2,356 research outputs found
Double- Order in a Frustrated Random Spin System
We use the three-dimensional Heisenberg model with site randomness as an
effective model of the compound Sr(FeMn)O. The model consists
of two types of ions that correspond to Fe and Mn ions. The nearest-neighbor
interactions in the ab-plane are antiferromagnetic. The nearest-neighbor
interactions along the c-axis between Fe ions are assumed to be
antiferromagnetic, whereas other interactions are assumed to be ferromagnetic.
From Monte Carlo simulations, we confirm the existence of the
double- ordered phase characterized by two wave numbers,
and . We also identify the spin ordering pattern in
the double- ordered phase.Comment: 5pages, 3figure
Dissipation-induced pure Gaussian state
This paper provides some necessary and sufficient conditions for a
generalMarkovian Gaussian master equation to have a unique pure steady state.
The conditions are described by simple matrix equations; thus the so-called
environment engineering problem for pure-Gaussian-state preparation can be
straightforwardly dealt with in the linear algebraic framework. In fact, based
on one of those conditions, for an arbitrary given pure Gaussian state,we
obtain a complete parametrization of the Gaussian master equation having that
state as a unique steady state; this leads to a systematic procedure for
engineering a desired dissipative system.We demonstrate some examples including
Gaussian cluster states.Comment: 8 page
Strong-coupling expansion for the momentum distribution of the Bose Hubbard model with benchmarking against exact numerical results
A strong-coupling expansion for the Green's functions, self-energies and
correlation functions of the Bose Hubbard model is developed. We illustrate the
general formalism, which includes all possible inhomogeneous effects in the
formalism, such as disorder, or a trap potential, as well as effects of thermal
excitations. The expansion is then employed to calculate the momentum
distribution of the bosons in the Mott phase for an infinite homogeneous
periodic system at zero temperature through third-order in the hopping. By
using scaling theory for the critical behavior at zero momentum and at the
critical value of the hopping for the Mott insulator to superfluid transition
along with a generalization of the RPA-like form for the momentum distribution,
we are able to extrapolate the series to infinite order and produce very
accurate quantitative results for the momentum distribution in a simple
functional form for one, two, and three dimensions; the accuracy is better in
higher dimensions and is on the order of a few percent relative error
everywhere except close to the critical value of the hopping divided by the
on-site repulsion. In addition, we find simple phenomenological expressions for
the Mott phase lobes in two and three dimensions which are much more accurate
than the truncated strong-coupling expansions and any other analytic
approximation we are aware of. The strong-coupling expansions and scaling
theory results are benchmarked against numerically exact QMC simulations in two
and three dimensions and against DMRG calculations in one dimension. These
analytic expressions will be useful for quick comparison of experimental
results to theory and in many cases can bypass the need for expensive numerical
simulations.Comment: 48 pages 14 figures RevTe
Supernova Explosions in the Early Universe: Evolution of Radiative Remnants and the Halo Destruction Efficiency
We study the evolution of supernova (SN) remnants of the first stars, taking
proper account of the radiative feedback of the progenitor stars on the
surroundings. We carry out a series of one-dimensional hydrodynamic simulations
with radiative cooling, starting from initial configurations that are drawn
from the results of our earlier radiation hydrodynamic simulations of the first
HII regions. In low-mass (< 10^6 M_sun) halos, the stellar radiation
significantly reduces the ambient gas density prior to the SN explosion. The
blastwave quickly propagates over the halo's virial radius, leading to complete
evacuation of the gas even with the input energy of 10^50 erg. We find that a
large fraction of the remnant's thermal energy is lost in 0.1-10 Myr by line
cooling, whereas, for larger explosion energies, the remnant expands even more
rapidly with decreasing interior density, and cools predominantly via inverse
Compton process. In higher mass halos, the gas density near the explosion site
remains high and the SN shock is heavily confined; the thermal energy of the
remnant is quickly radiated away by free-free emission, even if the total input
energy exceeds the binding energy of halos by two orders of magnitude. We show
that the efficiency of halo destruction is determined not only by the explosion
energy but also by the gas density profile, and thus controlled by radiative
feedback prior to the explosion. Several implications of our results for the
formation of first quasars and second-generation stars in the universe are also
discussed.Comment: 13 pages, 11 embedded figures. Accepted for publication in Ap
Formation of Massive Primordial Stars in a Reionized Gas
We use cosmological hydrodynamic simulations with unprecedented resolution to
study the formation of primordial stars in an ionized gas at high redshifts.
Our approach includes all the relevant atomic and molecular physics to follow
the thermal evolution of a prestellar gas cloud to very high densities of
~10^{18} cm^{-3}. We locate a star-forming gas cloud within a reionized region
in our cosmological simulation. The first run-away collapse is triggered when
the gas cloud's mass is ~40 Msun. We show that the cloud core remains stable
against chemo-thermal instability and also against gravitational deformation
throughout its evolution. Consequently, a single proto-stellar seed is formed,
which accretes the surrounding hot gas at the rate ~10^{-3} Msun/year. We carry
out proto-stellar evolution calculations using the inferred accretion rate. The
resulting mass of the star when it reaches the zero-age main sequence is M_ZAMS
~40 Msun. We argue that, since the obtained M_ZAMS is as large as the mass of
the collapsing parent cloud, the final stellar mass should be close to this
value. Such massive, rather than exceptionally massive, primordial stars are
expected to cause early chemical enrichment of the Universe by exploding as
black hole-forming super/hypernovae, and may also be progenitors of high
redshift gamma-ray bursts. The elemental abundance patterns of recently
discovered hyper metal-poor stars suggest that they might have been born from
the interstellar medium that was metal-enriched by supernovae of these massive
primordial stars.Comment: Revised version. To appear in ApJ
Analysis of scale-free networks based on a threshold graph with intrinsic vertex weights
Many real networks are complex and have power-law vertex degree distribution,
short diameter, and high clustering. We analyze the network model based on
thresholding of the summed vertex weights, which belongs to the class of
networks proposed by Caldarelli et al. (2002). Power-law degree distributions,
particularly with the dynamically stable scaling exponent 2, realistic
clustering, and short path lengths are produced for many types of weight
distributions. Thresholding mechanisms can underlie a family of real complex
networks that is characterized by cooperativeness and the baseline scaling
exponent 2. It contrasts with the class of growth models with preferential
attachment, which is marked by competitiveness and baseline scaling exponent 3.Comment: 5 figure
The Era of Massive Population III Stars: Cosmological Implications and Self-Termination
The birth and death of the first generation of stars have important
implications for the thermal state and chemical properties of the intergalactic
medium (IGM) in the early universe. Sometime after recombination, the neutral,
chemically pristine gas was reionized by ultraviolet photons emitted from the
first stars, but also enriched with heavy elements when these stars ended their
lives as energetic supernovae. Using the results from previous high-resolution
cosmological simulations of early structure formation that include radiative
transfer, we show that a significant volume fraction of the IGM can be
metal-polluted, as well as ionized, by massive Population III stars formed in
small-mass (10^6-10^7 Msun) halos early on. If most of the early generation
stars die as pair-instability supernovae with energies up to 10^{53} ergs, the
volume-averaged mean metallicity will quickly reach Z ~ 10^{-4}Zsun by a
redshift of 15-20, possibly causing a prompt transition to the formation of a
stellar population that is dominated by low-mass stars. In this scenario, the
early chemical enrichment history should closely trace the reionization history
of the IGM, and the end of the Population III era is marked by the completion
of reionization and pre-enrichment by z=15. We conclude that, while the
pre-enrichment may partially account for the ``metallicity-floor'' in
high-redshift Lyman-alpha clouds, it does not significantly affect the
elemental abundance in the intracluster medium.Comment: Version accepted by ApJ. Minor revisions and a few citations adde
Fast and stable method for simulating quantum electron dynamics
A fast and stable method is formulated to compute the time evolution of a
wavefunction by numerically solving the time-dependent Schr{\"o}dinger
equation. This method is a real space/real time evolution method implemented by
several computational techniques such as Suzuki's exponential product, Cayley's
form, the finite differential method and an operator named adhesive operator.
This method conserves the norm of the wavefunction, manages periodic conditions
and adaptive mesh refinement technique, and is suitable for vector- and
parallel-type supercomputers. Applying this method to some simple electron
dynamics, we confirmed the efficiency and accuracy of the method for simulating
fast time-dependent quantum phenomena.Comment: 10 pages, 35 eps figure
A renormalization procedure for tensor models and scalar-tensor theories of gravity
Tensor models are more-index generalizations of the so-called matrix models,
and provide models of quantum gravity with the idea that spaces and general
relativity are emergent phenomena. In this paper, a renormalization procedure
for the tensor models whose dynamical variable is a totally symmetric real
three-tensor is discussed. It is proven that configurations with certain
Gaussian forms are the attractors of the three-tensor under the renormalization
procedure. Since these Gaussian configurations are parameterized by a scalar
and a symmetric two-tensor, it is argued that, in general situations, the
infrared dynamics of the tensor models should be described by scalar-tensor
theories of gravity.Comment: 20 pages, 3 figures, references added, minor correction
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