1,859 research outputs found
Absence of long-range superconducting correlations in the frustrated 1/2-filled band Hubbard model
We present many-body calculations of superconducting pair-pair correlations
in the ground state of the half-filled band Hubbard model on large anisotropic
triangular lattices. Our calculations cover nearly the complete range of
anisotropies between the square and isotropic triangular lattice limits. We
find that the superconducting pair-pair correlations decrease monotonically
with increasing onsite Hubbard interaction U for inter-pair distances greater
than nearest neighbor. For the large lattices of interest here the distance
dependence of the correlations approaches that for noninteracting electrons.
Both these results are consistent with the absence of superconductivity in this
model in the thermodynamic limit. We conclude that the effective 1/2-filled
band Hubbard model, suggested by many authors to be appropriate for the
kappa-(BEDT-TTF)-based organic charge-transfer solids, does not explain the
superconducting transition in these materials.Comment: 9 pages, 7 figures. Revised version to appear in Phys. Rev.
Symmetry-Adapted Phonon Analysis of Nanotubes
The characteristics of phonons, i.e. linearized normal modes of vibration,
provide important insights into many aspects of crystals, e.g. stability and
thermodynamics. In this paper, we use the Objective Structures framework to
make concrete analogies between crystalline phonons and normal modes of
vibration in non-crystalline but highly symmetric nanostructures. Our strategy
is to use an intermediate linear transformation from real-space to an
intermediate space in which the Hessian matrix of second derivatives is
block-circulant. The block-circulant nature of the Hessian enables us to then
follow the procedure to obtain phonons in crystals: namely, we use the Discrete
Fourier Transform from this intermediate space to obtain a block-diagonal
matrix that is readily diagonalizable. We formulate this for general Objective
Structures and then apply it to study carbon nanotubes of various chiralities
that are subjected to axial elongation and torsional deformation. We compare
the phonon spectra computed in the Objective Framework with spectra computed
for armchair and zigzag nanotubes. We also demonstrate the approach by
computing the Density of States. In addition to the computational efficiency
afforded by Objective Structures in providing the transformations to
almost-diagonalize the Hessian, the framework provides an important conceptual
simplification to interpret the phonon curves.Comment: To appear in J. Mech. Phys. Solid
Simulating the assembly of galaxies at redshifts z = 6 - 12
We use state-of-the-art simulations to explore the physical evolution of
galaxies in the first billion years of cosmic time. First, we demonstrate that
our model reproduces the basic statistical properties of the observed
Lyman-break galaxy (LBG) population at z = 6 - 8, including the evolving
ultra-violet (UV) luminosity function (LF), the stellar-mass density (SMD), and
the average specific star-formation rates (sSFR) of LBGs with M_{UV} < -18 (AB
mag). Encouraged by this success we present predictions for the behaviour of
fainter LBGs extending down to M_{UV} <= -15 (as will be probed with the James
Webb Space Telescope) and have interrogated our simulations to try to gain
insight into the physical drivers of the observed population evolution. We find
that mass growth due to star formation in the mass-dominant progenitor builds
up about 90% of the total z ~ 6 LBG stellar mass, dominating over the mass
contributed by merging throughout this era. Our simulation suggests that the
apparent "luminosity evolution" depends on the luminosity range probed: the
steady brightening of the bright end of the LF is driven primarily by genuine
physical luminosity evolution and arises due to a fairly steady increase in the
UV luminosity (and hence star-formation rates) in the most massive LBGs.
However, at fainter luminosities the situation is more complex, due in part to
the more stochastic star-formation histories of lower-mass objects; at this
end, the evolution of the UV LF involves a mix of positive and negative
luminosity evolution (as low-mass galaxies temporarily brighten then fade)
coupled with both positive and negative density evolution (as new low-mass
galaxies form, and other low-mass galaxies are consumed by merging). We also
predict the average sSFR of LBGs should rise from sSFR = 4.5 Gyr^-1 at z = 6 to
about 11 Gyr^-1 by z = 9.Comment: Accepted for publication in MNRA
Lyman Alpha Emitter Evolution in the Reionization Epoch
Combining cosmological SPH simulations with a previously developed Lyman
Alpha production/transmission model and the Early Reionization Model (ERM,
reionization ends at redshift z~7), we obtain Lyman Alpha and UV Luminosity
Functions (LFs) for Lyman Alpha Emitters (LAEs) for redshifts between 5.7 and
7.6. Matching model results to observations at z~5.7 requires escape fractions
of Lyman Alpha, f_alpha=0.3, and UV (non-ionizing) continuum photons, f_c=0.22,
corresponding to a color excess, E(B-V)=0.15. We find that (i) f_c increases
towards higher redshifts, due the decreasing mean dust content of galaxies,
(ii) the evolution of f_alpha/f_c hints at the dust content of the ISM becoming
progressively inhomogeneous/clumped with decreasing redshift. The clustering
photoionization boost is important during the initial reionization phases but
has little effect on the Lyman Alpha LF for a highly ionized IGM. Halo
(stellar) masses are in the range 10.0 < \log M_h < 11.8 (8.1 < \log M_* <
10.4) with M_h \propto M_*^{0.64}. The star formation rates are between 3-120
solar masses per year, mass-weighted mean ages are greater than 20 Myr at all
redshifts, while the mean stellar metallicity increases from Z=0.12 to 0.22
solar metallicity from z~7.6 to z~5.7; both age and metallicity positively
correlate with stellar mass. The brightest LAEs are all characterized by large
star formation rates and intermediate ages (~200 Myr), while objects in the
faint end of the Lyman Alpha LF show large age and star formation rate spreads.
With no more free parameters, the Spectral Energy Distributions of three LAE at
z~5.7 observed by Lai et al. (2007) are well reproduced by an intermediate age
(182-220 Myr) stellar population and the above E(B-V) value.Comment: 13 pages, 9 figures, accepted to MNRA
Nutritional requirements of Asian seabass, Lates calcarifer
Asian seabass (Lates calcarifer) has emerged as an
important candidate finfish species for aquaculture in
many parts of the world. Availability of seed and
appropriate feed are the two important prerequisites for
development and propagation of aquaculture of any fish
species. After considerable efforts and extensive research,
the Central Institute of Brackishwater Aquaculture (CIBA)
has succeeded in developing captive brood stock and seed
production technology for Asian seabass. Research efforts
on nutritional requirements and development of suitable
formulated feeds have been in progress simultaneously
at CIBA. The nutritional requirements of fish vary with
different growth stages and depend upon the feeding
habits that change according to the morphology of
digesting system. Considerable effort has been made in
Australia, Thailand, Philippines and more recently Israel,
in defining the nutritional requirements of this species in
order to improve production. Understanding the
nutritional requirements of the candidate species is the
first and essential pre requisite for development of cost
effective, efficient and eco friendly feeds
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