1,545 research outputs found
Atomic oxygen adsorption and incipient oxidation of the Pb(111) surface: A density-functional theory study
We study the atomic oxygen adsorption on Pb(111) surface by using
density-functional theory within the generalized gradient approximation and a
supercell approach. The atomic and energetic properties of purely on-surface
and subsurface oxygen structures at the Pb(111) surface are systematically
investigated for a wide range of coverages and adsorption sites. The fcc and
tetra-II sites (see the text for definition) are found to be energetically
preferred for the on-surface and subsurface adsorption, respectively, in the
whole range of coverage considered. The on-surface and subsurface oxygen
binding energies monotonically increase with the coverage, and the latter is
always higher than the former, thus indicating the tendency to the formation of
oxygen islands (clusters) and the higher stability of subsurface adsorption.
The on-surface and subsurface diffusion-path energetics of atomic oxygen, and
the activation barriers for the O penetration from the on-surface to the
subsurface sites are presented at low and high coverages. In particular, it is
shown that the penetration barrier from the on-surface hcp to the subsurface
tetra-I site is as small as 65 meV at low coverage (=0.25). The other
properties of the O/Pb(111) system, including the charge distribution, the
lattice relaxation, the work function, and the electronic density of states,
are also studied and discussed in detail, which consistently show the gradually
stabilizing ionic O-Pb bond with increase of the oxygen coverage.Comment: 31 pages, 16 figure
Single Transverse Spin Asymmetries at Parton Level
Two factorization approaches have been proposed for single transverse spin
asymmetries. One is the collinear factorization, another is the
transverse-momentum-dependent factorization. They have been previously derived
in a formal way by using diagram expansion at hadron level. If the two
factorizations hold or can be proven, they should also hold when we replace
hadrons with parton states. We examine these two factorizations at parton level
with massless partons. It is nontrivial to generate these asymmetries at parton
level with massless partons because the asymmetries require helicity-flip and
nonzero absorptive parts in scattering amplitudes. By constructing suitable
parton states with massless partons we derive the two factorizations for the
asymmetry in Drell-Yan processes. It is found from our results that the
collinear factorization derived at parton level is not the same as that derived
at hadron level. Our results with massless partons confirm those derived with
single massive parton state in our previous works.Comment: shortened version to match published versio
The conserved aromatic residue W-122 is a determinant of potyviral coat protein stability, replication, and cell-to-cell movement in plants
Coat proteins (CPs) play critical roles in potyvirus cell-to-cell movement. However, the underlying mechanism controlling them remains unclear. Here, we show that substitutions of alanine, glutamic acid, or lysine for the conserved residue tryptophan at position 122 (W-122) in tobacco vein banding mosaic virus (TVBMV) CP abolished virus cell-to-cell movement in Nicotiana benthamiana plants. In agroinfiltrated N. benthamiana leaf patches, both the CP and RNA accumulation levels of three W-122 mutant viruses were significantly reduced compared with those of wild-type TVBMV, and CP accumulated to a low level similar to that of a replication-deficient mutant. The results of polyprotein transient expression experiments indicated that CP instability was responsible for the significantly low CP accumulation levels of the three W-122 mutant viruses. The substitution of W-122 did not affect CP plasmodesmata localization or virus particle formation; however, the substitution significantly reduced the number of virus particles. The wild-type TVBMV CP could complement the reduced replication and abolished cell-to-cell movement of the mutant viruses. When the codon for W-122 was mutated to that for a different aromatic residue, phenylalanine or tyrosine, the resultant mutant viruses moved systemically and accumulated up to 80% of the wild-type TVBMV level. Similar results were obtained for the corresponding amino acids of W-122 in the watermelon mosaic virus and potato virus Y CPs. Therefore, we conclude that the aromatic ring in W-122 in the core domain of the potyviral CP is critical for cell-to-cell movement through the effects on CP stability and viral replication.Peer reviewe
Coulomb-enhanced dynamic localization and Bell state generation in coupled quantum dots
We investigate the dynamics of two interacting electrons in coupled quantum
dots driven by an AC field. We find that the two electrons can be trapped in
one of the dots by the AC field, in spite of the strong Coulomb repulsion. In
particular, we find that the interaction may enhance the localization effect.
We also demonstrate the field excitation procedure to generate the maximally
entangled Bell states. The generation time is determined by both analytic and
numerical solutions of the time dependent Schrodinger equation.Comment: 12 pages, 5 figure
Quantum Entanglement of Excitons in Coupled Quantum Dots
Optically-controlled exciton dynamics in coupled quantum dots is studied. We
show that the maximally entangled Bell states and Greenberger-Horne-Zeilinger
(GHZ) states can be robustly generated by manipulating the system parameters to
be at the avoided crossings in the eigenenergy spectrum. The analysis of
population transfer is systematically carried out using a dressed-state
picture. In addition to the quantum dot configuration that have been discussed
by Quiroga and Johnson [Phys. Rev. Lett. \QTR{bf}{83}, 2270 (1999)], we show
that the GHZ states also may be produced in a ray of three quantum dots with a
shorter generation time.Comment: 16 pages, 7 figures, to appear in Phys. Rev.
The relative importance of physicochemical factors and crustacean zooplankton as determinants of rotifer density and species distribution in lakes adjacent to the Yangtze River, China
The planktonic community of freshwater Rotifera in 27 subtropical lakes was studied to assess the relative importance of physicochemical factors and crustacean zooplankton as determinants of rotifer density and species distribution. Factor analysis and multiple linear regressions showed that 21.9% and 29.9% of the variance in rotifer density was explained by physicochemical factors and crustaceans, respectively. Larger rotifer density was possible in shallower lakes with higher concentration of inorganic nitrogen and less herbivorous crustaceans such as Sinocalanus dorrii and Daphnia. Redundancy analysis showed that the variances of rotifer species distribution explained by crustaceans and physicochemical factors were 26.9% and 31.0%, respectively. Further analysis demonstrated that the variances explained by pure crustaceans and pure physicochemical factors were 12.5% and 16.6%, respectively. However, these two percentages were not statistically different. Rotifer species distribution was strongly associated with Chl a and Moina micrura. Their coexistence with Crustaceans seemed to be determined by their defense against potential predators and competitors. (C) 2009 Elsevier GmbH. All rights reserved
Localization of interacting electrons in quantum dot arrays driven by an ac-field
We investigate the dynamics of two interacting electrons moving in a
one-dimensional array of quantum dots under the influence of an ac-field. We
show that the system exhibits two distinct regimes of behavior, depending on
the ratio of the strength of the driving field to the inter-electron Coulomb
repulsion. When the ac-field dominates, an effect termed coherent destruction
of tunneling occurs at certain frequencies, in which transport along the array
is suppressed. In the other, weak-driving, regime we find the surprising result
that the two electrons can bind into a single composite particle -- despite the
strong Coulomb repulsion between them -- which can then be controlled by the
ac-field in an analogous way. We show how calculation of the Floquet
quasienergies of the system explains these results, and thus how ac-fields can
be used to control the localization of interacting electron systems.Comment: 7 pages, 6 eps figures V2. Minor changes, this version to be
published in Phys. Rev.
Molecular Basis for the Recognition of Adenomatous Polyposis Coli by the Discs Large 1 Protein
The human Discs Large 1 (DLG1) protein uses two of its three PDZ domains to interact with the C-terminal peptide of the Adenomatous Polyposis Coli (APC) tumor suppressor protein. The DLG1/APC complex inhibits the cell cycle progression from the G0/G1 to the S phase, regulates epithelial cell migration and morphogenesis, and is required for polarization of the microtubule cytoskeleton. However, the molecular details of how DLG1 recognizes APC is not clear. In this study, we performed biochemical and biophysical assays to investigate the interactions between PDZ domains of DLG1 and the C-terminal peptide of APC. In addition, we determined the crystal structures of the PDZ1 and PDZ2 domains of DLG1 each in complex with the C-terminal 11-residue peptide of APC. Our biochemical, biophysical, and structural results revealed structural elements and residues on PDZ1 and PDZ2 domains of DLG1 and on APC crucial for their mutual interaction. In particular, our results show that the β2/β3 loops of PDZ1 and PDZ2 play important roles in contributing to the binding affinities between PDZ domains and APC, through interacting with the residues upstream of the canonical PDZ-binding S/T-X-V motif. The results provide new insights into the binding mode of a defined C-terminal segment of APC by the PDZ domains of DLG1
Observation of a charged charmoniumlike structure in at GeV
We study the process at a
center-of-mass energy of 4.26GeV using a 827pb data sample obtained with
the BESIII detector at the Beijing Electron Positron Collider. Based on a
partial reconstruction technique, the Born cross section is measured to be
pb. We observe a structure near the
threshold in the recoil mass spectrum, which we denote as the
. The measured mass and width of the structure are
MeV/c and MeV, respectively. Its
production ratio is determined to be . The first uncertainties
are statistical and the second are systematic.Comment: 7 pages, 4 figures, 1 table; version accepted to be published in PR
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