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Caltech Authors (99,974 recursos)
Repository of works by Caltech published authors.

Type = Report or Paper

Mostrando recursos 1 - 20 de 2,735

1. Mobility of Basal Dislocations Interacting with Non Basal Dislocations in Zinc - Nagata, Norio; Vreeland, T., Jr.
Studies of the mobility of basal dislocations in zinc (1, 2) by use of a torsion stress pulsing technique (3) have shown that the maximum velocity of dislocations, V_(max), is a linear function of applied shear stress, τ, at stresses above 10^6 dyne/cm^2 as expressed by B V_(max) = τ b (1) where B is a drag coefficient and b is the basal Burgers vector. It has been concluded that some type of dislocation-phonon interaction is responsible for the damping of dislocation motion in an otherwise perfect crystal. Flow stress measurements have indicated a strong interaction between basal dislocations and forest dislocations (4). Hence, the scatter observed in...

2. The legacy of ADM - Deser, S.
This tribute to the memory of my old friend and collaborator, Richard Arnowitt, focuses on the history, results and physical significance of the Arnowitt-Deser-Misner formulation of General Relativity, starting from its birth in 1958-9 through its completion, in a series of over a dozen papers, in 1962-3. A few of its later applications are also reviewed.

3. Effects of Two Inert Scalar Doublets on Higgs Interactions and Electroweak Phase Transition - Ahriche, Amine; Faisal, Gaber; Ho, Shu-Yu; Nasri, Salah; Tandean, Jusak
We study some implications of the presence of two inert scalar doublets which are charged under a dark Abelian gauge symmetry. Specifically, we investigate the effects of the new scalars on oblique electroweak parameters and on the interactions of the 125 GeV Higgs boson, especially its decay modes h→γγ,γZ, and trilinear coupling, all of which will be probed with improved precision in future Higgs measurements. Moreover, we explore how the inert scalars may give rise to strongly first-order electroweak phase transition and also show its correlation with sizable modifications to the Higgs trilinear coupling.

4. Carbon in Red Giants in Globular Clusters and Dwarf Spheroidal Galaxies - Kirby, Evan N.; Guo, Michelle; Zhang, Andrew J.; Deng, Michelle; Cohen, Judith G.; Guhathakurta, Puragra; Shetrone, Matthew D.; Lee, Young Sun; Rizzi , Luca
We present carbon abundances of red giants in Milky Way globular clusters and dwarf spheroidal galaxies (dSphs). Our sample includes measurements of carbon abundances for 154 giants in the clusters NGC 2419, M68, and M15 and 398 giants in the dSphs Sculptor, Fornax, Ursa Minor, and Draco. This sample doubles the number of dSph stars with measurements of [C/Fe]. The [C/Fe] ratio in the clusters decreases with increasing luminosity above log(L/L_⊙)≃1.6, which can be explained by deep mixing in evolved giants. The same decrease is observed in dSphs, but the initial [C/Fe] of the dSph giants is not uniform. Stars...

5. Breaking through the Thresholds: an Analysis for Iterative Reweighted ℓ_1 Minimization via the Grassmann Angle Framework - Xu, Weiyu; Khajehnejad, M. Amin; Avestimehr, A. Salman; Hassibi, Babak
It is now well understood that ℓ_1 minimization algorithm is able to recover sparse signals from incomplete measurements [2], [1], [3] and sharp recoverable sparsity thresholds have also been obtained for the ℓ_1 minimization algorithm. However, even though iterative reweighted ℓ_1 minimization algorithms or related algorithms have been empirically observed to boost the recoverable sparsity thresholds for certain types of signals, no rigorous theoretical results have been established to prove this fact. In this paper, we try to provide a theoretical foundation for analyzing the iterative reweighted ℓ_1 algorithms. In particular, we show that for a nontrivial class of signals,...

6. Sparse Recovery of Positive Signals with Minimal Expansion - Khajehnejad, M. Amin; Dimakis, Alexandros G.; Xu, Weiyu; Hassibi, Babak
We investigate the sparse recovery problem of reconstructing a high-dimensional non-negative sparse vector from lower dimensional linear measurements. While much work has focused on dense measurement matrices, sparse measurement schemes are crucial in applications, such as DNA microarrays and sensor networks, where dense measurements are not practically feasible. One possible construction uses the adjacency matrices of expander graphs, which often leads to recovery algorithms much more efficient than ℓ_1 minimization. However, to date, constructions based on expanders have required very high expansion coefficients which can potentially make the construction of such graphs difficult and the size of the recoverable sets...

7. Compressive Sensing over the Grassmann Manifold: a Unified Geometric Framework - Xu, Weiyu; Hassibi, Babak
ℓ_1 minimization is often used for finding the sparse solutions of an under-determined linear system. In this paper we focus on finding sharp performance bounds on recovering approximately sparse signals using ℓ_1 minimization, possibly under noisy measurements. While the restricted isometry property is powerful for the analysis of recovering approximately sparse signals with noisy measurements, the known bounds on the achievable sparsity (The "sparsity" in this paper means the size of the set of nonzero or significant elements in a signal vector.) level can be quite loose. The neighborly polytope analysis which yields sharp bounds for ideally sparse signals cannot...

8. New Null Space Results and Recovery Thresholds for Matrix Rank Minimization - Oymak, Samet; Hassibi, Babak
Nuclear norm minimization (NNM) has recently gained significant attention for its use in rank minimization problems. Similar to compressed sensing, using null space characterizations, recovery thresholds for NNM have been studied in. However simulations show that the thresholds are far from optimal, especially in the low rank region. In this paper we apply the recent analysis of Stojnic for compressed sensing to the null space conditions of NNM. The resulting thresholds are significantly better and in particular our weak threshold appears to match with simulation results. Further our curves suggest for any rank growing linearly with matrix size n we...

9. Mobility of Basal Dislocations in Zinc - Pope, D. P.; Vreeland, T., Jr.
This paper reports the results of an experimental study in which basal dislocation velocities were measured in zinc as a function of stress, temperature and dislocation orientation. The velocities were measured using the direct or Gilman-Johnston technique in which the individual dislocations themselves are observed. Tests were performed on 99.999% purity monocrystals. The applied resolved shear stress ranged from 0 to about 20 x 10^6 dynes/cm^2, the load durations were in the microsecond range, the test temperatures were 300, 223, 173 and 123 °K, and the measured velocities ranged from about 200 to 2000 cm/sec. Since the velocities are a linear function of stress and the velocity at a given...

10. On the Damping of Screw Dislocation Motion in FCC Crystals by Phonon Viscosity - Jassby, K. M.; Vreeland, T., Jr.
The phonon viscosity mechanism, as it applies to screw dislocations moving on the {111} planes of face-centered-cubic crystals, is examined. Formulas are derived for changes in the elastic stiffnesses of cubic crystals with the impression of an arbitrary elastic strain. The result is specialized to the case of the strain field of a screw dislocation in a face-centered-cubic crystal. Lattice energy absorption from a moving screw dislocation is then considered through the relaxation of the elastic stiffnesses. Using the formula for energy absorption, a dislocation damping coefficient, B, is found. B is independent of temperature above the Debye temperature, and at lower temperatures has different forms for anisotropic and isotropic crystals....

11. Crystallographic Dependence of a Dislocation Etch for Zinc Single Crystals - Adams, K. H.; Vreeland, T., Jr.
Additional observations have been made on the crystallographic dependence of a dislocation etch for zinc single crystals originally developed by Brandt, et al. The original work confirmed that the etch technique was suitable for revealing basal and nonbasal dislocation intersections with {1010} prism planes. A new set of crystal surface orientations has been found where etch pits are revealed. A 2 in. diameter hemispherical crystal of 99.999 per cent purity zinc was etched and etch pits were found on all surfaces located between 3° and 12.2° to the [0001] axis. Figure 1 shows the regions of good, marginal and poor etching. Attempts to etch the (0001)...

12. Dislocation Velocity Measurements in Copper and Zinc - Vreeland, T., Jr.
Studies of dislocation dynamics in a number of different materials have been reported, but no direct measurements in FCC or HCP metals have appeared in the literature. Studies of slip band growth in the basal system of zinc at this laboratory have indicated that individual dislocations achieve relatively high velocities at the yield stress. This implies that only very short duration loading (µsec) of low dislocation density crystals (<1000 cm^(-2)) will permit direct measurement of the dynamics of motion of individual basal dislocations in zinc. Suitable single crystal specimens of 99.999+% copper and zinc were prepared for this study. A torsion loading system was developed to apply single, short duration stress...

13. Finding Dense Clusters via "Low Rank + Sparse" Decomposition - Oymak, Samet; Hassibi, Babak
Finding "densely connected clusters" in a graph is in general an important and well studied problem in the literature. It has various applications in pattern recognition, social networking and data mining. Recently, Ames and Vavasis have suggested a novel method for finding cliques in a graph by using convex optimization over the adjacency matrix of the graph. Also, there has been recent advances in decomposing a given matrix into its "low rank" and "sparse" components. In this paper, inspired by these results, we view "densely connected clusters" as imperfect cliques, where imperfections correspond missing edges, which are relatively sparse. We analyze...

14. A study of the mechanism of the delayed yield phenomenon - Vreeland, T., Jr.; Wood, D. S.; Clark, D. S.
This paper presents the results of an experimental investigation of the behavior of an annealed low carbon steel subjected to a rapidly applied constant stress and to repeated short-duration stress-pulses. The test stresses were greater than the upper yield stress. The material was aged at various temperatures between stress-pulses, and the effect of the time of aging on the number of stress-pulses to induce yielding was determined. Plastic and anelastic microstrain of the order of 30 X 10^(-6) in./in. is observed prior to the onset of yielding in rapidly applied constant stress tests and in repeated stress-pulse tests. Aging of the specimens for a sufficient length of time at a given temperature...

15. A study of dislocation mobility and density in metallic crystals - Vreeland, Thad, Jr.; Lau, Silvanus S.
This report summarizes the research accomplishments under the Atomic Energy Commission contracts, CALT-473 and CALT-767-P3b for the ten-year period, November 1, 1963 to October 31, 1973. The research was stimulated by technological advances which required improvements in our ability to predict the deformation behavior of materials. In the mid 1930's, theoreticians first recognized that crystal defects could play a central role in plastic deformation, and since that time a number of experiments have conclusively demonstrated the one-to-one correspondence between the motion of line defects (dislocations) and plastic deformation. Before the existence and significance of dislocations was recognized, theoreticians faced a puzzling...

16. A simple technique for the preparation of ultra-thin carbon films for electron microscopy - Arnal, Henri J.; Horgen, Henri M.
Ultra-thin supporting membranes of carbon are valuable in electron microscopy of biological samples and for some applications as examination of fine particles or thin metallic films obtained by vacuum evaporation which require high resolution. A simple method has been developed for production of support films thinner than 20 Å. These specimen supports show a good mechanical stability and a total open area of a 400-mesh grid is usable.

17. Improving the Thresholds of Sparse Recovery: An Analysis of a Two-Step Reweighted Basis Pursuit Algorithm - Khajehnejad, M. Amin; Xu, Weiyu; Avestimehr, A. Salman; Hassibi, Babak
It is well known that ℓ_1 minimization can be used to recover sufficiently sparse unknown signals from compressed linear measurements. In fact, exact thresholds on the sparsity, as a function of the ratio between the system dimensions, so that with high probability almost all sparse signals can be recovered from i.i.d. Gaussian measurements, have been computed and are referred to as "weak thresholds". In this paper, we introduce a reweighted ℓ_1 recovery algorithm composed of two steps: a standard ℓ_1 minimization step to identify a set of entries where the signal is likely to reside, and a weighted ℓ_1 minimization step where...

18. On the Entropy Region of Gaussian Random Variables - Shadbakht, Sormeh; Hassibi, Babak
Given n (discrete or continuous) random variables X_i, the (2^n-1)-dimensional vector obtained by evaluating the joint entropy of all non-empty subsets of {X_1,...,X_n} is called an entropic vector. Determining the region of entropic vectors is an important open problem with many applications in information theory. Recently, it has been shown that the entropy regions for discrete and continuous random variables, though different, can be determined from one another. An important class of continuous random variables are those that are vector-valued and jointly Gaussian. In this paper we give a full characterization of the convex cone of the entropy region of three jointly Gaussian vector-valued...

19. Error Correcting Codes for Distributed Control - Sukhavasi, Ravi Teja; Hassibi, Babak
The problem of stabilizing an unstable plant over a noisy communication link is an increasingly important one that arises in applications of networked control systems. Although the work of Schulman and Sahai over the past two decades, and their development of the notions of "tree codes" and "anytime capacity", provides the theoretical framework for studying such problems, there has been scant practical progress in this area because explicit constructions of tree codes with efficient encoding and decoding did not exist. To stabilize an unstable plant driven by bounded noise over a noisy channel one needs real-time encoding and real-time decoding and a reliability...

20. Anatomy of the AGN in NGC 5548: III. The high-energy view with NuSTAR and INTEGRAL - Ursini, F.; Boissay, R.; Petrucci, P.-O.; Matt, G.; Cappi, M.; Bianchi, S.; Kaastra, J.; Harrison, F.; Walton, D. J.; di Gesu, L.; Constantini, E.; De Marco, B.; Kriss, G. A.; Mehdipour, M.; Paltani, S.; Peterson, B. M.; Ponti, G.; Steenbrugge, K. C.
We describe the analysis of the seven broad-band X-ray continuum observations of the archetypal Seyfert 1 galaxy NGC 5548 that were obtained with XMM-Newton or Chandra, simultaneously with high-energy (> 10 keV) observations with NuSTAR and INTEGRAL. These data were obtained as part of a multiwavelength campaign undertaken from the summer of 2013 till early 2014. We find evidence of a high-energy cut-off in at least one observation, which we attribute to thermal Comptonization, and a constant reflected component that is likely due to neutral material at least a few light months away from the continuum source. We confirm the...

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