3,872 research outputs found
Role of the first coordination shell in determining the equilibrium structure and dynamics of simple liquids
The traditional view that the physical properties of a simple liquid are
determined primarily by its repulsive forces was recently challenged by
Berthier and Tarjus, who showed that in some cases ignoring the attractions
leads to large errors in the dynamics [L. Berthier and G. Tarjus, Phys. Rev.
Lett. 103, 170601 (2009); J. Chem. Phys. 134, 214503 (2011)]. We present
simulations of the standard Lennard-Jones liquid at several condensed-fluid
state points, including a fairly low density state and a very high density
state, as well as simulations of the Kob-Andersen binary Lennard-Jones mixture
at several temperatures. By varying the range of the forces, results for the
thermodynamics, dynamics, and structure show that the determining factor for
getting the correct statics and dynamics is not whether or not the attractive
forces {\it per se} are included in the simulations. What matters is whether or
not interactions are included from all particles within the first coordination
shell (FCS) - the attractive forces can thus be ignored, but only at extremely
high densities. The recognition of the importance of a local shell in condensed
fluids goes back to van der Waals; our results confirm this idea and thereby
the basic picture of the old hole- and cell theories for simple condensed
fluids
Quantum trajectory phase transitions in the micromaser
We study the dynamics of the single atom maser, or micromaser, by means of
the recently introduced method of thermodynamics of quantum jump trajectories.
We find that the dynamics of the micromaser displays multiple space-time phase
transitions, i.e., phase transitions in ensembles of quantum jump trajectories.
This rich dynamical phase structure becomes apparent when trajectories are
classified by dynamical observables that quantify dynamical activity, such as
the number of atoms that have changed state while traversing the cavity. The
space-time transitions can be either first-order or continuous, and are
controlled not just by standard parameters of the micromaser but also by
non-equilibrium "counting" fields. We discuss how the dynamical phase behavior
relates to the better known stationary state properties of the micromaser.Comment: 7 pages, 5 figure
Isomorphic classical molecular dynamics model for an excess electron in a supercritical fluid
Ring polymer molecular dynamics (RPMD) is used to directly simulate the
dynamics of an excess electron in a supercritical fluid over a broad range of
densities. The accuracy of the RPMD model is tested against numerically exact
path integral statistics through the use of analytical continuation techniques.
At low fluid densities, the RPMD model substantially underestimates the
contribution of delocalized states to the dynamics of the excess electron.
However, with increasing solvent density, the RPMD model improves, nearly
satisfying analytical continuation constraints at densities approaching those
of typical liquids. In the high density regime, quantum dispersion
substantially decreases the self-diffusion of the solvated electron.
In this regime where the dynamics of the electron is strongly coupled to the
dynamics of the atoms in the fluid, trajectories that can reveal diffusive
motion of the electron are long in comparison to .Comment: 24 pages, 4 figure
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A Galerkin boundary element method for high frequency scattering by convex polygons
In this paper we consider the problem of time-harmonic acoustic scattering in two dimensions by convex polygons. Standard boundary or finite element methods for acoustic scattering problems have a computational cost that grows at least linearly as a function of the frequency of the incident wave. Here we present a novel Galerkin boundary element method, which uses an approximation space consisting of the products of plane waves with piecewise polynomials supported on a graded mesh, with smaller elements closer to the corners of the polygon. We prove that the best approximation from the approximation space requires a number of degrees of freedom to achieve a prescribed level of accuracy that grows only logarithmically as a function of the frequency. Numerical results demonstrate the same logarithmic dependence on the frequency for the Galerkin method solution. Our boundary element method is a discretization of a well-known second kind combined-layer-potential integral equation. We provide a proof that this equation and its adjoint are well-posed and equivalent to the boundary value problem in a Sobolev space setting for general Lipschitz domains
Effects of tunnelling and asymmetry for system-bath models of electron transfer
We apply the newly derived nonadiabatic golden-rule instanton theory to
asymmetric models describing electron-transfer in solution. The models go
beyond the usual spin-boson description and have anharmonic free-energy
surfaces with different values for the reactant and product reorganization
energies. The instanton method gives an excellent description of the behaviour
of the rate constant with respect to asymmetry for the whole range studied. We
derive a general formula for an asymmetric version of Marcus theory based on
the classical limit of the instanton and find that this gives significant
corrections to the standard Marcus theory. A scheme is given to compute this
rate based only on equilibrium simulations. We also compare the rate constants
obtained by the instanton method with its classical limit to study the effect
of tunnelling and other quantum nuclear effects. These quantum effects can
increase the rate constant by orders of magnitude.Comment: 10 pages, 3 figure
Clinical Comparison of Retinopathy-Positive and Retinopathy-Negative Cerebral Malaria
AbstractCerebral malaria (CM) is a severe and often lethal complication of falciparum malaria. A classic malaria retinopathy is seen in some (retinopathy-positive [RP]) children but not others (retinopathy-negative [RN]), and is associated with increased parasite sequestration. It is unclear whether RN CM is a severe nonmalarial illness with incidental parasitemia or a less severe form of the same malarial illness as RP CM. Understanding the clinical differences between RP and RN CM may help shed light on the pathophysiology of malarial retinopathy. We compared clinical history, physical examination, laboratory findings, and outcomes of RP (N = 167) and RN (N = 87) children admitted to Mulago Hospital, Kampala, Uganda. Compared with RN children, RP children presented with a longer history of illness, as well as physical examination and laboratory findings indicative of more severe disease and organ damage. The hospital course of RP children was complicated by longer coma duration and a greater transfusion burden than RN children. Mortality did not differ significantly between RP and RN children (14.4% versus 8.0%, P = 0.14). Further, severity of retinal hemorrhage correlated with the majority of variables that differed between RP and RN children. The data suggest that RP and RN CM may reflect the spectrum of illness in CM, and that RN CM could be an earlier, less severe form of disease
DE 1 RIMS operational characteristics
The Retarding Ion Mass Spectrometer (RIMS) on the Dynamics Explorer 1 spacecraft observes both the thermal and superthermal (50 eV) ions of the ionosphere and inner magnetosphere. It is capable of measuring the detailed species distribution function of these ions in many cases. It was equipped with an integral electrometer to permit in-flight calibration of the detector sensitivities and variations thereof. A guide to understanding the RIMS data set is given. The reduction process from count rates to physical quantities is discussed in some detail. The procedure used to establish in-flight calibration is described, and results of a comparison with densities from plasma wave measurements are provided. Finally, a discussion is provided of various anomalies in the data set, including changes of channeltron efficiency with time, spin modulation of the axial sensor heads, apparent potential differences between the sensor heads, and failures of the radial head retarding potential sweep and of the -Z axial head aperture plane bias. Studies of the RIMS data set should be conducted only with a thorough awareness of the material presented here, or in collaboration with one of the scientists actively involved with RIMS data analysis
"Even if the test result is negative, they should be able to tell us what is wrong with us": a qualitative study of patient expectations of rapid diagnostic tests for malaria.
BACKGROUND: The debate on rapid diagnostic tests (RDTs) for malaria has begun to shift from whether RDTs should be used, to how and under what circumstances their use can be optimized. This has increased the need for a better understanding of the complexities surrounding the role of RDTs in appropriate treatment of fever. Studies have focused on clinician practices, but few have sought to understand patient perspectives, beyond notions of acceptability. METHODS: This qualitative study aimed to explore patient and caregiver perceptions and experiences of RDTs following a trial to assess the introduction of the tests into routine clinical care at four health facilities in one district in Ghana. Six focus group discussions and one in-depth interview were carried out with those who had received an RDT with a negative test result. RESULTS: Patients had high expectations of RDTs. They welcomed the tests as aiding clinical diagnoses and as tools that could communicate their problem better than they could, verbally. However, respondents also believed the tests could identify any cause of illness, beyond malaria. Experiences of patients suggested that RDTs were adopted into an existing system where patients are both physically and intellectually removed from diagnostic processes and where clinicians retain authority that supersedes tests and their results. In this situation, patients did not feel able to articulate a demand for test-driven diagnosis. CONCLUSIONS: Improvements in communication between the health worker and patient, particularly to explain the capabilities of the test and management of RDT negative cases, may both manage patient expectations and promote patient demand for test-driven diagnoses
Excitations are localized and relaxation is hierarchical in glass-forming liquids
For several atomistic models of glass formers, at conditions below their
glassy dynamics onset temperatures, , we use importance
sampling of trajectory space to study the structure, statistics and dynamics of
excitations responsible for structural relaxation. Excitations are detected in
terms of persistent particle displacements of length . At supercooled
conditions, for of the order of or smaller than a particle diameter, we
find that excitations are associated with correlated particle motions that are
sparse and localized, occupying a volume with an average radius that is
temperature independent and no larger than a few particle diameters. We show
that the statistics and dynamics of these excitations are facilitated and
hierarchical. Excitation energy scales grow logarithmically with .
Excitations at one point in space facilitate the birth and death of excitations
at neighboring locations, and space-time excitation structures are microcosms
of heterogeneous dynamics at larger scales. This nature of dynamics becomes
increasingly dominant as temperature is lowered. We show that slowing of
dynamics upon decreasing temperature below is the result of a
decreasing concentration of excitations and concomitant growing hierarchical
length scales, and further that the structural relaxation time follows
the parabolic law, , for , where , and
can be predicted quantitatively from dynamics at short time
scales. Particle motion is facilitated and directional, and we show this
becomes more apparent with decreasing . We show that stringlike motion is a
natural consequence of facilitated, hierarchical dynamics.Comment: 15 pages, 6 figures, + links to movies; To appear in Phys. Rev.
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The cleft ion fountain
Low-energy (below approximately 50 eV) ionospheric ions, injected into the magnetosphere at the dayside cleft, are studied using data from the retarding ion mass spectrometer (RIMS) experiment on the Dynamics Explorer 1 satellite. It is concluded that upwelling ions at the cleft form an ion fountain and are blown into the polar cap by antisunward convection. At high Kp (>4), convection is generally strong enough to fill the entire polar magnetosphere with low-energy O+ ions, whereas at low kp (<2) they are largely restricted to the dayside half of the cap. Using a two-dimensional kinetic ion trajectory model, the locations where RIMS detected O+ within the cap are shown to be consistent with the spatial distributions of O+ density, predicted for an upwelling ion source at the cleft and various dawn-dusk convection electric fields. A detailed study is made of one polar pass of DE 1, during which RIMS detected He+, N+, O+, and O++ ions, the ion trajectory model being used to trace all these ions back to a common source at an observed upwelling ion event near the cleft. All observed species are deduced to be falling earthward in the nightside of the cap, as predicted from the model, indicating the dominance of gravity over upward field-aligned acceleration (such as by the ambipolar electric field). Comparison of field-aligned velocities observed for O+ and O++ ions defines a maximum limit to the upward electrostatic acceleration present within the cap which was only sufficient to eject ionospheric H+ ions, all heavier ions being supplied from the dayside by the cleft ion fountain
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