Showing papers by "International School for Advanced Studies published in 2005"

On the nature of surface roughness with application to contact mechanics, sealing, rubber friction and adhesion.

Abstract: Surface roughness has a huge impact on many important phenomena. The most important property of rough surfaces is the surface roughness power spectrum C(q). We present surface roughness power spectra of many surfaces of practical importance, obtained from the surface height profile measured using optical methods and the atomic force microscope. We show how the power spectrum determines the contact area between two solids. We also present applications to sealing, rubber friction and adhesion for rough surfaces, where the power spectrum enters as an important input.

The Genome of the Basidiomycetous Yeast and Human Pathogen Cryptococcus Neoformans

Abstract: Cryptococcus neoformans is a basidionnycetous yeast ubiquitous in the environment, a model for fungal pathogenesis, and an opportunistic human pathogen of global importance. We have sequenced its similar to20-megabase genome, which contains similar to6500 intron-rich gene structures and encodes a transcriptome abundant in alternatively spliced and antisense messages. The genome is rich in transposons, many of which cluster at candidate centromeric regions. The presence of these transposons may drive karyotype instability and phenotypic variation. C. neoformans encodes unique genes that may contribute to its unusual virulence properties, and comparison of two phenotypically distinct strains reveals variation in gene content in addition to sequence polymorphisms between the genomes.

Carbon nanotube substrates boost neuronal electrical signaling.

Abstract: We demonstrate the possibility of using carbon nanotubes (CNTs) as potential devices able to improve neural signal transfer while supporting dendrite elongation and cell adhesion. The results strongly suggest that the growth of neuronal circuits on a CNT grid is accompanied by a significant increase in network activity. The increase in the efficacy of neural signal transmission may be related to the specific properties of CNT materials, such as the high electrical conductivity.

Morphing Marilyn into Maggie dissociates physical and identity face representations in the brain

Abstract: How the brain represents different aspects of faces remains controversial. Here we presented subjects with stimuli drawn from morph continua between pairs of famous faces. In the paired presentations, a second face could be identical to the first, could share perceived identity but differ physically (30% along the morph continuum), or could differ physically by the same distance along the continuum (30%) but in the other direction. We show that, behaviorally, subjects are more likely to classify face pairs in the third paired presentation as different and that this effect is more pronounced for subjects who are more familiar with the faces. In functional magnetic resonance imaging (fMRI), inferior occipital gyrus (IOG) shows sensitivity to physical rather than to identity changes, whereas right fusiform gyrus (FFG) shows sensitivity to identity rather than to physical changes. Bilateral anterior temporal regions show sensitivity to identity change that varies with the subjects' pre-experimental familiarity with the faces. These findings provide neurobiological support for a hierarchical model of face perception.

Thermal and fragmentation properties of star-forming clouds in low-metallicity environments

Abstract: The thermal and chemical evolution of star-forming clouds is studied for different gas metallicities, Z, using the model of Omukai, updated to include deuterium chemistry and the effects of cosmic microwave background (CMB) radiation. HD-line cooling dominates the thermal balance of clouds when Z ~ 10-5 to 10-3 Z☉ and density ≈105 cm-3. Early on, CMB radiation prevents the gas temperature from falling below TCMB, although this hardly alters the cloud thermal evolution in low-metallicity gas. From the derived temperature evolution, we assess cloud/core fragmentation as a function of metallicity from linear perturbation theory, which requires that the core elongation ≡ (b - a)/a > NL ~ 1, where a (b) is the short (long) core axis length. The fragment mass is given by the thermal Jeans mass at = NL. Given these assumptions and the initial (Gaussian) distribution of , we compute the fragment mass distribution as a function of metallicity. We find that (1) for Z = 0, all fragments are very massive, 103 M☉, consistent with previous studies; (2) for Z > 10-6 Z☉ a few clumps go through an additional high-density (1010 cm-3) fragmentation phase driven by dust cooling, leading to low-mass fragments; (3) the mass fraction in low-mass fragments is initially very small, but at Z ~ 10-5 Z☉ it becomes dominant and continues to grow as Z is increased; (4) as a result of the two fragmentation modes, a bimodal mass distribution emerges in 0.01 < Z/Z☉ < 0.1; and (5) for 0.1 Z☉, the two peaks merge into a single-peaked mass function, which might be regarded as the precursor of the ordinary Salpeter-like initial mass function.

Fundamental Algebraic Geometry

Abstract: Grothendieck topologies, fibered categories and descent theory: Introduction Preliminary notions Contravariant functors Fibered categories Stacks Construction of Hilbert and Quot schemes: Construction of Hilbert and Quot schemes Local properties and Hilbert schemes of points: Introduction Elementary deformation theory Hilbert schemes of points Grothendieck's existence theorem in formal geometry with a letter of Jean-Pierre Serre: Grothendieck's existence theorem in formal geometry The Picard scheme: The Picard scheme Bibliography Index

Quasistatic Crack Growth in Nonlinear Elasticity

Abstract: In this paper, we prove a new existence result for a variational model of crack growth in brittle materials proposed in [19]. We consider the case of n-dimensional nonlinear elasticity, for an arbitrary n≧1, with a quasiconvex bulk energy and with prescribed boundary deformations and applied loads, both depending on time.

Three-dimensional relativistic simulations of rotating neutron-star collapse to a Kerr black hole

Abstract: We present a new three-dimensional fully general-relativistic hydrodynamics code using high-resolution shock-capturing techniques and a conformal traceless formulation of the Einstein equations. Besides presenting a thorough set of tests which the code passes with very high accuracy, we discuss its application to the study of the gravitational collapse of uniformly rotating neutron stars to Kerr black holes. The initial stellar models are modeled as relativistic polytropes which are either secularly or dynamically unstable and with angular velocities which range from slow rotation to the mass-shedding limit. We investigate the gravitational collapse by carefully studying not only the dynamics of the matter, but also that of the trapped surfaces, i.e., of both the apparent and event horizons formed during the collapse. The use of these surfaces, together with the dynamical horizon framework, allows for a precise measurement of the black-hole mass and spin. The ability to successfully perform these simulations for sufficiently long times relies on excising a region of the computational domain which includes the singularity and is within the apparent horizon. The dynamics of the collapsing matter is strongly influenced by the initial amount of angular momentum in the progenitor star and, for initial models with sufficiently high angular velocities, the collapse can lead to the formation of an unstable disc in differential rotation. All of the simulations performed with uniformly rotating initial data and a polytropic or ideal-fluid equation of state show no evidence of shocks or of the presence of matter on stable orbits outside the black hole.

The First Cosmic Structures and Their Effects

Abstract: Despite much recent theoretical and observational progress in our knowledge of the early universe, many fundamental questions remain only partially answered. Here, we review the latest achievements and persisting problems in the understanding of first cosmic structure formation.

Predictions for high-frequency radio surveys of extragalactic sources

Abstract: We present detailed predictions of the contributions of the various source populations to the counts at frequen- cies of tens of GHz. New evolutionary models are worked out for flat-spectrum radio quasars, BL Lac objects, and steep- spectrum sources. Source populations characterized by spectra peaking at high radio frequencies, such as extreme GPS sources, ADAF/ADIOS sources and early phases of γ-ray burst afterglows are also dealt with. The counts of different populations of star-forming galaxies (normal spirals, starbursts, high-z galaxies detected by SCUBA and MAMBO surveys, interpreted as proto-spheroidal galaxies) are estimated taking into account both synchrotron and free-free emission, and dust re-radiation. Our analysis is completed by updated counts of Sunyaev-Zeldovich effects in clusters of galaxies and by a preliminary estimate of galactic-scale Sunyaev-Zeldovich signals associated to proto-galactic plasma.

Neuronal encoding of texture in the whisker sensory pathway.

Abstract: A major challenge of sensory systems neuroscience is to quantify brain activity underlying perceptual experiences and to explain this activity as the outcome of elemental neuronal response properties. Rats make extremely fine discriminations of texture by “whisking” their vibrissae across an object's surface, yet the neuronal coding underlying texture sensations remains unknown. Measuring whisker vibrations during active whisking across surfaces, we found that each texture results in a unique “kinetic signature” defined by the temporal profile of whisker velocity. We presented these texture-induced vibrations as stimuli while recording responses of first-order sensory neurons and neurons in the whisker area of cerebral cortex. Each texture is encoded by a distinctive, temporally precise firing pattern. To look for the neuronal coding properties that give rise to texture-specific firing patterns, we delivered horizontal and vertical whisker movements that varied randomly in time (“white noise”) and found that the response probabilities of first-order neurons and cortical neurons vary systematically according to whisker speed and direction. We applied the velocity-tuned spike probabilities derived from white noise to the sequence of velocity features in the texture to construct a simulated texture response. The close match between the simulated and real responses indicates that texture coding originates in the selectivity of neurons to elemental kinetic events.

Computations of primordial black-hole formation

Abstract: Results are presented from general relativistic numerical computations of primordial black-hole formation during the radiation-dominated era of the universe. Growing-mode perturbations are specified within the linear regime and their subsequent evolution is followed as they become nonlinear. We use a spherically symmetric Lagrangian code and study both super-critical perturbations, which go on to produce black holes, and sub-critical perturbations, for which the overdensity eventually disperses into the background medium. For super-critical perturbations, we confirm the results of previous work concerning scaling laws but note that the threshold amplitude for a perturbation to lead to black-hole formation is substantially reduced when the initial conditions are taken to represent purely growing modes. For sub-critical cases, where an initial collapse is followed by a subsequent re-expansion, strong compressions and rarefactions are seen for perturbation amplitudes near to the threshold. We have also investigated the effect of including a significant component of vacuum energy and have calculated the resulting changes in the threshold and in the slope of the scaling law.

Cosmological parameters from supernova observations: A critical comparison of three data sets

Abstract: We extend our previous analysis of cosmological supernova type Ia data (Padmanabhan & Choudhury 2003) to include three recent compilation of data sets. Our analysis ignores the possible correlations and systematic effects present in the data and concentrates mostly on some key theoretical issues. Among the three data sets, the first set consists of 194 points obtained from various observations while the second discards some of the points from the first one because of large uncertainties and thus consists of 142 points. The third data set is obtained from the second by adding the latest 14 points observed through HST. A careful comparison of these different data sets help us to draw the following conclusions: (i) All the three data sets strongly rule out non-accelerating models. Interestingly, the first and the second data sets favour a closed universe; if Ωtot ≡ Ωm +Ω Λ, then the probability of obtaining models with Ωtot > 1i s>0.97. Hence these data sets are in mild disagreement with the "concordance" flat model. However, this disagreement is reduced (the probability of obtaining models with Ωtot > 1 being ≈0.9) for the third data set, which includes the most recent points observed by HST around 1 0.34) redshift supernova, it turns out that these two subsets, individually, admit non-accelerating models with zero dark energy because of different magnitude zero-point values for the different subsets. This can also be seen when the data is analysed while allowing for possibly different magnitude zero-points for the two redshift subsets. However, the non-accelerating models seem to be ruled out using only the low redshift data for the other two data sets, which have less uncertainties. (iii) We have also found that it is quite difficult to measure the evolution of the dark energy equation of state wX(z) though its present value can be constrained quite well. The best-fit value seems to mildly favour a dark energy component with current equation of state wX 0.2.

Merger of binary neutron stars with realistic equations of state in full general relativity

Abstract: We present numerical results of three-dimensional simulations for the merger of binary neutron stars in full general relativity. Hybrid equations of state are adopted to mimic realistic nuclear equations of state. In this approach, we divide the equations of state into two parts as $P={P}_{\mathrm{cold}}+{P}_{\mathrm{th}}$. ${P}_{\mathrm{cold}}$ is the cold part for which we assign a fitting formula for realistic equations of state of cold nuclear matter slightly modifying the formula developed by Haensel and Potekhin. We adopt the SLy and FPS equations of state for which the maximum allowed Arnowitt-Deser-Misner (ADM) mass of cold and spherical neutron stars is $\ensuremath{\approx}2.04{M}_{\ensuremath{\bigodot}}$ and $1.80{M}_{\ensuremath{\bigodot}}$, respectively. ${P}_{\mathrm{th}}$ denotes the thermal part which is written as ${P}_{\mathrm{th}}=({\ensuremath{\Gamma}}_{\mathrm{th}}\ensuremath{-}1)\ensuremath{\rho}(\ensuremath{\epsilon}\ensuremath{-}{\ensuremath{\epsilon}}_{\mathrm{cold}})$, where $\ensuremath{\rho}$, $\ensuremath{\epsilon}$, ${\ensuremath{\epsilon}}_{\mathrm{cold}}$, and ${\ensuremath{\Gamma}}_{\mathrm{th}}$ are the baryon rest-mass density, total specific internal energy, specific internal energy of the cold part, and the adiabatic constant, respectively. Simulations are performed for binary neutron stars of the total ADM mass in the range between $2.4{M}_{\ensuremath{\bigodot}}$ and $2.8{M}_{\ensuremath{\bigodot}}$ with the rest-mass ratio ${Q}_{M}$ to be in the range $0.9\ensuremath{\lesssim}{Q}_{M}\ensuremath{\le}1$. It is found that if the total ADM mass of the system is larger than a threshold ${M}_{\mathrm{thr}}$, a black hole is promptly formed in the merger irrespective of the mass ratios. In the other case, the outcome is a hypermassive neutron star of a large ellipticity, which results from the large adiabatic index of the realistic equations of state adopted. The value of ${M}_{\mathrm{thr}}$ depends on the equation of state: ${M}_{\mathrm{thr}}\ensuremath{\sim}2.7{M}_{\ensuremath{\bigodot}}$ and $\ensuremath{\sim}2.5{M}_{\ensuremath{\bigodot}}$ for the SLy and FPS equations of state, respectively. Gravitational waves are computed in terms of a gauge-invariant wave extraction technique. In the formation of the hypermassive neutron star, quasiperiodic gravitational waves of a large amplitude and of frequency between 3 and 4 kHz are emitted. The estimated emission time scale is $\ensuremath{\lesssim}100\text{ }\text{ }\mathrm{ms}$, after which the hypermassive neutron star collapses to a black hole. Because of the long emission time, the effective amplitude may be large enough to be detected by advanced laser interferometric gravitational wave detectors if the distance to the source is smaller than $\ensuremath{\sim}100\text{ }\text{ }\mathrm{Mpc}$. Thermal properties of the outcome formed after the merger are also analyzed to approximately estimate the neutrino emission energy.

Ground states of nonlinear Schrödinger equations with potentials vanishing at infinity

Abstract: We deal with a class on nonlinear Schr\"odinger equations \eqref{eq:1} with potentials $V(x)\sim |x|^{-\a}$, $0 0$. Working in weighted Sobolev spaces, the existence of ground states $v_{\e}$ belonging to $W^{1,2}(\Rn)$ is proved under the assumption that $p$ satisfies \eqref{eq:p}. Furthermore, it is shown that $v_{\e}$ are {\em spikes} concentrating at a minimum of ${\cal A}=V^{\theta}K^{-2/(p-1)}$, where $\theta= (p+1)/(p-1)-1/2$.

Linguistic Constraints on Statistical Computations The Role of Consonants and Vowels in Continuous Speech Processing

Abstract: Speech is produced mainly in continuous streams containing several words. Listeners can use the transitional probability (TP) between adjacent and non- adjacent syllables to segment ''words'' from a continuous stream of artificial speech, much as they use TPs to or- ganize a variety of perceptual continua. It is thus possible that a general-purpose statistical device exploits any speech unit to achieve segmentation of speech streams. Alterna- tively, language may limit what representations are open to statistical investigation according to their specific lin- guistic role. In this article, we focus on vowels and con- sonants in continuous speech. We hypothesized that vowels and consonants in words carry different kinds of infor- mation, the latter being more tied to word identification and the former to grammar. We thus predicted that in a word identification task involving continuous speech, learners would track TPs among consonants, but not among vowels. Our results show a preferential role for consonants in word identification.

Orbital magnetization in periodic insulators.

Abstract: Working in the Wannier representation, we derive an expression for the orbital magnetization of a periodic insulator. The magnetization is shown to be comprised of two contributions, an obvious one associated with the internal circulation of bulklike Wannier functions in the interior, and an unexpected one arising from net currents carried by Wannier functions near the surface. Each contribution can be expressed as a bulk property in terms of Bloch functions in a gauge-invariant way. Our expression is verified by comparing numerical tight-binding calculations for finite and periodic samples.

Annual fishes of the genus Nothobranchius as a model system for aging research

Abstract: Summary Aging research in vertebrates is hampered by the lack of short- lived models. Annual fishes of the genus Nothobranchius live in East African seasonal ponds. Their life expectancy in the wild is limited by the duration of the wet season and their lifespan in captivity is also short. Nothobranchius are popular aquarium fishes and many different species are kept as captive strains, providing rich material for comparative studies. The present paper aims at reviving the interest in these fishes by reporting that: (1) Nothobranchius can be cultured, and their eggs stored dry at room temperature for months or years, offering inexpensive methods of embryo storage; (2) Nothobranchius show accelerated growth and expression of aging biomarkers at the level of histology and behaviour; (3) the species Nothobranchius furzeri has a maximum lifespan of only 3 months and offers the possibility to perform investigations thus far unthinkable in a vertebrate, such as drug screening with life-long pharmacological treatments and experimental evolution; (4) when the lifespan of different species is compared, a general correlation is found between wet season duration in their natural habitat and longevity in captivity; and (5) vertebrate aging-related genes, such as p66Shc and MTP , can be easily isolated in Nothobranchius by homology cloning. These fishes can become excellent models for aging studies. They can be employed to test the effects of experimental manipulation on aging at a pace com- parable with that of Drosophila and to probe the effects of natural selection on the evolution of aging-related genes.

Metal Work-Function Changes Induced by Organic Adsorbates: A Combined Experimental and Theoretical Study

Abstract: The role of molecular dipole moment, charge transfer, and Pauli repulsion in determining the work-function change (Deltaphi) at organic-metal interfaces has been elucidated by a combined experimental and theoretical study of (CH(3)S)(2)/Au(111) and CH(3)S/Au(111). Comparison between experiment and theory allows us to determine the origin of the interface dipole layer for both phases. For CH(3)S/Au(111), Deltaphi can be ascribed almost entirely to the dipole moment of the CH(3)S layer. For (CH(3)S)(2)/Au(111), a Pauli repulsion mechanism occurs. The implications of these results on the interpretation of Deltaphi in the presence of strongly and weakly adsorbed molecules is discussed.

Wave function optimization in the variational Monte Carlo method

Abstract: An appropriate iterative scheme for the minimization of the energy, based on the variational Monte Carlo (VMC) technique, is introduced and compared with existing stochastic schemes. We test the various methods for the one-dimensional Heisenberg ring and the two-dimensional $t\text{\ensuremath{-}}J$ model and show that, with the present scheme, very accurate and efficient calculations are possible, even for several variational parameters. Indeed, by using a very efficient statistical evaluation of the first and the second energy derivatives, it is possible to define a very rapidly converging iterative scheme that, within VMC, is much more convenient than the standard Newton method. It is also shown how to optimize simultaneously both the Jastrow and the determinantal part of the wave function.

Coarse-grained model of proteins incorporating atomistic detail of the active site.

Abstract: We present a novel approach to explore the conformational space of globular proteins near their native state. It combines the advantages of coarse-grained models with those of all-atoms simulations, required to treat molecular recognition processes. The comparison between calculated structural properties with those obtained with all-atoms molecular dynamics simulations establishes the accuracy of the model. Our method has the potential to be extended to molecular recognition processes in systems whose characteristic size and time scale prevent an analysis based on all-atoms molecular dynamics.

The Dwarf Galaxy DDO 47 as a Dark Matter Laboratory: Testing Cusps Hiding in Triaxial Halos

Abstract: We present an analysis of high-resolution H I data of the dwarf galaxy DDO 47, aimed at testing the hypothesis that dark halo triaxiality might induce noncircular motions resulting in rotation curves best fitted by cored halos, even if the dark matter halo is intrinsically cuspy. This hypothesis could be invoked in order to reconcile the predictions of the standard ΛCDM theory with the rotation curves of disk galaxies. DDO 47 is an ideal case to test this hypothesis because it has a very regular velocity field, its rotation curve is best fitted by a cored halo, and an NFW halo is inconsistent with the data. We analyze the velocity field through the higher order harmonic terms in order to search for kinematical signatures of alleged noncircular motions needed to "hide" a cusp: the result is that globally noncircular motions are at a level of 2-3 km s-1, and they are more likely to be associated with the presence of some spiral structure than with a global elongated potential (e.g., a triaxial halo). These noncircular motions are far from being sufficient to account for the discrepancy with the ΛCDM predictions. We therefore conclude that the dark matter halo around the dwarf galaxy DDO 47 is truly cored and that a cusp cannot be hidden by noncircular motions.

The Detectability of Pair-Production Supernovae at z 6

Abstract: Nonrotating, zero-metallicity stars with initial masses 140 M 260 M☉ are expected to end their lives as pair-production supernovae (PPSNe), in which an electron-positron pair-production instability triggers explosive nuclear burning Interest in such stars has been rekindled by recent theoretical studies that suggest primordial molecular clouds preferentially form stars with these masses Since metal enrichment is a local process, the resulting PPSNe could occur over a broad range of redshifts, in pockets of metal-free gas Using the implicit hydrodynamics code KEPLER, we have calculated a set of PPSN light curves that addresses the theoretical uncertainties and allows us to assess observational strategies for finding these objects at intermediate redshifts The peak luminosities of typical PPSNe are only slightly greater than those of Type Ia, but they remain bright much longer (~1 yr) and have hydrogen lines Ongoing supernova searches may soon be able to limit the contribution of these very massive stars to 1% of the total star formation rate density out to z ≈ 2, which already provides useful constraints for theoretical models The planned Joint Dark Energy Mission satellite will be able to extend these limits out to z ≈ 6

Variational description of Mott insulators.

Abstract: The Gutzwiller wave function for a strongly correlated model can, if supplemented with a long-range Jastrow factor, provide a proper variational description of Mott insulators, so far unavailable. We demonstrate this concept in the prototypical one-dimensional $t\ensuremath{-}{t}^{\ensuremath{'}}$ Hubbard model, where at half-filling we reproduce all known phases, namely, the ordinary Mott undimerized insulator with power-law spin correlations at small ${t}^{\ensuremath{'}}/t$, the spin-gapped metal above a critical ${t}^{\ensuremath{'}}/t$ and small $U$, and the dimerized Mott insulator at large repulsion.

Common and Differential Neural Mechanisms Supporting Imitation of Meaningful and Meaningless Actions

Abstract: Neuropsychological studies indicate that, after brain damage, the ability to imitate meaningful or meaningless actions can be selectively impaired. However, the neural bases supporting the imitation of these two types of action are still poorly understood. Using PET, we investigated in 10 healthy individuals the neural mechanisms of imitating novel, meaningless actions and familiar, meaningful actions. Data were analyzed using SPM99. During imitation, a significant positive correlation (p < .05, corrected) of regional cerebral blood flow with the amount of meaningful actions was observed in the left inferior temporal gyrus only. In contrast, a significant positive correlation (p < .05, corrected) with the amount of meaningless movements was observed in the right parieto-occipital junction. The direct categorical comparison of imitating meaningful (100%) relative to meaningless (100%) actions showed differential increases in neural activity (p < .001, uncorrected) in the left inferior temporal gyrus, the left parahippocampal gyrus, and the left angular gyrus. The reverse categorical comparison of imitating meaningless (100%) relative to meaningful (100%) actions revealed differential increases in neural activity (p < .001, uncorrected) in the superior parietal cortex bilaterally, in the right parieto-occipital junction, in the right occipital–temporal junction (MT, V5), and in the left superior temporal gyrus. Increased neural activity common to imitation of meaningless and meaningful actions compared to action observation was observed in a network of areas known to be involved in imitation of actions including the primary sensorimotor cortex, the supplementary motor area, and the ventral premotor cortex. These results are compatible with the two-route model of action imitation which suggests that there are at least two mechanisms involved in imitation of actions: a direct mechanism transforming a novel action into a motor output, and a semantic mechanism, on the basis of stored memories, that allows reproductions of known actions. Our results indicate that, in addition to shared neural processes, the direct and the semantic mechanisms that underlie action imitation also draw upon differential neural mechanisms. The direct mechanism underlying imitation of meaningless actions differentially involves visuospatial transformation processes as evidenced by activation of areas belonging to the dorsal stream. In contrast, imitation of meaningful actions differentially involves semantic processing as evidenced by activation of areas belonging to the ventral stream.