5,295 research outputs found
Hybrid stars with reactive interfaces: analysis within the Nambu-Jona-Lasinio model
It has been shown recently that quark-hadron conversions at the interface of
a hybrid star may have a key role on the dynamic stability of the compact
object. In this work we perform a systematic study of hybrid stars with
reactive interfaces using a model-agnostic piecewise-polytropic hadronic
equation of state and the Nambu-Jona-Lasinio model for three-flavor quark
matter. For the hadronic phase we use a soft, an intermediate and a stiff
parametrization that match at {with predictions} based on chiral
effective field theory (cEFT) interactions. In the NJL Lagrangian we include
scalar, vector and 't Hooft interactions. The vector coupling constant
is treated as a free parameter. We also consider that there is a split between
the deconfinement and the chiral phase transitions which is controlled by
changing the conventional value of the vacuum pressure in the NJL
thermodynamic potential by , being
a free parameter. We analyze the mass-radius (-)
relation in the case of rapid () and slow () conversions, being the reaction timescale. In the
case of slow interface reactions we find - curves with a cusp at the
maximum mass point where a pure hadronic branch and a slow-stable hybrid star
(SSHS) branch coincide. We find that the length of the slow-stable branch grows
with the increase of the transition density and the energy density jump at the
hadron-quark interface. We calculate the tidal deformabilities of SSHSs and
analyse them in the light of the GW170817 event.Comment: 12 pages, 4 figures, to be published in Physical Review
Hyperelastic cloaking theory: Transformation elasticity with pre-stressed solids
Transformation elasticity, by analogy with transformation acoustics and
optics, converts material domains without altering wave properties, thereby
enabling cloaking and related effects. By noting the similarity between
transformation elasticity and the theory of incremental motion superimposed on
finite pre-strain it is shown that the constitutive parameters of
transformation elasticity correspond to the density and moduli of
small-on-large theory. The formal equivalence indicates that transformation
elasticity can be achieved by selecting a particular finite (hyperelastic)
strain energy function, which for isotropic elasticity is semilinear strain
energy. The associated elastic transformation is restricted by the requirement
of statically equilibrated pre-stress. This constraint can be cast as \tr
{\mathbf F} = constant, where is the deformation gradient,
subject to symmetry constraints, and its consequences are explored both
analytically and through numerical examples of cloaking of anti-plane and
in-plane wave motion.Comment: 20 pages, 5 figure
Controlling the exchange interaction using the spin-flip transition of antiferromagnetic spins in NiFe / -FeO
We report studies of exchange bias and coercivity in ferromagnetic
NiFe layers coupled to antiferromagnetic (AF) (0001),
(110), and (112) -FeO layers. We show that AF
spin configurations which permit spin-flop coupling give rise to a strong
uniaxial anisotropy and hence a large coercivity, and that by annealing in
magnetic fields parallel to specific directions in the AF we can control either
coercivity or exchange bias. In particular, we show for the first time that a
reversible temperature-induced spin reorientation in the AF can be used to
control the exchange interaction.Comment: 15 pages, 5 figures, submitted to Phys. Rev. Let
How light can the lightest neutralino be?
In this talk we summarize previous work on mass bounds of a light neutralino
in the Minimal Supersymmetric Standard Model. We show that without the GUT
relation between the gaugino mass parameters M_1 and M_2, the mass of the
lightest neutralino is essentially unconstrained by collider bounds and
precision observables. We conclude by considering also the astrophysics and
cosmology of a light neutralino.Comment: 6 pages, 3 figures, to appear in the proceedings of the 16th
International Symposium on Particles, Strings and Cosmology (PASCOS2010),
Valencia (Spain), July 19th - 23rd, 201
Enhancement of near-cloaking. Part II: the Helmholtz equation
The aim of this paper is to extend the method of improving cloaking
structures in the conductivity to scattering problems. We construct very
effective near-cloaking structures for the scattering problem at a fixed
frequency. These new structures are, before using the transformation optics,
layered structures and are designed so that their first scattering coefficients
vanish. Inside the cloaking region, any target has near-zero scattering cross
section for a band of frequencies. We analytically show that our new
construction significantly enhances the cloaking effect for the Helmholtz
equation.Comment: 16pages, 12 fugure
Dissipative Dynamics of Collisionless Nonlinear Alfven Wave Trains
The nonlinear dynamics of collisionless Alfven trains, including resonant
particle effects is studied using the kinetic nonlinear Schroedinger (KNLS)
equation model. Numerical solutions of the KNLS reveal the dynamics of Alfven
waves to be sensitive to the sense of polarization as well as the angle of
propagation with respect to the ambient magnetic field. The combined effects of
both wave nonlinearity and Landau damping result in the evolutionary formation
of stationaryOA S- and arc-polarized directional and rotational
discontinuities. These waveforms are freqently observed in the interplanetary
plasma.Comment: REVTeX, 6 pages (including 5 figures). This and other papers may be
found at http://sdphpd.ucsd.edu/~medvedev/papers.htm
Transformation elastodynamics and active exterior acoustic cloaking
This chapter consists of three parts. In the first part we recall the
elastodynamic equations under coordinate transformations. The idea is to use
coordinate transformations to manipulate waves propagating in an elastic
material. Then we study the effect of transformations on a mass-spring network
model. The transformed networks can be realized with "torque springs", which
are introduced here and are springs with a force proportional to the
displacement in a direction other than the direction of the spring terminals.
Possible homogenizations of the transformed networks are presented, with
potential applications to cloaking. In the second and third parts we present
cloaking methods that are based on cancelling an incident field using active
devices which are exterior to the cloaked region and that do not generate
significant fields far away from the devices. In the second part, the exterior
cloaking problem for the Laplace equation is reformulated as the problem of
polynomial approximation of analytic functions. An explicit solution is given
that allows to cloak larger objects at a fixed distance from the cloaking
device, compared to previous explicit solutions. In the third part we consider
the active exterior cloaking problem for the Helmholtz equation in 3D. Our
method uses the Green's formula and an addition theorem for spherical outgoing
waves to design devices that mimic the effect of the single and double layer
potentials in Green's formula.Comment: Submitted as a chapter for the volume "Acoustic metamaterials:
Negative refraction, imaging, lensing and cloaking", Craster and Guenneau
ed., Springe
X-ray Photoemission Study of MgB2
A c-axis oriented thin film and a high density sintered pellet of MgB2 have
been studied by x-ray photoemission spectroscopy, and compared to measurements
from MgO and MgF2 single crystals. The as-grown surface has a layer which is
Mg-rich and oxidized, which is effectively removed by a nonaqueous etchant. The
subsurface region of the pellet is Mg-deficient. This nonideal near-surface
region may explain varied scanning tunneling spectroscopy results. The MgB2
core level and Auger signals are similar to measurements from metallic Mg and
transition metal diborides, and the measured valence band is consistent with
the calculated density of states.Comment: 19 pages, 6 figures; Submitted to Phys. Rev. B; added references and
new data on film
Genome-wide association study of regional brain volume suggests involvement of known psychiatry candidate genes, identified new candidates for psychiatric disorders and points to potential modes of their action
Though most psychiatric disorders are highly heritable, it has been hard to identify genetic risk factors involved, which are most likely of small individual effect size. A possible way to aid identification of risk genes is the use of intermediate phenotypes. These are supposed to be closer to the biological substrate(s) of the disorder than psychiatric diagnoses, and therefore less genetically complex. Intermediate phenotypes can be defined e. g. at the level of brain function and of regional brain structure. Both are highly heritable, and regional brain structure is linked to brain function. Within the Brain Imaging Genetics (BIG) study at the Radboud University Nijmegen (Medical Centre) we performed a genome-wide association study (GWAS) in 1000 of the currently 1400 healthy study participants. For all BIG participants, structural MRI brain images were available. Gray and white matter volumes were determined by brain segmentation using SPM software. FSL-FIRST was used to assess volumes of specific brain structures. Genotyping was performed on Affymetrix 6.0 arrays. Results implicate known candidates from earlier GWAS and candidate gene studies in mental disorders in the regulation of regional brain structure. E. g. polymorphisms in CDH13, featuring among the top-findings of GWAS in disorders including ADHD, addiction and schizophrenia, were found associated with amygdala volume. The ADHD candidate gene SNAP25 was found associated with total brain volume. In conclusion, the use of intermediate phenotypes based on (subcortical) brain volumes may shed more light on pathways from genes to diseases, but can also be expected to facilitate gene identification in psychiatric disorders
The Effect of Proton Temperature Anisotropy on the Solar Minimum Corona and Wind
A semi-empirical, axisymmetric model of the solar minimum corona is developed
by solving the equations for conservation of mass and momentum with prescribed
anisotropic temperature distributions. In the high-latitude regions, the proton
temperature anisotropy is strong and the associated mirror force plays an
important role in driving the fast solar wind; the critical point where the
outflow velocity equals the parallel sound speed is reached already at 1.5 Rsun
from Sun center. The slow wind arises from a region with open field lines and
weak anisotropy surrounding the equatorial streamer belt. The model parameters
were chosen to reproduce the observed latitudinal extent of the equatorial
streamer in the corona and at large distance from the Sun. We find that the
magnetic cusp of the closed-field streamer core lies at about 1.95 Rsun. The
transition from fast to slow wind is due to a decrease in temperature
anisotropy combined with the non-monotonic behavior of the non-radial expansion
factor in flow tubes that pass near the streamer cusp. In the slow wind, the
plasma beta is of order unity and the critical point lies at about 5 Rsun, well
beyond the magnetic cusp. The predicted outflow velocities are consistent with
OVI Doppler dimming measurements from UVCS/SOHO. We also find good agreement
with polarized brightness (pB) measurements from LASCO/SOHO and HI Ly-alpha
images from UVCS/SOHO.Comment: 36 pages, 13 figures. AAS LaTeX Macros v5.0. To appear in The
Astrophysical Journal, Vol. 598, No. 2, Issue December 1, 200
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