430 research outputs found
Self-consistent electronic subband structure of undoped InAs/GaSb-based type II and broken-gap quantum well systems
Motivated by a very recent experimental work on investigating electronic properties of InAs/GaSb-based type II and broken-gap quantum well structures, in this article we present a simple and transparent theoretical approach to calculate electronic subband structure in such device systems. The theoretical model is developed on the basis of solving self-consistently the Schrödinger equation for the eigenfunctions and eigenvalues coupled with the Poisson equation for the confinement potentials, in which the effects such as charge distribution and depletion are considered. In particular, we examine the effect of a GaSb cap layer on electronic properties of the quantum well systems in conjunction with experiments and experimental findings. The results obtained from the proposed self-consistent calculation can be used to understand important experimental findings and are in line with those measured experimentally.One of us W.X. was supported by the Australian Research
Council and Chinese Academy of Sciences
Magneto-infrared modes in InAs-AlSb-GaSb coupled quantum wells
We have studied a series of InAs/GaSb coupled quantum wells using
magneto-infrared spectroscopy for high magnetic fields up to 33T within
temperatures ranging from 4K to 45K in both Faraday and tilted field
geometries. This type of coupled quantum wells consists of an electron layer in
the InAs quantum well and a hole layer in the GaSb quantum well, forming the
so-called two dimensional electron-hole bilayer system. Unlike the samples
studied in the past, the hybridization of the electron and hole subbands in our
samples is largely reduced by having narrower wells and an AlSb barrier layer
interposed between the InAs and the GaSb quantum wells, rendering them weakly
hybridized. Previous studies have revealed multiple absorption modes near the
electron cyclotron resonance of the InAs layer in moderately and strongly
hybridized samples, while only a single absorption mode was observed in the
weakly hybridized samples. We have observed a pair of absorption modes
occurring only at magnetic fields higher than 14T, which exhibited several
interesting phenomena. Among which we found two unique types of behavior that
distinguishes this work from the ones reported in the literature. This pair of
modes is very robust against rising thermal excitations and increasing magnetic
fields alligned parallel to the heterostructures. While the previous results
were aptly explained by the antilevel crossing gap due to the hybridization of
the electron and hole wavefunctions, i.e. conduction-valence Landau level
mixing, the unique features reported in this paper cannot be explained within
the same concept. The unusual properties found in this study and their
connection to the known models for InAs/GaSb heterostructures will be
disccused; in addition, several alternative ideas will be proposed in this
paper and it appears that a spontaneous phase separation can account for most
of the observed features
Structural and Photometric Classification of Galaxies - I. Calibration Based on a Nearby Galaxy Sample
In this paper we define an observationally robust, multi-parameter space for
the classification of nearby and distant galaxies. The parameters include
luminosity, color, and the image-structure parameters: size, image
concentration, asymmetry, and surface brightness. Based on an initial
calibration of this parameter space using the ``normal'' Hubble-types surveyed
by Frei et al. (1996), we find that only a subset of the parameters provide
useful classification boundaries for this sample. Interestingly, this subset
does not include distance-dependent scale parameters, such as size or
luminosity. The essential ingredient is the combination of a spectral index
(e.g., color) with parameters of image structure and scale: concentration,
asymmetry, and surface-brightness. We refer to the image structure parameters
(concentration and asymmetry) as indices of ``form.'' We define a preliminary
classification based on spectral index, form, and surface-brightness (a scale)
that successfully separates normal galaxies into three classes. We
intentionally identify these classes with the familiar labels of Early,
Intermediate, and Late. This classification, or others based on the above four
parameters can be used reliably to define comparable samples over a broad range
in redshift. The size and luminosity distribution of such samples will not be
biased by this selection process except through astrophysical correlations
between spectral index, form, and surface-brightness.Comment: to appear in AJ (June, 2000); 34 pages including 4 tables and 12
figure
Automatic Redshift Determination by use of Principal Component Analysis --- I: Fundamentals
With the advent of very large redshift surveys of tens to hundreds of
thousands of galaxies reliable techniques for automatically determining galaxy
redshifts are becoming increasingly important. The most common technique
currently in common use is the cross-correlation of a galactic spectrum with a
set of templates. This series of papers presents a new method based on
Principal Component Analysis. The method generalizes the cross-correlation
approach by replacing the individual templates by a simultaneous linear
combination of orthogonal templates. This effectively eliminates the mismatch
between templates and data and provides for the possibility of better error
estimates. In this paper, the first of a series, the basic mathematics are
presented along with a simple demonstration of the application.Comment: 23 pages, 9 Figures, minor revisions, accepted for publication in
Astrophysical Journa
VVV High proper motion stars I. The catalogue of bright Ks stars
The final, definitive version of this paper has been published in Monthly Notices of the Royal Astronomical Society, Vol. 464(1): 1247-1258, January 2017, DOI: 10.1093/mnras/stw2357, first published on line September 16, 2016, published by Oxford University Press on behalf of MNRAS.Knowledge of the stellar content near the Sun is important for a broad range of topics ranging from the search for planets to the study of Milky Way structure. The most powerful method for identifying potentially nearby stars is proper motion (PM) surveys. All old optical surveys avoid, or are at least substantially incomplete, near the Galactic plane. The depth and breadth of the "Vista Variables in Via Lactea" (VVV) near-IR survey significantly improves this situation. Taking advantage of the VVV survey database, we have measured PMs in the densest regions of the MW bulge and southern plane in order to complete the census of nearby objects. We have developed a custom PM pipeline based on VVV catalogues from the Cambridge Astronomy Survey Unit (CASU), by comparing the first epoch of JHKs with the multi-epoch Ks-bands acquired later. Taking advantage of the large time baseline between the 2MASS and the VVV observations, we also obtained 2MASS-VVV PMs. We present a near-IR proper motion catalogue for the whole area of the VVV survey, which includes 3003 moving stellar sources. All of these have been visually inspected and are real PM objects. Our catalogue is in very good agreement with the proper motion data supplied in IR catalogues outside the densest zone of the MW. The majority of the PM objects in our catalogue are nearby M-dwarfs, as expected. This new database allow us to identify 57 common proper motion binary candidates, among which are two new systems within ~30pc of the Sun.Peer reviewe
Spectral Classification and Luminosity Function of Galaxies in the Las Campanas Redshift Survey
We construct a spectral classification scheme for the galaxies of the Las
Campanas Redshift Survey (LCRS) based on a principal component analysis of the
measured galaxy spectra. We interpret the physical significance of our six
spectral types and conclude that they are sensitive to morphological type and
the amount of active star formation. In this first analysis of the LCRS to
include spectral classification, we estimate the general luminosity function,
expressed as a weighted sum of the type-specific luminosity functions. In the
R-band magnitude range of -23 < M <= -16.5, this function exhibits a broad
shoulder centered near M = -20, and an increasing faint-end slope which
formally converges on an alpha value of about -1.8 in the faint limit. The
Schechter parameterization does not provide a good representation in this case,
a fact which may partly explain the reported discrepancy between the luminosity
functions of the LCRS and other redshift catalogs such as the Century Survey
(Geller et al. 1997). The discrepancy may also arise from environmental effects
such as the density-morphology relationship for which we see strong evidence in
the LCRS galaxies. However, the Schechter parameterization is more effective
for the luminosity functions of the individual spectral types. The data show a
significant, progressive steepening of the faint-end slope, from alpha = +0.5
for early-type objects, to alpha = -1.8 for the extreme late-type galaxies. The
extreme late-type population has a sufficiently high space density that its
contribution to the general luminosity function is expected to dominate fainter
than M = -16. We conclude that an evaluation of type-dependence is essential to
any assessment of the general luminosity function.Comment: 21 pages (LaTeX), 7 figures (Postscript). To appear in the
Astrophysical Journal. The discussion of environmental dependence of
luminosity functions has been shortened; the material from the earlier
version now appears in a separate manuscript (astro-ph/9805197
The discovery of an M4+T8.5 binary system
The original article can be found at: http://www3.interscience.wiley.com Copyright Blackwell Publishing / Royal Astronomical Society. DOI: 10.1111/j.1365-2966.2009.14620.xWe report the discovery of a T8.5 dwarf, which is a companion to the M4 dwarf Wolf 940. [Please see original online abstract for complete version with correct notation]Peer reviewe
Tree structure of the percolating Universe
We present a numerical study of topological descriptors of initially Gaussian
and scale-free density perturbations evolving via gravitational instability in
an expanding universe. We carefully evaluate and avoid numerical contamination
in making accurate measurements on simulated fields on a grid in a finite box.
Independent of extent of non linearity, the measured Euler number of the
excursion set at the percolation threshold, , is positive and nearly
equal to the number of isolated components, suggesting that these structures
are trees. Our study of critical point counts reconciles the clumpy appearance
of the density field at with measured filamentary local curvature.
In the Gaussian limit, we measure in contrast to widely
held belief that , where is the variance of
the density field.Comment: 4 pages, 2 figures, Accepted for publication in Phys. Rev. Let
The First Hour of Extra-galactic Data of the Sloan Digital Sky Survey Spectroscopic Commissioning: The Coma Cluster
On 26 May 1999, one of the Sloan Digital Sky Survey (SDSS) fiber-fed
spectrographs saw astronomical first light. This was followed by the first
spectroscopic commissioning run during the dark period of June 1999. We present
here the first hour of extra-galactic spectroscopy taken during these early
commissioning stages: an observation of the Coma cluster of galaxies. Our data
samples the Southern part of this cluster, out to a radius of 1.5degrees and
thus fully covers the NGC 4839 group. We outline in this paper the main
characteristics of the SDSS spectroscopic systems and provide redshifts and
spectral classifications for 196 Coma galaxies, of which 45 redshifts are new.
For the 151 galaxies in common with the literature, we find excellent agreement
between our redshift determinations and the published values. As part of our
analysis, we have investigated four different spectral classification
algorithms: spectral line strengths, a principal component decomposition, a
wavelet analysis and the fitting of spectral synthesis models to the data. We
find that a significant fraction (25%) of our observed Coma galaxies show signs
of recent star-formation activity and that the velocity dispersion of these
active galaxies (emission-line and post-starburst galaxies) is 30% larger than
the absorption-line galaxies. We also find no active galaxies within the
central (projected) 200 h-1 Kpc of the cluster. The spatial distribution of our
Coma active galaxies is consistent with that found at higher redshift for the
CNOC1 cluster survey. Beyond the core region, the fraction of bright active
galaxies appears to rise slowly out to the virial radius and are randomly
distributed within the cluster with no apparent correlation with the potential
merger of the NGC 4839 group. [ABRIDGED]Comment: Accepted in AJ, 65 pages, 20 figures, 5 table
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