460,934 research outputs found
Transmission through a n interacting quantum dot in the Coulomb blockade regime
The influence of electron-electron (e-e) interactions on the transmission
through a quantum dot is investigated numerically for the Coulomb blockade
regime. For vanishing magnetic fields, the conductance peak height statistics
is found to be independent of the interactions strength. It is identical to the
statistics predicted by constant interaction single electron random matrix
theory and agrees well with recent experiments. However, in contrast to these
random matrix theories, our calculations reproduces the reduced sensitivity to
magnetic flux observed in many experiments. The relevant physics is traced to
the short range Coulomb correlations providing thus a unified explanation for
the transmission statistics as well as for the large conductance peak spacing
fluctuations observed in other experiments.Comment: Final version as publishe
Left-invariant evolutions of wavelet transforms on the Similitude Group
Enhancement of multiple-scale elongated structures in noisy image data is
relevant for many biomedical applications but commonly used PDE-based
enhancement techniques often fail at crossings in an image. To get an overview
of how an image is composed of local multiple-scale elongated structures we
construct a multiple scale orientation score, which is a continuous wavelet
transform on the similitude group, SIM(2). Our unitary transform maps the space
of images onto a reproducing kernel space defined on SIM(2), allowing us to
robustly relate Euclidean (and scaling) invariant operators on images to
left-invariant operators on the corresponding continuous wavelet transform.
Rather than often used wavelet (soft-)thresholding techniques, we employ the
group structure in the wavelet domain to arrive at left-invariant evolutions
and flows (diffusion), for contextual crossing preserving enhancement of
multiple scale elongated structures in noisy images. We present experiments
that display benefits of our work compared to recent PDE techniques acting
directly on the images and to our previous work on left-invariant diffusions on
orientation scores defined on Euclidean motion group.Comment: 40 page
Theory of spin-polarized transport in ferromagnet-semiconductor structures: Unified description of ballistic and diffusive transport
A theory of spin-polarized electron transport in ferromagnet-semiconductor
heterostructures, based on a unified semiclassical description of ballistic and
diffusive transport in semiconductors, is outlined. The aim is to provide a
framework for studying the interplay of spin relaxation and transport mechanism
in spintronic devices. Transport inside the (nondegenerate) semiconductor is
described in terms of a thermoballistic current, in which electrons move
ballistically in the electric field arising from internal and external
electrostatic potentials, and are thermalized at randomly distributed
equilibration points. Spin relaxation is allowed to take place during the
ballistic motion. For arbitrary potential profile and arbitrary values of the
momentum and spin relaxation lengths, an integral equation for a spin transport
function determining the spin polarization in the semiconductor is derived. For
field-driven transport in a homogeneous semiconductor, the integral equation
can be converted into a second-order differential equation that generalizes the
spin drift-diffusion equation. The spin-polarization in ferromagnet
semiconductor structures is obtained by matching the spin-resolved chemical
potentials at the interfaces, with allowance for spin-selective interface
resistances. Illustrative examples are considered.Comment: 11 pages, 4 figures; to appear in Materials Science and Engineering
The mass of Albireo Aa and the nature of Albireo AB: New aspects from Gaia DR2
Aims: We aim to clarify the nature of Albireo AB and specifically to decipher
whether it is an optical or physical pair. We also try to determine the mass of
Albireo Aa. Methods: We scrutinize and compare the available absolute
astrometric data (from Hipparcos and Gaia DR2) of Albireo A and B, and we
investigate the relative orbit of the pair Albireo Aa,Ac using orbit solutions
based on ground-based interferometric measurements. Results: The mass of
Albireo Aa (K3 II) is surprisingly small; only an upper limit of about 0.7
solar masses could be derived. The systemic proper motion of Aa,Ac differs from
that of component B by about 10 mas/year with an uncertainty of less than 2
mas/year. Albireo AB is therefore most probably an optical double. Conclusions:
Specific astrometric and spectroscopic follow-up observations clarifying the
surprising mass estimate for Albireo Aa are recommended.Comment: 4 pages, 3 figures, accepted by Astron. Astrophy
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