9,621 research outputs found
Pairing of 1-hexyl-3-methylimidazolium and tetrafluoroborate ions in n-pentanol
Molecular dynamics simulations are obtained and analyzed to study pairing of
1-hexyl-3-methylimidazolium and tetrafluoroborate ions in n-pentanol, in
particular by evaluating the potential-of-mean-force between counter ions. The
present molecular model and simulation accurately predicts the dissociation
constant Kd in comparison to experiment, and thus the behavior and magnitudes
for the ion-pair pmf at molecular distances, even though the dielectric
constant of the simulated solvent differs from the experimental value by about
30%. A naive dielectric model does not capture molecule structural effects such
as multiple conformations and binding geometries of the Hmim+ and BF4-
ion-pairs. Mobilities identify multiple time-scale effects in the
autocorrelation of the random forces on the ions, and specifically a slow,
exponential time-decay of those long-ranged forces associated here with
dielectric friction effects.Comment: 5 pages, 7 figures. V2: Figs. 4 & 7 redrawn for better visual clarity
with log-scales. No change in results. In press J. Chem. Phys. 201
Waves and instabilities in an anisotropic universe
The excitation of low frequency plasma waves in an expanding anisotropic cosmological model which contains a magnetic field frozen into the matter and pointing in the longitudinal direction is discussed. Using the exact equations governing finite-amplitude wave propagation in hydromagnetic media within the framework of general theory of relativity, we show that a spectrum of magnetized sound waves will be excited and form large scale ``\textit{damped oscillations}'' on the expanding universe. The characteristic frequency of the excited waves is slightly shifted away from the sound frequency and the shift depends on the strength of the primordial magnetic field. This magnetic field dependent shift may have an effect on the acoustic peaks of the CM
The Effects of Shortening the Distance between Hurdles during Training
The high hurdle events consists of succesive changes in cycling (sprinting) and non-cycling (hurdle clearing) movements.
This characteristic is the basic element of the events correct technique.
It is acceptable that in jumping events, the high performance athletes follow a distinct preparatory phase just before the actual jump in order to achieve a position that facilitates a better transition from running to jumping conditions (Nixdorf. 1983, Tsarouchas. 1986).
The purpose of the present study was to investigate shorter distances between the hurdles during training, as a means to improve the preparation phase of hurdle clearance
Scalar field induced oscillations of neutron stars and gravitational collapse
We study the interaction of massless scalar fields with self-gravitating
neutron stars by means of fully dynamic numerical simulations of the
Einstein-Klein-Gordon perfect fluid system. Our investigation is restricted to
spherical symmetry and the neutron stars are approximated by relativistic
polytropes. Studying the nonlinear dynamics of isolated neutron stars is very
effectively performed within the characteristic formulation of general
relativity, in which the spacetime is foliated by a family of outgoing light
cones. We are able to compactify the entire spacetime on a computational grid
and simultaneously impose natural radiative boundary conditions and extract
accurate radiative signals. We study the transfer of energy from the scalar
field to the fluid star. We find, in particular, that depending on the
compactness of the neutron star model, the scalar wave forces the neutron star
either to oscillate in its radial modes of pulsation or to undergo
gravitational collapse to a black hole on a dynamical timescale. The radiative
signal, read off at future null infinity, shows quasi-normal oscillations
before the setting of a late time power-law tail.Comment: 12 pages, 13 figures, submitted to Phys. Rev.
Invariant Killing spinors in 11D and type II supergravities
We present all isotropy groups and associated groups, up to discrete
identifications of the component connected to the identity, of spinors of
eleven-dimensional and type II supergravities. The groups are products
of a Spin group and an R-symmetry group of a suitable lower dimensional
supergravity theory. Using the case of SU(4)-invariant spinors as a paradigm,
we demonstrate that the groups, and so the R-symmetry groups of
lower-dimensional supergravity theories arising from compactifications, have
disconnected components. These lead to discrete symmetry groups reminiscent of
R-parity. We examine the role of disconnected components of the groups
in the choice of Killing spinor representatives and in the context of
compactifications.Comment: 22 pages, typos correcte
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