1,495 research outputs found
A self-consistent first-principles calculation scheme for correlated electron systems
A self-consistent calculation scheme for correlated electron systems is
created based on the density-functional theory (DFT). Our scheme is a
multi-reference DFT (MR-DFT) calculation in which the electron charge density
is reproduced by an auxiliary interacting Fermion system. A short-range
Hubbard-type interaction is introduced by a rigorous manner with a residual
term for the exchange-correlation energy. The Hubbard term is determined
uniquely by referencing the density fluctuation at a selected localized
orbital. This strategy to obtain an extension of the Kohn-Sham scheme provides
a self-consistent electronic structure calculation for the materials design.
Introducing an approximation for the residual exchange-correlation energy
functional, we have the LDA+U energy functional. Practical self-consistent
calculations are exemplified by simulations of Hydrogen systems, i.e. a
molecule and a periodic one-dimensional array, which is a proof of existence of
the interaction strength U as a continuous function of the local fluctuation
and structural parameters of the system.Comment: 23 pages, 8 figures, to appear in J. Phys. Condens. Matte
Updated constraint on a primordial magnetic field during big bang nucleosynthesis and a formulation of field effects
A new upper limit on the amplitude of primordial magnetic field (PMF) is
derived by a comparison between a calculation of elemental abundances in big
bang nucleosynthesis (BBN) model and the latest observational constraints on
the abundances. Updated nuclear reaction rates are adopted in the calculation.
Effects of PMF on the abundances are consistently taken into account in the
numerical calculation with the precise formulation of changes in physical
variables. We find that abundances of 3He and 6Li increase while that of 7Li
decreases when the PMF amplitude increases, in the case of the baryon-to-photon
ratio determined from the measurement of cosmic microwave background radiation.
We derive a constraint on the present amplitude of PMF, i.e., B(0)<1.5 micro G
[corresponding to the amplitude less than 2.0x10^{11} G at BBN temperature of
T=10^9 K] based on the rigorous calculation.Comment: 26 pages, 4 figures, new observation of D/H ratio adopted, tighter
constraint derived, Sec. IV modified, accepted for publication in PR
Theorems on ground-state phase transitions in Kohn-Sham models given by the Coulomb density functional
Some theorems on derivatives of the Coulomb density functional with respect
to the coupling constant are given. Consider an electron density
given by a ground state. A model Fermion system with the
reduced coupling constant, , is defined to reproduce and the ground state energy. Fixing the charge density, possible phase
transitions as level crossings detected in a value of the reduced density
functional happen only at discrete points along the axis. If the
density is -representable also for , accumulation of phase
transition points is forbidden when . Relevance of the
theorems for the multi-reference density functional theory is discussed.Comment: 19 page
Spin-twist driven persistent current in a strongly correlated two-dimensional electron system: a manifestation of the gauge field
A persistent current, coupled with the spin state, of purely many-body origin
is shown to exist in Nagaoka's ferromagnetic state in two dimensions (2D). This
we regard as a manifestation of a gauge field, which comes from the surrounding
spin configuration and acts on the hole motion, being coupled to the
Aharonov-Bohm flux. This provides an example where the electron-electron
interaction exerts a profound effect involving the spins in clean
two-dimensional lattice systems in sharp contrast to continuum or spinless
fermion systems.Comment: 11 pages, typeset using Revtex 3.0, Phys. Rev. B in press, 2 figures
available upon request at [email protected]
Pregalactic LiBeB Production by Supernova Cosmic Rays
I calculate the evolution of Be and B abundances produced by cosmic rays
generated by massive stars in the pregalactic phase of the universe. The inputs
for calculation, i.e. the star formation rate and the nuclear abundances of
cosmic rays, which I assume to be the same as those of the ISM, are taken from
the results of a detailed cosmic chemical evolution model with its parameters
best fitted from several items of observational information including an early
reionization of the IGM by . I found that when the Li plateau
abundance observed in metal-poor halo stars originated in the pregalactic
cosmological cosmic ray nucleosynthesis, Be and B simultaneously produced with
Li amount to the lowest levels ever detected in metal-poor halo stars. It
is desirable to observe Be and B abundances in metal-poor halo stars with
[Fe/H] in order to elucidate the possibility of early LiBeB
production by pregalactic supernova cosmic ray nucleosynthesis.Comment: 13 pages, 8 figures, ApJ accepte
JHKs time-series observations of a few ultracool dwarfs
The M8.5 object SSSPM J0109−5101 has recently been shown to be both a periodic and a
flaring variable, based on optical observations in the extreme red. More than 16 h of monitoring
in the near-infrared (NIR) reported here failed to show any variability. Similarly, no NIR
variability could be detected in intensive monitoring of three other suspected optical variables.
This paper also reports on photometry of half a dozen targets monitored over a few weeks, and
on the comparison of intensive monitoring at different epochs. In only one case, that of the
T dwarf binary Indi Bab, is there good evidence for variability. Our results allow stringent
limits to be placed on the NIR variability levels in a large sample of ultracool dwarfs.Web of Scienc
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