SUDV). Here, we describe three naturally...

  7. Single Day Construction of Multigene Circuits with 3G Assembly

    Halleran, Andrew D.; Swaminathan, Anandh; Murray, Richard M.
    The ability to rapidly design, build, and test prototypes is of key importance to every engineering discipline. DNA assembly often serves as a rate limiting step of the prototyping cycle for synthetic biology. Recently developed DNA assembly methods such as isothermal assembly and type IIS restriction enzyme systems take different approaches to accelerate DNA construction. We introduce a hybrid method, Golden Gate-Gibson (3G), that takes advantage of modular part libraries introduced by type IIS restriction enzyme systems and isothermal assembly’s ability to build large DNA constructs in single pot reactions. Our method is highly efficient and rapid, facilitating construction of...

  8. Exoplanet Biosignatures: A Review of Remotely Detectable Signs of Life

    Schwieterman, Edward W.; Kiang, Nancy Y.; Parenteau, Mary N.; Harman, Chester E.; DasSarma, Shiladitya; Fisher, Theresa M.; Arney, Giada N.; Hartnett, Hilairy E.; Reinhard, Christopher T.; Olson, Stephanie L.; Meadows, Victoria S.; Cockell, Charles S.; Walker, Sara I.; Grenfell, John Lee; Hegde, Siddharth; Rugheimer, Sarah; Hu, Renyu; Lyons, Timothy W.
    In the coming years and decades, advanced space- and ground-based observatories will allow an unprecedented opportunity to probe the atmospheres and surfaces of potentially habitable exoplanets for signatures of life. Life on Earth, through its gaseous products and reflectance and scattering properties, has left its fingerprint on the spectrum of our planet. Aided by the universality of the laws of physics and chemistry, we turn to Earth's biosphere, both in the present and through geologic time, for analog signatures that will aid in the search for life elsewhere. Considering the insights gained from modern and ancient Earth, and the broader...

  9. Harnessing bistability for directional propulsion of soft, untethered robots

    Chen, Tian; Bilal, Osama R.; Shea, Kristina; Daraio, Chiara
    In most macroscale robotic systems, propulsion and controls are enabled through a physical tether or complex onboard electronics and batteries. A tether simplifies the design process but limits the range of motion of the robot, while onboard controls and power supplies are heavy and complicate the design process. Here, we present a simple design principle for an untethered, soft swimming robot with preprogrammed, directional propulsion without a battery or onboard electronics. Locomotion is achieved by using actuators that harness the large displacements of bistable elements triggered by surrounding temperature changes. Powered by shape memory polymer (SMP) muscles, the bistable elements...

  10. Machine Learning Seismic Wave Discrimination: Application to Earthquake Early Warning

    Li, Zefeng; Meier, Men-Andrin; Hauksson, Egill; Zhan, Zhongwen; Andrews, Jennifer
    Performance of earthquake early warning (EEW) systems suffers from false alerts caused by local impulsive noise from natural or anthropogenic sources. To mitigate this problem, we train a Generative Adversarial Network (GAN) to learn the characteristics of first‐arrival earthquake P waves, using 300,000 waveforms recorded in southern California and Japan. We apply the GAN critic as an automatic feature extractor and train a Random Forest classifier with about 700,000 earthquake and noise waveforms. We show that the discriminator can recognize 99.2% of the earthquake P waves and 98.4% of the noise signals. This state‐of‐the‐art performance is expected to reduce significantly...

  11. Foraging for foundations in decision neuroscience: insights from ethology

    Mobbs, Dean; Trimmer, Pete C.; Blumstein, Daniel T.; Dayan, Peter
    Modern decision neuroscience offers a powerful and broad account of human behaviour using computational techniques that link psychological and neuroscientific approaches to the ways that individuals can generate near-optimal choices in complex controlled environments. However, until recently, relatively little attention has been paid to the extent to which the structure of experimental environments relates to natural scenarios, and the survival problems that individuals have evolved to solve. This situation not only risks leaving decision-theoretic accounts ungrounded but also makes various aspects of the solutions, such as hard-wired or Pavlovian policies, difficult to interpret in the natural world. Here, we suggest...

  12. In silico discovery of new dopants for Fe-doped Ni oxyhydroxide (Ni_(1-x)Fe_xOOH) catalysts for oxygen evolution reaction

    Shin, Hyeyoung; Xiao, Hai; Goddard, William A., III
    The oxygen evolution reaction (OER) is critical to efficient water splitting to produce the H_2 fuel for sustainable energy production. Currently, the best non-noble metal OER electrocatalyst in base conditions is the Fe-doped NiOOH (Ni_(1-x)Fe_xOOH), with η=0.4, but much lower values are desired. We use density functional theory to determine the overall mechanism for the OER of Ni_(1-x)Fe_xOOH, concluding that promoting radical character on the metal-oxo bond is critical to efficient OER. Then we consider replacing Fe with 17 other transition metals of the Fe, Ru, and Os rows, where we find 3 new promising candidates: Co, Rh, and Ir,...

  13. Model-theoretic aspects of the Gurarij operator system

    Goldbring, Isaac; Lupini, Martino
    We establish some of the basic model theoretic facts about the Gurarij operator system GS recently constructed by the second-named author. In particular, we show: (1) GS is the unique separable 1-exact existentially closed operator system; (2) GS is the unique separable nuclear model of its theory; (3) every embedding of GS into its ultrapower is elementary; (4) GS is the prime model of its theory; and (5) GS does not have quantifier-elimination, whence the theory of operator systems does not have a model companion. We also show that, for any q ∈ N, the theories of M_q -spaces and...

  14. Small-Pore Zeolites: Synthesis and Catalysis

    Dusselier, Michiel; Davis, Mark E.
    In the past decade or so, small-pore zeolites have received greater attention than large- and medium-pore molecular sieves that have historically dominated the literature. This is primarily due to the commercialization of two major catalytic processes, NOx exhaust removal and methanol conversion to light olefins, that take advantage of the properties of these materials with smaller apertures. Small-pore zeolites possess pores that are constructed of eight tetrahedral atoms (Si^(4+) and Al^(3+)), each time linked by a shared oxygen These eight-member ring pores (8MR) provide small molecules access to the intracrystalline void space, e.g., to NOx during car exhaust cleaning (NOx...

  15. Template-Free Synthesis of Periodic Three-Dimensional PbSe Nanostructures via Photoelectrodeposition

    Carim, Azhar I.; Hamann, Kathryn R.; Batara, Nicolas A.; Thompson, Jonathan R.; Atwater, Harry A.; Lewis, Nathan S.
    Highly periodic, geometrically directed, anisotropic Se-Pb films have been synthesized at room temperature from an isotropic aqueous solution without the use of physical templates, by photoelectrodeposition using a series of discrete input illumination polarizations and wavelengths from an unstructured, uncorrelated, incoherent light source. Dark growth did not generate deposits with substantial long-range order, but growth using unpolarized illumination resulted in an ordered, nanoscale, mesh-type morphology. Linearly polarized illumination generated Se-Pb deposits that displayed an ordered, highly anisotropic lamellar pattern wherein the long axes of the lamellae were aligned parallel to the light polarization vector. The pitch of the lamellar features...

  16. Transient reactivation of a deep-seated landslide by undrained loading captured with repeat airborne and terrestrial lidar

    Booth, Adam M.; McCarley, Justin; Hinkle, Jason; Shaw, Susan; Ampuero, Jean-Paul; Lamb, Michael P.
    Landslides reactivate due to external environmental forcing or internal mass redistribution, but the process is rarely documented quantitatively. We capture the three‐dimensional, 1‐m resolution surface deformation field of a transiently reactivated landslide with image correlation of repeat airborne lidar. Undrained loading by two debris flows in the landslide's head, rather than external forcing, triggered reactivation. After that loading, the lower 2 km of the landslide advanced by up to 14 m in 2 yrs before completely stopping. The displacement field over those 2 yrs implies that the slip surface gained 1 kPa of shear strength, which was likely accomplished by...

  17. Mixing, scalar boundedness, and numerical dissipation in large-eddy simulations

    Sharan, Nek; Matheou, Georgios; Dimotakis, Paul E.
    Numerical schemes for scalar transport and mixing in turbulent flows must be high-order accurate, and observe conservation and boundedness constraints. Discretization accuracy can be evaluated from the truncation error, and assessed by its dispersion and dissipation properties. Dispersion errors can cause violation of physical scalar bounds, whereas numerical dissipation is key to mitigating those violations. Numerical dissipation primarily alters the energy at small scales that are critical to turbulent mixing. Influence of additional dissipation on scalar mixing in large-eddy simulations (LES) of incompressible temporally evolving shear flow is examined in terms of the resolved passive-scalar field, z. Scalar fields in...

  18. Secondary fluorescence effects in microbeam analysis and their impacts on geospeedometry and geothermometry

    Borisova, Anastassia Y.; Zagrtdenov, Nail R.; Toplis, Michael J.; Donovan, John J.; Llovet, Xavier; Asimow, Paul D.; de Parseval, Philippe; Gouy, Sophie
    Characteristic and bremsstrahlung X-ray emission during electron-specimen interactions in electron microprobe (EPMA) and scanning electron microscope (SEM) instruments causes secondary fluorescence X-ray effects from adjacent (boundary) phases. This is well-known, yet the impact of such effects in microbeam analysis of natural mineral-hosted inclusions and adjacent to mineral-mineral and mineral-glass boundaries are frequently neglected, especially in geospeedometry and geothermometry applications. To demonstrate the important influence of the secondary fluorescence effect on the measured concentration of elements and its consequences for geochemical applications, we consider the effect of mineral-mineral and mineral-glass boundaries in microanalysis of Cr, Zr and Ti both experimentally, using...

  19. A Smartphone Based in‐Gel Loop Mediated Isothermal Amplification (gLAMP) System Enables Rapid Coliphage MS2 Quantification in Environmental Waters

    Huang, Xiao; Lin, Xingyu; Urmann, Katharina; Li, Lijie; Xie, Xing; Jiang, Sunny; Hoffmann, Michael R.
    Model coliphages (e.g., ΦX174, MS2, and PRD1) have been widely used in place of pathogenic viruses to study their fate and transport in the environment and during wastewater treatment. Two groups of coliphages (F-specific and somatic) are being explored as indicators of viral fecal pollution in ambient water. However, the detection and quantification of coliphages still largely relies on time-consuming culture-based plaque assays. In this study, we developed an in-gel loop mediated isothermal amplification (gLAMP) system enabling coliphage MS2 quantification within 30 minutes using standard laboratory devices. Viral particles (MS2) were immobilized with LAMP reagents in polyethylene glycol hydrogel, and...

  20. Observation of valence band crossing: the thermoelectric properties of CaZn_2Sb_2–CaMg_2Sb_2 solid solution

    Wood, Max; Aydemir, Umut; Ohno, Saneyuki; Snyder, G. Jeffrey
    CaAl_2Si_2 type Zintl phases have long been known to be promising thermoelectric materials. Here we report for the first time on the thermoelectric properties of CaMg_2Sb_2 along with the transport properties of CaZn_2Sb_2–CaMg_2Sb_2 solid solution. The charge carrier tuning in this system was carried out by substituting divalent Ca^(2+) with monovalent Na^+. To check a possible band convergence, we applied an effective mass analysis to our samples and found an abrupt doubling of the samples' effective masses as the composition switches from Zn-rich to Mg-rich. We further analyzed the effect that alloy scattering plays in the lattice thermal conductivity of...

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