874 research outputs found
Proximity Effect, Andreev Reflections, and Charge Transport in Mesoscopic Superconducting-Semiconducting Heterostructures
In the quasi-twodimensional (Q2D) electron gas of an InAs channel between an
AlSb substrate and superconducting Niobium layers the proximity effect induces
a pair potential so that a Q2D mesoscopic
superconducting-normal-superconducting (SNS) junction forms in the channel. The
pair potential is calculated with quasiclassical Green's functions in the clean
limit. For such a junction alternating Josephson currents and current-voltage
characteristics (CVCs) are computed, using the non-equilibrium quasiparticle
wavefunctions which solve the time-dependent Bogoliubov-de Gennes Equations.
The CVCs exhibit features found experimentally by the Kroemer group: A steep
rise of the current at small voltages ("foot") changes at a "corner current" to
a much slower increase of current with higher voltages, and the zero-bias
differential resistance increases with temperature. Phase-coherent multiple
Andreev reflections and the associated Cooper pair transfers are the physical
mechanisms responsible for the oscillating Josephson currents and the CVCs.
Additional experimental findings not reproduced by the theory require model
improvements, especially a consideration of the external current leads which
should give rise to hybrid quasiparticle/collective mode excitations.Comment: 8 pages, 4 figures (consisting of 5 .ps-files), added referenc
On the challenge to improve the density response with unusual gradient approximations
Certain excitations, especially ones of long-range charge transfer character,
are poorly described by time-dependent density functional theory (TDDFT) when
typical (semi-)local functionals are used. A proper description of these
excitations would require an exchange-correlation response differing
substantially from the usual (semi-)local one. It has recently been shown that
functionals of the generalized gradient approximation (GGA) type can yield
unusual potentials, mimicking features of the exact exchange derivative
discontinuity and showing divergences on orbital nodal surfaces. We here
investigate whether these unusual potential properties translate into
beneficial response properties. Using the Sternheimer formalism we closely
investigate the response obtained with the 2013 exchange approximation by
Armiento and K\"ummel (AK13) and the 1988 exchange approximation by Becke
(B88), both of which show divergences on orbital nodal planes. Numerical
calculations for Na2 as well as analytical and numerical calculations for the
hydrogen atom show that the response of AK13 behaves qualitatively different
from usual semi local functionals. However, the AK13 functional leads to
fundamental instabilities in the asymptotic region that prevent its practical
application in TDDFT. Our findings may help the development of future improved
functionals, and corroborate that the frequency-dependent Sternheimer formalism
is excellently suited for running and analyzing TDDFT calculations
The Hubble Legacy Archive ACS Grism Data
A public release of slitless spectra, obtained with ACS/WFC and the G800L
grism, is presented. Spectra were automatically extracted in a uniform way from
153 archival fields (or "associations") distributed across the two Galactic
caps, covering all observations to 2008. The ACS G800L grism provides a
wavelength range of 0.55-1.00 \mu40 \ \AA / pixel\sim 80\ \AA32,149i_{\rm
AB}0.2-4.6$.Comment: Accepted for publication in Astronomy and Astrophysics; 29 pages, 16
Figures, 4 Tables in text and 3Tables in Appendi
Effect of growth conditions on optical properties of CdSe/ZnSe single quantum dots
In this work, we have investigated the optical properties of two samples of
CdSe quantum dots by using submicro-photoluminescence spectroscopy. The effect
of vicinal-surface GaAs substrates on their properties has been also assessed.
The thinner sample, grown on a substrate with vicinal surface, includes only
dots with a diameter of less than 10 nm (type A islands). Islands of an average
diameter of about 16 nm (type B islands) that are related to a phase transition
via a Stranski-Krastanow growth process are also distributed in the thicker
sample grown on an oriented substrate. We have studied the evolution of
lineshapes of PL spectra for these two samples by improving spatial resolution
that was achieved using nanoapertures or mesa structures. It was found that the
use of a substrate with the vicinal surface leads to the suppression of
excitonic PL emitted from a wetting layer.Comment: 2pages, 2 figures, Proceedings of International Conference On
Superlattices Nano-Structures And Nano-Devices, July, Toulouse, France, to
appear in the special issue of Physica
Simple iterative construction of the optimized effective potential for orbital functionals, including exact exchange
For exchange-correlation functionals that depend explicitly on the Kohn-Sham
orbitals, the potential V_{\mathrm{xc}\sigma}(\re) must be obtained as the
solution of the optimized effective potential (OEP) integral equation. This is
very demanding and has limited the use of orbital functionals like exact
exchange. We demonstrate that the OEP can be obtained iteratively by solving a
system of partial differential equations instead of an integral equation. This
amounts to calculating the orbital shifts that exactify the Krieger-Li-Iafrate
(KLI) approximation. Unoccupied orbitals do not need to be calculated. Accuracy
and efficiency of the method are shown for atoms and clusters using the exact
exchange energy. Counter-intuitive asymptotic limits of the exact OEP, not
accessible from previous constructions, are presented.Comment: Physical Review Letters, accepted for publication. 4 pages, 1 figur
Dynamics of conversion of supercurrents into normal currents, and vice versa
The generation and destruction of the supercurrent in a superconductor (S)
between two resistive normal (N) current leads connected to a current source is
computed from the source equation for the supercurrent density. This equation
relates the gradient of the pair potential's phase to electron and hole
wavepackets that create and destroy Cooper pairs in the N/S interfaces. Total
Andreev reflection and supercurrent transmission of electrons and holes are
coupled together by the phase rigidity of the non-bosonic Cooper-pair
condensate. The calculations are illustrated by snapshots from a computer film.Comment: 8 pages, 1 figure, accepted by Phys. Rev.
Two Avenues to Self-Interaction Correction within Kohn-Sham Theory: Unitary Invariance is the Shortcut
The most widely-used density functionals for the exchange-correlation energy
are inexact for one-electron systems. Their self-interaction errors can be
severe in some applications. The problem is not only to correct the
self-interaction error, but to do so in a way that will not violate
size-consistency and will not go outside the standard Kohn-Sham density
functional theory. The solution via the optimized effective potential (OEP)
method will be discussed, first for the Perdew-Zunger self-interaction
correction (whose performance for molecules is briefly summarized) and then for
the more modern self-interaction corrections based upon unitarily-invariant
indicators of iso-orbital regions. For the latter approaches, the OEP
construction is greatly simplified. The kinetic-energy-based iso-orbital
indicator \tau^W_\sigma(\re)/\tau_\sigma(\re) will be discussed and plotted,
along with an alternative exchange-based indicator
The Hubble Legacy Archive NICMOS Grism Data
The Hubble Legacy Archive (HLA) aims to create calibrated science data from
the Hubble Space Telescope archive and make them accessible via user-friendly
and Virtual Observatory (VO) compatible interfaces. It is a collaboration
between the Space Telescope Science Institute (STScI), the Canadian Astronomy
Data Centre (CADC) and the Space Telescope - European Coordinating Facility
(ST-ECF). Data produced by the Hubble Space Telescope (HST) instruments with
slitless spectroscopy modes are among the most difficult to extract and
exploit. As part of the HLA project, the ST-ECF aims to provide calibrated
spectra for objects observed with these HST slitless modes. In this paper, we
present the HLA NICMOS G141 grism spectra. We describe in detail the
calibration, data reduction and spectrum extraction methods used to produce the
extracted spectra. The quality of the extracted spectra and associated direct
images is demonstrated through comparison with near-IR imaging catalogues and
existing near-IR spectroscopy. The output data products and their associated
metadata are publicly available through a web form at http://hla.stecf.org and
via VO interfaces. In total, 2470 spectra of 1923 unique targets are included
in the current release.Comment: 18 pages, 21 figures, accepted for publication in Astronomy &
Astrophysic
Equilibrium and time-dependent Josephson current in one-dimensional superconducting junctions
We investigate the transport properties of a one-dimensional
superconductor-normal metal-superconductor (S-N-S) system described within the
tight-binding approximation. We compute the equilibrium dc Josephson current
and the time-dependent oscillating current generated after the switch-on of a
constant bias. In the first case an exact embedding procedure to calculate the
Nambu-Gorkov Keldysh Green's function is employed and used to derive the
continuum and bound states contributions to the dc current. A general formalism
to obtain the Andreev bound states (ABS) of a normal chain connected to
superconducting leads is also presented. We identify a regime in which all
Josephson current is carried by the ABS and obtain an analytic formula for the
current-phase relation in the limit of long chains. In the latter case the
condition for perfect Andreev reflections is expressed in terms of the
microscopic parameters of the model, showing a limitation of the so called
wide-band-limit (WBL) approximation. When a finite bias is applied to the S-N-S
junction we compute the exact time-evolution of the system by solving
numerically the time-dependent Bogoliubov-deGennes equations. We provide a
microscopic description of the electron dynamics not only inside the normal
region but also in the superconductors, thus gaining more information with
respect to WBL-based approaches. Our scheme allows us to study the ac regime as
well as the transient dynamics whose characteristic time-scale is dictated by
the velocity of multiple Andreev reflections
Active Brownian Motion Tunable by Light
Active Brownian particles are capable of taking up energy from their
environment and converting it into directed motion; examples range from
chemotactic cells and bacteria to artificial micro-swimmers. We have recently
demonstrated that Janus particles, i.e. gold-capped colloidal spheres,
suspended in a critical binary liquid mixture perform active Brownian motion
when illuminated by light. In this article, we investigate in some more details
their swimming mechanism leading to active Brownian motion. We show that the
illumination-borne heating induces a local asymmetric demixing of the binary
mixture generating a spatial chemical concentration gradient, which is
responsible for the particle's self-diffusiophoretic motion. We study this
effect as a function of the functionalization of the gold cap, the particle
size and the illumination intensity: the functionalization determines what
component of the binary mixture is preferentially adsorbed at the cap and the
swimming direction (towards or away from the cap); the particle size determines
the rotational diffusion and, therefore, the random reorientation of the
particle; and the intensity tunes the strength of the heating and, therefore,
of the motion. Finally, we harness this dependence of the swimming strength on
the illumination intensity to investigate the behaviour of a micro-swimmer in a
spatial light gradient, where its swimming properties are space-dependent
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