Spin-polarized stable phases of the 2-D electron fluid at finite temperatures

The Helmholtz free energy F of the interacting 2-D electron fluid is calculated nonperturbatively using a mapping of the quantum fluid to a classical Coulomb fluid [Phys. Rev. Letters, vol. 87, 206404 (2001)]. For density parameters rs such that rs<~25, the fluid is unpolarized at all temperatures t=T/EF where EF is the Fermi energy. For lower densities, the system becomes fully spin polarized for t<~0.35, and partially polarized for 0.35<t< 2, depending on the density. At rs ~25-30, and t ~0.35, an ''ambispin'' phase where F is almost independent of the spin polarization is found. These results support recent claims, based on quantum Monte Carlo results, for a stable, fully spin-polarized fluid phase at T = 0 for rs larger than about 25-26.Comment: Latex manuscript (4-5 pages) and two postscript figures; see also http://nrcphy1.phy.nrc.ca/ims/qp/chandre/chnc

Total energies from variational functionals of the Green function and the renormalized four-point vertex

We derive variational expressions for the grand potential or action in terms of the many-body Green function $G$ which describes the propagation of particles and the renormalized four-point vertex $\Gamma$ which describes the scattering of two particles in many-body systems. The main ingredient of the variational functionals is a term we denote as the $\Xi$-functional which plays a role analogously to the usual $\Phi$-functional studied by Baym (G.Baym, Phys.Rev. 127, 1391 (1962)) in connection with the conservation laws in many-body systems. We show that any $\Xi$-derivable theory is also $\Phi$-derivable and therefore respects the conservation laws. We further set up a computational scheme to obtain accurate total energies from our variational functionals without having to solve computationally expensive sets of self-consistent equations. The input of the functional is an approximate Green function $\tilde{G}$ and an approximate four-point vertex $\tilde{\Gamma}$ obtained at a relatively low computational cost. The variational property of the functional guarantees that the error in the total energy is only of second order in deviations of the input Green function and vertex from the self-consistent ones that make the functional stationary. The functionals that we will consider for practical applications correspond to infinite order summations of ladder and exchange diagrams and are therefore particularly suited for applications to highly correlated systems. Their practical evaluation is discussed in detail.Comment: 21 pages, 10 figures. Physical Review B (accepted

A charged particle in a magnetic field - Jarzynski Equality

We describe some solvable models which illustrate the Jarzynski theorem and related fluctuation theorems. We consider a charged particle in the presence of magnetic field in a two dimensional harmonic well. In the first case the centre of the harmonic potential is translated with a uniform velocity, while in the other case the particle is subjected to an ac force. We show that Jarzynski identity complements Bohr-van Leeuwen theorem on the absence of diamagnetism in equilibrium classical system.Comment: 5 pages, minor corrections made and journal reference adde

Correlated errors in Hipparcos parallaxes towards the Pleiades and the Hyades

We show that the errors in the Hipparcos parallaxes towards the Pleiades and the Hyades open clusters are spatially correlated over angular scales of 2 to 3 deg, with an amplitude of up to 2 mas. This correlation is stronger than expected based on the analysis of the Hipparcos catalog. We predict the parallaxes of individual cluster members, pi_pm, from their Hipparcos proper motions, assuming that all cluster members have the same space velocity. We compare pi_pm with their Hipparcos parallaxes, pi_Hip, and find that there are significant spatial correlations in pi_Hip. We derive a distance modulus to the Pleiades of 5.58 +- 0.18 mag using the radial-velocity gradient method. This value, agrees very well with the distance modulus of 5.60 +- 0.04 mag determined using the main-sequence fitting technique, compared with the value of 5.33 +- 0.06 inferred from the average of the Hipparcos parallaxes of the Pleiades members. We show that the difference between the main-sequence fitting distance and the Hipparcos parallax distance can arise from spatially correlated errors in the Hipparcos parallaxes of individual Pleiades members. Although the Hipparcos parallax errors towards the Hyades are spatially correlated in a manner similar to those of the Pleiades, the center of the Hyades is located on a node of this spatial structure. Therefore, the parallax errors cancel out when the average distance is estimated, leading to a mean Hyades distance modulus that agrees with the pre-Hipparcos value. We speculate that these spatial correlations are also responsible for the discrepant distances that are inferred using the mean Hipparcos parallaxes to some open clusters. Finally, we note that our conclusions are based on a purely geometric method and do not rely on any models of stellar isochrones.Comment: 33 pages including 10 Figures, revised version accepted for publication in Ap

Time correlations in a confined magnetized free-electron gas

The time-dependent pair correlation functions for a degenerate ideal quantum gas of charged particles in a uniform magnetic field are studied on the basis of equilibrium statistics. In particular, the influence of a flat hard wall on the correlations is investigated, both for a perpendicular and a parallel orientation of the wall with respect to the field. The coherent and incoherent parts of the time-dependent structure function in position space are determined from an expansion in terms of the eigenfunctions of the one-particle Hamiltonian. For the bulk of the system, the intermediate scattering function and the dynamical structure factor are derived by taking successive Fourier transforms. In the vicinity of the wall the time-dependent coherent structure function is found to decay faster than in the bulk. For coinciding positions near the wall the form of the structure function turns out to be independent of the orientation of the wall. Numerical results are shown to corroborate these findings.Comment: 25 pages, 14 figures, to be published in Journal of Physics

Investigating the Effect of a Visual Search Task for Simulator-Based Driver Training

Novice drivers tend to direct their gaze to the road ahead and not scan the environment properly. This study investigated the training effectiveness of a visual search task in a driving simulator, aimed at increasing young drivers\u27 spread of visual search. Two groups of inexperienced drivers were instructed to drive as accurately as possible in the center of the right lane in a self-paced driving task of four 6-min sessions in a rural environment. While driving, one group performed a visual search task, consisting of detecting and fixating on visual stimuli in the peripheral area. The stimuli were purple dots that faded in slowly and disappeared when fixated by the participant. After training, both groups drove a transfer session in an urban environment, in which various hazardous situations occurred. Results showed that both groups improved their lane keeping performance, whereas the training group became more proficient in the visual search task. However, in the transfer session no group differences were detected. In conclusion, despite improvements in visual search performance during a relatively short training period, the visual search training did not detectibly influence the spread of visual search of novice drivers during a post training transfer session

Food sharing with friends and acquaintances:A study in preschool boys and girls

IntroductionThe current study examined whether preschoolers in a (semi-)natural situation shared more food with friends or acquaintances, and whether this was different between boys and girls, older and younger children, and for preferred and non- preferred food. In order to do so, we replicated and extended the classical work of Birch and Billman in a Dutch sample.MethodsParticipants included 91 children aged between 3 to 6 years (52.7% boys, 93.4% Western European) from a middle- to upper-middle-class neighborhood in the Netherlands.ResultsThe results revealed that children shared more non-preferred than preferred food with others. Girls gave more non-preferred food to acquaintances than to friends, whereas boys gave more to friends than to acquaintances. No effect of relationship was found for preferred food. Older children shared more food than younger children. Compared to acquaintances, friends made more active attempts to get food. Moreover, children who were not shared with were just as likely to share food as children who were shared with.DiscussionOverall, only a small degree of agreement with the original study was found: Some significant findings could not be replicated, and some unconfirmed hypotheses of the original study were supported. The results underscore both the need for replications and studying the effect of social-contextual factors in natural settings

Conserving approximations in time-dependent quantum transport: Initial correlations and memory effects

We study time-dependent quantum transport in a correlated model system by means of time-propagation of the Kadanoff-Baym equations for the nonequilibrium many-body Green function. We consider an initially contacted equilibrium system of a correlated central region coupled to tight-binding leads. Subsequently a time-dependent bias is switched on after which we follow in detail the time-evolution of the system. Important features of the Kadanoff-Baym approach are 1) the possibility of studying the ultrafast dynamics of transients and other time-dependent regimes and 2) the inclusion of exchange and correlation effects in a conserving approximation scheme. We find that initial correlation and memory terms due to many-body interactions have a large effect on the transient currents. Furthermore the value of the steady state current is found to be strongly dependent on the approximation used to treat the electronic interactions.Comment: 5 pages, 2 figure

Phase Coexistence of a Stockmayer Fluid in an Applied Field

We examine two aspects of Stockmayer fluids which consists of point dipoles that additionally interact via an attractive Lennard-Jones potential. We perform Monte Carlo simulations to examine the effect of an applied field on the liquid-gas phase coexistence and show that a magnetic fluid phase does exist in the absence of an applied field. As part of the search for the magnetic fluid phase, we perform Gibbs ensemble simulations to determine phase coexistence curves at large dipole moments, $\mu$. The critical temperature is found to depend linearly on $\mu^2$ for intermediate values of $\mu$ beyond the initial nonlinear behavior near $\mu=0$ and less than the $\mu$ where no liquid-gas phase coexistence has been found. For phase coexistence in an applied field, the critical temperatures as a function of the applied field for two different $\mu$ are mapped onto a single curve. The critical densities hardly change as a function of applied field. We also verify that in an applied field the liquid droplets within the two phase coexistence region become elongated in the direction of the field.Comment: 23 pages, ReVTeX, 7 figure

Tool use for corpse cleaning in chimpanzees

For the first time, chimpanzees have been observed using tools to clean the corpse of a deceased group member. A female chimpanzee sat down at the dead body of a young male, selected a firm stem of grass, and started to intently remove debris from his teeth. This report contributes novel behaviour to the chimpanzee’s ethogram, and highlights how crucial information for reconstructing the evolutionary origins of human mortuary practices may be missed by refraining from developing adequate observation techniques to capture non-human animals’ death responses.Publisher PDFPeer reviewe