218 research outputs found
Ab initio study of mirages and magnetic interactions in quantum corrals
The state of the art ab initio calculations of quantum mirages,the
spin-polarization of surface-state electrons and the exchange interaction
between magnetic adatoms in Cu and Co corrals on Cu(111) are presented. We find
that the spin-polarization of the surface-state electrons caused by magnetic
adatoms can be projected to a remote location and can be strongly enhanced in
corrals compared to an open surface.Our studies give a clear evidence that
quantum corrals could permit to tailor the exchange interaction between
magnetic adatoms at large separations. The spin-polarization of surface-state
electrons at the empty focus in the Co corral used in the experimental setup of
Manoharan et al., (Nature 403, 512 (2000)) is revealed.Comment: Submitted to Physical Review Letter
Zn(O, S) layers for chalcoyprite solar cells sputtered from a single target
A simplified Cu(In, Ga)(S, Se)2/Zn(O, S)/ZnO:Al stack for chalcopyrite thin-
film solar cells is proposed. In this stack the Zn(O, S) layer combines the
roles of the traditional CdS buffer and undoped ZnO layers. It will be shown
that Zn(O, S) films can be sputtered in argon atmosphere from a single mixed
target without substrate heating. The photovoltaic performance of the
simplified stack matches that of the conventional approach. Replacing the ZnO
target with a ZnO/ZnS target may therefore be sufficient to omit the CdS
buffer layer and avoid the associated complexity, safety and recycling issues,
and to lower production cost
The de novo centriole assembly pathway in HeLa cells: cell cycle progression and centriole assembly/maturation
It has been reported that nontransformed mammalian cells become arrested during G1 in the absence of centrioles (Hinchcliffe, E., F. Miller, M. Cham, A. Khodjakov, and G. Sluder. 2001. Science. 291:1547–1550). Here, we show that removal of resident centrioles (by laser ablation or needle microsurgery) does not impede cell cycle progression in HeLa cells. HeLa cells born without centrosomes, later, assemble a variable number of centrioles de novo. Centriole assembly begins with the formation of small centrin aggregates that appear during the S phase. These, initially amorphous “precentrioles” become morphologically recognizable centrioles before mitosis. De novo–assembled centrioles mature (i.e., gain abilities to organize microtubules and replicate) in the next cell cycle. This maturation is not simply a time-dependent phenomenon, because de novo–formed centrioles do not mature if they are assembled in S phase–arrested cells. By selectively ablating only one centriole at a time, we find that the presence of a single centriole inhibits the assembly of additional centrioles, indicating that centrioles have an activity that suppresses the de novo pathway
Kinetochores Use a Novel Mechanism for Coordinating the Dynamics of Individual Microtubules
SummaryChromosome alignment during mitosis is frequently accompanied by a dynamic switching between elongation and shortening of kinetochore fibers (K-fibers) that connect kinetochores and spindle poles [1, 2]. In higher eukaryotes, mature K-fibers consist of 10–30 kinetochore microtubules (kMTs) whose plus ends are embedded in the kinetochore [1–3]. A critical and long-standing question is how the dynamics of individual kMTs within the K-fiber are coordinated [1–5]. We have addressed this question by using electron tomography to determine the polymerization/depolymerization status of individual kMTs in the K-fibers of PtK1 and Drosophila S2 cells. Surprisingly, we find that the plus ends of two-thirds of kMTs are in a depolymerizing state, even when the K-fiber exhibits net tubulin incorporation at the plus end [6–8]. Furthermore, almost all individual K-fibers examined had a mixture of kMTs in the polymerizing and depolymerizing states. Therefore, although K-fibers elongate and shrink as a unit, the dynamics of individual kMTs within a K-fiber are not coordinated at any given moment. Our results suggest a novel control mechanism through which attachment to the kinetochore outer plate prevents shrinkage of kMTs. We discuss the ramifications of this new model on the regulation of chromosome movement and the stability of K-fibers
Optimization of generator coordinate method with machine-learning algorithms for nuclear spectra and neutrinoless double-beta decay
The generator coordinate method (GCM) is an important tool of choice for
modeling large-amplitude collective motion in atomic nuclei. Recently, it has
attracted increasing interest as it can be exploited to extend ab initio
methods to the description of collective excitations of medium-mass and heavy
deformed nuclei, as well as the nuclear matrix elements (NME) of candidates for
neutrinoless double-beta (NLDBD) decay. The computational complexity of the GCM
increases rapidly with the number of collective coordinates. It imposes a
strong restriction on the applicability of the method. We aim to exploit
machine learning (ML) algorithms to speed up GCM calculations and ultimately
provide a more efficient description of nuclear energy spectra and other
observables such as the NME of NLDBD decay without loss of accuracy. To speed
up GCM calculations, we propose a subspace reduction algorithm that employs
optimized ML models as surrogates for exact quantum-number projection
calculations for norm and Hamiltonian kernels. The model space of the original
GCM is reduced to a subspace relevant for nuclear low energy spectra and the
NME of ground state to ground state decay based on the
orthogonality condition (OC) and the energy transition-orthogonality procedure
(ENTROP), respectively. For simplicity, a polynomial regression algorithm is
used to learn the norm and Hamiltonian kernels. The efficiency and accuracy of
this algorithm are illustrated for 76Ge and 76Se by comparing results obtained
using the ML models to direct GCM calculations. The results show that the
performance of the GCM+OC/ENTROP+ML is more robust than that of the GCM+ML
alone, and the former can reproduce the results of the original GCM calculation
rather accurately with a significantly reduced computational cost.Comment: 14 pages with 18 figure
Cover Crop Effect on Subsequent Wheat Yield in the Central Great Plains
Crop production systems in the water-limited environment
of the semiarid central Great Plains may not have potential to
profitably use cover crops because of lowered subsequent wheat
(Triticum asestivum L.) yields following the cover crop. Mixtures
have reportedly shown less yield-reducing effects on subsequent
crops than single-species plantings. This study was conducted
to determine winter wheat yields following both mixtures
and single-species plantings of spring-planted cover crops. The
study was conducted at Akron, CO, and Sidney, NE, during
the 2012–2013 and 2013–2014 wheat growing seasons under
both rainfed and irrigated conditions. Precipitation storage
efficiency before wheat planting, wheat water use, biomass, and
yield were measured and water use efficiency and harvest index
were calculated for wheat following four single-species cover
crops (flax [Linum usitatissimum L.], oat [Avena sativa L.], pea
[Pisum sativum ssp. arvense L. Poir], rapeseed [Brassica napus
L.]), a 10-species mixture, and a fallow treatment with proso
millet (Panicum miliaceum L.) residue. There was an average 10%
reduction in wheat yield following a cover crop compared with
following fallow, regardless of whether the cover crop was grown
in a mixture or in a single-species planting. Yield reductions
were greater under drier conditions. The slope of the wheat
water use–yield relationship was not significantly different for
wheat following the mixture (11.80 kg ha–1 mm–1) than for wheat
following single-species plantings (12.32–13.57 kg ha–1 mm–1).
The greater expense associated with a cover crop mixture
compared with a single species is not justified
Study of Zn O,S Films grown by Aerosol Assisted Chemical Vapour Deposition and their Application as Buffer Layers in Cu In,Ga S,Se 2 Solar Cells
To reduce the use of toxic and expensive elements in chalcopyrite thin film solar cells, materials such as cadmium or indium used in buffer layers need to be substituted. Zn O,S is considered to be a potential buffer layer material when deposited with a fast and inexpensive method. Zn O,S layers have been prepared by aerosol assisted chemical vapour deposition AACVD technique. AACVD technique is a simple non vacuum process where the thin film deposition temperatures do not exceed 250 C. 10 mM spray solution was made by dissolving zinc II acetylacetonate monohydrate in ethanol. The films were grown on Mo substrate at 225 C film growth temperature . The effect of deposition parameters spray solution concentration, N2 flow rate, H2S flow rate on Zn O,S thin film properties were studied with SEM and XRD. Thereupon optimizing the deposition parameters, homogeneous and compact Zn O,S thin films were obtained and the films were employed in the chalcopyrite thin film solar cell structure by growing films on Cu In,Ga S,Se 2 substrates industrially produced by BOSCH Solar CISTech GmbH. The resulting cells were studied using current voltage and quantum efficiency analysis and compared with solar cell references that include In2S3 and CdS as buffer layer deposited by ion layer gas reaction and chemical bath deposition, respectively. The best output of the solar cell containing Zn O,S as buffer layer and without intrinsic ZnO under standard test conditions AM 1.5G, 100 mW cm2, 25 C is Voc 573 mV, Jsc 39.2 mA cm2, FF 68.4 and efficiency of 15.4 being slightly better than the In2S3 or CdS containing solar cell reference
Testing the density matrix expansion against ab initio calculations of trapped neutron drops
Microscopic input to a universal nuclear energy density functional can be
provided through the density matrix expansion (DME), which has recently been
revived and improved. Several DME implementation strategies are tested for
neutron drop systems in harmonic traps by comparing to Hartree-Fock (HF) and ab
initio no-core full configuration (NCFC) calculations with a model interaction
(Minnesota potential). The new DME with exact treatment of Hartree
contributions is found to best reproduce HF results and supplementing the
functional with fit Skyrme-like contact terms shows systematic improvement
toward the full NCFC results.Comment: 10 pages, 5 figure
Measuring the photon distribution by ON/OFF photodectors
Reconstruction of photon statistics of optical states provide fundamental
information on the nature of any optical field and find various relevant
applications. Nevertheless, no detector that can reliably discriminate the
number of incident photons is available. On the other hand the alternative of
reconstructing density matrix by quantum tomography leads to various technical
difficulties that are particular severe in the pulsed regime (where mode
matching between signal an local oscillator is very challenging). Even if
on/off detectors, as usual avalanche PhotoDiodes operating in Geiger mode, seem
useless as photocounters, recently it was shown how reconstruction of photon
statistics is possible by considering a variable quantum efficiency. Here we
present experimental reconstructions of photon number distributions of both
continuous-wave and pulsed light beams in a scheme based on on/off avalanche
photodetection assisted by maximum-likelihood estimation. Reconstructions of
the distribution for both semiclassical and quantum states of light (as single
photon, coherent, pseudothermal and multithermal states) are reported for
single-mode as well as for multimode beams. The stability and good accuracy
obtained in the reconstruction of these states clearly demonstrate the
interesting potentialities of this simple technique.Comment: 6 pages, 7 figures, to appear on Laser Physic
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