94 research outputs found
Berry's phase and Quantum Dynamics of Ferromagnetic Solitons
We study spin parity effects and the quantum propagation of solitons (Bloch
walls) in quasi-one dimensional ferromagnets. Within a coherent state path
integral approach we derive a quantum field theory for nonuniform spin
configurations. The effective action for the soliton position is shown to
contain a gauge potential due to the Berry phase and a damping term caused by
the interaction between soliton and spin waves. For temperatures below the
anisotropy gap this dissipation reduces to a pure soliton mass renormalization.
The gauge potential strongly affects the quantum dynamics of the soliton in a
periodic lattice or pinning potential. For half-integer spin, destructive
interference between soliton states of opposite chirality suppresses nearest
neighbor hopping. Thus the Brillouin zone is halved, and for small mixing of
the chiralities the dispersion reveals a surprising dynamical correlation: Two
subsequent band minima belong to different chirality states of the soliton. For
integer spin, the Berry phase is inoperative and a simple tight-binding
dispersion is obtained. Finally it is shown that external fields can be used to
interpolate continuously between the Bloch wall dispersions for half-integer
and integer spin.Comment: 20 pages, RevTex 3.0 (twocolumn), to appear in Phys. Rev. B 53, 3237
(1996), 4 PS figures available upon reques
Real-time non-equilibrium dynamics of quantum glassy systems
We develop a systematic analytic approach to aging effects in quantum
disordered systems in contact with an environment. Within the closed-time
path-integral formalism we include dissipation by coupling the system to a set
of independent harmonic oscillators that mimic a quantum thermal bath. After
integrating over the bath variables and averaging over disorder we obtain an
effective action that determines the real-time dynamics of the system. The
classical limit yields the Martin-Siggia-Rose generating functional associated
to a colored noise. We apply this general formalism to a prototype model
related to the spin-glass. We show that the model has a dynamic phase
transition separating the paramagnetic from the spin-glass phase and that
quantum fluctuations depress the transition temperature until a quantum
critical point is reached. We show that the dynamics in the paramagnetic phase
is stationary but presents an interesting crossover from a region controlled by
the classical critical point to another one controlled by the quantum critical
point. The most characteristic property of the dynamics in a glassy phase,
namely aging, survives the quantum fluctuations. In the sub-critical region the
quantum fluctuation-dissipation theorem is modified in a way that is consistent
with the notion of effective temperatures introduced for the classical case. We
discuss these results in connection with recent experiments in dipolar quantum
spin-glasses and the relevance of the effective temperatures with respect to
the understanding of the low temperature dynamics.Comment: 56 pages, Revtex, 17 figures include
High school drinking mediates the relationship between parental monitoring and college drinking: A longitudinal analysis
<p>Abstract</p> <p>Background</p> <p>College drinking is a significant public health problem. Although parental monitoring and supervision reduces the risk for alcohol consumption among younger adolescents, few studies have investigated the impact of earlier parental monitoring on later college drinking. This study examined whether parental monitoring indirectly exerts a protective effect on college drinking by reducing high school alcohol consumption.</p> <p>Methods</p> <p>A longitudinal cohort of 1,253 male and female students, ages 17 to 19, attending a large, public, mid-Atlantic university was studied at two time points. First, data on high school parental monitoring and alcohol consumption were gathered via questionnaire during the summer prior to college entry. Second, during the first year of college, past-year alcohol consumption was measured via a personal interview. Multiple regression models tested the relationship between parental monitoring and past year alcohol use (i.e., number of drinks per drinking day).</p> <p>Results</p> <p>Holding constant demographics, SAT score, and religiosity, parental monitoring had a significant protective effect on both high school and college drinking level. However, the association between parental monitoring and college drinking level became non-significant once high school drinking level was held constant.</p> <p>Conclusion</p> <p>While parental monitoring did not directly influence college alcohol consumption, evidence for mediation was observed, whereby parental monitoring had an indirect influence on college drinking through reductions in high school drinking. Initiatives that promote effective parenting might be an important strategy to curb high-risk drinking among older adolescents. More research is needed to understand the nature and degree of parent-child communication that is necessary to extend the protective influence of parents into the college years.</p
Differential Responses of Calcifying and Non-Calcifying Epibionts of a Brown Macroalga to Present-Day and Future Upwelling pCO2
Seaweeds are key species of the Baltic Sea benthic ecosystems. They are the substratum of numerous fouling epibionts like bryozoans and tubeworms. Several of these epibionts bear calcified structures and could be impacted by the high pCO2 events of the late summer upwellings in the Baltic nearshores. Those events are expected to increase in strength and duration with global change and ocean acidification. If calcifying epibionts are impacted by transient acidification as driven by upwelling events, their increasing prevalence could cause a shift of the fouling communities toward fleshy species. The aim of the present study was to test the sensitivity of selected seaweed macrofoulers to transient elevation of pCO2 in their natural microenvironment, i.e. the boundary layer covering the thallus surface of brown seaweeds. Fragments of the macroalga Fucus serratus bearing an epibiotic community composed of the calcifiers Spirorbis spirorbis (Annelida) and Electra pilosa (Bryozoa) and the non-calcifier Alcyonidium hirsutum (Bryozoa) were maintained for 30 days under three pCO2 conditions: natural 460±59 ”atm, present-day upwelling1193±166 ”atm and future upwelling 3150±446 ”atm. Only the highest pCO2 caused a significant reduction of growth rates and settlement of S. spirorbis individuals. Additionally, S. spirorbis settled juveniles exhibited enhanced calcification of 40% during daylight hours compared to dark hours, possibly reflecting a day-night alternation of an acidification-modulating effect by algal photosynthesis as opposed to an acidification-enhancing effect of algal respiration. E. pilosa colonies showed significantly increased growth rates at intermediate pCO2 (1193 ”atm) but no response to higher pCO2. No effect of acidification on A. hirsutum colonies growth rates was observed. The results suggest a remarkable resistance of the algal macro-epibionts to levels of acidification occurring at present day upwellings in the Baltic. Only extreme future upwelling conditions impacted the tubeworm S. spirorbis, but not the bryozoans
Relativistic Brownian Motion
Stimulated by experimental progress in high energy physics and astrophysics,
the unification of relativistic and stochastic concepts has re-attracted
considerable interest during the past decade. Focusing on the framework of
special relativity, we review, here, recent progress in the phenomenological
description of relativistic diffusion processes. After a brief historical
overview, we will summarize basic concepts from the Langevin theory of
nonrelativistic Brownian motions and discuss relevant aspects of relativistic
equilibrium thermostatistics. The introductory parts are followed by a detailed
discussion of relativistic Langevin equations in phase space. We address the
choice of time parameters, discretization rules, relativistic
fluctuation-dissipation theorems, and Lorentz transformations of stochastic
differential equations. The general theory is illustrated through analytical
and numerical results for the diffusion of free relativistic Brownian
particles. Subsequently, we discuss how Langevin-type equations can be obtained
as approximations to microscopic models. The final part of the article is
dedicated to relativistic diffusion processes in Minkowski spacetime. Due to
the finiteness of velocities in relativity, nontrivial relativistic Markov
processes in spacetime do not exist; i.e., relativistic generalizations of the
nonrelativistic diffusion equation and its Gaussian solutions must necessarily
be non-Markovian. We compare different proposals that were made in the
literature and discuss their respective benefits and drawbacks. The review
concludes with a summary of open questions, which may serve as a starting point
for future investigations and extensions of the theory.Comment: review article, 159 pages, references updated, misprints corrected,
App. A.4. correcte
Interactive effects of global climate change and pollution on marine microbes: the way ahead
Global climate change has the potential to seriously and adversely affect marine ecosystem functioning. Numerous experimental and modeling studies have demonstrated how predicted ocean acidification and increased ultraviolet radiation (UVR) can affect marine microbes. However, researchers have largely ignored interactions between ocean acidification, increased UVR and anthropogenic pollutants in marine environments. Such interactions can alter chemical speciation and the bioavailability of several organic and inorganic pollutants with potentially deleterious effects, such as modifying microbial-mediated detoxification processes. Microbes mediate major biogeochemical cycles, providing fundamental ecosystems services such as environmental detoxification and recovery. It is, therefore, important that we understand how predicted changes to oceanic pH, UVR, and temperature will affect microbial pollutant detoxification processes in marine ecosystems. The intrinsic characteristics of microbes, such as their short generation time, small size, and functional role in biogeochemical cycles combined with recent advances in molecular techniques (e.g., metagenomics and metatranscriptomics) make microbes excellent models to evaluate the consequences of various climate change scenarios on detoxification processes in marine ecosystems. In this review, we highlight the importance of microbial microcosm experiments, coupled with high-resolution molecular biology techniques, to provide a critical experimental framework to start understanding how climate change, anthropogenic pollution, and microbiological interactions may affect marine ecosystems in the future.The authors acknowledge support from Centre for Environmental and Marine Studies (CESAM) and Foundation for Science and Technology (FCT, Portugal) PTDC/AAC -CLI/107916/2008 (http://alfa.fct.mctes.pt) and the European Regional Development Fund (ERDF) through COMPETE-(FCOMP-01-0124-FEDER-008657). Francisco J. R. C. Coelho and Ana L. Santos were supported by Ph.D. scholarships (SFRH/BD/46322/2008 and SFR/BD/ 40160/2007, respectively) funded by Fundação para a CiĂȘncia e Tecnologia, Portugal (QREN-POPH â Type 4.1 â Advanced Training, subsidized by the European Social Fund and national funds MCTES).publishe
Estimating the impact of the cryptic degassing of Large Igneous Provinces: A mid-Miocene case-study
Development of zinc-based architectured composite electrodes for rechargeable alkaline batteries
Ces travaux de thĂšse rĂ©sultent dâune Ă©tude multidisciplinaire dont lâobjectif final Ă©tait dâĂ©laborer une Ă©lectrode nĂ©gative Ă base de zinc pour accumulateur alcalin rechargeable. Lâorigine de lâĂ©tude tient en la dĂ©couverte surprenante, par la sociĂ©tĂ© EASYL, dâun nouveau procĂ©dĂ© de synthĂšse du zincate de calcium (CAZN), matiĂšre Ă©lectrochimiquement active et connue pour ses bonnes caractĂ©ristiques de cyclabilitĂ© en gĂ©nĂ©rateur alcalin rechargeable. LâintĂ©rĂȘt de cette dĂ©couverte rĂ©side dans ses caractĂ©ristiques avantageuses : la synthĂšse ultra-rapide se fait en continu, nâutilise aucun systĂšme de chauffe ni de solution alcaline et conduit Ă une granulomĂ©trie et une puretĂ© contrĂŽlĂ©e du zincate de calcium ; la rendant compatible avec une production industrielle de ce matĂ©riau.Lâutilisation de CAZN en batterie prismatique de 4 Ah a permis la dĂ©couverte dâun fonctionnement des Ă©lectrodes de type cĆur-coquille, leur cĆur actif Ă©tant riche en zinc et leur surface jouant le rĂŽle de couche protectrice ; si la capacitĂ© nominale est choisie infĂ©rieure Ă la capacitĂ© thĂ©orique de la batterie, lâactivitĂ© Ă cĆur de lâĂ©lectrode est maintenue sans que sa surface ne soit altĂ©rĂ©e, ce qui permet dâĂ©viter (ou de ralentir) la formation de dendrite, un effet trĂšs bĂ©nĂ©fique sur la cyclabilitĂ© de lâĂ©lectrode.Cependant, lâutilisation du zincate de calcium comme seule source de matiĂšre active ne semble pas appropriĂ©e. En effet, la formation du cĆur de zinc conduit Ă lâapparition dâune couche rĂ©sistive dâhydroxyde de calcium Ă sa pĂ©riphĂ©rie, diminuant les performances Ă©lectrochimiques des Ă©lectrodes. Aussi surprenant que cela puisse paraitre, il est cependant possible de rĂ©gĂ©nĂ©rer une Ă©lectrode vieillie ayant formĂ© une couche riche en hydroxyde de calcium par un simple repos, soit un arrĂȘt pur et simple de la batterie. La formation de cette couche rĂ©sistive peut en outre ĂȘtre Ă©vitĂ©e par lâajout dâoxyde de zinc sacrificiel au zincate de calcium, additif actif qui sâest avĂ©rĂ©e efficace tant dâun point de vue morphologique quâĂ©lectrochimique.En revanche, la formation contrĂŽlĂ©e dâun cĆur riche en zinc conduit Ă la densification du zinc sur lui-mĂȘme, et diminue la surface de contact matiĂšre active/Ă©lectrolyte et donc les performances Ă©lectrochimiques. Partant de ce constat, la structure de lâĂ©lectrode a Ă©tĂ© intĂ©gralement repensĂ©e pour permettre la formation, non pas dâun cĆur de zinc, mais de plusieurs dâentre eux, par lâemploi de collecteurs de courant multicouches ; cette mĂ©thodologie, aussi simple quâefficace, conduit Ă dâexcellentes performances pratiques et une cyclabilitĂ© optimale de la batterie.The work presented in this document results from a multidisciplinary study, the unique goal of which is to develop a negative electrode for alkaline rechargeable batteries. At the origin of this thesis, is the surprising discovery by EASYL of a new way to synthesize calcium zincate (CAZN), an electrochemically active material known for its good cycling characteristics in alkaline batteries. The advantage of such a discovery resides in its unique characteristics: the ultra-fast synthesis is carried out continuously, uses neither heating system nor alkaline solutions, yields pure and tailored CAZN crystals; it is therefore compatible with an industrial production of this material.Its use in a 4 Ah prismatic batteries allowed to unveil a core-shell operation mechanism, in which the electrode evolves towards an active zinc-core surrounded by a protective shell. So, if the nominal capacity remains below the theoretical one, the core of the electrode can be kept active while the surface is maintained, thus avoiding (or at least slowing down) possible dendrite formation and yielding prolonged cycle life.However, the use of calcium zincate as the only active material source is not appropriate, because the formation of the zinc-core leads to the appearance of a resistive layer of calcium hydroxide at its periphery, which reduces the overall electrochemical performance. As surprising as it may seem, it is possible to regenerate an electrode having formed such a calcium hydroxide-rich layer by a simple rest such as a stop of the battery. Nevertheless, it is preferable to avoid the formation of this resistive layer and to do so, the use of a mixture of sacrificial zinc oxide combined with calcium zincate has proven very effective, both from a morphological and an electrochemical point-of-view.However, the controlled formation of a zinc-rich core leads to zinc densification on itself; this decreases the surface of contact between the active material and the electrolyte, and thus the electrochemical performance. This negative effect has been overcome by drastically rethinking the structure of the electrode, in order to allow the formation of multiple and tailored zinc cores. To that goal, multilayers of current collector were employed, which proved simple and effective to reach high-performance and high cyclability zinc electrodes for alkaline batteries
Electrode à réseau multiple de collecte de courant
Lâinvention concerne une Ă©lectrode pour un dispositif rechargeable de stockage de lâĂ©nergie, comprenant plusieurs couches de matĂ©riau dâĂ©lectrode et plusieurs couches poreuses de collecteur de courant, lesdites couches de matĂ©riau dâĂ©lectrode et de collecteur de courant Ă©tant disposĂ©es de façon spĂ©cifique, un dispositif rechargeable de stockage de lâĂ©nergie comprenant ladite Ă©lectrode, ainsi que les utilisations de ladite Ă©lectrode
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