1,393 research outputs found
Signatures of electron correlations in the transport properties of quantum dots
The transition matrix elements between the correlated and
electron states of a quantum dot are calculated by numerical diagonalization.
They are the central ingredient for the linear and non--linear transport
properties which we compute using a rate equation. The experimentally observed
variations in the heights of the linear conductance peaks can be explained. The
knowledge of the matrix elements as well as the stationary populations of the
states allows to assign the features observed in the non--linear transport
spectroscopy to certain transition and contains valuable information about the
correlated electron states.Comment: 4 pages (revtex,27kB) + 3 figures in one file ziped and uuencoded
(postscript,33kB), to appear in Phys.Rev.B as Rapid Communicatio
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
Length-dependent oscillations of the conductance through atomic chains: The importance of electronic correlations
We calculate the conductance of atomic chains as a function of their length.
Using the Density Matrix Renormalization Group algorithm for a many-body model
which takes into account electron-electron interactions and the shape of the
contacts between the chain and the leads, we show that length-dependent
oscillations of the conductance whose period depends on the electron density in
the chain can result from electron-electron scattering alone. The amplitude of
these oscillations can increase with the length of the chain, in contrast to
the result from approaches which neglect the interactions.Comment: 7 pages, 4 figure
Electron Transport through Disordered Domain Walls: Coherent and Incoherent Regimes
We study electron transport through a domain wall in a ferromagnetic nanowire
subject to spin-dependent scattering. A scattering matrix formalism is
developed to address both coherent and incoherent transport properties. The
coherent case corresponds to elastic scattering by static defects, which is
dominant at low temperatures, while the incoherent case provides a
phenomenological description of the inelastic scattering present in real
physical systems at room temperature. It is found that disorder scattering
increases the amount of spin-mixing of transmitted electrons, reducing the
adiabaticity. This leads, in the incoherent case, to a reduction of conductance
through the domain wall as compared to a uniformly magnetized region which is
similar to the giant magnetoresistance effect. In the coherent case, a
reduction of weak localization, together with a suppression of spin-reversing
scattering amplitudes, leads to an enhancement of conductance due to the domain
wall in the regime of strong disorder. The total effect of a domain wall on the
conductance of a nanowire is studied by incorporating the disordered regions on
either side of the wall. It is found that spin-dependent scattering in these
regions increases the domain wall magnetoconductance as compared to the effect
found by considering only the scattering inside the wall. This increase is most
dramatic in the narrow wall limit, but remains significant for wide walls.Comment: 23 pages, 12 figure
Spin-Blockade in Single and Double Quantum Dots in Magnetic Fields: a Correlation Effect
The total spin of correlated electrons in a quantum dot changes with magnetic
field and this effect is generally linked to the change in the total angular
momentum from one magic number to another, which can be understood in terms of
an `electron molecule' picture for strong fields. Here we propose to exploit
this fact to realize a spin blockade, i.e., electrons are prohibited to tunnel
at specific values of the magnetic field. The spin-blockade regions have been
obtained by calculating both the ground and excited states. In double dots the
spin-blockade condition is found to be less stringent than in single dots.Comment: 4pages, to be published in Phys. Rev. B (Rapid Communication
Spin blockade in ground state resonance of a quantum dot
We present measurements on spin blockade in a laterally integrated quantum
dot. The dot is tuned into the regime of strong Coulomb blockade, confining ~
50 electrons. At certain electronic states we find an additional mechanism
suppressing electron transport. This we identify as spin blockade at zero bias,
possibly accompanied by a change in orbital momentum in subsequent dot ground
states. We support this by probing the bias, magnetic field and temperature
dependence of the transport spectrum. Weak violation of the blockade is
modelled by detailed calculations of non-linear transport taking into account
forbidden transitions.Comment: 4 pages, 4 figure
Urban Material Classification Using Spectral and Textural Features Retrieved from Autoencoders
Classification of urban materials using remote sensing data, in particular hyperspectral data, is common practice. Spectral libraries can be utilized to train a classifier since they provide spectral features about selected urban materials. However, urban materials can have similar spectral characteristic features due to high inter-class correlation which can lead to misclassification. Spectral libraries rarely provide imagery of their samples, which disables the possibility of classifying urban materials with additional textural information. Thus, this paper conducts material classification comparing the benefits of using close-range acquired spectral and textural features. The spectral features consist of either the original spectra, a PCA-based encoding or the compressed spectral representation of the original spectra retrieved using a deep autoencoder. The textural features are generated using a deep denoising convolutional autoencoder. The spectral and textural features are gathered from the recently published spectral library KLUM. Three classifiers are used, the two well-established Random Forest and Support Vector Machine classifiers in addition to a Histogram-based Gradient Boosting Classification Tree. The achieved overall accuracy was within the range of 70–80% with a standard deviation between 2–10% across all classification approaches. This indicates that the amount of samples still is insufficient for some of the material classes for this classification task. Nonetheless, the classification results indicate that the spectral features are more important for assigning material labels than the textural features
--Oscillations for Correlated Electron Pairs in Disordered Mesoscopic Rings
The full spectrum of two interacting electrons in a disordered mesoscopic
one--dimensional ring threaded by a magnetic flux is calculated numerically.
For ring sizes far exceeding the one--particle localization length we
find several --periodic states whose eigenfunctions exhibit a pairing
effect. This represents the first direct observation of interaction--assisted
coherent pair propagation, the pair being delocalized on the scale of the whole
ring.Comment: 4 pages, uuencoded PostScript, containing 5 figures
Nuclear spin relaxation probed by a single quantum dot
We present measurements on nuclear spin relaxation probed by a single quantum
dot in a high-mobility electron gas. Current passing through the dot leads to a
spin transfer from the electronic to the nuclear spin system. Applying electron
spin resonance the transfer mechanism can directly be tuned. Additionally, the
dependence of nuclear spin relaxation on the dot gate voltage is observed. We
find electron-nuclear relaxation times of the order of 10 minutes
The Importance of Satellite Quenching for the Build-Up of the Red Sequence of Present Day Galaxies
In the current paradigm, red sequence galaxies are believed to have formed as
blue disk galaxies that subsequently had their star formation quenched. Since
red-sequence galaxies typically have an early-type morphology, the transition
from the blue to the red sequence also involves a morphological transformation.
In this paper we study the impact of transformation mechanisms that operate
only on satellite galaxies, such as strangulation, ram-pressure stripping and
galaxy harassment. Using a large galaxy group catalogue constructed from the
SDSS, we compare the colors and concentrations of satellites galaxies to those
of central galaxies of the same stellar mass, adopting the hypothesis that the
latter are the progenitors of the former. On average, satellites are redder and
more concentrated than central galaxies of the same stellar mass.
Central-satellite pairs that are matched in both stellar mass and color,
however, show no average concentration difference, indicating that the
transformation mechanisms affect color more than morphology. The color and
concentration differences of matched central-satellite pairs are completely
independent of the halo mass of the satellite galaxy, indicating that
satellite-specific transformation mechanisms are equally efficient in haloes of
all masses. This strongly favors strangulation as the main quenching mechanism
for satellite galaxies. Finally, we determine the relative importance of
satellite quenching for the build-up of the red sequence. We find that roughly
70 percent of red sequence satellite galaxies with a stellar mass of 10^9 Msun
had their star formation quenched as satellites. This drops rapidly to zero
with increasing stellar mass, indicating that a significant fraction of red
satellites were already quenched before they became a satellite.Comment: 14 pages, 10 figures. Submitted for publication in MNRA
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