49,464 research outputs found
Nearly-logarithmic decay in the colloidal hard-sphere system
Nearly-logarithmic decay is identified in the data for the mean-squared
displacement of the colloidal hard-sphere system at the liquid-glass transition
[v. Megen et. al, Phys. Rev. E 58, 6073(1998)]. The solutions of mode-coupling
theory for the microscopic equations of motion fit the experimental data well.
Based on these equations, the nearly-logarithmic decay is explained as the
equivalent of a beta-peak phenomenon, a manifestation of the critical
relaxation when the coupling between of the probe variable and the density
fluctuations is strong. In an asymptotic expansion, a Cole-Cole formula
including corrections is derived from the microscopic equations of motion,
which describes the experimental data for three decades in time.Comment: 4 pages, 3 figure
Shear Viscosity of Quark Matter
We consider the shear viscosity of a system of quarks and its ratio to the
entropy density above the critical temperature for deconfinement. Both
quantities are derived and computed for different modeling of the quark
self-energy, also allowing for a temperature dependence of the effective mass
and width. The behaviour of the viscosity and the entropy density is argued in
terms of the strength of the coupling and of the main characteristics of the
quark self-energy. A comparison with existing results is also discussed.Comment: 15 pages, 4 figure
Warm Ice Giant GJ 3470b. I. A Flat Transmission Spectrum Indicates a Hazy, Low-methane, and/or Metal-rich Atmosphere
We report our spectroscopic investigation of the transiting ice giant GJ
3470b's atmospheric transmission, and the first results of extrasolar planet
observations from the new Keck/MOSFIRE spectrograph. We measure a planet/star
radius ratio of Rp/Rs = 0.0789 +/- 0.0020 in a bandpass from 2.09-2.36 micron
and in six narrower bands across this wavelength range. When combined with
existing broadband photometry, these measurements rule out cloud-free
atmospheres in chemical equilibrium assuming either solar abundances (5.4 sigma
confidence) or a moderate level of metal enrichment (50x solar abundances, 3.8
sigma), confirming previous results that such models are not representative for
cool, low-mass, externally irradiated extrasolar planets. Current measurements
are consistent with a flat transmission spectrum, which suggests that the
atmosphere is explained by high-altitude clouds and haze, disequilibrium
chemistry, unexpected abundance patterns, or the atmosphere is extremely
metal-rich (>200x solar). Because GJ 3470b's low bulk density sets an upper
limit on the planet's atmospheric enrichment of <300x solar, the atmospheric
mean molecular weight must be <9. Thus, if the atmosphere is cloud-free its
spectral features should be detectable with future observations. Transit
observations at shorter wavelengths will provide the best opportunity to
discriminate between plausible scenarios. We obtained optical spectroscopy with
the GMOS spectrograph, but these observations exhibit large systematic
uncertainties owing to thin, persistent cirrus conditions. Finally, we also
provide the first detailed look at the steps necessary for well-calibrated
MOSFIRE observations, and provide advice for future observations with this
instrument.Comment: Accepted to A&A. Light curves will be available at CDS (or download
arXiv tarball
Mesonic correlation functions at finite temperature and density in the Nambu-Jona-Lasinio model with a Polyakov loop
We investigate the properties of scalar and pseudo-scalar mesons at finite
temperature and quark chemical potential in the framework of the
Nambu-Jona-Lasinio (NJL) model coupled to the Polyakov loop (PNJL model) with
the aim of taking into account features of both chiral symmetry breaking and
deconfinement. The mesonic correlators are obtained by solving the
Schwinger-Dyson equation in the RPA approximation with the Hartree (mean field)
quark propagator at finite temperature and density. In the phase of broken
chiral symmetry a narrower width for the sigma meson is obtained with respect
to the NJL case; on the other hand, the pion still behaves as a Goldstone
boson. When chiral symmetry is restored, the pion and sigma spectral functions
tend to merge. The Mott temperature for the pion is also computed.Comment: 24 pages, 9 figures, version to appear in Phys. Rev.
How the Polyakov loop and the regularization affect strangeness and restoration of symmetries at finite T
The effects of the Polyakov loop and of a regularization procedure that
allows the presence of high momentum quark states at finite temperature is
investigated within the Polyakov-loop extended Nambu--Jona-Lasinio model. The
characteristic temperatures, as well as the behavior of observables that signal
deconfinement and restoration of chiral and axial symmetries, are analyzed,
paying special attention to the behavior of strangeness degrees of freedom. We
observe that the cumulative effects of the Polyakov loop and of the
regularization procedure contribute to a better description of the
thermodynamics, as compared with lattice estimations. We find a faster partial
restoration of chiral symmetry and the restoration of the axial symmetry
appears as a natural consequence of the full recovering of the chiral symmetry
that was dynamically broken. These results show the relevance of the effects of
the interplay among the Polyakov loop dynamics, the high momentum quark sates
and the restoration of the chiral and axial symmetries at finite temperature.Comment: Talk given at XIII International Conference on Hadron Spectroscopy
(Hadron 2009), Tallahassee, Florida, USA, 29 Nov - 4 Dec, 200
Are polar liquids less simple?
Strong correlation between equilibrium fluctuations of the potential energy,
U, and the virial, W, is a characteristic of a liquid that implies the presence
of certain dynamic properties, such as density scaling of the relaxation times
and isochronal superpositioning of the relaxation function. In this work we
employ molecular dynamics simulations (mds) on methanol and two variations,
lacking hydrogen bonds and a dipole moment, to assess the connection between
the correlation of U and W and these dynamic properties. We show, in accord
with prior results of others [T.S. Ingebrigtsen, T.B. Schroder, J.C. Dyre,
Phys. Rev. X 2, 011011 (2012).], that simple van der Waals liquids exhibit both
strong correlations and the expected dynamic behavior. However, for polar
liquids this correspondence breaks down - weaker correlation between U and W is
not associated with worse conformance to density scaling or isochronal
superpositioning. The reason for this is that strong correlation between U and
W only requires their proportionality, whereas the expected dynamic behavior
depends primarily on constancy of the proportionality constant for all state
points. For hydrogen-bonded liquids, neither strong correlation nor adherence
to the dynamic properties is observed; however, this nonconformance is not
directly related to the concentration of hydrogen bonds, but rather to the
greater deviation of the intermolecular potential from an inverse power law
(IPL). Only (hypothetical) liquids having interactions governed strictly by an
IPL are perfectly correlating and exhibit the consequent dynamic properties
over all thermodynamic conditions.Comment: 14 pages, 8 figure
Kinetic Theory for Electron Dynamics Near a Positive Ion
A theoretical description of time correlation functions for electron
properties in the presence of a positive ion of charge number Z is given. The
simplest case of an electron gas distorted by a single ion is considered. A
semi-classical representation with a regularized electron - ion potential is
used to obtain a linear kinetic theory that is asymptotically exact at short
times. This Markovian approximation includes all initial (equilibrium) electron
- electron and electron - ion correlations through renormalized pair
potentials. The kinetic theory is solved in terms of single particle
trajectories of the electron - ion potential and a dielectric function for the
inhomogeneous electron gas. The results are illustrated by a calculation of the
autocorrelation function for the electron field at the ion. The dependence on
charge number Z is shown to be dominated by the bound states of the effective
electron - ion potential. On this basis, a very simple practical representation
of the trajectories is proposed and shown to be accurate over a wide range
including strong electron - ion coupling. This simple representation is then
used for a brief analysis of the dielectric function for the inhomogeneous
electron gas.Comment: 30 pages, 5 figures, submitted to Journal of Statistical Mechanics:
Theory and Experimen
On the frequency and remnants of Hypernovae
Under the hypothesis that some fraction of massive stellar core collapses
give rise to unusually energetic events, termed hypernovae, I examine the
required rates assuming some fraction of such events yield gamma ray bursts. I
then discuss evidence from studies of pulsars and r-process nucleosynthesis
that independently suggests the existence of a class of unusually energetic
events. Finally I describe a scenario which links these different lines of
evidence as supporting the hypernova hypothesis.Comment: TeX, To appear in ApJ Letter
Tunable effective g-factor in InAs nanowire quantum dots
We report tunneling spectroscopy measurements of the Zeeman spin splitting in
InAs few-electron quantum dots. The dots are formed between two InP barriers in
InAs nanowires with a wurtzite crystal structure grown by chemical beam
epitaxy. The values of the electron g-factors of the first few electrons
entering the dot are found to strongly depend on dot size and range from close
to the InAs bulk value in large dots |g^*|=13 down to |g^*|=2.3 for the
smallest dots. These findings are discussed in view of a simple model.Comment: 4 pages, 3 figure
- …