2,720 research outputs found

    Ferromagnetic resonance in periodic particle arrays

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    We report measurements of the ferromagnetic resonance (FMR) spectra of arrays of submicron size periodic particle arrays of permalloy produced by electron-beam lithography. In contrast to plane ferromagnetic films, the spectra of the arrays show a number of additional resonance peaks, whose position depends strongly on the orientation of the external magnetic field and the interparticle interaction. Time-dependent micromagnetic simulation of the ac response show that these peaks are associated with coupled exchange and dipolar spin wave modesComment: 4 pages, 4 figure

    Comparisons of physique, body composition, and somatotype by weight division between male and female collegiate taekwondo athletes

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    The aim of the study was to compare the physique, body composition and somatotype between male and female collegiate taekwondo athletes and specially focus on differences by weight division. 60 collegiate taekwondo athletes (male: 29, female: 31) voluntarily participated in the study. They were divided into four Olympic weight divisions (male for -58 kg, -68 kg, -80 kg, +80 kg, female for -49 kg, -57 kg, -67 kg, +67 kg). Anthropometric measurements included body weight, height, sitting height, body circumferences (relaxed arm, flexed arm, chest, waist, hip, thigh, and calf), bone widths (humerus and femur), and skinfold thicknesses (triceps, subscapular, suprailiac, thigh, and calf) were measured. The three somatotype components were assessed by Heath-Carter anthropometric method (Carter & Heath, 1990). Independent t-test and one-way ANOVA were applied to analyze difference of dependent variables. Significant level was set at .05. Male athletes were taller and heavier than female athletes. However, sum of skinfold thickness was significantly higher in female athletes than male athletes. The three somatotype components for male athletes were 3.4-3.5-3.1 and characterized with balanced mesomorphy. On the other hand, the somatotype of female athletes were 6.1-3.4-2.6 and characterized with mesomorphic endomorph. In male athletes -80 kg and +80 kg weight divisions were higher mesomorphy, but lower ectomorphy than -58 kg and -68 kg weight divisions. In female, -57 kg, -67 kg and +67 kg weight divisions were higher endomorphy and mesomorphy, but lower ectomorphy than -49 kg weight divisions. In conclusion, male athletes had higher anthropometric characteristics than female athletes except for the skinfold thickness. Female athletes had higher endomorphy, whereas male athletes had higher ectomorphy. Physique and somatotype were different between weight divisions both male and female athletes. This study provides a reference data of morphological characteristics of collegiate elite taekwondo athletes

    Real-time detection of malicious network activity using stochastic models

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    Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2006.Includes bibliographical references (p. 115-122).This dissertation develops approaches to rapidly detect malicious network traffic including packets sent by portscanners and network worms. The main hypothesis is that stochastic models capturing a host's particular connection-level behavior provide a good foundation for identifying malicious network activity in real-time. Using the models, the dissertation shows that a detection problem can be formulated as one of observing a particular "trajectory" of arriving packets and inferring from it the most likely classification for the given host's behavior. This stochastic approach enables us not only to estimate an algorithm's performance based on the measurable statistics of a host's traffic but also to balance the goals of promptness and accuracy in detecting malicious network activity. This dissertation presents three detection algorithms based on Wald's mathematical framework of sequential analysis. First, Threshold Random Walk (TRW) rapidly detects remote hosts performing a portscan to a target network. TRW is motivated by the empirically observed disparity between the frequency with which connections to newly visited local addresses are successful for benign hosts vs. for portscanners. Second, it presents a hybrid approach that accurately detects scanning worm infections quickly after the infected local host begins to engage in worm propagation.(cont.) Finally, it presents a targeting worm detection algorithm, Rate-Based Sequential Hypothesis Testing (RBS), that promptly identifies high-fan-out behavior by hosts (e.g., targeting worms) based on the rate at which the hosts initiate connections to new destinations. RBS is built on an empirically-driven probability model that captures benign network characteristics. It then presents RBS+TRW, a unified framework for detecting fast-propagating worms independently of their target discovery strategy. All these schemes have been implemented and evaluated using real packet traces collected from multiple network vantage points.by Jaeyeon Jung.Ph.D

    Acoustic articulatory evidence for quantal vowel categories : the features [low] and [back]

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    Thesis (Ph. D.)--Harvard-MIT Division of Health Sciences and Technology, 2009.Cataloged from student-submitted PDF version of thesis.Includes bibliographical references (p. 139-142).In recent years, research in human speech communication suggested that the inventory of sound units that are observed in vowels across languages is strongly influenced by the acoustic properties of the human subglottal system. That is, there is a discrete set of possible vowel features that are constrained by the interaction of the acoustic/articulatory properties of the vowels and a small set of attributes that are observed in the subglottal region. This thesis tests the hypothesis that subglottal resonances govern vowel feature boundaries for three populations: adult speakers of English; adult speakers of Korean; and children learning English. First, we explored the relations among F1 of vowels, the first subglottal resonances (SubF1) and the feature [low] in English. For the diphthong [??], F1 peaks for vowels showed an acoustic irregularity near the speaker' s SubF1. For monophthongs, analysis of F1 frequency distributions shows a boundary between [+low] and [-low] vowels at the speakers' SubF1. Second, we studied the relations among F2 of Korean vowels, SubF2 and the feature [back], to test whether the relation between subglottal resonances and the feature boundary, demonstrated earlier for English, also can be applied to other languages. Results show that the F2 boundary between [back] and [front] vowels was placed near SubF2 in Korean, as in English. Third, we explored the development of vowel formants in relation to subglottal resonances for 10 children in the age range of 2;6-3;9 years using the database of Imbrie (2005). Results show that at the earlier ages, formant values deviated from the expected relations, but during the six month period in which the measurements were made, there was considerable movement toward the expected values.(cont.)The transition to the expected relations appeared to occur by the age of 3 years for most of these children, in a developmental pattern that was inconsistent with an account in terms of simple anatomical increase. These three sets of observations provide evidence that subglottal resonances play a role in defining vowel feature boundaries, as predicted by Stevens' (1972) hypothesis that contrastive phonological features in human languages have arisen from quantal discontinuities in articulatory-acoustic space.by Youngsook Jung.Ph.D

    Atomic Scale Memory at a Silicon Surface

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    The limits of pushing storage density to the atomic scale are explored with a memory that stores a bit by the presence or absence of one silicon atom. These atoms are positioned at lattice sites along self-assembled tracks with a pitch of 5 atom rows. The writing process involves removal of Si atoms with the tip of a scanning tunneling microscope. The memory can be reformatted by controlled deposition of silicon. The constraints on speed and reliability are compared with data storage in magnetic hard disks and DNA.Comment: 13 pages, 5 figures, accepted by Nanotechnolog

    Fast methods for inverse wave scattering problems

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    Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2008.Includes bibliographical references (p. 125-137).Inverse wave scattering problems arise in many applications including computerized/diffraction tomography, seismology, diffraction/holographic grating design, object identification from radar singals, and semiconductor quality control. Efficient algorithms exist for some inverse wave scattering problems in the low- and high-frequency regime or with weak scatterers. However, inverse wave scattering problems in the resonance regime with strong scatterers still pose many challenges. This thesis proposes algorithms for inverse wave scattering problems in the resonance regime with strong scatterers. These problems are part of, for instance, grating design, object identification, and semiconductor quality control. The proposed methods are (a) a spectrally convergent Nyström method for periodic structures in 2-D; (b) a fast Jacobian approximation method accompanying a Nyström method; (c) a fast and accurate method for evaluating the potential integrals in the 3-D mixed-potential integral operator with the Rao-Wilton-Glisson basis function; and (d) optimization with parameterized reduced-order models. The Nyström method and the method to evaluate the potential integrals accelerate scattered field evaluations by solving integral equations efficiently. The Jacobian approximation method and optimization with parameterized reduced-order models efficiently couple algorithms to evaluate scattered fields due to a guess of the scatterer and optimization methods to improve the guess. The Nyström and the Jacobian approximation methods are used to identify the parameters characterizing a periodic dielectric grating in 2-D. The method to evaluate the potential integrals and optimization with parameterized reduced-order models are applied to the problem of identifying simple discrete geometries in 3-D.by Jung Hoon Lee.Ph.D

    LHC Predictions from a Tevatron Anomaly in the Top Quark Forward-Backward Asymmetry

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    We examine the implications of the recent CDF measurement of the top-quark forward-backward asymmetry, focusing on a scenario with a new color octet vector boson at 1-3 TeV. We study several models, as well as a general effective field theory, and determine the parameter space which provides the best simultaneous fit to the CDF asymmetry, the Tevatron top pair production cross section, and the exclusion regions from LHC dijet resonance and contact interaction searches. Flavor constraints on these models are more subtle and less severe than the literature indicates. We find a large region of allowed parameter space at high axigluon mass and a smaller region at low mass; we match the latter to an SU(3)xSU(3)/SU(3) coset model with a heavy vector-like fermion. Our scenario produces discoverable effects at the LHC with only 1-2 inverse femtobarns of luminosity at 7-8 TeV. Lastly, we point out that a Tevatron measurement of the b-quark forward-backward asymmetry would be very helpful in characterizing the physics underlying the top-quark asymmetry.Comment: 35 pages, 10 figures, 4 table
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