15,985 research outputs found
Numerical simulation of the twist-grain-boundary phase of chiral liquid crystals
We study the core structure of the twist-grain-boundary (TGB) phase of chiral
liquid crystals by numerically minimizing the Landau-de Gennes free energy. We
analyze the morphology of layers at the grain boundary, to better understand
the mechanism of frustration between the smectic layer order and chirality. As
the chirality increases, the effective bending rigidity of layers is reduced
due to unlocking of the layer orientation and the director. This results in
large deviation of the layer morphology from that of Scherk's first minimal
surface and linear stack of screw dislocations (LSD).Comment: 4 pages and 6 figure
Linear-response theory of the longitudinal spin Seebeck effect
We theoretically investigate the longitudinal spin Seebeck effect, in which
the spin current is injected from a ferromagnet into an attached nonmagnetic
metal in a direction parallel to the temperature gradient. Using the fact that
the phonon heat current flows intensely into the attached nonmagnetic metal in
this particular configuration, we show that the sign of the spin injection
signal in the longitudinal spin Seebeck effect can be opposite to that in the
conventional transverse spin Seebeck effect when the electron-phonon
interaction in the nonmagnetic metal is sufficiently large. Our linear-response
approach can explain the sign reversal of the spin injection signal recently
observed in the longitudinal spin Seebeck effect.Comment: Proc. of ICM 2012 (Accepted for publication in J. Korean Phys. Soc.),
typos correcte
Chemical evolution of the bulge of M31: predictions about abundance ratios
We aim at reproducing the chemical evolution of the bulge of M31 by means of
a detailed chemical evolution model, including radial gas flows coming from the
disk. We study the impact of the initial mass function, the star formation rate
and the time scale for bulge formation on the metallicity distribution function
of stars. We compute several models of chemical evolution using the metallicity
distribution of dwarf stars as an observational constraint for the bulge of
M31. Then, by means of the model which best reproduces the metallicity
distribution function, we predict the [X/Fe] vs. [Fe/H] relations for several
chemical elements (O, Mg, Si, Ca, C, N). Our best model for the bulge of M31 is
obtained by means of a robust statistical method and assumes a Salpeter initial
mass function, a Schmidt-Kennicutt law for star formation with an exponent
k=1.5, an efficiency of star formation of , and an
infall timescale of Gyr. Our results suggest that the bulge
of M31 formed very quickly by means of an intense star formation rate and an
initial mass function flatter than in the solar vicinity but similar to that
inferred for the Milky Way bulge. The [/Fe] ratios in the stars of the
bulge of M31 should be high for most of the [Fe/H] range, as is observed in the
Milky Way bulge. These predictions await future data to be proven.Comment: Accepted for publication by MNRA
The Three Component Electronic Structure of the Cuprates Derived from SI-STM
We present a phenomenological model that describes the low energy electronic
structure of the cuprate high temperature superconductor Bi2Sr2CaCu2O8+x as
observed by Spectroscopic Imagining Scanning Tunneling Microscopy (SI-STM). Our
model is based on observations from Quasiparticle Interference (QPI)
measurements and Local Density of States (LDOS) measurements that span a range
of hole densities from critical doping, p~0.19, to extremely underdoped,
p~0.06. The model presented below unifies the spectral density of states
observed in QPI studies with that of the LDOS. In unifying these two separate
measurements, we find that the previously reported phenomena, the Bogoliubov
QPI termination, the checkerboard conductance modulations, and the pseudogap
are associated with unique energy scales that have features present in both the
q-space and LDOS(E) data sets
Asymmetric Supernovae from Magneto-Centrifugal Jets
Strong toroidal magnetic fields generated in stellar collapse can generate
magneto-centrifugal jets in analogy to those found in simulations of black hole
accretion and explain why all core collapse supernovae are found to be
substantially asymmetric and predominantly bi-polar. We describe two phases:
the initial LeBlanc-Wilson jet and a subsequent protopulsar or toroidal jet
that propagates at about the core escape velocity. The jets will produce bow
shocks that tend to expel matter, including iron and silicon, into equatorial
tori, accounting for observations of the element distribution in Cas A. A
magnetic ``switch'' mechanism may apply in instances of low density and large
magnetic field with subsequent increase in the speed and collimation of the
toroidal jet, depositing relatively little momentum. The result could be enough
infall to form a black hole with a third, highly relativistic jet that could
catch up to the protopulsar jet after it has emerged from the star. The
interaction of these two jets could generate internal shocks and explain the
presence of iron lines in the afterglow. Recent estimates that typical
gamma-ray burst energy is about 3x10^50 erg imply either a very low efficiency
for conversion of rotation into jets, or a rather rapid turnoff of the jet
process even though the black hole still rotates rapidly. Magnetars and
``hypernovae'' might arise in an intermediate parameter regime of energetic
jets that yield larger magnetic fields and provide more energy than the routine
case, but that are not so tightly collimated that they yield failed supernova.
(slightly abridged)Comment: AASTeX, 29 pages, 2 postscript figures, accepted by ApJ, November 20,
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