Isotopic dependence of the fragments' internal temperatures observed in multifragment emission

The internal temperatures of fragments produced by an excited nuclear source are investigated using the microcanonical version of the Statistical Multifragmentation Model, with discrete energy. We focus on the fragments' properties at the breakup stage, before they have time to deexcite by particle emission. Since the adopted model provides the excitation energy distribution of these primordial fragments, it allows one to calculate the temperatures of different isotope families and infer on the sensitivity to their isospin composition. It is found that, due to the functional form of the nuclear density of states and the excitation energy distribution of the fragments, proton rich isotopes are hotter than neutron rich ones. This property has been taken to be an indication of earlier emission of the former from a source that cools down as it expands and emits fragments. Although this scenario is incompatible with the prompt breakup of a thermally equilibrated source, our results reveal that the latter framework also provides the same qualitative features just mentioned. Therefore they suggest that this property cannot be taken as evidence for non-equilibrium emission. We also found that this sensitivity to the isotopic composition of the fragments depends on the isospin composition of the source, and that it is weakened as the excitation energy of the source increases.Comment: 5 pages, 3 figure

Ergodic and Thermodynamic Games

Let $T:X\to X$ and $S:Y \to Y$ be continuous maps defined on compact sets. Let $$\varphi_i(\mu,\nu)=\int_{X \times Y} A_i(x,y) d\mu(x) d\nu(y)\;\;{for} \;\; i=1,2,$$ where $\mu$ is $T$-invariant and $\nu$ is $S$-invariant, be pay-off functions for a game (in the usual sense of game theory) between players that have the set of invariant measures for $T$ (player 1) and $S$ (player 2) as possible strategies. Our goal here is to establish the notion of Nash equilibrium point for the game defined by this pay-offs and strategies. The main tools came from ergodic optimization (as we are optimizing over the set of invariant measures) and thermodynamic formalism (when we add to the integrals above the entropy of measures in order to define a second case to be explored). Both cases are ergodic versions of non-cooperative games. We show the existence of Nash equilibrium points with two independent arguments. One of the arguments works for the case with entropy, and uses only tools of thermodynamical formalism, while the other, that works in the case without entropy but can be adapted to deal with both cases, uses the Kakutani fixed point. We also present examples and briefly discuss uniqueness (or lack of uniqueness). In the end we present a different example where players are allowed to collaborate. This final example show connections between cooperative games and ergodic transport

The relation between velocity dispersions and chemical abundances in RAVE giants

We developed a Bayesian framework to determine in a robust way the relation between velocity dispersions and chemical abundances in a sample of stars. Our modelling takes into account the uncertainties in the chemical and kinematic properties. We make use of RAVE DR5 radial velocities and abundances together with Gaia DR1 proper motions and parallaxes (when possible, otherwise UCAC4 data is used). We found that, in general, the velocity dispersions increase with decreasing [Fe/H] and increasing [Mg/Fe]. A possible decrease in velocity dispersion for stars with high [Mg/Fe] is a property of a negligible fraction of stars and hardly a robust result. At low [Fe/H] and high [Mg/Fe] the sample is incomplete, affected by biases, and likely not representative of the underlying stellar population.Comment: 2 pages, to appear in Proceedings of the IAU Symposium 330, "Astrometry and Astrophysics in the Gaia Sky", held in April 2017, Nice, Franc

A generalization of convergence actions

Let a group $G$ act properly discontinuously and cocompactly on a locally compact space $X$. A Hausdorff compact space $Z$ that contains $X$ as an open subspace has the perspectivity property if the action $G\curvearrowright X$ extends to an action $G\curvearrowright Z$, by homeomorphisms, such that for every compact $K\subseteq X$ and every element $u$ of the unique uniform structure compatible with the topology of $Z$, the set $\{gK: g \in G\}$ has finitely many non $u$-small sets. We describe a correspondence between the compact spaces with the perspectivity property with respect to $X$ (and the fixed action of $G$ on it) and the compact spaces with the perspectivity property with respect to $G$ (and the left multiplication on itself). This generalizes a similar result for convergence group actions.Comment: 57 page

Low-cost educational robotics applied to physics teaching in Brazil

In this paper we propose strategies and methodologies of teaching topics in high school physics, through a show of Educational Robotics. The Exhibition was part of a set of actions promoted by a brazilian government program of incentive for teaching activities (PIBID) and whose primary focus is the training of teachers, improvement of teaching in public schools, dissemination of science and formation of new scientists and researchers. By means of workshops, banners and prototyping of robotics, we are able to create a connection between the study areas and their surrounding, making learning meaningful and accessible for the students involved and contributing to their cognitive development.Comment: 5 pages and 10 figure

Many-particle correlations and Coulomb effects on temperatures from fragment momentum fluctuations

We investigate correlations in the fragment momentum distribution due to the propagation of fragments under the influence of their mutual Coulomb field, after the breakup of an excited nuclear source.The magnitude of the effects on the nuclear temperatures obtained from such distributions is estimated with the help of a simple approach in which a charged fragment interacts with a homogeneous charged sphere. The resuslts are used to correct the temperatures obtained from the asymptotic momentum distributions of fragments produced by a Monte-Carlo simulation in which the system's configuration at breakup is provided by the canonical version of the Statistical Multifragmentation Model. In a separate calculation, the dynamics of this many-particle charged system is followed in a molecular dynamics calculation until the fragments are far away from the breakup volume. The results suggest that, although the magnitude of the corrections is similar in both models, many-particle correlations present in the second approach are non-negligible and should be taken into account in order to minimize ambiguities in such studies.Comment: 7 pages, 4 figure

Finite size effects in isobaric ratios

The properties of isobaric ratios, between nuclei produced in the same reaction, are investigated using the canonical and grand-canonical statistical ensembles. Although the grand-canonical for- mulae furnish a means to correlate the ratios with the liquid drop parameters, finite size effects make it difficult to obtain their actual values from fitting nuclear collision data.Comment: 4 pages, 2 figure

The possible role of van Hove singularities in the high TcT_c of superconducting H3_3S

We observe that H$_3$S has a bcc structure and, with nearest neighbour hopping only, a strong singularity occurs at zero energy. This singularity is accompanied with a highly nested Fermi surface, which is {\it not} conducive to a stable superconducting instability. Introduction of next-nearest-neighbour hopping removes the singularity, but a `robust' peak remains in the electron density of states. Solution of the BCS equations shows an enhanced superconducting $T_c$ due to this peak. Furthermore, nesting is no longer present, so other instabilities will not compete effectively with superconductivity. We find high critical temperatures are possible, even with very modest coupling strengths. We also examine a limit of the $T=0$ equations (in an Appendix) where an analytical solution is possible over the entire range of coupling strengths, and therefore the BCS-BEC crossover is fully covered.Comment: 5 pages 2 figure

A systematic study of the superconducting critical temperature in two and three dimensional tight-binding models: a possible scenario for superconducting H3_3S ?

Ever since BCS theory was first formulated it was recognized that a large electronic density of states at the Fermi level was beneficial to enhancing $T_c$. The A15 compounds and the high temperature cuprate materials both have had an enormous amount of effort devoted to studying the possibility that such peaks play an important role in the high critical temperatures existing in these compounds. Here we provide a systematic study of the effect of these peaks on the superconducting transition temperature for a variety of tight-binding models of simple structures, both in two and three dimensions. In three dimensions large enhancements in $T_c$ can occur, due to van Hove singularities that result in divergences in the density of states. Furthermore, even in more realistic structures, where the van Hove singularity disappears, large enhancements in $T_c$ continue due to the presence of `robust' peaks in the densities of states. Such a peak, recently identified in the bcc structure of H$_3$S, is likely the result of such a van Hove singularity. In certain regimes, anomalies in the isotope coefficient are also expected.Comment: 15 pages, 15 figures, changes reflect more emphasis on implications for the 200K superconductor, H3S. A couple of additional changes have been made to comply with the published versio

Open quantum random walks: ergodicity, hitting times, gambler's ruin and potential theory

In this work we study certain aspects of Open Quantum Random Walks (OQRWs), a class of quantum channels described by S. Attal et al. \cite{attal}. As a first objective we consider processes which are nonhomogeneous in time, i.e., at each time step, a possibly distinct evolution kernel. Inspired by a spectral technique described by L. Saloff-Coste and J. Z\'u\~niga \cite{saloff}, we define a notion of ergodicity for finite nonhomogeneous quantum Markov chains and describe a criterion for ergodicity of such objects in terms of singular values. As a second objective, and based on a quantum trajectory approach, we study a notion of hitting time for OQRWs and we see that many constructions are variations of well-known classical probability results, with the density matrix degree of freedom on each site giving rise to systems which are seen to be nonclassical. In this way we are able to examine open quantum versions of the gambler's ruin, birth-and-death chain and a basic theorem on potential theory.Comment: Revised version. arXiv admin note: substantial text overlap with arXiv:1504.05398, arXiv:1506.0832