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Caltech Authors (90,284 recursos)
Repository of works by Caltech published authors.

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Mostrando recursos 1 - 20 de 2,481

1. Experimental determination of the microscopic origin of magnetism in parent iron pnictides - Hsieh, D.; Xia, Y.; Wray, L.; Qian, D.; Gomes, K.; Yazdani, A.; Chen, G. F.; Luo, J. L.; Wang, N. L.; Hasan, M. Z.
Like high T_c cuprates, the newly discovered iron based superconductors lie in close proximity to a magnetically ordered parent phase. However, while the magnetic order in parent cuprates is known to derive from a spin-spin local superexchange interaction, a plethora of experiments including neutron scattering have so far been unable to conclusively resolve whether a local moment Heisenberg description applies in parent iron based compounds, or whether magnetism arises from a collective SDW order instability. These two alternatives can in principle be distinguished by measuring the low energy momentum-resolved bulk-representative electronic structure of the magnetically ordered phase. Using a combination of polarization dependent ARPES and STM, we have isolated the...

2. Tail Bounds for All Eigenvalues of a Sum of Random Matrices - Gittens, Alex A.; Tropp, Joel A.
This work introduces the minimax Laplace transform method, a modification of the cumulant-based matrix Laplace transform method developed in [Tro11c] that yields both upper and lower bounds on each eigenvalue of a sum of random self-adjoint matrices. This machinery is used to derive eigenvalue analogs of the classical Chernoff, Bennett, and Bernstein bounds. Two examples demonstrate the efficacy of the minimax Laplace transform. The first concerns the effects of column sparsification on the spectrum of a matrix with orthonormal rows. Here, the behavior of the singular values can be described in terms of coherence-like quantities. The second example addresses the question of relative accuracy...

3. Error Bounds for Random Matrix Approximation Schemes - Gittens, A.; Tropp, J. A.
Randomized matrix sparsification has proven to be a fruitful technique for producing faster algorithms in applications ranging from graph partitioning to semidefinite programming. In the decade or so of research into this technique, the focus has been—with few exceptions—on ensuring the quality of approximation in the spectral and Frobenius norms. For certain graph algorithms, however, the ∞→1 norm may be a more natural measure of performance. This paper addresses the problem of approximating a real matrix A by a sparse random matrix X with respect to several norms. It provides the first results on approximation error in the ∞→1 and ∞→2 norms, and it uses a result...

4. Anatomy of the Amplituhedron - Franco, Sebastián; Galloni, Daniele; Mariotti, Alberto; Trnka, Jaroslav
We initiate a comprehensive investigation of the geometry of the amplituhedron, a recently found geometric object whose volume calculates the integrand of scattering amplitudes in planar N = 4 SYM theory. We do so by introducing and studying its stratification, focusing on four-point amplitudes. The new stratification exhibits interesting combinatorial properties and positivity is neatly captured by permutations. As explicit examples, we find all boundaries for the two and three loop amplitudes and related geometries. We recover the stratifications of some of these geometries from the singularities of the corresponding integrands, providing a non-trivial test of the amplituhedron/scattering amplitude correspondence. We finally...

5. Herschel HIFI observations of O_2 toward Orion: special conditions for shock enhanced emission - Chen, Jo-Hsin; Goldsmith, Paul F.; Viti, Serena; Snell, Ronald; Lis, Dariusz C.; Benz, Arnold; Bergin, Edwin; Black, John; Caselli, Paola; Encrenaz, Pierre; Falgarone, Edith; Goicoechea, Javier R.; Hjalmarson, Åke; Hollenbach, David; Kaufman, Michael; Melnick, Gary; Neufeld, David; Pagani, Laurent; van der Tak, Floris; Van Dishoeck, Ewine; Yildiz, Umut A.
We report observations of molecular oxygen (O_2) rotational transitions at 487 GHz, 774 GHz, and 1121 GHz toward Orion Peak A. The O_2 lines at 487 GHz and 774 GHz are detected at velocities of 10-12 km s^(-1) with line widths ~3 km s^(-3); however, the transition at 1121 GHz is not detected. The observed line characteristics, combined with the results of earlier observations, suggest that the region responsible for the O_2 emission is ≃9" (6x10^(16) cm) in size, and is located close to the H_2 Peak 1 position (where vibrationally-excited H_2 emission peaks), and not at Peak A, 23"...

6. Dispersion of Long-Period Love Waves in a Spherical Earth - Kovach, Robert L.; Anderson, Don L.
Periods of torsional eigenvibrations have been computed for heterogeneous spheres corresponding to a variety of Earth models, and the periods of oscillation are used to calculate phase and group velocities for the fundamental and first higher modes of Love waves. A comparison is made between velocities for different spherical models, with the velocities calculated by use of equivalent flat Earth structures. The comparison shows that (1) the effect of sphericity on fundamental-mode Love waves is more complicated than for Rayleigh waves because of the efficient channeling of waves by low-velocity layers, and (2) the first higher Love mode is more affected by curvature than the fundamental mode. The variation with...

7. Influence of Aerosol Scattering on the Retrieval of CO_2 Mixing Ratios: A Case Study Using Measurements from the California Laboratory for Atmospheric Remote Sensing (CLARS) - Zhang, Qiong; Natraj, Vijay; Li, King-Fai; Shia, Run-Lie; Fu, Dejian; Pongetti, Thomas J.; Sander, Stanley P.; Yung, Yuk L.
Column abundances of greenhouse gases in the Los Angeles (LA) basin have recently been measured in the near-infrared spectral region using the California Laboratory for Atmospheric Remote Sensing Fourier Transform Spectrometer (CLARS-FTS) deployed at Mt. Wilson, California since August 2011. In the presence of haze, aerosol scattering causes a variable bias in the measured slant column densities (SCDs). We apply an analytic radiative transfer model and a numerical two-stream model to estimate the impact of neglecting aerosol scattering on the CO_2 and O_2 SCDs operationally retrieved from CLARS-FTS measurements. A retrieval scheme has been developed, and tested using synthetic data....

8. Pressure Line Broadening and Feasibility of CO_2 Profile Retrieval using Near Infrared Observations of an Absorption Line - Shia, Run-Lie; Kuai, Le; Line, Michael R.; Trauger, John T.; Yung, Yuk L.
Analytic expressions are derived for the transmittance and reflectance of sunlight and their Jacobians for an absorption line with Lorentz line broadening. Rodgers information analysis is applied to calculate the information content, the degrees of freedom and the averaging kernel for a simple atmospheric model to investigate the feasibility of retrieving the profile of CO_2 using near-infrared (NIR) measurements over a single absorption line. The results have implications for the design of future space instruments with high spectral resolution and high signal to noise ratios to obtain global scale information on the CO_2 vertical distribution which is important for inferring the sources, sinks, and transport of CO_2.

9. Non-Axisymmetric Flows on Hot Jupiters with Oblique Magnetic Fields - Batygin, Konstantin; Stanley, Sabine
Giant planets that reside in close proximity to their host stars are subject to extreme irradiation, which gives rise to thermal ionization of trace Alkali metals in their atmospheres. On objects where the atmospheric electrical conductivity is substantial, the global circulation couples to the background magnetic field, inducing supplementary fields and altering the nature of the flow. To date, a number of authors have considered the influence of a spin-pole aligned dipole magnetic field on the dynamical state of a weakly-ionized atmosphere and found that magnetic breaking may lead to significantly slower winds than predicted within a purely hydrodynamical framework....

10. Model Realization and Numerical Studies of a Three-Dimensional Bosonic Topological Insulator and Symmetry-Enriched Topological Phases - Geraedts, Scott D.; Motrunich, Olexei I.
We study a topological phase of interacting bosons in (3+1) dimensions which is protected by charge conservation and time-reversal symmetry. We present an explicit lattice model which realizes this phase and which can be studied in sign-free Monte Carlo simulations. The idea behind our model is to bind bosons to topological defects called hedgehogs. We determine the phase diagram of the model and identify a phase where such bound states are proliferated. In this phase we observe a Witten effect in the bulk whereby an external monopole binds half of the elementary boson charge, which confirms that it is a...

11. MORE THAN SMART: A Framework to Make the Distribution Grid More Open, Efficient and Resilient - De Martini, Paul
This paper is the result of a series of workshops with industry, government and nonprofit leaders focused on helping guide future utility investments and planning for a new distributed generation system. The distributed grid is the final stage in the delivery of electric power linking electricity sub-stations to customers. To date, no state has initiated a comprehensive effort that includes the planning, design-build and operational requirements for large scale integration of DER into state-wide distributed generation systems. This paper provides a framework and guiding principles for how to initiate such a system and can be used to implement California law AB 327 passed in 2013 requiring investor...

12. Covariant constraints for generic massive gravity and analysis of its characteristics - Deser, S.; Sandora, M.; Waldron, A.; Zahariade, G.
We perform a covariant constraint analysis of massive gravity valid for its entire parameter space, demonstrating that the model generically propagates five degrees of freedom; this is also verified by a new and streamlined Hamiltonian description. The constraint's covariant expression permits computation of the model's caustics. Although new features such as the dynamical Riemann tensor appear in the characteristic matrix, the model still exhibits the pathologies uncovered in earlier work: superluminality and likely acausalities.

13. BPS Soliton Solutions of a D3-brane Action - Schwarz, John H.
The world-volume action of a probe D3-brane in AdS_5 X S^5 with N = 4 units of flux has the field content, symmetries, and dualities of the U(1) factor of N = 4U(N+1) super Yang--Mills theory, spontaneously broken to U(N) x U(1) by being on the Coulomb branch, with the massive fields integrated out. Thus, it might be the exact effective action (a highly effective action), or else a useful approximation to it. We construct an SL(2,Z) multiplet of BPS soliton solutions of the D3-brane action and show that in the N = 1 case they correspond to the electrically charged states that have been integrated out as...

14. Banjo timbre from string stretching and frequency modulation - Politzer, David
The geometry of a floating bridge on a drumhead soundboard produces string stretching that is first order in the amplitude of the bridge motion. This stretching modulates the string tension and consequently modulates string frequencies at acoustic frequencies. Early work in electronic sound synthesis identified such modulation as a source of bell-like and metallic timbre. And increasing string stretching by adjusting banjo string-tailpiece-head geometry enhances characteristic banjo tone. Hence, this mechanism is likely a significant source of the ring, ping, clang, and plunk common to the family of instruments that share floating-bridge/drumhead construction.

15. The plucked string: an example of non-normal dynamics - Politzer, David
Motion of a single Fourier mode of the plucked string is an example of transient, free decay of coupled, damped oscillators. It shares the rarely discussed features of the generic case, e.g., possessing a complete set of non-orthogonal eigenvectors and no normal modes, but it can be analyzed and solved analytically by hand in an approximation that is appropriate to musical instruments' plucked strings.

16. Disrupting Entanglement of Black Holes - Leichenauer, Stefan
We study entanglement in thermofield double states of strongly coupled CFTs by analyzing two-sided Reissner-Nordstrom solutions in AdS. The central object of study is the mutual information between a pair of regions, one on each asymptotic boundary of the black hole. For large regions the mutual information is positive and for small ones it vanishes; we compute the critical length scale, which goes to infinity for extremal black holes, of the transition. We also generalize the butterfly effect of Shenker and Stanford to a wide class of charged black holes, showing that mutual information is disrupted upon perturbing the system and waiting for a time of order log...

17. Unconstrained canonical action for, and positive energy of, massive spin 2 - Deser, S.
Filling a much-needed gap, we exhibit the D = 4 Fierz-Pauli (FP) massive s = 2 action, and its -- manifestly positive -- energy, in terms of its 2s + 1 = 5 unconstrained helicity (2,1,0) excitations, after reducing and diagonalizing the troublesome helicity-0 sector.

18. Gauge Theories on the Coulomb branch - Schwarz, John H.
We construct the world-volume action of a probe D3-brane in AdS_5×S^5 with N units of flux. It has the field content, symmetries, and dualities of the U(1) factor of N = 4 U(N + 1) super Yang–Mills theory, spontaneously broken to U(N)×U(1) by being on the Coulomb branch, with the massive fields integrated out. This motivates the conjecture that it is the exact effective action, called a highly effective action (HEA). We construct an SL(2,ℤ) multiplet of BPS soliton solutions of the D3-brane theory (the conjectured HEA) and show that they reproduce the electrically charged massive states that have been integrated out as well as magnetic monopoles and...

19. Infrared Consistency and the Weak Gravity Conjecture - Cheung, Clifford; Remmen, Grant N.
The weak gravity conjecture (WGC) asserts that an Abelian gauge theory coupled to gravity is inconsistent unless it contains a particle of charge q and mass m such that q ≥ m/m_(Pl). This criterion is obeyed by all known ultraviolet completions and is needed to evade pathologies from stable black hole remnants. In this paper, we explore the WGC from the perspective of low-energy effective field theory. Below the charged particle threshold, the effective action describes a photon and graviton interacting via higher-dimension operators. We derive infrared consistency conditions on the parameters of the effective action using i) analyticity of light-by-light scattering, ii) unitarity of the dynamics of an...

20. Final Report - DuMond, Jesse W. M.
This final report on Contract N6onr - 244, Task Order IV, (NR 017-602), covering the period from its beginning, March 1, 1947, to its termination, November 30 1954, includes a brief discussion of the entire research carried out under its auspices . References are made to a bibliography which includes all Special Technical Reports and published articles pertaining to accomplishments of this Task. Work financed by other organizations is referred to in this report wherever this work has been instrumental in the pursuit of the objectives of the Task.

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