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MIT Open Access Articles

Mostrando recursos 41 - 60 de 23.296

  1. Broadband angular selectivity of light at the nanoscale: Progress, applications, and outlook

    Shen, Yichen; Hsu, Chia Wei; Yeng, Yi Xiang; Joannopoulos, John D.; Soljačić, Marin; Yeng, YiXiang; Joannopoulos, John; Soljacic, Marin
    Humankind has long endeavored to control the propagation direction of light. Since time immemorial, shades, lenses, and mirrors have been used to control the flow of light. In modern society, with the rapid development of nanotechnology, the control of light is moving toward devices at micrometer and even nanometer scales. At such scales, traditional devices based on geometrical optics reach their fundamental diffraction limits and cease to work. Nano-photonics, on the other hand, has attracted wide attention from researchers, especially in the last decade, due to its ability to manipulate light at the nanoscale. This review focuses on the nano-photonics...

  2. The effects of main-ion dilution on turbulence in low q[subscript 95] C-Mod ohmic plasmas, and comparisons with nonlinear GYRO

    Candy, J.; Staebler, G.; Reinke, M. L; Ennever, Paul Chappell; Porkolab, Miklos; Rice, John E; Rost, Jon C; Ernst, Darin R; Hughes Jr, Jerry; Baek, Seung Gyou
    Recent experiments on C-mod seeding nitrogen into ohmic plasmas with [subscript q]95 = 3.4 found that the seeding greatly reduced long-wavelength (ITG-scale) turbulence. The long-wavelength turbulence that was reduced by the nitrogen seeding was localized to the region of r/a≈0.85, where the turbulence is well above marginal stability (as evidenced by Q[subscript i]/Q[subscript GB]≫1). The nonlinear gyrokinetic code GYRO was used to simulate the expected turbulence in these plasmas, and the simulated turbulent density fluctuations and turbulent energy fluxes quantitatively agreed with the experimental measurements both before and after the nitrogen seeding. Unexpectedly, the intrinsic rotation of the plasma was also found...

  3. Validation of full-wave simulations for mode conversion of waves in the ion cyclotron range of frequencies with phase contrast imaging in Alcator C-Mod

    Tsujii, N.; Jaeger, E. F.; Green, D. L.; Harvey, R. W.; Porkolab, Miklos; Bonoli, Paul T; Edlund, Eric Matthias; Ennever, Paul Chappell; Lin, Yijun; Wright, John C; Wukitch, Stephen James
    Mode conversion of fast waves in the ion cyclotron range of frequencies (ICRF) is known to result in current drive and flow drive under optimised conditions, which may be utilized to control plasma profiles and improve fusion plasma performance. To describe these processes accurately in a realistic toroidal geometry, numerical simulations are essential. Quantitative comparison of these simulations and the actual experimental measurements is important to validate their predictions and to evaluate their limitations. The phase contrast imaging (PCI) diagnostic has been used to directly detect the ICRF waves in the Alcator C-Mod tokamak. The measurements have been compared with...

  4. A New Event Builder for CMS Run II

    Darlea, G.-L.; Gomez-Ceballos, Guillelmo; Paus, Christoph M. E.; Sumorok, Konstanty C; Veverka, Jan
    The data acquisition system (DAQ) of the CMS experiment at the CERN Large Hadron Collider (LHC) assembles events at a rate of 100 kHz, transporting event data at an aggregate throughput of 100GB/s to the high-level trigger (HLT) farm. The DAQ system has been redesigned during the LHC shutdown in 2013/14. The new DAQ architecture is based on state-of-the-art network technologies for the event building. For the data concentration, 10/40 Gbps Ethernet technologies are used together with a reduced TCP/IP protocol implemented in FPGA for a reliable transport between custom electronics and commercial computing hardware. A 56 Gbps Infiniband FDR...

  5. Track Reconstruction Progress from the DMTPC Directional Dark Matter Experiment

    Druitt, Gabriela; Eggleston, Richard; Lopez, Jeremy; Monroe, Jocelyn; Deaconu, Cosmin Stefan; Fisher, Peter H; Tomita, Hidefumi; Zayas, Evan M.
    he Dark Matter Time Projection Chamber (DMTPC) collaboration is developing prototype detectors to measure both the energies and directions of nuclear recoils. The intended application is to exploit the expected directional anisotropy of dark matter velocities at Earth to unambiguously observe dark matter induced recoils. The detector consist of low-pressure CF[subscript 4] TPC's with CCD cameras, PMT's, and charge amplifiers for readout. This talk gives an overview of the experiment and describes recent advances in hardware and analysis.

  6. A Quantum Version of Schoening's Algorithm Applied to Quantum 2-Sat

    Kimmel, Shelby; Temme, Kristan; Farhi, Edward H
    We study a quantum algorithm that consists of a simple quantum Markov process, and we analyze its behavior on restricted versions of Quantum 2-SAT. We prove that the algorithm solves this decision problem with high probability for n qubits, L clauses, and promise gap c in time O(n[superscript 2]L[superscript 2]c[superscript −2]). If the Hamiltonian is additionally polynomially gapped, our algorithm efficiently produces a state that has high overlap with the satisfying subspace. The Markov process we study is a quantum analogue of Sch¨oning’s probabilistic algorithm for k-SAT.

  7. The Design, Fabrication, and Performance of the East African Trial Leveraged Freedom Chair

    Pearlman, Jonathan L.; Winter, Amos G.; Bollini, Mario A; DeLatte, Danielle M; Judge, Benjamin M; O'Hanley, Harrison F; Scolnik, Natasha
    The Leveraged Freedom Chair (LFC) is a lever-powered, wheelchair-based mobility aid designed specifically for use in the developing world. Its drivetrain optimally converts upper body power in a wide range of terrains, giving the LFC operational capabilities that extend beyond those of currently available mobility products. In this work we present the design and analysis process used to create an LFC for trial in East Africa. All of the moving parts in the LFC are made from bicycle components and the entire chair can be fabricated without any machining processes. This allows the LFC to be manufactured for the same...

  8. Light scattering and transmission measurement using digital imaging for online analysis of constituents in milk

    Jain, Pranay; Sarma, Sanjay E
    Milk is an emulsion of fat globules and casein micelles dispersed in an aqueous medium with dissolved lactose, whey proteins and minerals. Quantification of constituents in milk is important in various stages of the dairy supply chain for proper process control and quality assurance. In field-level applications, spectrophotometric analysis is an economical option due to the low-cost of silicon photodetectors, sensitive to UV/Vis radiation with wavelengths between 300 - 1100 nm. Both absorption and scattering are witnessed as incident UV/Vis radiation interacts with dissolved and dispersed constituents in milk. These effects can in turn be used to characterize the chemical...

  9. Interferometric analysis of laser-driven cylindrically focusing shock waves in a thin liquid layer

    Pezeril, Thomas; Veysset, David Georges; Maznev, Alexei; Kooi, Steven E; Nelson, Keith Adam
    Shock waves in condensed matter are of great importance for many areas of science and technology ranging from inertially confined fusion to planetary science and medicine. In laboratory studies of shock waves, there is a need in developing diagnostic techniques capable of measuring parameters of materials under shock with high spatial resolution. Here, time-resolved interferometric imaging is used to study laser-driven focusing shock waves in a thin liquid layer in an all-optical experiment. Shock waves are generated in a 10 µm-thick layer of water by focusing intense picosecond laser pulses into a ring of 95 µm radius. Using a Mach-Zehnder interferometer and...

  10. Making the Cut: Using Status-Based Countertactics to Block Social Movement Implementation and Microinstitutional Change in Surgery

    Kellogg, Katherine C.
    Much of the change that social movements try to accomplish requires changing practices inside organizations, yet reform implementation is difficult to achieve. This comparative case study of two hospitals demonstrates that implementing reform inside organizations may require internal reformers not only to mobilize with one another but also to stand up to internal defenders' countertactics in everyday encounters. Because reformer alliances across identity lines often require reformers with different statuses to collaborate with one another, defenders can divide reformer coalitions by linking reform practices to a status characteristic associated with lower-status reformers, denigrating higher-status reformers by associating them with these...

  11. DELAY SPECTRUM WITH PHASE-TRACKING ARRAYS: EXTRACTING THE H i POWER SPECTRUM FROM THE EPOCH OF REIONIZATION

    de Oliveira Costa, Angelica; Ewall-Wice, Aaron Michael; Feng, Lu; Hewitt, Jacqueline N; Neben, Abraham Richard; Tegmark, Max Erik; Morgan, Eric R.; Williams, Christopher Leigh
    The detection of redshifted 21 cm emission from the epoch of reionization (EoR) is a challenging task owing to strong foregrounds that dominate the signal. In this paper, we propose a general method, based on the delay spectrum approach, to extract H i power spectra that are applicable to tracking observations using an imaging radio interferometer ("Delay Spectrum with Imaging Arrays"). Our method is based on modeling the H i signal taking into account the impact of wide field effects such as the w-term, which are then used as appropriate weights in cross-correlating the measured visibilities. Our method is applicable...

  12. Asymmetric collapse by dissolution or melting in a uniform flow

    Rycroft, Chris H.; Bazant, Martin Z
    An advection–diffusion-limited dissolution model of an object being eroded by a two-dimensional potential flow is presented. By taking advantage of the conformal invariance of the model, a numerical method is introduced that tracks the evolution of the object boundary in terms of a time-dependent Laurent series. Simulations of a variety of dissolving objects are shown, which shrink and collapse to a single point in finite time. The simulations reveal a surprising exact relationship, whereby the collapse point is the root of a non-analytic function given in terms of the flow velocity and the Laurent series coefficients describing the initial shape....

  13. Massively parallel sequencing of single cells by epicPCR links functional genes with phylogenetic markers

    Guo, Mira T; Briggs, Adrian W; A Weitz, David; Pitkänen, Leena K; Vigneault, Francois; Virta, Marko PJuhani; Spencer, Sarah J.; Tamminen, Manu V.; Preheim, Sarah P.; Brito, Ilana Lauren
    Many microbial communities are characterized by high genetic diversity. 16S ribosomal RNA sequencing can determine community members, and metagenomics can determine the functional diversity, but resolving the functional role of individual cells in high throughput remains an unsolved challenge. Here, we describe epicPCR (Emulsion, Paired Isolation and Concatenation PCR), a new technique that links functional genes and phylogenetic markers in uncultured single cells, providing a throughput of hundreds of thousands of cells with costs comparable to one genomic library preparation. We demonstrate the utility of our technique in a natural environment by profiling a sulfate-reducing community in a freshwater lake,...

  14. Formation mechanism of guided resonances and bound states in the continuum in photonic crystal slabs

    Gao, Xingwei; Chen, Hongsheng; Hsu, Chia Wei; Zhen, Bo; Lin, Xiao; Joannopoulos, John; Soljacic, Marin
    We develop a formalism, based on the mode expansion method, to describe the guided resonances and bound states in the continuum (BICs) in photonic crystal slabs with one-dimensional periodicity. This approach provides analytic insights to the formation mechanisms of these states: the guided resonances arise from the transverse Fabry–Pérot condition, and the divergence of the resonance lifetimes at the BICs is explained by a destructive interference of radiation from different propagating components inside the slab. We show BICs at the center and on the edge of the Brillouin zone protected by symmetry, BICs at generic wave vectors not protected by...

  15. Origin of the low critical observing temperature of the quantum anomalous Hall effect in V-doped (Bi, Sb)[subscript 2]Te[subscript 3] film

    Li, W.; Claassen, M.; Moritz, B.; Jia, T.; Zhang, C.; Rebec, S.; Lee, J. J.; Hashimoto, M.; Lu, D.-H.; Moore, R. G.; Devereaux, T. P.; Shen, Z.-X.; Chang, Cui-zu; Moodera, Jagadeesh
    The experimental realization of the quantum anomalous Hall (QAH) effect in magnetically-doped (Bi, Sb)[subscript 2]Te[subscript 3] films stands out as a landmark of modern condensed matter physics. However, ultra-low temperatures down to few tens of mK are needed to reach the quantization of Hall resistance, which is two orders of magnitude lower than the ferromagnetic phase transition temperature of the films. Here, we systematically study the band structure of V-doped (Bi, Sb)[subscript 2]Te[subscript 3] thin films by angle-resolved photoemission spectroscopy (ARPES) and show unambiguously that the bulk valence band (BVB) maximum lies higher in energy than the surface state Dirac...

  16. Search for metastable heavy charged particles with large ionisation energy loss in pp collisions at √s = 8  TeV using the ATLAS experiment

    Taylor, Frank E
    Many extensions of the Standard Model predict the existence of charged heavy long-lived particles, such as R-hadrons or charginos. These particles, if produced at the Large Hadron Collider, should be moving non-relativistically and are therefore identifiable through the measurement of an anomalously large specific energy loss in the ATLAS pixel detector. Measuring heavy long-lived particles through their track parameters in the vicinity of the interaction vertex provides sensitivity to metastable particles with lifetimes from 0.6 ns to 30 ns. A search for such particles with the ATLAS detector at the Large Hadron Collider is presented, based on a data sample...

  17. Parkinson-associated risk variant in distal enhancer of α-synuclein modulates target gene expression

    Soldner, Frank; Stelzer, Yonatan; Abraham, Brian J.; Latourelle, Jeanne C.; Barrasa, M. Inmaculada; Goldmann, Johanna; Myers, Richard H.; Young, Richard A; Jaenisch, Rudolf; Shivalila, Chikdu Shakti
    Genome-wide association studies (GWAS) have identified numerous genetic variants associated with complex diseases, but mechanistic insights are impeded by a lack of understanding of how specific risk variants functionally contribute to the underlying pathogenesis. It has been proposed that cis-acting effects of non-coding risk variants on gene expression are a major factor for phenotypic variation of complex traits and disease susceptibility. Recent genome-scale epigenetic studies have highlighted the enrichment of GWAS-identified variants in regulatory DNA elements of disease-relevant cell types. Furthermore, single nucleotide polymorphism (SNP)-specific changes in transcription factor binding are correlated with heritable alterations in chromatin state and considered...

  18. Shanghai Gone: Domicide and Defiance in a Chinese Megacity. By QinShao. Lanham, Md.: Rowman & Littlefield, 2013. xviii, 307 pp. $79.00 (cloth); $29.95 (paper); $28.99 (e-book)

    Leighton, Christopher R.
    Attention to the physical transformation of urban China across the last decades often fixates on feats of construction: ever more ambitious infrastructure, lofty skyscrapers, and opulent entertainments. Shanghai, the country’s premier megacity, usually sets the pace, and will soon add the eighteenth track to its metro, a 2,000 foot tower crowning its skyline, and a Disneyland triple the size of Hong Kong’s (itself a grand project only a decade old). In Shanghai Gone, Qin Shao redirects us to consider instead the equally impressive process of destruction that precedes and propels the city’s continuing reconfiguration.

  19. Synthesis of Molybdenum and Tungsten Alkylidene Complexes that Contain a tert-Butylimido Ligand

    Jeong, Hyangsoo; Schrock, Richard Royce; Mueller, Peter
    National Science Foundation (U.S.) (CHE-1111133)

  20. Stereospecific Ring-Opening Metathesis Polymerization (ROMP) of endo-Dicyclopentadiene by Molybdenum and Tungsten Catalysts

    Axtell, Jonathan C.; Ota, Antje; Lehr, Thomas; Buchmeiser, Michael R.; Autenrieth, Benjamin; Jeong, Hyangsoo; Forrest Jr, William P; Axtell, Jonathan; Schrock, Richard Royce
    United States. Department of Energy (DE-FG02-86ER13564)

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