Lifetime of dynamic heterogeneity in strong and fragile kinetically constrained spin models

Kinetically constrained spin models are schematic coarse-grained models for the glass transition which represent an efficient theoretical tool to study detailed spatio-temporal aspects of dynamic heterogeneity in supercooled liquids. Here, we study how spatially correlated dynamic domains evolve with time and compare our results to various experimental and numerical investigations. We find that strong and fragile models yield different results. In particular, the lifetime of dynamic heterogeneity remains constant and roughly equal to the alpha relaxation time in strong models, while it increases more rapidly in fragile models when the glass transition is approached.Comment: Submitted to the proceedings of the 6th EPS Liquid Matter Conference, Utrecht 2-6 July 200

Phase ordering and roughening on growing films

We study the interplay between surface roughening and phase separation during the growth of binary films. Already in 1+1 dimension, we find a variety of different scaling behaviors depending on how the two phenomena are coupled. In the most interesting case, related to the advection of a passive scalar in a velocity field, nontrivial scaling exponents are obtained in simulations.Comment: 4 pages latex, 6 figure

Negative differential resistance in nanotube devices

Carbon nanotube junctions are predicted to exhibit negative differential resistance, with very high peak-to-valley current ratios even at room temperature. We treat both nanotube p-n junctions and undoped metal-nanotube-metal junctions, calculating quantum transport through the self-consistent potential within a tight-binding approximation. The undoped junctions in particular may be suitable for device integration.Comment: 4 pages, 4 figures, to appear in Physical Review Letter

Theme Analysis of Flora Nwapa’s Never Again: A Systemic Functional Approach

This paper deals with theme analysis of Flora Nwapa’s Never Again. The theory that underpins this study is systemic functional linguistics, which is one of the approaches proposed by scholars such as M.A.K Halliday, S. Eggins, J. R. Martin, R. Fowler, J. D. Benson to mention but a very few, for the study of language and its function(s). In this system, the study of language involves three functional labels: experiential, interpersonal and textual meanings. This study aims at describing and analysing linguistic features which connote textual meaning in Never Again so as to have a deep understanding of the novel and provide a new interpretation of it. It also aims at uncovering the deep messages conveyed through the novel. Both quantitative and qualitative methods have been used in this study. Thus, two extracts have been selected from Never Again and systemic functional linguistic theories have been applied to each of them for the purpose of exploring how language is used to organise itself. The study of the internal organisation and communicative nature of the selected Texts has enabled the researchers to come up with valuable findings. Indeed, through Mood adjuncts that conflate interpersonal themes, participants in Text 1 have expressed particular attitudes and judgments about the limited power of women and the roles of women and children during war whereas those in Text 2 have expressed a high degree of certainty and inclination. This study enables the researchers to provide a deeper understanding of Never Again by contending that the deep messages conveyed through the novel are the Nigerian civil war and the roles of women and children in our societies especially during war time. Keywords: Metafunctions, register variables, systemic functional linguistics, theme patterns

Epitaxial growth in dislocation-free strained alloy films: Morphological and compositional instabilities

The mechanisms of stability or instability in the strained alloy film growth are of intense current interest to both theorists and experimentalists. We consider dislocation-free, coherent, growing alloy films which could exhibit a morphological instability without nucleation. We investigate such strained films by developing a nonequilibrium, continuum model and by performing a linear stability analysis. The couplings of film-substrate misfit strain, compositional stress, deposition rate, and growth temperature determine the stability of film morphology as well as the surface spinodal decomposition. We consider some realistic factors of epitaxial growth, in particular the composition dependence of elastic moduli and the coupling between top surface and underlying bulk of the film. The interplay of these factors leads to new stability results. In addition to the stability diagrams both above and below the coherent spinodal temperature, we also calculate the kinetic critical thickness for the onset of instability as well as its scaling behavior with respect to misfit strain and deposition rate. We apply our results to some real growth systems and discuss the implications related to some recent experimental observations.Comment: 26 pages, 13 eps figure

Measuring objective fatigability and autonomic dysfunction in clinical populations: How and why?

Fatigue is a major symptom in many diseases, often among the most common and severe ones and may last for an extremely long period. Chronic fatigue impacts quality of life, reduces the capacity to perform activities of daily living, and has socioeconomical consequences such as impairing return to work. Despite the high prevalence and deleterious consequences of fatigue, little is known about its etiology. Numerous causes have been proposed to explain chronic fatigue. They encompass psychosocial and behavioral aspects (e.g., sleep disorders) and biological (e.g., inflammation), hematological (e.g., anemia) as well as physiological origins. Among the potential causes of chronic fatigue is the role of altered acute fatigue resistance, i.e. an increased fatigability for a given exercise, that is related to physical deconditioning. For instance, we and others have recently evidenced that relationships between chronic fatigue and increased objective fatigability, defined as an abnormal deterioration of functional capacity (maximal force or power), provided objective fatigability is appropriately measured. Indeed, in most studies in the field of chronic diseases, objective fatigability is measured during single-joint, isometric exercises. While those studies are valuable from a fundamental science point of view, they do not allow to test the patients in ecological situations when the purpose is to search for a link with chronic fatigue. As a complementary measure to the evaluation of neuromuscular function (i.e., fatigability), studying the dysfunction of the autonomic nervous system (ANS) is also of great interest in the context of fatigue. The challenge of evaluating objective fatigability and ANS dysfunction appropriately (i.e.,. how?) will be discussed in the first part of the present article. New tools recently developed to measure objective fatigability and muscle function will be presented. In the second part of the paper, we will discuss the interest of measuring objective fatigability and ANS (i.e. why?). Despite the beneficial effects of physical activity in attenuating chronic fatigue have been demonstrated, a better evaluation of fatigue etiology will allow to personalize the training intervention. We believe this is key in order to account for the complex, multifactorial nature of chronic fatigue

Color Detection Using Chromophore-Nanotube Hybrid Devices

We present a nanoscale color detector based on a single-walled carbon nanotube functionalized with azobenzene chromophores, where the chromophores serve as photoabsorbers and the nanotube as the electronic read-out. By synthesizing chromophores with specific absorption windows in the visible spectrum and anchoring them to the nanotube surface, we demonstrate the controlled detection of visible light of low intensity in narrow ranges of wavelengths. Our measurements suggest that upon photoabsorption, the chromophores isomerize from the ground state trans configuration to the excited state cis configuration, accompanied by a large change in dipole moment, changing the electrostatic environment of the nanotube. All-electron ab initio calculations are used to study the chromophore-nanotube hybrids, and show that the chromophores bind strongly to the nanotubes without disturbing the electronic structure of either species. Calculated values of the dipole moments support the notion of dipole changes as the optical detection mechanism.Comment: Accepted by Nano Letter

Modeling a Schottky-barrier carbon nanotube field-effect transistor with ferromagnetic contacts

In this study, a model of a Schottky-barrier carbon nanotube field- effect transistor (CNT-FET), with ferromagnetic contacts, has been developed. The emphasis is put on analysis of current-voltage characteristics as well as shot (and thermal) noise. The method is based on the tight-binding model and the non- equilibrium Green's function technique. The calculations show that, at room temperature, the shot noise of the CNT FET is Poissonian in the sub-threshold region, whereas in elevated gate and drain/source voltage regions the Fano factor gets strongly reduced. Moreover, transport properties strongly depend on relative magnetization orientations in the source and drain contacts. In particular, one observes quite a large tunnel magnetoresistance, whose absolute value may exceed 50%.Comment: 8 pages, 4 figure

Deciphering the pH-dependence of ground- and excited-state equilibria of thienoguanine

The thienoguanine nucleobase (thGb) is an isomorphic fluorescent analogue of guanine. In aqueous buffer at neutral pH, thGb exists as a mixture of two ground-state H1 and H3 keto-amino tautomers with distinct absorption and emission spectra and high quantum yield. In this work, we performed the first systematic photophysical characterization of thGb as a function of pH (2 to 12). Steady-state and time-resolved fluorescence spectroscopies, supplemented with theoretical calculations, enabled us to identify three additional thGb forms, resulting from pH-dependent ground-state and excited-state reactions. Moreover, a thorough analysis allowed us to retrieve their individual absorption and emission spectra as well as the equilibrium constants which govern their interconversion. From these data, the complete photoluminescence pathway of thGb in aqueous solution and its dependence as a function of pH was deduced. As the identified forms differ by their spectra and fluorescence lifetime, thGb could be used as a probe for sensing local pH changes under acidic conditions

Stress-driven instability in growing multilayer films

We investigate the stress-driven morphological instability of epitaxially growing multilayer films, which are coherent and dislocation-free. We construct a direct elastic analysis, from which we determine the elastic state of the system recursively in terms of that of the old states of the buried layers. In turn, we use the result for the elastic state to derive the morphological evolution equation of surface profile to first order of perturbations, with the solution explicitly expressed by the growth conditions and material parameters of all the deposited layers. We apply these results to two kinds of multilayer structures. One is the alternating tensile/compressive multilayer structure, for which we determine the effective stability properties, including the effect of varying surface mobility in different layers, its interplay with the global misfit of the multilayer film, and the influence of asymmetric structure of compressive and tensile layers on the system stability. The nature of the asymmetry properties found in stability diagrams is in agreement with experimental observations. The other multilayer structure that we study is one composed of stacked strained/spacer layers. We also calculate the kinetic critical thickness for the onset of morphological instability and obtain its reduction and saturation as number of deposited layers increases, which is consistent with recent experimental results. Compared to the single-layer film growth, the behavior of kinetic critical thickness shows deviations for upper strained layers.Comment: 27 pages, 11 figures; Phys. Rev. B, in pres