32,211 research outputs found

    Quantum Monte-Carlo methods and exact treatment of the two-body problem with Hartree-Fock Bogoliubov states

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    In this article, we show that the exact two-body problem can be replaced by quantum jumps between densities written as D=| \Psi_a \right> \left< \Psi_b | where | \Psi_a \right> and | \Psi_b \right> are vacuum for different quasi-particles operators. It is shown that the stochastic process can be written as a Stochastic Time-Dependent Hartree-Fock Bogoliubov theory (Stochastic TDHFB) for the generalized density R{\cal R} associated to DD where R2=R{\cal R}^2 = {\cal R} along each stochastic trajectory.Comment: 5 page

    Introduction - Strong interaction in the nuclear medium: new trends

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    Recent achievements in nuclear forces theory open new perspectives for the next decade of low energy nuclear physics, bringing together people from very different communities. Although many developments remain to be done, the possibility to directly use QCD to describe nuclear system is a major challenge that is within reach. In this introduction to the 2009 International Joliot-Curie School (EJC2009), new trends in the strong nuclear interaction are summarized starting from quarks and ending with finite or infinite nuclear systems. At different energy scales, selected new concepts and ideas have been discussed in a rather simple way. Recent advances in the theory of nuclear forces, thanks to chiral perturbation and effective field theories, have led to a new generation of strong nuclear interaction particularly suited to low energy nuclear physics. The interesting aspects of new interactions compared to conventional forces are underlined. Recent achievements in ab initio theories that directly start from the bare nucleon-nucleon interaction and their key role to understand the three-body force are illustrated. Finally, future perspectives for standard nuclear physics theories, namely Shell Model and Energy Density Functional, are discussed.Comment: 26 pages, 28 figures--Introduction Lecture to the 2009 International Joliot-Curie School, Lacanau, France, 27 Sept.-3 Oct. 200

    Heavy fermion behavior of itinerant frustrated systems: beta-Mn, Y(Sc)Mn_{2} and LiV_{2}O_{4}$

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    These three metallic systems do not exhibit any magnetic ordering despite experiments show the existence of localized moments with large antiferromagnetic exchange: this is a consequence of the strong geometric frustration (Y(Sc)Mn_{2} and LiV_{2}O_{4} have the Pyrochlore structure, while beta-Mn has a more complicated frustrated structure). Another common feature is their very large specific heat coefficient \gamma =C/T (420 mJ mole^{-1} K^{-2} for LiV_{2}O_{4}). Several explanations have been proposed for this ''3d heavy fermion behavior'', including a 3d-Kondo effect. However the similarities between the three compounds indicate that frustration plays a big role. We propose a new model which takes into account the existence of two types of 3d-electrons (localized and itinerant) and a frustrated antiferromagnetic exchange between the localized 3d electrons.Comment: Proceedings of HFM-Conference, June 2000, Waterloo, Ontario, Canada To appear in Can. J. Phy

    Large amplitude collective dynamic beyond the independent particle/quasiparticle picture

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    In the present note, a summary of selected aspects of time-dependent mean-field theory is first recalled. This approach is optimized to describe one-body degrees of freedom. A special focus is made on how this microscopic theory can be reduced to a macroscopic dynamic for a selected set of collective variables. Important physical phenomena like adiabaticity/diabaticity, one-body dissipation or memory effect are discussed. Special aspects related to the use of a time-dependent density functional instead of a time-dependent Hartree-Fock theory based on a bare hamiltonian are underlined. The absence of proper description of complex internal correlations however strongly impacts the predictive power of mean-field. A brief overview of theories going beyond the independent particles/quasi-particles theory is given. Then, a special attention is paid for finite fermionic systems at low internal excitation. In that case, quantum fluctuations in collective space that are poorly treated at the mean-field level, are important. Several approaches going beyond mean-field, that are anticipated to improve the description of quantum fluctuations, are discussed: the Balian-V\'en\'eroni variational principle, the Time-Dependent Random Phase Approximation and the recently proposed Stochastic Mean-Field theory. Relations between these theories are underlined as well as their advantages and shortcomings.Comment: To be published in the ebook "Progress of time-dependent nuclear reaction theory" honoring Prof. Joachim Maruhn's retiremen

    Gender Differences in Parents\u27 Assessment of Language Development

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    This study aims to determine if there is a statistically significant difference between genders in the reporting of their child’s communication abilities. Possible variances between men and women are important to consider because parent-based questionnaires are often used to help identify children who are in need and eligible for the services of a speech-language pathologist. Literature surrounding parental stereotypes and the impact of the parent in development, as well as stigma in relation to the need for speech therapy services is explored. A survey based off of the Ages and Stages Questionnaire, Third Edition was distributed to parents of children between 24 and 36 months old. While the results of the survey did not show a statistically significant difference between men and women, overarching patterns were found within the data. The most notable trends showed that males consistently reported higher average scores for expressive language-based questions while females reported scores either the same or higher than males for all receptive language-based questions. However, because no statistically difference was found it can be concluded that having only one parent take a questionnaire would not have a large impact on the overall results and subsequent recommendations of a speech-language pathologist

    Density Matrix Functional Theory for the Lipkin model

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    A Density Matrix Functional theory is constructed semi-empirically for the two-level Lipkin model. This theory, based on natural orbitals and occupation numbers, is shown to provide a good description for the ground state energy of the system as the two-body interaction and particle number vary. The application of Density Matrix Functional theory to the Lipkin model illustrates that it could be a valuable tool for systems presenting a shape phase-transition such as nuclei. The improvement of one-body observables description as well as the interest for Energy Density Functional theory are discussed.Comment: 9 pages, 7 figures, to appear in Physical Review

    Optimizing stochastic trajectories in exact quantum jump approaches of interacting systems

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    The quantum jump approach, where pairs of state vectors follow Stochastic Schroedinger Equation (SSE) in order to treat the exact quantum dynamics of two interacting systems, is first described. In this work the non-uniqueness of such stochastic Schroedinger equations is investigated to propose strategies to optimize the stochastic paths and reduce statistical fluctuations. In the proposed method, called the 'adaptative noise method', a specific SSE is obtained for which the noise depends explicitly on both the initial state and on the properties of the interaction Hamiltonian. It is also shown that this method can be further improved by introduction of a mean-field dynamics. The different optimization procedures are illustrated quantitatively in the case of interacting spins. A significant reduction of the statistical fluctuations is obtained. Consequently a much smaller number of trajectories is needed to accurately reproduce the exact dynamics as compared to the SSE without optimization.Comment: 12 pages, 5 figures, revised versio

    From microscopic to macroscopic dynamics in mean-field theory: effect of neutron skin on fusion barrier and dissipation

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    In this work, we introduce a new method to reduce the microscopic mean-field theory to a classical macroscopic dynamics during the initial stage of fusion reactions. We show that TDHF (Time-dependent Hartree-Fock) could be a useful tool to gain information on fusion barriers as well as on one-body dissipation effects. We apply the mean-field theory to the case of head-on reaction between 16^{16}O and 16,22,24,28^{16,22,24,28}O in order to quantify the effect of neutron skin on fusion. We show that the determination of fusion barrier requires, in addition to a precise knowledge of the relative distance between the center of mass of the two fusing nuclei, the introduction of an additional collective coordinate that explicitly breaks the neutron-proton symmetry. In this context, we estimate the position, height and diffuseness of the barrier as well as the one-body friction and show that a global enhancement of the fusion cross-section is expected in neutron rich nuclei.Comment: 9 ps pages including 9 figure

    Assessing the Impact of a Wage Subsidy for Single Parents on Social Assistance

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    In 2002 the Quebec government implemented the “Action Emploi" (AE) program aimed at making work pay for long-term social assistance recipients (SA). AE offered a generous income supplement that could last up to three years to recipients who found a full-time job within twelve months. The program was implemented for a trial period of one year. Based on little empirical evidence, a slightly modified version of the program was implemented on a permanent basis in May 2008. The paper investigates the impact of the temporary program by focusing on the labour market transitions of the targeted population starting one year before the implementation of the program and up until the end of 2005. We use a multi-state multi-episode model. The endogeneity of the participation status is accounted for by treating AE as a distinct state and by allowing correlated unobserved factors to affect the transitions. The model is estimated by the method of simulated moments. Our results show that AE has indeed increased the duration of Off-SA spells and decreased the duration of SA spells slightly. There is also some evidence that the response to the program varies considerably with unobserved individual characteristics. En 2002, le gouvernement du Québec a mis sur pied le programme Action emploi (AE) qui visait à mieux rémunérer le travail des prestataires de l’aide sociale (AS) de longue durée. AE offrait un supplément de revenu généreux pouvant s’échelonner sur une période d’au plus trois ans aux prestataires ayant trouvé un emploi à temps plein à l’intérieur de 12 mois. Le programme a été mis en œuvre pendant une période d’essai d’un an. Sur la base d’une faible évidence empirique, une version légèrement modifiée du programme a été adoptée de façon permanente en mai 2008. Le document examine l’incidence du programme temporaire en mettant l’accent sur les transitions de la population ciblée sur le marché du travail, à compter de l’année précédant la mise en œuvre du programme jusqu’à la fin de 2005. Nous utilisons un modèle multi-états et multi-épisodes. Afin de prendre en compte l’endogénéité du statut de participation, nous considérons que le programme AE est un état distinct et nous permettons à des facteurs latents corrélés d’influencer les transitions. Le modèle est évalué par la méthode du maximum de vraisemblance simulée. Nos résultats démontrent que le programme a effectivement augmenté la durée des périodes de sortie de l’AS et diminué légèrement la durée des périodes de recours à l’AS. Le document montre également que la réponse au programme varie considérablement en fonction des caractéristiques individuelles latentes., assistance sociale, supplément de revenu, modèle de transition multi-états et multi-épisodes.
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