1,441 research outputs found
Spin Polarized Transport Through a Single-Molecule Magnet: Current-Induced Magnetic Switching
Magnetic switching of a single-molecule magnet (SMM) due to spin-polarized
current is investigated theoretically. The charge transfer between the
electrodes takes place via the lowest unoccupied molecular orbital (LUMO) of
the SMM. Generally, the double occupancy of the LUMO level, and a finite
on-site Coulomb repulsion, is taken into account. Owing to the exchange
interaction between electrons in the LUMO level and the SMM's spin, the latter
can be reversed. The perturbation approach (Fermi golden rule) is applied to
calculate current-voltage characteristics. The influence of Coulomb
interactions on the switching process is also analyzed.Comment: 5 pages with 4 EPS figures; version as accepted for publication in
Phys. Rev. B (more general model introduced
Theory of scanning gate microscopy
A systematic theory of the conductance measurements of non-invasive (weak
probe) scanning gate microscopy is presented that provides an interpretation of
what precisely is being measured. A scattering approach is used to derive
explicit expressions for the first and second order conductance changes due to
the perturbation by the tip potential in terms of the scattering states of the
unperturbed structure. In the case of a quantum point contact, the first order
correction dominates at the conductance steps and vanishes on the plateaus
where the second order term dominates. Both corrections are non-local for a
generic structure. Only in special cases, such as that of a centrally symmetric
quantum point contact in the conductance quantization regime, can the second
order correction be unambiguously related with the local current density. In
the case of an abrupt quantum point contact we are able to obtain analytic
expressions for the scattering eigenfunctions and thus evaluate the resulting
conductance corrections.Comment: 19 pages, 7 figure
Imaging magnetoelectric subbands in ballistic constrictions
We perform scanning gate experiments on ballistic constrictions in the
presence of small perpendicular magnetic fields. The constrictions form the
entrance and exit of a circular gate-defined ballistic stadium. Close to
constrictions we observe sets of regular fringes creating a checker board
pattern. Inside the stadium conductance fluctuations governed by chaotic
dynamics of electrons are visible. The checker board pattern allows us to
determine the number of transmitted modes in the constrictions forming between
the tip-induced potential and gate-defined geometry. Spatial investigation of
the fringe pattern in a perpendicular magnetic field shows a transition from
electrostatic to magnetic depopulation of magnetoelectric subbands. Classical
and quantum simulations agree well with different aspects of our observations.Comment: 18 pages, 7 figure
Electron backscattering in a cavity: ballistic and coherent effects
Numerous experimental and theoretical studies have focused on low-dimensional
systems locally perturbed by the biased tip of a scanning force microscope. In
all cases either open or closed weakly gate-tunable nanostructures have been
investigated, such as quantum point contacts, open or closed quantum dots, etc.
We study the behaviour of the conductance of a quantum point contact with a
gradually forming adjacent cavity in series under the influence of a scanning
gate. Here, an initially open quantum point contact system gradually turns into
a closed cavity system. We observe branches and interference fringes known from
quantum point contacts coexisting with irregular conductance fluctuations.
Unlike the branches, the fluctuations cover the entire area of the cavity. In
contrast to previous studies, we observe and investigate branches under the
influence of the confining stadium potential, which is gradually built up. We
find that the branches exist only in the area surrounded by cavity top gates.
As the stadium shrinks, regular fringes originate from tip-induced
constrictions leading to quantized conduction. In addition, we observe arc-like
areas reminiscent of classical electron trajectories in a chaotic cavity. We
also argue that electrons emanating from the quantum point contact spread out
like a fan leaving branch-like regions of enhanced backscattering.Comment: 7 pages, 4 figure
Properties of Galaxy Groups in the SDSS: I.-- The Dependence of Colour, Star Formation, and Morphology on Halo Mass
Using a large galaxy group catalogue constructed from the SDSS, we
investigate the correlation between various galaxy properties and halo mass. We
split the population of galaxies in early types, late types, and intermediate
types, based on their colour and specific star formation rate. At fixed
luminosity, the early type fraction increases with increasing halo mass. Most
importantly, this mass dependence is smooth and persists over the entire mass
range probed, without any break or feature at any mass scale. We argue that the
previous claim of a characteristic feature on galaxy group scales is an
artefact of the environment estimators used. At fixed halo mass, the luminosity
dependence of the type fractions is surprisingly weak: galaxy type depends more
strongly on halo mass than on luminosity. We also find that the early type
fraction decreases with increasing halo-centric radius. Contrary to previous
studies, we find that this radial dependence is also present in low mass
haloes. The properties of satellite galaxies are strongly correlated with those
of their central galaxy. In particular, the early type fraction of satellites
is significantly higher in a halo with an early type central galaxy than in a
halo of the same mass but with a late type central galaxy. This phenomenon,
which we call `galactic conformity', is present in haloes of all masses and for
satellites of all luminosities. Finally, the fraction of intermediate type
galaxies is always ~20 percent, independent of luminosity, independent of halo
mass, independent of halo-centric radius, and independent of whether the galaxy
is a central galaxy or a satellite galaxy. We discuss the implications of all
these findings for galaxy formation and evolution.Comment: 28 pages, 15 figures. Submitted for publication in MNRA
Semantic 3D scene interpretation: A framework combining optimal neighborhood size selection with relevant features
3D scene analysis by automatically assigning 3D points a semantic label has become an issue of major interest in recent years. Whereas the tasks of feature extraction and classification have been in the focus of research, the idea of using only relevant and more distinctive features extracted from optimal 3D neighborhoods has only rarely been addressed in 3D lidar data processing. In this paper, we focus on the interleaved issue of extracting relevant, but not redundant features and increasing their distinctiveness by considering the respective optimal 3D neighborhood of each individual 3D point. We present a new, fully automatic and versatile framework consisting of four successive steps: (i) optimal neighborhood size selection, (ii) feature extraction, (iii) feature selection, and (iv) classification. In a detailed evaluation which involves 5 different neighborhood definitions, 21 features, 6 approaches for feature subset selection and 2 different classifiers, we demonstrate that optimal neighborhoods for individual 3D points significantly improve the results of scene interpretation and that the selection of adequate feature subsets may even further increase the quality of the derived results
The clustering of SDSS galaxy groups: mass and color dependence
We use a sample of galaxy groups selected from the SDSS DR 4 with an adaptive
halo-based group finder to probe how the clustering strength of groups depends
on their masses and colors. In particular, we determine the relative biases of
groups of different masses, as well as that of groups with the same mass but
with different colors. In agreement with previous studies, we find that more
massive groups are more strongly clustered, and the inferred mass dependence of
the halo bias is in good agreement with predictions for the CDM
cosmology. Regarding the color dependence, we find that groups with red
centrals are more strongly clustered than groups of the same mass but with blue
centrals. Similar results are obtained when the color of a group is defined to
be the total color of its member galaxies. The color dependence is more
prominent in less massive groups and becomes insignificant in groups with
masses \gta 10^{14}\msunh. We construct a mock galaxy redshift survey
constructed from the large Millenium simulation that is populated with galaxies
according to the semi-analytical model of Croton et al. Applying our group
finder to this mock survey, and analyzing the mock data in exactly the same way
as the true data, we are able to accurately recover the intrinsic mass and
color dependencies of the halo bias in the model. This suggests that our group
finding algorithm and our method of assigning group masses do not induce
spurious mass and/or color dependencies in the group-galaxy correlation
function. The semi-analytical model reveals the same color dependence of the
halo bias as we find in our group catalogue. In halos with M\sim
10^{12}\msunh, though, the strength of the color dependence is much stronger
in the model than in the data.Comment: 16 pages, 14 figures, Accepted for publication in ApJ. In the new
version, we add the bias of the shuffled galaxy sample. The errors are
estimated according to the covariance matrix of the GGCCF, which is then
diagonalize
Observational Evidence for an Age Dependence of Halo Bias
We study the dependence of the cross-correlation between galaxies and galaxy
groups on group properties. Confirming previous results, we find that the
correlation strength is stronger for more massive groups, in good agreement
with the expected mass dependence of halo bias. We also find, however, that for
groups of the same mass, the correlation strength depends on the star formation
rate (SFR) of the central galaxy: at fixed mass, the bias of galaxy groups
decreases as the SFR of the central galaxy increases. We discuss these findings
in light of the recent findings by Gao et al (2005) that halo bias depends on
halo formation time, in that halos that assemble earlier are more strongly
biased. We also discuss the implication for galaxy formation, and address a
possible link to galaxy conformity, the observed correlation between the
properties of satellite galaxies and those of their central galaxy.Comment: 4 pages, 4 figures, Accepted for publication in ApJ Letters. Figures
3 and 4 replaced. The bias dependence on the central galaxy luminosity is
omitted due to its sensitivity to the mass mode
Effect of Absence of Developing Grain on Carbohydrate Content and Senescence of Maize Leaves
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