13,227 research outputs found
Construction of Wannier functions from localized atomic-like orbitals
The problem of construction of the Wannier functions (WFs) in a restricted
Hilbert space of eigenstates of the one-electron Hamiltonian (forming
the so-called low-energy part of the spectrum) can be formulated in several
different ways. One possibility is to use the projector-operator techniques,
which pick up a set of trial atomic orbitals and project them onto the given
Hilbert space. Another possibility is to employ the downfolding method, which
eliminates the high-energy part of the spectrum and incorporates all related to
it properties into the energy-dependence of an effective Hamiltonian. We show
that by modifying the high-energy part of the spectrum of the original
Hamiltonian , which is rather irrelevant to the construction of WFs in
the low-energy part of the spectrum, these two methods can be formulated in an
absolutely exact and identical form, so that the main difference between them
is reduced to the choice of the trial orbitals. Concerning the latter part of
the problem, we argue that an optimal choice for trial orbitals can be based on
the maximization of the site-diagonal part of the density matrix. The main idea
is illustrated for a simple toy model, consisting of only two bands, as well as
for a more realistic example of bands in VO. An analogy with
the search of the ground state of a many-electron system is also discussed.Comment: 13 pages, 6 figure
First-principle Wannier functions and effective lattice fermion models for narrow-band compounds
We propose a systematic procedure for constructing effective lattice fermion
models for narrow-band compounds on the basis of first-principles electronic
structure calculations. The method is illustrated for the series of
transition-metal (TM) oxides: SrVO, YTiO, VO, and
YMoO. It consists of three parts, starting from LDA. (i)
construction of the kinetic energy Hamiltonian using downfolding method. (ii)
solution of an inverse problem and construction of the Wannier functions (WFs)
for the given kinetic energy Hamiltonian. (iii) calculation of screened Coulomb
interactions in the basis of \textit{auxiliary} WFs, for which the
kinetic-energy term is set to be zero. The last step is necessary in order to
avoid the double counting of the kinetic-energy term, which is included
explicitly into the model. The screened Coulomb interactions are calculated in
a hybrid scheme. First, we evaluate the screening caused by the change of
occupation numbers and the relaxation of the LMTO basis functions, using the
conventional constraint-LDA approach, where all matrix elements of
hybridization involving the TM orbitals are set to be zero. Then, we switch
on the hybridization and evaluate the screening associated with the change of
this hybridization in RPA. The second channel of screening is very important,
and results in a relatively small value of the effective Coulomb interaction
for isolated bands. We discuss details of this screening and consider
its band-filling dependence, frequency dependence, influence of the lattice
distortion, proximity of other bands, and the dimensionality of the model
Hamiltonian.Comment: 35 pages, 25 figure
An Assessment of Grass Regeneration Nurseries at the Western Regional Plant Introduction Station, 1994-1997
The Western Regional Plant Introduction Station (WRPIS), Pullman, WA, USA, maintains over 17,000 accessions of forage and turf grasses that are mostly wind cross-pollinated and highly heterogenic. Regeneration procedures have been refined over the past nine years to include improved isolation distance and increased plant populations for regeneration. The grass regeneration nurseries planted from 1994 through 1997 were evaluated using data recorded in the Germplasm Resources Information Network (GRIN) and it was found that approximately 78% of the regenerations were successful. Reasons for failures were contributed to inadequate plant number, presence of disease, seed shattering, and unsuitable growing environment. Several solutions have been identified and implemented
Conservation of Grass Collections at the Western Regional Plant Introduction Station
Germplasm managers at the Western Regional Plant Introduction Station (WRPIS) have focused on improving seed regeneration in forage and turf grass species through studies of diversity maintenance, using isozyme markers in annual ryegrass (Lolium multiflorum Lam.) and through studies of pollen isolation, using strains of dominant pubescent and recessive glabrous smooth bromegrass (Bromus inermis Leyss.). Balanced samples of annual ryegrass (an equal number of seeds per plant) from field plots were best for maintaining allelic frequencies, but genetic diversity (heterozygosity and allelic richness) was maintained nearly as well in bulk samples (seeds proportionally assembled according to seeds per plant) during early regeneration cycles. In 1995, bromegrass marker plots integrated into WRPIS seed-regeneration nurseries at two locations resulted in average contamination of 4.2% at distances between 22 and 27 m. Diversity maintenance in early regeneration cycles and a relatively low level of pollen contamination appear possible in bulk samples and with modest isolation distances
Explosion of a collapsing Bose-Einstein condensate
We show that elastic collisions between atoms in a Bose-Einstein condensate
with attractive interactions lead to an explosion that ejects a large fraction
of the collapsing condensate. We study variationally the dynamics of this
explosion and find excellent agreement with recent experiments on magnetically
trapped Rubidium-85. We also determine the energy and angular distribution of
the ejected atoms during the collapse.Comment: Four pages of ReVTeX and five postscript figure
The Surface of a Bose-Einstein Condensed Atomic Cloud
We investigate the structure and collective modes of a planar surface of a
trapped Bose-Einstein condensed gas at zero temperature. In the long-wavelength
limit we find a mode similar to the gravity wave on the surface of a fluid with
the frequency and the wavenumber related by . Here
is the force due to the confining potential at the surface and is the
particle mass. At shorter wavelengths we use a variational approach and find
corrections to of order . We demonstrate the usefulness
of the concept of an effective surface tension for describing both static and
dynamic properties of condensed atomic clouds.Comment: 8 pages, REVTEX, submitted to Phys. Rev.
Fingerprints of Spin-Orbital Physics in Crystalline O
The alkali hyperoxide KO is a molecular analog of strongly-correlated
systems, comprising of orbitally degenerate magnetic O ions. Using
first-principles electronic structure calculations, we set up an effective
spin-orbital model for the low-energy \textit{molecular} orbitals and argue
that many anomalous properties of KO replicate the status of its orbital
system in various temperature regimes.Comment: 4 pages, 2 figures, 1 tabl
Initial Stages of Bose-Einstein Condensation
We present the quantum theory for the nucleation of Bose-Einstein
condensation in a dilute atomic Bose gas. This quantum theory comfirms the
results of the semiclassical treatment, but has the important advantage that
both the kinetic and coherent stages of the nucleation process can now be
described in a unified way by a single Fokker-Planck equation.Comment: Four pages of ReVTeX and no figure
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Recovery of critically endangered Nassau grouper (Epinephelus striatus) in the Cayman Islands following targeted conservation actions.
Many large-bodied marine fishes that form spawning aggregations, such as the Nassau grouper (Epinephelus striatus), have suffered regional overfishing due to exploitation during spawning. In response, marine resource managers in many locations have established marine protected areas or seasonal closures to recover these overfished stocks. The challenge in assessing management effectiveness lies largely in the development of accurate estimates to track stock size through time. For the past 15 y, the Cayman Islands government has taken a series of management actions aimed at recovering collapsed stocks of Nassau grouper. Importantly, the government also partnered with academic and nonprofit organizations to establish a research and monitoring program (Grouper Moon) aimed at documenting the impacts of conservation action. Here, we develop an integrated population model of 2 Cayman Nassau grouper stocks based on both diver-collected mark-resight observations and video censuses. Using both data types across multiple years, we fit parameters for a state-space model for population growth. We show that over the last 15 y the Nassau grouper population on Little Cayman has more than tripled in response to conservation efforts. Census data from Cayman Brac, while more sparse, show a similar pattern. These findings demonstrate that spatial and seasonal closures aimed at rebuilding aggregation-based fisheries can foster conservation success
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