2,001 research outputs found
The Centaurus Group and the Outer Halo of NGC 5128: Are they Dynamically Connected?
NGC 5128, a giant elliptical galaxy only Mpc away, is the dominant
member of a galaxy group of over 80 probable members. The Centaurus group
provides an excellent sample for a kinematic comparison between the halo of NGC
5128 and its surrounding satellite galaxies. A new study, presented here, shows
no kinematic difference in rotation amplitude, rotation axis, and velocity
dispersion between the halo of NGC 5128, determined from over of its
globular clusters, and those of the Centaurus group as a whole. These results
suggest NGC 5128 could be behaving in part as the inner component to the galaxy
group, and could have begun as a large initial seed galaxy, gradually built up
by minor mergers and satellite accretions, consistent with simple cold dark
matter models. The mass and mass-to-light ratios in the B-band, corrected for
projection effects, are determined to be
M_{\sun} and M_{\sun}/L_{\sun} for NGC 5128 out to a
galactocentric radius of 45 kpc, and M_{\sun}
and M_{\sun}/L_{\sun} for the Centaurus group, consistent with
previous studies.Comment: 14 pages, 3 tables, 7 figures, Accepted for publication in A
A Circuit Model for Domain Walls in Ferromagnetic Nanowires: Application to Conductance and Spin Transfer Torques
We present a circuit model to describe the electron transport through a
domain wall in a ferromagnetic nanowire. The domain wall is treated as a
coherent 4-terminal device with incoming and outgoing channels of spin up and
down and the spin-dependent scattering in the vicinity of the wall is modelled
using classical resistances. We derive the conductance of the circuit in terms
of general conductance parameters for a domain wall. We then calculate these
conductance parameters for the case of ballistic transport through the domain
wall, and obtain a simple formula for the domain wall magnetoresistance which
gives a result consistent with recent experiments. The spin transfer torque
exerted on a domain wall by a spin-polarized current is calculated using the
circuit model and an estimate of the speed of the resulting wall motion is
made.Comment: 10 pages, 5 figures; submitted to Physical Review
Total Generalized Variation for Manifold-valued Data
In this paper we introduce the notion of second-order total generalized
variation (TGV) regularization for manifold-valued data in a discrete setting.
We provide an axiomatic approach to formalize reasonable generalizations of TGV
to the manifold setting and present two possible concrete instances that
fulfill the proposed axioms. We provide well-posedness results and present
algorithms for a numerical realization of these generalizations to the manifold
setup. Further, we provide experimental results for synthetic and real data to
further underpin the proposed generalization numerically and show its potential
for applications with manifold-valued data
Transmission phase of a quantum dot and statistical fluctuations of partial-width amplitudes
Experimentally, the phase of the amplitude for electron transmission through
a quantum dot (transmission phase) shows the same pattern between consecutive
resonances. Such universal behavior, found for long sequences of resonances, is
caused by correlations of the signs of the partial-width amplitudes of the
resonances. We investigate the stability of these correlations in terms of a
statistical model. For a classically chaotic dot, the resonance eigenfunctions
are assumed to be Gaussian distributed. Under this hypothesis, statistical
fluctuations are found to reduce the tendency towards universal phase
evolution. Long sequences of resonances with universal behavior only persist in
the semiclassical limit of very large electron numbers in the dot and for
specific energy intervals. Numerical calculations qualitatively agree with the
statistical model but quantitatively are closer to universality.Comment: 8 pages, 4 figure
Interacting electron systems between Fermi leads: effective one-body transmissions and correlation clouds
In order to extend the Landauer formulation of quantum transport to
correlated fermions, we consider a spinless system in which charge carriers
interact, connected to two reservoirs by non-interacting one-dimensional leads.
We show that the mapping of the embedded many-body scatterer onto an effective
one-body scatterer with interaction-dependent parameters requires to include
parts of the attached leads where the interacting region induces power law
correlations. Physically, this gives a dependence of the conductance of a
mesoscopic scatterer upon the nature of the used leads which is due to electron
interactions inside the scatterer. To show this, we consider two identical
correlated systems connected by a non-interacting lead of length
. We demonstrate that the effective one-body transmission of the
ensemble deviates by an amount from the behavior obtained
assuming an effective one-body description for each element and the combination
law of scatterers in series. is maximum for the interaction strength
around which the Luttinger liquid becomes a Mott insulator in the used model,
and vanishes when and . Analogies with the Kondo
problem are pointed out.Comment: 5 pages, 6 figure
Partial local density of states from scanning gate microscopy
Scanning gate microscopy images from measurements made in the vicinity of
quantum point contacts were originally interpreted in terms of current flow.
Some recent work has analytically connected the local density of states to
conductance changes in cases of perfect transmission, and at least
qualitatively for a broader range of circumstances. In the present paper, we
show analytically that in any time-reversal invariant system there are
important deviations that are highly sensitive to imperfect transmission.
Nevertheless, the unperturbed partial local density of states can be extracted
from a weakly invasive scanning gate microscopy experiment, provided the
quantum point contact is tuned anywhere on a conductance plateau. A
perturbative treatment in the reflection coefficient shows just how sensitive
this correspondence is to the departure from the quantized conductance value
and reveals the necessity of local averaging over the tip position. It is also
shown that the quality of the extracted partial local density of states
decreases with increasing tip radius.Comment: 16 pages, 9 figure
Spin-orbit effects in nanowire-based wurtzite semiconductor quantum dots
We study the effect of the Dresselhaus spin-orbit interaction on the
electronic states and spin relaxation rates of cylindrical quantum dots defined
on quantum wires having wurtzite lattice structure. The linear and cubic
contributions of the bulk Dresselhaus spin-orbit coupling are taken into
account, along with the influence of a weak external magnetic field. The
previously found analytic solution for the electronic states of cylindrical
quantum dots with zincblende lattice structures with Rashba interaction is
extended to the case of quantum dots with wurtzite lattices. For the electronic
states in InAs dots, we determine the spin texture and the effective g-factor,
which shows a scaling collapse when plotted as a function of an effective
renormalized dot-size dependent spin-orbit coupling strength. The
acoustic-phonon-induced spin relaxation rate is calculated and the transverse
piezoelectric potential is shown to be the dominant one.Comment: 12 pages, 5 figure
Mesoscopic behavior of the transmission phase through confined correlated electronic systems
We investigate the effect of electronic correlations on the transmission
phase of quantum coherent scatterers, considering quantum dots in the Coulomb
blockade regime connected to two single-channel leads. We focus on transmission
zeros and the associated \pi-phase lapses that have been observed in
interferometric experiments. We numerically explore two types of models for
quantum dots: (i) lattice models with up to eight sites, and (ii) resonant
level models with up to six levels. We identify different regimes of parameters
where the presence of electronic correlations is responsible for the increase
or the decrease of the number of transmission zeros vs. electrochemical
potential on the dot. However, we show that interaction effects cannot
reproduce the universal behavior of alternating resonances and phase lapses,
experimentally observed in many-electron Coulomb blockaded dots. Our numerical
results strongly suggest that the main experimentally observed features are
captured by the theory for chaotic ballistic dots of Molina et al., [Phys. Rev.
Lett. 108, 076803 (2012)] incorporating one-particle wave-function correlations
but ignoring many-particle electronic correlations.Comment: 17 pages, 14 figure
Spin relaxation near the metal-insulator transition: dominance of the Dresselhaus spin-orbit coupling
We identify the Dresselhaus spin-orbit coupling as the source of the dominant
spin-relaxation mechanism in the impurity band of doped semiconductors. The
Dresselhaus-type (i.e. allowed by bulk-inversion asymmetry) hopping terms are
derived and incorporated into a tight-binding model of impurity sites, and they
are shown to unexpectedly dominate the spin relaxation, leading to
spin-relaxation times in good agreement with experimental values. This
conclusion is drawn from two complementary approaches employed to extract the
spin-relaxation time from the effective Hamiltonian: an analytical
diffusive-evolution calculation and a numerical finite-size scaling.Comment: 4 pages, 2 figures, submitted to Phys. Rev. Let
Delocalization effects and charge reorganizations induced by repulsive interactions in strongly disordered chains
We study the delocalization effect of a short-range repulsive interaction on
the ground state of a finite density of spinless fermions in strongly
disordered one dimensional lattices. The density matrix renormalization group
method is used to explore the charge density and the sensitivity of the ground
state energy with respect to the boundary condition (the persistent current)
for a wide range of parameters (carrier density, interaction and disorder).
Analytical approaches are developed and allow to understand some mechanisms and
limiting conditions. For weak interaction strength, one has a Fermi glass of
Anderson localized states, while in the opposite limit of strong interaction,
one has a correlated array of charges (Mott insulator). In the two cases, the
system is strongly insulating and the ground state energy is essentially
invariant under a twist of the boundary conditions. Reducing the interaction
strength from large to intermediate values, the quantum melting of the solid
array gives rise to a more homogeneous distribution of charges, and the ground
state energy changes when the boundary conditions are twisted. In individual
chains, this melting occurs by abrupt steps located at sample-dependent values
of the interaction where an (avoided) level crossing between the ground state
and the first excitation can be observed. Important charge reorganizations take
place at the avoided crossings and the persistent currents are strongly
enhanced around the corresponding interaction value. These large delocalization
effects become smeared and reduced after ensemble averaging. They mainly
characterize half filling and strong disorder, but they persist away of this
optimal condition.Comment: 18 pages, 15 figures, accepted for publication in Eur. Phys. J.
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