8,374 research outputs found
Absence of charge backscattering in the nonequilibrium current of normal-superconductor structures
We study the nonequilibrium transport properties of a
normal-superconductor-normal structure, focussing on the effect of adding an
impurity in the superconducting region. Current conservation requires the
superfluid velocity to be nonzero, causing a distortion of the quasiparticle
dispersion relation within the superconductor. For weakly reflecting interfaces
we find a regime of intermediate voltages in which Andreev transmission is the
only permitted mechanism for quasiparticles to enter the superconductor.
Impurities in the superconductor can only cause Andreev reflection of these
quasiparticles and thus cannot degrade the current. At higher voltages, a state
of gapless superconductivity develops which is sensitive to the presence of
impurities.Comment: Latex file, 11 pages, 2 figures available upon request
[email protected], to be published in Journal of Physics: Condensed Matte
PMAS: The Potsdam Multi Aperture Spectrophotometer. II. The Wide Integral Field Unit PPak
PPak is a new fiber-based Integral Field Unit (IFU), developed at the
Astrophysical Institute Potsdam, implemented as a module into the existing PMAS
spectrograph. The purpose of PPak is to provide both an extended field-of-view
with a large light collecting power for each spatial element, as well as an
adequate spectral resolution. The PPak system consists of a fiber bundle with
331 object, 36 sky and 15 calibration fibers. The object and sky fibers collect
the light from the focal plane behind a focal reducer lens. The object fibers
of PPak, each 2.7 arcseconds in diameter, provide a contiguous hexagonal
field-of-view of 74 times 64 arcseconds on the sky, with a filling factor of
60%. The operational wavelength range is from 400 to 900nm. The PPak-IFU,
together with the PMAS spectrograph, are intended for the study of extended,
low surface brightness objects, offering an optimization of total
light-collecting power and spectral resolution. This paper describes the
instrument design, the assembly, integration and tests, the commissioning and
operational procedures, and presents the measured performance at the telescope.Comment: 14 pages, 21 figures, accepted at PAS
Anderson localization in a periodic photonic lattice with a disordered boundary
We investigate experimentally the light evolution inside a two-dimensional
finite periodic array of weakly- coupled optical waveguides with a disordered
boundary. For a completely localized initial condition away from the surface,
we find that the disordered boundary induces an asymptotic localization in the
bulk, centered around the initial position of the input beam.Comment: 3 pages, 4 figure
Optimization by thermal cycling
Thermal cycling is an heuristic optimization algorithm which consists of
cyclically heating and quenching by Metropolis and local search procedures,
respectively, where the amplitude slowly decreases. In recent years, it has
been successfully applied to two combinatorial optimization tasks, the
traveling salesman problem and the search for low-energy states of the Coulomb
glass. In these cases, the algorithm is far more efficient than usual simulated
annealing. In its original form the algorithm was designed only for the case of
discrete variables. Its basic ideas are applicable also to a problem with
continuous variables, the search for low-energy states of Lennard-Jones
clusters.Comment: Submitted to Proceedings of the Workshop "Complexity, Metastability
and Nonextensivity", held in Erice 20-26 July 2004. Latex, 7 pages, 3 figure
Diffusion of single active-dipolar cubes in applied fields
"Active matter" refers to a class of out-of-equilibrium systems whose ability
to transform environmental energy to kinetic energy is sought after in multiple
fields of science and at very different length scales. At microscopic scales,
an important challenge lies in overpowering the particles reorientation due to
thermal fluctuations, especially in nano-sized systems, to create non-random,
directed motion, needed for a wide range of possible applications. In this
article, we employ molecular dynamics simulations to show that the diffusion of
a self-propelling dipolar nanocube can be enhanced in a pre-defined direction
with the help of a moderately strong applied magnetic field, overruling the
effect of the thermal fluctuations. Furthermore, we show that the direction of
diffusion is given by the orientation of the net internal magnetisation of the
cube. This can be used to determine experimentally the latter in synthetically
crafted active cobalt ferrite nanocubes.Comment: 10 pages, 7 Figures, 1 Tabl
Optimizing omnidirectional reflection by multilayer mirrors
Periodic layered media can reflect strongly for all incident angles and
polarizations in a given frequency range. Quarter-wave stacks at normal
incidence are commonplace in the design of such omnidirectional reflectors. We
discuss alternative design criteria to optimize these systems.Comment: 9 pages, 6 figures. To be published in J. Opt. A: Pure and Applied
Optic
Relationship between Hubble type and spectroscopic class in local galaxies
We compare the Hubble type and the spectroscopic class of the galaxies with
spectra in SDSS/DR7. As it is long known, elliptical galaxies tend to be red
whereas spiral galaxies tend to be blue, however, this relationship presents a
large scatter, which we measure and quantify in detail. We compare the
Automatic Spectroscopic K-means based classification (ASK) with most of the
commonly used morphological classifications. All of them provide consistent
results. Given a spectral class, the morphological type wavers with a standard
deviation between 2 and 3 T types, and the same large dispersion characterizes
the variability of spectral classes fixed the morphological type. The
distributions of Hubble types given an ASK class are very skewed -- they
present long tails that go to the late morphological types for the red
galaxies, and to the early morphological types for the blue spectroscopic
classes. The scatter is not produced by problems in the classification, and it
remains when particular subsets are considered. A considerable fraction of the
red galaxies are spirals (40--60 %), but they never present very late Hubble
types (Sd or later). Even though red spectra are not associated with
ellipticals, most ellipticals do have red spectra: 97 % of the ellipticals in
the morphological catalog by Nair & Abraham, used here for reference, belong to
ASK 0, 2 or 3. It contains only a 3 % of blue ellipticals. The galaxies in the
green valley class (ASK~5) are mostly spirals, and the AGN class (ASK 6)
presents a large scatter of Hubble types from E to Sd. From redshift 0.25 to
now the galaxies redden from ASK 2 to ASK 0, as expected from the passive
evolution of their stellar populations. Two of the ASK classes (1 and 4) gather
edge-on spirals, a property of interest in studies requiring knowing the
intrinsic shape of a galaxy (e.g., weak lensing calibration).Comment: Accepted for publication in ApJ. 16 pages. 12 Figs. 2 summary table
Self-consistent scattering description of transport in normal-superconductor structures
We present a scattering description of transport in several
normal-superconductor structures. We show that the related requirements of
self-consistency and current conservation introduce qualitative changes in the
transport behavior when the current in the superconductor is not negligible.
The energy thresholds for quasiparticle propagation in the superconductor are
sensitive to the existence of condensate flow (). This dependence is
responsible for a rich variety of transport regimes, including a voltage range
in which only Andreev transmission is possible at the interfaces, and a state
of gapless superconductivity which may survive up to high voltages if
temperature is low. The two main effects of current conservation are a shift
towards lower voltages of the first peak in the differential conductance and an
enhancement of current caused by the greater availability of charge
transmitting scattering channels.Comment: 31 pages, 10 PS figures, Latex file, psfig.sty file is added. To
appear in Phys. Rev. B (Jan 97
Non Abelian TQFT and scattering of self dual field configuration
A non-abelian topological quantum field theory describing the scattering of
self-dual field configurations over topologically non-trivial Riemann surfaces,
arising from the reduction of 4-dim self-dual Yang-Mills fields, is introduced.
It is shown that the phase space of the theory can be exactly quantized in
terms of the space of holomorphic structures over stable vector bundles of
degree zero over Riemann surfaces. The Dirac monopoles are particular static
solutions of the field equations. Its relation to topological gravity is
discussed.Comment: 13 pages, Late
Supercurrent flow through an effective double barrier structure
Supercurrent flow is studied in a structure that in the Ginzburg-Landau
regime can be described in terms of an effective double barrier potential. In
the limit of strongly reflecting barriers, the passage of Cooper pairs through
such a structure may be viewed as a realization of resonant tunneling with a
rigid wave function. For interbarrier distances smaller than no
current-carrying solutions exist. For distances between and , four
solutions exist. The two symmetric solutions obey a current-phase relation of
, while the two asymmetric solutions satisfy
for all allowed values of the current. As the distance
exceeds , a new group of four solutions appears, each contaning
soliton-type oscillations between the barriers. We prove the inexistence of a
continuous crossover between the physical solutions of the nonlinear
Ginzburg-Landau equation and those of the corresponding linearized
Schr\"odinger equation. We also show that under certain conditions a repulsive
delta function barrier may quantitatively describe a SNS structure. We are thus
able to predict that the critical current of a SNSNS structure vanishes as
, where is lower than the bulk critical temperature.Comment: 20 pages, RevTex, to appear in Phys. Rev. B, 6 figures on request at
[email protected]
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