4,661 research outputs found
Macroscopic approximation to relativistic kinetic theory from a nonlinear closure
We use a macroscopic description of a system of relativistic particles based
on adding a nonequilibrium tensor to the usual hydrodynamic variables. The
nonequilibrium tensor is linked to relativistic kinetic theory through a
nonlinear closure suggested by the Entropy Production Principle; the evolution
equation is obtained by the method of moments, and together with
energy-momentum conservation closes the system. Transport coefficients are
chosen to reproduce second order fluid dynamics if gradients are small. We
compare the resulting formalism to exact solutions of Boltzmann's equation in
0+1 dimensions and show that it tracks kinetic theory better than second order
fluid dynamics.Comment: v2: 6 two-column pages, 2 figures. Corrected typos and a numerical
error, and added reference
A hydrodynamic approach to QGP instabilities
We show that the usual linear analysis of QGP Weibel instabilities based on
the Maxwell-Boltzmann equation may be reproduced in a purely hydrodynamic
model. The latter is derived by the Entropy Production Variational Method from
a transport equation including collisions, and can describe highly
nonequilibrium flow. We find that, as expected, collisions slow down the growth
of Weibel instabilities. Finally, we discuss the strong momentum anisotropy
limit.Comment: 11 pages, no figures. v2: minor changes, added references. Accepted
in Phys. Rev.
Edge Effects in Finite Elongated Graphene Nanoribbons
We analyze the relevance of finite-size effects to the electronic structure
of long graphene nanoribbons using a divide and conquer density functional
approach. We find that for hydrogen terminated graphene nanoribbons most of the
physical features appearing in the density of states of an infinite graphene
nanoribbon are recovered at a length of 40 nm. Nevertheless, even for the
longest systems considered (72 nm long) pronounced edge effects appear in the
vicinity of the Fermi energy. The weight of these edge states scales inversely
with the length of the ribbon and they are expected to become negligible only
at ribbons lengths of the order of micrometers. Our results indicate that
careful consideration of finite-size and edge effects should be applied when
designing new nanoelectronic devices based on graphene nanoribbons. These
conclusions are expected to hold for other one-dimensional systems such as
carbon nanotubes, conducting polymers, and DNA molecules.Comment: 4 pages, 4 figure
Heavy quark collisional energy loss in the quark-gluon plasma including finite relaxation time
In this paper, we calculate the soft-collisional energy loss of heavy quarks
traversing the viscous quark-gluon plasma including the effects of a finite
relaxation time on the energy loss. We find that the collisional
energy loss depends appreciably on . In particular, for typical
values of the viscosity-to-entropy ratio, we show that the energy loss obtained
using = 0 can be 10 larger than the one obtained using
= 0. Moreover, we find that the energy loss obtained using the
kinetic theory expression for is much larger that the one obtained
with the derived from the Anti de Sitter/Conformal Field Theory
correspondence. Our results may be relevant in the modeling of heavy quark
evolution through the quark-gluon plasma.Comment: v2: 5 pages, 4 figures, added references. Accepted for publication in
Phys. Rev.
Enhanced Half-Metallicity in Edge-Oxidized Zigzag Graphene Nanoribbons
We present a novel comprehensive first-principles theoretical study of the
electronic properties and relative stabilities of edge-oxidized zigzag graphene
nanoribbons. The oxidation schemes considered include hydroxyl, carboxyl,
ether, and ketone groups. Using screened exchange density functional theory, we
show that these oxidized ribbons are more stable than hydrogen-terminated
nanoribbons except for the case of the etheric groups. The stable oxidized
configurations maintain a spin-polarized ground state with antiferromagnetic
ordering localized at the edges, similar to the fully hydrogenated
counterparts. More important, edge oxidation is found to lower the onset
electric field required to induce half-metallic behavior and extend the overall
field range at which the systems remain half-metallic. Once the half-metallic
state is reached, further increase of the external electric field intensity
produces a rapid decrease in the spin magnetization up to a point where the
magnetization is quenched completely. Finally, we find that oxygen containing
edge groups have a minor effect on the energy difference between the
antiferromagnetic ground state and the above-lying ferromagnetic state.Comment: 5 pages,5 figures, 1 tabl
"Frictions in financial and labor markets": a summary of the 35th Annual Economic Policy Conference
This article contains synopses of the papers presented at the 35th Annual Economic Policy Conference of the Federal Reserve Bank of St. Louis held October 21-22, 2010. The conference theme was “Frictions in Financial and Labor Markets.” Leading participants in this field presented their research and commentary.Labor market ; Financial markets
Magnetic Exchange Couplings from Noncollinear Spin Density Functional Perturbation Theory
We propose a method for the evaluation of magnetic exchange couplings based
on noncollinear spin-density functional calculations. The method employs the
second derivative of the total Kohn-Sham energy of a single reference state, in
contrast to approximations based on Kohn-Sham total energy differences. The
advantage of our approach is twofold: It provides a physically motivated
picture of the transition from a low-spin to a high-spin state, and it utilizes
a perturbation scheme for the evaluation of magnetic exchange couplings. The
latter simplifies the way these parameters are predicted using
first-principles: It avoids the non-trivial search for different spin-states
that needs to be carried out in energy difference methods and it opens the
possibility of "black-boxifying" the extraction of exchange couplings from
density functional theory calculations. We present proof of concept
calculations of magnetic exchange couplings in the H--He--H model system and in
an oxovanadium bimetallic complex where the results can be intuitively
rationalized.Comment: J.Chem. Phys. (accepted
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