114 research outputs found

    Phase transitions of quasistationary states in the Hamiltonian Mean Field model

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    The out-of-equilibrium dynamics of the Hamiltonian Mean Field (HMF) model is studied in presence of an externally imposed magnetic field h. Lynden-Bell's theory of violent relaxation is revisited and shown to adequately capture the system dynamics, as revealed by direct Vlasov based numerical simulations in the limit of vanishing field. This includes the existence of an out-of-equilibrium phase transition separating magnetized and non magnetized phases. We also monitor the fluctuations in time of the magnetization, which allows us to elaborate on the choice of the correct order parameter when challenging the performance of Lynden-Bell's theory. The presence of the field h removes the phase transition, as it happens at equilibrium. Moreover, regions with negative susceptibility are numerically found to occur, in agreement with the predictions of the theory.Comment: 6 pages, 7 figure

    Experimental perspectives for systems based on long-range interactions

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    The possibility of observing phenomena peculiar to long-range interactions, and more specifically in the so-called Quasi-Stationary State (QSS) regime is investigated within the framework of two devices, namely the Free-Electron Laser (FEL) and the Collective Atomic Recoil Laser (CARL). The QSS dynamics has been mostly studied using the Hamiltonian Mean-Field (HMF) toy model, demonstrating in particular the presence of first versus second order phase transitions from magnetized to unmagnetized regimes in the case of HMF. Here, we give evidence of the strong connections between the HMF model and the dynamics of the two mentioned devices, and we discuss the perspectives to observe some specific QSS features experimentally. In particular, a dynamical analog of the phase transition is present in the FEL and in the CARL in its conservative regime. Regarding the dissipative CARL, a formal link is established with the HMF model. For both FEL and CARL, calculations are performed with reference to existing experimental devices, namely the FERMI@Elettra FEL under construction at Sincrotrone Trieste (Italy) and the CARL system at LENS in Florence (Italy)

    Models with short and long-range interactions: phase diagram and reentrant phase

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    We study the phase diagram of two different Hamiltonians with competiting local, nearest-neighbour, and mean-field couplings. The first example corresponds to the HMF Hamiltonian with an additional short-range interaction. The second example is a reduced Hamiltonian for dipolar layered spin structures, with a new feature with respect to the first example, the presence of anisotropies. The two examples are solved in both the canonical and the microcanonical ensemble using a combination of the min-max method with the transfer operator method. The phase diagrams present typical features of systems with long-range interactions: ensemble inequivalence, negative specific heat and temperature jumps. Moreover, in a given range of parameters, we report the signature of phase reentrance. This can also be interpreted as the presence of azeotropy with the creation of two first order phase transitions with ensemble inequivalence, as one parameter is varied continuously

    Solitons in Five Dimensional Minimal Supergravity: Local Charge, Exotic Ergoregions, and Violations of the BPS Bound

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    We describe a number of striking features of a class of smooth solitons in gauged and ungauged minimal supergravity in five dimensions. The solitons are globally asymptotically flat or asymptotically AdS without any Kaluza-Klein directions but contain a minimal sphere formed when a cycle pinches off in the interior of the spacetime. The solutions carry a local magnetic charge and many have rather unusual ergosurfaces. Perhaps most strikingly, many of the solitons have more electric charge or, in the asymptotically AdS case, more electric charge and angular momentum than is allowed by the usual BPS bound. We comment on, but do not resolve, the new puzzle this raises for AdS/CFT.Comment: 60 pages, 12 figures, 3 table

    On the effectiveness of mixing in violent relaxation

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    Relaxation processes in collisionless dynamics lead to peculiar behavior in systems with long-range interactions such as self-gravitating systems, non-neutral plasmas and wave-particle systems. These systems, adequately described by the Vlasov equation, present quasi-stationary states (QSS), i.e. long lasting intermediate stages of the dynamics that occur after a short significant evolution called "violent relaxation". The nature of the relaxation, in the absence of collisions, is not yet fully understood. We demonstrate in this article the occurrence of stretching and folding behavior in numerical simulations of the Vlasov equation, providing a plausible relaxation mechanism that brings the system from its initial condition into the QSS regime. Area-preserving discrete-time maps with a mean-field coupling term are found to display a similar behaviour in phase space as the Vlasov system.Comment: 10 pages, 11 figure

    G2 Dualities in D=5 Supergravity and Black Strings

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    Five dimensional minimal supergravity dimensionally reduced on two commuting Killing directions gives rise to a G2 coset model. The symmetry group of the coset model can be used to generate new solutions by applying group transformations on a seed solution. We show that on a general solution the generators belonging to the Cartan and nilpotent subalgebras of G2 act as scaling and gauge transformations, respectively. The remaining generators of G2 form a sl(2,R)+sl(2,R) subalgebra that can be used to generate non-trivial charges. We use these generators to generalize the five dimensional Kerr string in a number of ways. In particular, we construct the spinning electric and spinning magnetic black strings of five dimensional minimal supergravity. We analyze physical properties of these black strings and study their thermodynamics. We also explore their relation to black rings.Comment: typos corrected (26 pages + appendices, 2 figures

    Key Maternity Care Stakeholders' Views on Midwives' Professional Autonomy.

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    Advancement towards the professionalism of midwifery is closely linked to midwives' professional autonomy. Although the perspectives of Belgian midwives on their professional autonomy have been studied, the views of other maternity care stakeholders are a blind spot. The aim of this study, therefore, was to explore maternity care stakeholders' views on Belgian midwives' professional autonomy. A qualitative exploratory study was performed using focus group interviews. A heterogenous group of 27 maternity care stakeholders participated. The variation between midwives, with different levels of autonomy, was reported. The analysis of the data resulted in five themes: (1) The autonomous midwife is adequately educated and committed to continuous professional further education, (2) The autonomous midwife is competent, (3) The autonomous midwife is experienced, (4) The autonomous midwife assures safe and qualitative care, and (5) The autonomous midwife collaborates with all stakeholders in maternity care. A maternity collaborative framework, where all maternity care professionals respect each other's competences and autonomy, is crucial for providing safe and quality care. To achieve this, it is recommended to implement interprofessional education to establish strong foundations for interprofessional collaboration. Additionally, a regulatory body with supervisory powers can help ensure safe and quality care, while also supporting midwives' professional autonomy and professionalisation

    A General Black String and its Microscopics

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    Using G2(2) dualities we construct the most general black string solution of minimal five-dimensional ungauged supergravity. The black string has five independent parameters, namely, the magnetic one-brane charge, smeared electric zero-brane charge, boost along the string direction, energy above the BPS bound, and rotation in the transverse space. In one extremal limit it reduces to the three parameter supersymmetric string of five-dimensional minimal supergravity; in another extremal limit it reduces to the three parameter non-supersymmetric extremal string of five-dimensional minimal supergravity. It also admits an extremal limit when it has maximal rotation in the four-dimensional transverse space. The decoupling limit of our general black string is a BTZ black hole times a two sphere. The macroscopic entropy of the string is reproduced by the Maldacena-Strominger-Witten CFT in appropriate ranges of the parameters. When the pressureless condition is imposed, our string describes the infinite radius limit of the most general class of black rings of minimal supergravity. We discuss implications our solution has for extremal and non-extremal black rings of minimal supergravity.Comment: 35 pages; 3 figures; v2 section 4.1.1 rewritten + minor changes + ref adde

    Finite and infinite-dimensional symmetries of pure N=2 supergravity in D=4

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    We study the symmetries of pure N=2 supergravity in D=4. As is known, this theory reduced on one Killing vector is characterised by a non-linearly realised symmetry SU(2,1) which is a non-split real form of SL(3,C). We consider the BPS brane solutions of the theory preserving half of the supersymmetry and the action of SU(2,1) on them. Furthermore we provide evidence that the theory exhibits an underlying algebraic structure described by the Lorentzian Kac-Moody group SU(2,1)^{+++}. This evidence arises both from the correspondence between the bosonic space-time fields of N=2 supergravity in D=4 and a one-parameter sigma-model based on the hyperbolic group SU(2,1)^{++}, as well as from the fact that the structure of BPS brane solutions is neatly encoded in SU(2,1)^{+++}. As a nice by-product of our analysis, we obtain a regular embedding of the Kac-Moody algebra su(2,1)^{+++} in e_{11} based on brane physics.Comment: 70 pages, final version published in JHE

    How Do Midwives View Their Professional Autonomy, Now and in Future?

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    Background: Internationally, midwives’ professional autonomy is being challenged, resulting in their inability to practice to their full scope of practice. This situation contrasts with the increasing international calls for strengthening the midwifery profession. The aim of this study therefore is to explore Belgian midwives’ views on their current and future autonomy. Methods: An online survey among Belgian midwives was performed. Data were collected and analyzed using a quantitative approach, while quotes from respondents were used to contextualize the quantitative data. Results: Three hundred and twelve midwives from different regions and professional settings in Belgium completed the questionnaire. Eighty-five percentage of respondents believe that they are mostly or completely autonomous. Brussels’ midwives feel the most autonomous, while those in Wallonia feel the least. Primary care midwives feel more autonomous than hospital-based midwives. Older midwives and primary care midwives feel less recognized and respected by other professionals in maternity care. The majority of our respondents believe that in future midwives should be able to work more autonomously in constructive collaboration with other professionals. Conclusion: While Belgian midwives generally rated their own professional autonomy as high, a significant majority of respondents desire more autonomy in future. In addition, our respondents want to be recognized and respected by society and other health professionals in maternity care. It is recommended to prioritize efforts in enhancing midwives’ autonomy, while also addressing the need for increased recognition and respect from society and other maternity care professionals
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