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MIT Open Access Articles

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  1. Internal Friction and Nonequilibrium Unfolding of Polymeric Globules

    Wada, Hirofumi; Netz, Roland R.; Alexander-Katz, Alfredo
    The stretching response of a single collapsed homopolymer is studied using Brownian dynamic simulations. The irreversibly dissipated work is found to be dominated by internal friction effects below the collapse temperature, and the internal viscosity grows exponentially with the effective cohesive strength between monomers. These results explain friction effects of globular DNA and are relevant for dissipation at intermediate stages of protein folding.

  2. Computers, Conversation, Utilization and Commoditization: the 2008 Herb Abrams Lecture

    Levy, Frank
    I want to thank Steven Seltzer and the Brigham radiology community for the opportunity to speak today. The invitation is both an honor and a responsibility. It is an honor to follow the distinguished group of previous Abrams Lecturers: John Wennberg, Marcia Angel, Norman Shumway and Margaret Marshall, to name a few. The responsibility is to say something of interest to radiologists. We will know soon enough how that goes.

  3. Origin of transverse magnetization in epitaxial Cu/Ni/Cu nanowire arrays

    Ciria, M.; Castano, Fernando; Diez-Ferrer, J. L.; Arnaudas, J. I.; Ng, B. G.; O'Handley, Robert C.; Ross, Caroline A.
    The patterning-induced changes in the magnetic anisotropy and hysteresis of epitaxial (100)-oriented Cu/Ni(9, 10, 15 nm)/Cu planar nanowires have been quantified. When the Ni films are patterned into lines, strain relaxation leads to a thickness-dependent net in-plane anisotropy transverse to the lines. The magnetoelastic anisotropy was found from the three-dimensional strain state measured directly by synchrotron x-ray diffraction and has a value of −21 kJ/m[superscript 3] for 10-nm-thick nanowires. The angular dependence of the remanence of the nanowires indicates that the in-plane easy direction is the result of the competition between the cubic magnetocrystalline anisotropy and a uniaxial anisotropy that includes...

  4. Buffer-Gas Cooled Bose-Einstein Condensate

    Ketterle, Wolfgang; Connolly, Colin B.; Doyle, John M.; Doret, S. Charles
    We report the creation of a Bose-Einstein condensate using buffer-gas cooling, the first realization of Bose-Einstein condensation using a broadly general method which relies neither on laser cooling nor unique atom-surface properties. Metastable helium ([superscript 4]He[superscript *]) is buffer-gas cooled, magnetically trapped, and evaporatively cooled to quantum degeneracy. 10[superscript 11] atoms are initially trapped, leading to Bose-Einstein condensation at a critical temperature of 5  μK and threshold atom number of 1.1×10 [superscript 6]. This method is applicable to a wide array of paramagnetic atoms and molecules, many of which are impractical to laser cool and impossible to surface cool.

  5. Quenched-vacancy induced spin-glass order

    Gulpinar, Gul; Berker, A. Nihat
    The ferromagnetic phase of an Ising model in d=3, with any amount of quenched antiferromagnetic bond randomness, is shown to undergo a transition to a spin-glass phase under sufficient quenched bond dilution. This result, demonstrated here with the numerically exact global renormalization-group solution of a d=3 hierarchical lattice, is expected to hold true generally, for the cubic lattice and for quenched site dilution. Conversely, in the ferromagnetic–spin-glass–antiferromagnetic phase diagram, the spin-glass phase expands under quenched dilution at the expense of the ferromagnetic and antiferromagnetic phases. In the ferromagnetic–spin-glass phase transition induced by quenched dilution, reentrance as a function of temperature...

  6. Toric-boson model: Toward a topological quantum memory at finite temperature

    Hamma, Alioscia; Castelnovo, Claudio; Chamon, Claudio
    We discuss the existence of stable topological quantum memory at finite temperature. At stake here is the fundamental question of whether it is, in principle, possible to store quantum information for macroscopic times without the intervention from the external world, that is, without error correction. We study the toric code in two dimensions with an additional bosonic field that couples to the defects, in the presence of a generic environment at finite temperature: the toric-boson model. Although the coupling constants for the bare model are not finite in the thermodynamic limit, the model has a finite spectrum. We show that...

  7. Origin of Intrinsic Gilbert Damping

    Hickey, Mark; Moodera, Jagadeesh
    The damping of magnetization, represented by the rate at which it relaxes to equilibrium, is successfully modeled as a phenomenological extension in the Landau-Lifschitz-Gilbert equation. This is the damping torque term known as Gilbert damping and its direction is given by the vector product of the magnetization and its time derivative. Here we derive the Gilbert term from first-principles by a nonrelativistic expansion of the Dirac equation. We find that this term arises when one calculates the time evolution of the spin observable in the presence of the full spin-orbital coupling terms, while recognizing the relationship between the curl of...

  8. Observations of Electromagnetic Fields and Plasma Flow in Hohlraums with Proton Radiography

    Betti, R.; Back, C. A.; Frenje, Johan A.; Kilkenny, J. D.; Nikroo, A.; Town, R. P.; Seguin, Fredrick Hampton; Landen, O. L.; Amendt, P. A.; Petrasso, Richard D.; Rygg, J. R.; Soures, J. M.; Meyerhofer, D. D.; Knauer, J. P.; Li, Chikang
    We report on the first proton radiography of laser-irradiated hohlraums. This experiment, with vacuum gold (Au) hohlraums, resulted in observations of self-generated magnetic fields with peak values ~10[superscript 6]  G. Time-gated radiographs of monoenergetic protons with discrete energies (15.0 and 3.3 MeV) reveal dynamic pictures of field structures and plasma flow. Near the end of the 1-ns laser drive, a stagnating Au plasma (~10  mg cm [superscript -3]) forms at the center of the hohlraum. This is a consequence of supersonic, radially directed Au jets (~1000  μm ns[superscript -1], ~Mach  4) that arise from the interaction of laser-driven plasma bubbles expanding into one another.

  9. Quantum Solution to the Arrow-of-Time Dilemma

    Maccone, Lorenzo
    The arrow-of-time dilemma states that the laws of physics are invariant for time inversion, whereas the familiar phenomena we see everyday are not (i.e., entropy increases). I show that, within a quantum mechanical framework, all phenomena which leave a trail of information behind (and hence can be studied by physics) are those where entropy necessarily increases or remains constant. All phenomena where the entropy decreases must not leave any information of their having happened. This situation is completely indistinguishable from their not having happened at all. In the light of this observation, the second law of thermodynamics is reduced to...

  10. EMPIRICAL CONSTRAINTS ON TROJAN COMPANIONS AND ORBITAL ECCENTRICITIES IN 25 TRANSITING EXOPLANETARY SYSTEMS

    Madhusudhan, Nikku; Winn, Joshua Nathan
    We present a search for Trojan companions to 25 transiting exoplanets. We use the technique of Ford & Gaudi, in which a difference is sought between the observed transit time and the transit time that is calculated by fitting a two-body Keplerian orbit to the radial-velocity data. This technique is sensitive to the imbalance of mass at the L4/L5 points of the planet-star orbit. No companions were detected above 2σ confidence. The median 2σ upper limit is 56 M ⊕, and the most constraining limit is 2.8 M ⊕ for the case of GJ 436. A similar survey using forthcoming...

  11. Analytic approximations for transit light curve observables, uncertainties, and covariances

    Carter, Joshua Adam; Yee, Jennifer C.; Eastman, Jason; Gaudi, B. Scott; Winn, Joshua Nathan
    The light curve of an exoplanetary transit can be used to estimate the planetary radius and other parameters of interest. Because accurate parameter estimation is a non-analytic and computationally intensive problem, it is often useful to have analytic approximations for the parameters as well as their uncertainties and covariances. Here we give such formulas, for the case of an exoplanet transiting a star with a uniform brightness distribution. When limb darkening is significant, our parameter sets are still useful, although our analytic formulas underpredict the covariances and uncertainties.

  12. NEAR-INFRARED TRANSIT PHOTOMETRY OF THE EXOPLANET HD 149026b

    Carter, Joshua Adam; Winn, Joshua Nathan; Gilliland, Ronald L.; Holman, Matthew J.
    The transiting exoplanet HD 149026b is an important case for theories of planet formation and planetary structure, for the planet's relatively small size has been interpreted as evidence for a highly metal-enriched composition. We present observations of four transits with the Near-Infrared Camera and Multi-Object Spectrometer (NICMOS) on the Hubble Space Telescope within a wavelength range of 1.1-2.0 μm. Analysis of the light curve gives the most precise estimate yet of the stellar mean density, ρsstarf = 0.497[superscript +0.042] –0.057 g cm[superscript –3]. By requiring agreement between the observed stellar properties (including ρsstarf) and stellar evolutionary models, we refine the...

  13. Observation of ion cyclotron range of frequencies mode conversion plasma flow drive on Alcator C-Mod

    Lin, Yijun; Rice, John E.; Wukitch, Stephen James; Greenwald, Martin J.; Hubbard, Amanda E.; Alex, Ince-Cushman; Lin, Liang; Marmar, Earl S.; Porkolab, Miklos; Reinke, Matthew Logan; Tsujii, Naoto; Wright, John C.
    At modest [superscript 3]He levels (n[subscript 3He]/n[subscript e]~8%–12%), in relatively low density D([superscript 3]He) plasmas, [overline n]e<=1.3×10[superscript 20] m[superscript −3], heated with 50 MHz rf power at B[subscript t0]~5.1 T, strong (up to 90 km/s) toroidal rotation (Vphi) in the cocurrent direction has been observed by high-resolution x-ray spectroscopy on Alcator C-Mod. The change in central Vphi scales with the applied rf power (<=30 km s[superscript −1] MW[superscript −1]), and is generally at least a factor of 2 higher than the empirically determined intrinsic plasma rotation scaling. The rotation in the inner plasma (r/a<=0.3) responds to the rf power more...

  14. Eddy Transport and Mixing in a Wind- and Buoyancy-Driven Jet on the Sphere

    Cerovecki, Ivana; Plumb, R. Alan; Heres, William
    The baroclinically unstable wind- and buoyancy-driven flow in a zonally reentrant pie-shaped sector on a sphere is numerically modeled and then analyzed using the transformed Eulerian-mean (TEM) formalism. Mean fields are obtained by zonal and time averaging performed at fixed height. The very large latitudinal extent of the basin (50.7°S latitude to the equator) allows the latitude variation of the Coriolis parameter to strongly influence the flow. Persistent zonal jets are observed in the statistically steady state. Reynolds stress terms play an important role in redistributing zonal angular momentum: convergence of the lateral momentum flux gives rise to a strong...

  15. Note to Self: Examining Personal Information Keeping in a Lightweight Note-Taking Tool

    Van Kleek, Max G.; Bernstein, Michael S.; Panovich, Katrina Marie; Vargas, Gregory G.; Karger, David R.; schraefel, mc
    This paper describes a longitudinal field experiment in personal note-taking that examines how people capture and use information in short textual notes. Study participants used our tool, a simple browser-based textual note-taking utility, to capture personal information over the course of ten days. We examined the information they kept in notes using the tool, how this information was expressed, and aspects of note creation, editing, deletion, and search. We found that notes were recorded extremely quickly and tersely, combined information of multiple types, and were rarely revised or deleted. The results of the study demonstrate the need for a tool such as ours to support the rapid capture and retrieval of short...

  16. Application-Aware Deadlock-Free Oblivious Routing

    Kinsy, Michel A.; Cho, Myong Hyon; Wen, Tina; Suh, Edward; Van Dijk, Marten; Devadas, Srinivas
    Conventional oblivious routing algorithms are either not application-aware or assume that each flow has its own private channel to ensure deadlock avoidance. We present a framework for application-aware routing that assures deadlock-freedom under one or more channels by forcing routes to conform to an acyclic channel dependence graph. Arbitrary minimal routes can be made deadlock-free through appropriate static channel allocation when two or more channels are available. Given bandwidth estimates for flows, we present a mixed integer-linear programming (MILP) approach and a heuristic approach for producing deadlock-free routes that minimize maximum channel load. The heuristic algorithm is calibrated using the...

  17. An Integrated Proof Language for Imperative Programs

    Zee, Karen; Kuncak, Viktor; Rinard, Martin C.
    We present an integrated proof language for guiding the actions of multiple reasoning systems as they work together to prove complex correctness properties of imperative programs. The language operates in the context of a program verification system that uses multiple reasoning systems to discharge generated proof obligations. It is designed to 1) enable developers to resolve key choice points in complex program correctness proofs, thereby enabling automated reasoning systems to successfully prove the desired correctness properties; 2) allow developers to identify key lemmas for the reasoning systems to prove, thereby guiding the reasoning systems to find an effective proof decomposition;...

  18. A Nearly Optimal Oracle for Avoiding Failed Vertices and Edges

    Bernstein, Aaron; Karger, David R.
    We present an improved oracle for the distance sensitivity problem. The goal is to preprocess a directed graph G = (V,E) with non-negative edge weights to answer queries of the form: what is the length of the shortest path from x to y that does not go through some failed vertex or edge f. The previous best algorithm produces an oracle of size ~O(n[superscript 2]) that has an O(1) query time, and an ~O(nn[superscript 2]√m) construction time. It was a randomized Monte Carlo algorithm that worked with high probability. Our oracle also has a constant query time and an ~O(n[superscript...

  19. A Multiscale Ensemble Filtering System for Hydrologic Data Assimilation. Part I: Implementation and Synthetic Experiment

    McLaughlin, Dennis; Entekhabi, Dara; Wood, Eric F.; Pan, Ming; Luo, Lifeng
    The multiscale autoregressive (MAR) framework was introduced in the last decade to process signals that exhibit multiscale features. It provides the method for identifying the multiscale structure in signals and a filtering procedure, and thus is an efficient way to solve the optimal estimation problem for many high-dimensional dynamic systems. Later, an ensemble version of this multiscale filtering procedure, the ensemble multiscale filter (EnMSF), was developed for estimation systems that rely on Monte Carlo samples, making this technique suitable for a range of applications in geosciences. Following the prototype study that introduced EnMSF, a strategy is devised here to implement...

  20. Search for Dimuon Decays of a Light Scalar Boson in Radiative Transitions Υ→γA0

    Fisher, Peter H.; Spitznagel, M.; Dujmic, Denis; Yamamoto, R. K.; Sciolla, Gabriella; Henderson, Shawn Wesley; Zhao, M.; Cowan, Ray Franklin
    We search for evidence of a light scalar boson in the radiative decays of the Υ(2S) and Υ(3S) resonances: Υ(2S,3S)→γA[superscript 0], A[superscript 0]→μ[superscript +]μ[superscript -]. Such a particle appears in extensions of the standard model, where a light CP-odd Higgs boson naturally couples strongly to b quarks. We find no evidence for such processes in the mass range 0.212≤m[subscript A [superscript 0]]≤9.3  GeV in the samples of 99×106 Υ(2S) and 122×106 Υ(3S) decays collected by the BABAR detector at the SLAC PEP-II B factory and set stringent upper limits on the effective coupling of the b quark to the A[superscript 0]....

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