8,767 research outputs found
Chiral 3-exchange NN-potentials: Results for dominant next-to-leading order contributions
We calculate in (two-loop) chiral perturbation theory the local NN-potentials
generated by the three-pion exchange diagrams with one insertion from the
second order chiral effective pion-nucleon Lagrangian proportional to the
low-energy constants . The resulting isoscalar central potential
vanishes identically. In most cases these -exchange potentials are larger
than the ones generated by the diagrams involving only leading order vertices
due to the large values of (which mainly represent virtual
-excitation). A similar feature has been observed for the chiral
-exchange. We also give suitable (double-integral) representations for
the spin-spin and tensor potentials generated by the leading-order diagrams
proportional to involving four nucleon propagators. In these cases the
Cutkosky rule cannot be used to calculate the spectral-functions in the
infinite nucleon mass limit since the corresponding mass-spectra start with a
non-vanishing value at the -threshold. Altogether, one finds that chiral
-exchange leads to small corrections in the region fm where
- and chiral -exchange alone provide a very good strong NN-force as
shown in a recent analysis of the low-energy pp-scattering data-base.Comment: 11 pages, 7 figures, to be published in The Physical Review
Chiral -exchange NN-potentials: Results for diagrams proportional to g_A^4 and g_A^6
We calculate in (two-loop) chiral perturbation theory the local NN-potentials
generated by the three-pion exchange diagrams proportional to g_A^4 and g_A^6.
Surprisingly, we find that the total isoscalar central -exchange
potential vanishes identically. The individually largest -exchange
potentials are of isoscalar spin-spin, isovector central and isoscalar tensor
type. For these potentials simple analytical expressions can be given. The
strength of these dominant -exchange potentials at r=1.0 fm is 4.6 MeV,
2.9 MeV and 1.4 MeV, respectively. Furthermore, we observe that the spin-spin
and tensor potentials due to the diagrams proportional to g_A^6 do not exist in
the infinite nucleon mass limit.Comment: 8 pages, 5 figure
Chiral -exchange NN-potentials: Two-loop contributions
We calculate in heavy baryon chiral perturbation theory the local
NN-potentials generated by the two-pion exchange diagrams at two-loop order. We
give explicit expressions for the mass-spectra (or imaginary parts) of the
corresponding isoscalar and isovector central, spin-spin and tensor
NN-amplitudes. We find from two-loop two-pion exchange a sizeable isoscalar
central repulsion which amounts to MeV at fm. There is a
similarly strong isovector central attraction which however originates mainly
from the third order low energy constants entering the chiral -scattering amplitude. We also evaluate the one-loop -exchange diagram
with two second order chiral -vertices proportional to the low
energy constants as well as the first relativistic 1/M-correction
to the -exchange diagrams with one such vertex. The diagrammatic results
presented here are relevant components of the chiral NN-potential at
next-to-next-to-next-to-leading order.Comment: 6 pages, 2 figure
From Point Defects in Graphene to Two-Dimensional Amorphous Carbon
While crystalline two-dimensional materials have become an experimental
reality during the past few years, an amorphous 2-D material has not been
reported before. Here, using electron irradiation we create an sp2-hybridized
one-atom-thick flat carbon membrane with a random arrangement of polygons,
including four-membered carbon rings. We show how the transformation occurs
step-by-step by nucleation and growth of low-energy multi-vacancy structures
constructed of rotated hexagons and other polygons. Our observations, along
with first-principles calculations, provide new insights to the bonding
behavior of carbon and dynamics of defects in graphene. The created domains
possess a band gap, which may open new possibilities for engineering
graphene-based electronic devices.Comment: 10 pages, 10 figures including supplementary informatio
Comparisons of spectra determined using detector atoms and spatial correlation functions
We show how two level atoms can be used to determine the local time dependent
spectrum. The method is applied to a one dimensional cavity. The spectrum
obtained is compared with the mode spectrum determined using spatially filtered
second order correlation functions. The spectra obtained using two level atoms
give identical results with the mode spectrum. One benefit of the method is
that only one time averages are needed. It is also more closely related to a
realistic measurement scheme than any other definition of a time dependent
spectrum.Comment: 8 pages, 8 figure
Onsager's Wien Effect on a Lattice
The Second Wien Effect describes the non-linear, non-equilibrium response of
a weak electrolyte in moderate to high electric fields. Onsager's 1934
electrodiffusion theory along with various extensions has been invoked for
systems and phenomena as diverse as solar cells, surfactant solutions, water
splitting reactions, dielectric liquids, electrohydrodynamic flow, water and
ice physics, electrical double layers, non-Ohmic conduction in semiconductors
and oxide glasses, biochemical nerve response and magnetic monopoles in spin
ice. In view of this technological importance and the experimental ubiquity of
such phenomena, it is surprising that Onsager's Wien effect has never been
studied by numerical simulation. Here we present simulations of a lattice
Coulomb gas, treating the widely applicable case of a double equilibrium for
free charge generation. We obtain detailed characterisation of the Wien effect
and confirm the accuracy of the analytical theories as regards the field
evolution of the free charge density and correlations. We also demonstrate that
simulations can uncover further corrections, such as how the field-dependent
conductivity may be influenced by details of microscopic dynamics. We conclude
that lattice simulation offers a powerful means by which to investigate
system-specific corrections to the Onsager theory, and thus constitutes a
valuable tool for detailed theoretical studies of the numerous practical
applications of the Second Wien Effect.Comment: Main: 12 pages, 4 figures. Supplementary Information: 7 page
Determination of the chiral coupling constants c3 and c4 in new pp and np partial-wave analyses
As a first result of two new partial-wave analyses, one of the pp and another
one of the np scattering data below 500 MeV, we report a study of the
long-range chiral two-pion exchange interaction which contains the chiral
coupling constants c1, c3, and c4. By using as input a theoretical value for c1
we are able to determine in pp as well as in np scattering accurate values for
c3 and c4. The values determined from the pp data and independently from the np
data are in very good agreement, indicating the correctness of the chiral
two-pion exchange interaction. The weighted averages are c3 = -4.78(10) / GeV
and c4 = 3.96(22) / GeV, where the errors are statistical. The value of c3 is
best determined from the pp data and that of c4 from the np data.Comment: 9 pages, 1 figure. Accepted for publication in Phys. Rev.
Effects of Galaxy Formation on Thermodynamics of the Intracluster Medium
We present detailed comparisons of the intracluster medium (ICM) in
cosmological Eulerian cluster simulations with deep Chandra observations of
nearby relaxed clusters. To assess the impact of galaxy formation, we compare
two sets of simulations, one performed in the non-radiative regime and another
with radiative cooling and several physical processes critical to various
aspects of galaxy formation: star formation, metal enrichment and stellar
feedback. We show that the observed ICM properties outside cluster cores are
well-reproduced in the simulations that include cooling and star formation,
while the non-radiative simulations predict an overall shape of the ICM
profiles inconsistent with observations. In particular, we find that the ICM
entropy in our runs with cooling is enhanced to the observed levels at radii as
large as half of the virial radius. We also find that outside cluster cores
entropy scaling with the mean ICM temperature in both simulations and Chandra
observations is consistent with being self-similar within current error bars.
We find that the pressure profiles of simulated clusters are also close to
self-similar and exhibit little cluster-to-cluster scatter. The X-ray
observable-total mass relations for our simulated sample agree with the Chandra
measurements to \~10%-20% in normalization. We show that this systematic
difference could be caused by the subsonic gas motions, unaccounted for in
X-ray hydrostatic mass estimates. The much improved agreement of simulations
and observations in the ICM profiles and scaling relations is encouraging and
the existence of tight relations of X-ray observables, such as Yx, and total
cluster mass and the simple redshift evolution of these relations hold promise
for the use of clusters as cosmological probes.Comment: 14 pages, 6 figures. Matches version accepted to Ap
The large-scale jet-powered radio nebula of Circinus X-1
We present multi-epoch observations of the radio nebula around the neutron
star X-ray binary Circinus X-1 made at 1.4 and 2.5 GHz with the Australia
Telescope Compact Array between October 2000 and September 2004. The nebula can
be seen as a result of the interaction between the jet from the system and the
interstellar medium and it is likely that we are actually looking toward the
central X-ray binary system through the jet-powered radio lobe. The study of
the nebula thus offers a unique opportunity to estimate for the first time
using calorimetry the energetics of a jet from an object clearly identified as
a neutron star. An extensive discussion on the energetics of the complex is
presented: a first approach is based on the minimum energy estimation, while a
second one employs a self-similar model of the interaction between the jets and
the surrounding medium. The results suggest an age for the nebula of \leq 10^5
years and a corresponding time-averaged jet power \geq 10^{35} erg s^{-1}.
During periodic flaring episodes, the instantaneous jet power may reach values
of similar magnitude to the X-ray luminosity.Comment: Accepted to MNRA
- …