7,977,706 research outputs found
Theory of double resonance magnetometers based on atomic alignment
We present a theoretical study of the spectra produced by
optical-radio-frequency double resonance devices, in which resonant linearly
polarized light is used in the optical pumping and detection processes. We
extend previous work by presenting algebraic results which are valid for atomic
states with arbitrary angular momenta, arbitrary rf intensities, and arbitrary
geometries. The only restriction made is the assumption of low light intensity.
The results are discussed in view of their use in optical magnetometers
The Energetics of Li Off-Centering in KLiTaO; First Principles Calculations
KLiTaO (KLT) solid solutions exhibit a variety of
interesting physical phenomena related to large displacements of Li-ions from
ideal perovskite A-site positions. First-principles calculations for KLT
supercells were used to investigate these phenomena. Lattice dynamics
calculations for KLT exhibit a Li off-centering instability. The energetics of
Li-displacements for isolated Li-ions and for Li-Li pairs up to 4th neighbors
were calculated. Interactions between nearest neighbor Li-ions, in a Li-Li
pair, strongly favor ferroelectric alignment along the pair axis. Such Li-Li
pairs can be considered "seeds" for polar nanoclusters in KLT.
Electrostriction, local oxygen relaxation, coupling to the KT soft-mode, and
interactions with neighboring Li-ions all enhance the polarization from Li
off-centering. Calculated hopping barriers for isolated Li-ions and for nearest
neighbor Li-Li pairs are in good agreement with Arrhenius fits to experimental
dielectric data.Comment: 14 pages including 10 figures. To Physical Review B. Replaced after
corrections due to referees' remark
Geometric, electronic properties and the thermodynamics of pure and Al--doped Li clusters
The first--principles density functional molecular dynamics simulations have
been carried out to investigate the geometric, the electronic, and the finite
temperature properties of pure Li clusters (Li, Li) and Al--doped
Li clusters (LiAl, LiAl). We find that addition of two Al
impurities in Li results in a substantial structural change, while the
addition of one Al impurity causes a rearrangement of atoms. Introduction of
Al--impurities in Li establishes a polar bond between Li and nearby Al
atom(s), leading to a multicentered bonding, which weakens the Li--Li metallic
bonds in the system. These weakened Li--Li bonds lead to a premelting feature
to occur at lower temperatures in Al--doped clusters. In LiAl, Al
atoms also form a weak covalent bond, resulting into their dimer like behavior.
This causes Al atoms not to `melt' till 800 K, in contrast to the Li atoms
which show a complete diffusive behavior above 400 K. Thus, although one Al
impurity in Li cluster does not change its melting characteristics
significantly, two impurities results in `surface melting' of Li atoms whose
motions are confined around Al dimer.Comment: 9 pages, 7 figure
Thermodynamically stable lithium silicides and germanides from density-functional theory calculations
Density-functional-theory (DFT) calculations have been performed on the Li-Si
and Li-Ge systems. Lithiated Si and Ge, including their metastable phases, play
an important technological r\^ole as Li-ion battery (LIB) anodes. The
calculations comprise structural optimisations on crystal structures obtained
by swapping atomic species to Li-Si and Li-Ge from the X-Y structures in the
International Crystal Structure Database, where X={Li,Na,K,Rb,Cs} and
Y={Si,Ge,Sn,Pb}. To complement this at various Li-Si and Li-Ge stoichiometries,
ab initio random structure searching (AIRSS) was also performed. Between the
ground-state stoichiometries, including the recently found LiSi
phase, the average voltages were calculated, indicating that germanium may be a
safer alternative to silicon anodes in LIB, due to its higher lithium insertion
voltage. Calculations predict high-density LiSi and LiGe
layered phases which become the ground state above 2.5 and 5 GPa
respectively and reveal silicon and germanium's propensity to form dumbbells in
the LiSi, stoichiometry range. DFT predicts the stability of
the LiGe , LiGe and LiGe
phases and several new Li-Ge compounds, with stoichiometries LiGe,
LiGe, LiGe and LiGe.Comment: 10 pages, 5 figure
Learners’ continuance participation intention of collaborative group project in virtual learning environment: an extended TAM perspective
The aim of this study is to explore learners’ intention to return to the electronic environment through the use of wikipages. The survey is based on students’ participation in a collaborative group project over a one semester course on business information systems. A research model based on the extended Technology Acceptance Model (TAM) has been proposed to investigate what factors will influence learners’ continuance participation in the electronic learning environment. 75% of students returned the questionnaire and the data analysis results based on the extended TAM Shows that the learners’ intention to return to the electronic learning environment was highly associated with their attitude towards the electronic learning tool and the affection associated with the tool
New Keck Observations of Lithium in Very Metal-poor Stars
Lithium abundances have been determined in more than 100 metal-poor halo
stars both in the field and in clusters. From these data we find trends of Li
with both temperature and metallicity and a real dispersion in Li abundances in
the Spite Li plateau. We attribute this dispersion primarily to Li depletion
(presumably due to extra mixing induced by stellar rotation) and to Galactic
chemical evolution. We derive a primordial Li of 2.44 0.18 for A(Li) =
log N(Li/H) + 12.00. This agrees with the Li abundances predicted by the
results. For stars cooler than the Li plateau we have evidence that Li
depletion sets in at hotter temperatures for the higher metallicity stars than
for the low-metal stars. This is the opposite sense of predictions from stellar
models. The smooth transition of the Li content from the Li plateau stars to
the cool stars adds weight to the inference of Li depletion in the plateau
stars.Comment: Invited talk for IAU Symposium 228 "From Lithium to Uranium..." held
in Paris in May, 2005. 6 pages, 6 figure
Li spectrum from Li fragmentation
A recently developed time dependent model for the excitation of a nucleon
from a bound state to a continuum resonant state in the system n+core is
applied to the study of the population of the low energy continuum of the
unbound Li system obtained from Li fragmentation. Comparison of
the model results to new data from the GSI laboratory suggests that the
reaction mechanism is dominated by final state effects rather than by the
sudden process, but for the population of the l=0 virtual state, in which case
the two mechanisms give almost identical results. There is also, for the first
time, a clear evidence for the population of a d resonance in
Li.Comment: 15 pages, 4 figures, 3 tables. Accepted for publication in
Nucl.Phys.
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