1,134 research outputs found

    Quantum Ratchets at High Temperatures

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    Using the continued-fraction method we solve the Caldeira-Leggett master equation in the phase-space (Wigner) representation to study Quantum ratchets. Broken spatial symmetry, irreversibility and periodic forcing allows for a net current in these systems. We calculate this current as a function of the force under adiabatic conditions. Starting from the classical limit we make the system quantal. In the quantum regime tunnel events and over-barrier wave reflection phenomena modify the classical result. Finally, using the phase-space formalism we give some insights about the decoherence in these systems.Comment: submitted to Physia E (proceedings of conference "Frontiers of Quantum and Mesoscopic Thermodynamics", Prague 26-29 July 2004

    Spatially selective implementation of the adiabatic T2 prep sequence for magnetic resonance angiography of the coronary arteries.

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    In coronary magnetic resonance angiography, a magnetization-preparation scheme for T2 -weighting (T2 Prep) is widely used to enhance contrast between the coronary blood-pool and the myocardium. This prepulse is commonly applied without spatial selection to minimize flow sensitivity, but the nonselective implementation results in a reduced magnetization of the in-flowing blood and a related penalty in signal-to-noise ratio. It is hypothesized that a spatially selective T2 Prep would leave the magnetization of blood outside the T2 Prep volume unaffected and thereby lower the signal-to-noise ratio penalty. To test this hypothesis, a spatially selective T2 Prep was implemented where the user could freely adjust angulation and position of the T2 Prep slab to avoid covering the ventricular blood-pool and saturating the in-flowing spins. A time gap of 150 ms was further added between the T2 Prep and other prepulses to allow for in-flow of a larger volume of unsaturated spins. Consistent with numerical simulation, the spatially selective T2 Prep increased in vivo human coronary artery signal-to-noise ratio (42.3 ± 2.9 vs. 31.4 ± 2.2, n = 22, P < 0.0001) and contrast-to-noise-ratio (18.6 ± 1.5 vs. 13.9 ± 1.2, P = 0.009) as compared to those of the nonselective T2 Prep. Additionally, a segmental analysis demonstrated that the spatially selective T2 Prep was most beneficial in proximal and mid segments where the in-flowing blood volume was largest compared to the distal segments. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc

    Coexistence of single-mode and multi-longitudinal mode emission in the ring laser model

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    A homogeneously broadened unidirectonal ring laser can emit in several longitudinal modes for large enough pump and cavity length because of Rabi splitting induced gain. This is the so called Risken-Nummedal-Graham-Haken (RNGH) instability. We investigate numerically the properties of the multi-mode solution. We show that this solution can coexist with the single-mode one, and its stability domain can extend to pump values smaller than the critical pump of the RNGH instability. Morevoer, we show that the multi-mode solution for large pump values is affected by two different instabilities: a pitchfork bifurcation, which preserves phase-locking, and a Hopf bifurcation, which destroys it.Comment: 14 pages, 7 figure

    Offsprings of a point vortex

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    The distribution engendered by successive splitting of one point vortex are considered. The process of splitting a vortex in three using a reverse three-point vortex collapse course is analysed in great details and shown to be dissipative. A simple process of successive splitting is then defined and the resulting vorticity distribution and vortex populations are analysed

    What do young athletes implicitly understand about psychological skills?

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    One reason sport psychologists teach psychological skills is to enhance performance in sport; but the value of psychological skills for young athletes is questionable because of the qualitative and quantitative differences between children and adults in their understanding of abstract concepts such as mental skills. To teach these skills effectively to young athletes, sport psychologists need to appreciate what young athletes implicitly understand about such skills because maturational (e.g., cognitive, social) and environmental (e.g., coaches) factors can influence the progressive development of children and youth. In the present qualitative study, we explored young athletes’ (aged 10–15 years) understanding of four basic psychological skills: goal setting, mental imagery, self-talk, and relaxation. Young athletes (n = 118: 75 males and 43 females) completed an open-ended questionnaire to report their understanding of these four basic psychological skills. Compared with the older youth athletes, the younger youth athletes were less able to explain the meaning of each psychological skill. Goal setting and mental imagery were better understood than self-talk and relaxation. Based on these findings, sport psychologists should consider adapting interventions and psychoeducational programs to match young athletes’ age and developmental level

    Spreading and localization of wavepackets in disordered wires in a magnetic field

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    We study the diffusive and localization properties of wavepackets in disordered wires in a magnetic field. In contrast to a recent supersymmetry approach our numerical results show that the decay rate of the steady state changes {\em smoothly} at the crossover from preserved to broken time-reversal symmetry. Scaling and fluctuation properties are also analyzed and a formula, which was derived analytically only in the pure symmetry cases is shown to describe also the steady state wavefunction at the crossover regime. Finally, we present a scaling for the variance of the packet which shows again a smooth transition due to the magnetic field.Comment: 4 pages, 4 figure

    Dynamics of Tunneling Centers in Metallic Systems

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    Dynamics of tunneling centers (TC) in metallic systems is studied, using the technique of bosonization. The interaction of the TC with the conduction electrons of the metal involves two processes, namely, the screening of the TC by electrons, and the so-called electron assisted tunneling. The presence of the latter process leads to a different form of the renormalized tunneling frequency of the TC, and the tunneling motion is damped with a temperature dependent relaxation rate. As the temperature is lowered, the relaxation rate per temperature shows a steep rise as opposed to that in the absence of electron assisted process. It is expected that this behavior should be observed at very low temperatures in a careful experiment. The present work thus tries to go beyond the existing work on the {\it dynamics} of a two-level system in metals, by treating the electron assisted process.Comment: REVTeX twocolumn format, 5 pages, two PostScript figures available on request. Preprint # : imsc 94/3

    Low noise amplication of an optically carried microwave signal: application to atom interferometry

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    In this paper, we report a new scheme to amplify a microwave signal carried on a laser light at λ\lambda=852nm. The amplification is done via a semiconductor tapered amplifier and this scheme is used to drive stimulated Raman transitions in an atom interferometer. Sideband generation in the amplifier, due to self-phase and amplitude modulation, is investigated and characterized. We also demonstrate that the amplifier does not induce any significant phase-noise on the beating signal. Finally, the degradation of the performances of the interferometer due to the amplification process is shown to be negligible

    SUPEROVULAÇÃO OVARIANA E PRODUÇÃO DE EMBRIÕES EM Bubalus bubalis

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    The aim of the present research work was to optimize ovarian superovulation rate and the embryo production in Murrah and Mediterranean buffaloes around the 60th day after parturition. Fourteen animals were divided in two groups of 7 animals each, G1 (treated animals) and G2 (untreated animals). All animals were subjected to a step of oestrus synchronization by receiving at the 0 day (OD) a vaginal pressary with progestagen (CIDR-B, Intervet), followed by the administration, in the morning of the day 1 (D1), of 3 mg stradiol benzoate IM (Estrogin, Farmavet, SP). The ovarian superovulation (SO) was carried out daily, at intervals of 12 hours in the morning and the afternoon by means of the administration of follicule stimulant hormone (FSH) (Pluset, Serono, Italy) in both groups of experimental animals, during four days according to the following protocol: at the 6th day, 75 IU; at the 7th day, 40 IU; at the 8th day, 30 IU; and at the 9th day, 20 IU. At the afternoon of the 8th day, administration of 500 ”g of cloprostenol IM (Closin-Schering-Plough) was performed followed by the withdraw of the vaginal pessaries from all animals. Then, observations on the estrus of the experimental animals has been carried out. For this purpose the animals were inseminated twice at intervals of 12 hours. Additionaly, all animals from the G1, together with the first artificial insemination (AI) received a dose of 3000 UI of gonadotrophic chorionic hormone (GCH) (Vetecor, Calier, IV) while the G2 animals received 1 ml of saline as placebo. All animals from both G1 and G2 groups had their ovaries monitored by ultrasonography from the first day of the AI up to the day when they had their embryos collected. Ultrasography has been carried out by means of an Aloka SSD-550 (Japan) instrument aiming to evaluate the follicles, the ovulation, and the corpora lutea. At the 5.5th after the estrus observation a washing of the uterine cornus has been carried out for the embryos harvesting and evaluation. According to the present experiment, it was possible to conclude that the protocol used to promote superovulation (SO) in buffaloes by means of FSH showed to be efficient resulting in the production of significant number of antral follicles larger than 8.0 mm diameter at the estrus day and that the gonadotropohic corionic hormone showed significant efficiency by displaying larger percentual ovulations in the treated group (pO presente estudo objetivou otimizar a superovulação ovariana, a taxa de ovulação e a produção de embriĂ”es em bĂșfalas. Os animais eram da raça Murrah e MediterrĂąneo e estavam em torno do 60Âș dia pĂłs parto. Quatorze animais foram divididos em dois grupos, sendo um grupo tratado (G1=7 animais) e outro nĂŁo-tratado (G2=7 animais). Todos os animais foram submetidos Ă  sincronização do estro, recebendo no dia 0 (D0) um pessĂĄrio vaginal com progestĂĄgeno (CIDR-B, Intervet); no dia 1 (D1) pela manhĂŁ foi aplicado 3 mg de benzoato de estradiol IM (Estrogin Farmavet, SP). A superovulação ovariana (SO) foi executada pela manhĂŁ e Ă  tarde com hormĂŽnio folĂ­culo estimulante (FSH)(Pluset, Serono, ItĂĄlia)), administrado diariamente a intervalo de 12 horas entre cada aplicação, durante quatro dias seguidos nos dois grupos, conforme o protocolo: dia 6, 75 UI; dia 7, 40 UI; dia 8, 30 UI; dia 9, 20 UI. No dia 8 Ă  tarde administrou-se 500 microgramas de cloprostenol IM (Ciosin - Schering-Plough) e em seguida houve a retirada dos pessĂĄrios vaginais de todos os animais. Na sequĂȘncia, foram feitas as observaçÔes de cio; os animais foram inseminados duas vezes a intervalos de 12 horas um do outro. Adicionalmente, o G1 recebeu junto com a 1ÂȘ inseminação artificial (IA), uma dose de 3000 UI de hormĂŽnio coriĂŽnico gonadotrĂłfico (hCG) (Vetecor, Calier, IV) e o G2 recebeu 1 ml de solução fisiolĂłgica, como placebo. Todos os animais tiveram os ovĂĄrios monitorados mediante ultrassonografia, desde o dia da 1ÂȘ IA atĂ© o dia da coleta dos embriĂ”es, mediante aparelho Aloka SSD-550 (JapĂŁo), visando a avaliação de folĂ­culos, ovulação e corpos lĂșteos. No 5,5Âș dia pĂłs a observação de cio, procedeu-se Ă s lavagens dos cornos uterinos para a colheita dos embriĂ”es e avaliação. Concluiu-se que o protocolo utilizado para SO em bubalinos com FSH, foi eficiente, ao promover significativo numero de folĂ­culos antrais maiores que 8,0 mm de diĂąmetro no dia do cio e que o hCG mostrou-se significativamente eficiente, pois proporcionou maior % de ovulaçÔes no grupo tratado (
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