2,082 research outputs found
Evidence of Raleigh-Hertz surface waves and shear stiffness anomaly in granular media
Due to the non-linearity of Hertzian contacts, the speed of sound in granular
matter increases with pressure. Under gravity, the non-linear elastic
description predicts that acoustic propagation is only possible through surface
modes, called Rayleigh-Hertz modes and guided by the index gradient. Here we
directly evidence these modes in a controlled laboratory experiment and use
them to probe the elastic properties of a granular packing under vanishing
confining pressure. The shape and the dispersion relation of both transverse
and sagittal modes are compared to the prediction of non-linear elasticity that
includes finite size effects. This allows to test the existence of a shear
stiffness anomaly close to the jamming transition.Comment: 4 pages 4 figure
Detectability of dissipative motion in quantum vacuum via superradiance
We propose an experiment for generating and detecting vacuum-induced
dissipative motion. A high frequency mechanical resonator driven in resonance
is expected to dissipate energy in quantum vacuum via photon emission. The
photons are stored in a high quality electromagnetic cavity and detected
through their interaction with ultracold alkali-metal atoms prepared in an
inverted population of hyperfine states. Superradiant amplification of the
generated photons results in a detectable radio-frequency signal temporally
distinguishable from the expected background.Comment: 4 pages, 2 figure
Anomalies in electrostatic calibrations for the measurement of the Casimir force in a sphere-plane geometry
We have performed precision electrostatic calibrations in the sphere-plane
geometry and observed anomalous behavior. Namely, the scaling exponent of the
electrostatic signal with distance was found to be smaller than expected on the
basis of the pure Coulombian contribution and the residual potential found to
be distance dependent. We argue that these findings affect the accuracy of the
electrostatic calibrations and invite reanalysis of previous determinations of
the Casimir force.Comment: 4 pages, 4 figure
Differential rates of perinatal maturation of human primary and nonprimary auditory cortex
Abstract Primary and nonprimary cerebral cortex mature along different timescales; however, the differences between the rates of maturation of primary and nonprimary cortex are unclear. Cortical maturation can be measured through changes in tissue microstructure detectable by diffusion magnetic resonance imaging (MRI). In this study, diffusion tensor imaging (DTI) was used to characterize the maturation of Heschl’s gyrus (HG), which contains both primary auditory cortex (pAC) and nonprimary auditory cortex (nAC), in 90 preterm infants between 26 and 42 weeks postmenstrual age (PMA). The preterm infants were in different acoustical environments during their hospitalization: 46 in open ward beds and 44 in single rooms. A control group consisted of 15 term-born infants. Diffusion parameters revealed that (1) changes in cortical microstructure that accompany cortical maturation had largely already occurred in pAC by 28 weeks PMA, and (2) rapid changes were taking place in nAC between 26 and 42 weeks PMA. At term equivalent PMA, diffusion parameters for auditory cortex were different between preterm infants and term control infants, reflecting either delayed maturation or injury. No effect of room type was observed. For the preterm group, disturbed maturation of nonprimary (but not primary) auditory cortex was associated with poorer language performance at age two years
Pretend Play and Social Engagement in Toddlers at High and Low Genetic Risk for Autism Spectrum Disorder
Toddlers with an older sibling with autism spectrum disorder (ASD) and low risk (LR) toddlers with typically-developing older siblings were observed during free play with a parent and elicited pretend with an examiner at 22-months. Functional and pretend play, children’s social engagement, and parent sensitivity were assessed during free play. Complexity of play was assessed during the elicited pretend task. Toddlers with an ASD diagnosis showed less pretend play across contexts and less social engagement with parents or the examiner than either LR toddlers or high risk toddlers without a diagnosis (HR-noASD). Lower levels of pretend play and social engagement were associated with symptom severity within the high risk group, reflecting emerging ASD in toddlerhood
A zeta function approach to the relation between the numbers of symmetry planes and axes of a polytope
A derivation of the Ces\`aro-Fedorov relation from the Selberg trace formula
on an orbifolded 2-sphere is elaborated and extended to higher dimensions using
the known heat-kernel coefficients for manifolds with piecewise-linear
boundaries. Several results are obtained that relate the coefficients, ,
in the Shephard-Todd polynomial to the geometry of the fundamental domain. For
the 3-sphere we show that is given by the ratio of the volume of the
fundamental tetrahedron to its Schl\"afli reciprocal.Comment: Plain TeX, 26 pages (eqn. (86) corrected
Previous Migration Experience and Legal Immigration Status among Intending Mexican Migrants to the United States
We examine the size and composition of flows of Mexican migrants intending to cross the border and enter the United States. We focus on two characteristics which relate to eligibility for Social Security retirement benefits: legal immigration status and prior US migration history. We compare estimates from before and after the onset of the Great Recession in 2007. We analyze data from the Survey of Migration at the Northern Border of Mexico (EMIF-Norte), an innovative survey of migration flows fielded in border cities and towns in Mexico. We find that the declining overall level of migration from Mexico to the US since 2007 is driven entirely by declines in the level of undocumented immigration. The size of the legal immigrant flow from Mexico has actually increased since the recession began. We find an overall shift away from migrants with no previous trips to the US, toward migrants with 11 or more previous trips. There was no significant shift in the distribution of previous US trips among undocumented migrants, but among work-authorized migrants, the representation of both first time migrants and those with more than 11 previous US trips grew. The increasing share and absolute number of work-authorized Mexican immigrants with 11 or more past trips to the US seems likely to affect the share of Mexican immigrants qualifying for Social Security retirement benefits in the future, and it may be valuable to continue to examine these trends.Social Security Administrationhttp://deepblue.lib.umich.edu/bitstream/2027.42/106388/1/wp304.pd
Developmental dyslexia: predicting individual risk
Background: Causal theories of dyslexia suggest that it is a heritable disorder, which is the outcome of multiple risk factors. However, whether early screening for dyslexia is viable is not yet known. Methods: The study followed children at high risk of dyslexia from preschool through the early primary years assessing them from age 3 years and 6 months (T1) at approximately annual intervals on tasks tapping cognitive, language, and executive-motor skills. The children were recruited to three groups: children at family risk of dyslexia, children with concerns regarding speech, and language development at 3;06 years and controls considered to be typically developing. At 8 years, children were classified as 'dyslexic' or not. Logistic regression models were used to predict the individual risk of dyslexia and to investigate how risk factors accumulate to predict poor literacy outcomes. Results: Family-risk status was a stronger predictor of dyslexia at 8 years than low language in preschool. Additional predictors in the preschool years include letter knowledge, phonological awareness, rapid automatized naming, and executive skills. At the time of school entry, language skills become significant predictors, and motor skills add a small but significant increase to the prediction probability. We present classification accuracy using different probability cutoffs for logistic regression models and ROC curves to highlight the accumulation of risk factors at the individual level. Conclusions: Dyslexia is the outcome of multiple risk factors and children with language difficulties at school entry are at high risk. Family history of dyslexia is a predictor of literacy outcome from the preschool years. However, screening does not reach an acceptable clinical level until close to school entry when letter knowledge, phonological awareness, and RAN, rather than family risk, together provide good sensitivity and specificity as a screening battery
Bose-Einstein condensation in arbitrarily shaped cavities
We discuss the phenomenon of Bose-Einstein condensation of an ideal
non-relativistic Bose gas in an arbitrarily shaped cavity. The influence of the
finite extension of the cavity on all thermodynamical quantities, especially on
the critical temperature of the system, is considered. We use two main methods
which are shown to be equivalent. The first deals with the partition function
as a sum over energy levels and uses a Mellin-Barnes integral representation to
extract an asymptotic formula. The second method converts the sum over the
energy levels to an integral with a suitable density of states factor obtained
from spectral analysis. The application to some simple cavities is discussed.Comment: 10 pages, LaTeX, to appear in Physical Review
Integrating Teaching and Research in Undergraduate Biology Laboratory Education
A course recently designed and implemented at Stanford University applies practical suggestions for creating research-based undergraduate courses that benefit both teaching and research
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