8,408 research outputs found
InAs/InP single quantum wire formation and emission at 1.5 microns
Isolated InAs/InP self-assembled quantum wires have been grown using in situ
accumulated stress measurements to adjust the optimal InAs thickness. Atomic
force microscopy imaging shows highly asymmetric nanostructures with average
length exceeding more than ten times their width. High resolution optical
investigation of as-grown samples reveals strong photoluminescence from
individual quantum wires at 1.5 microns. Additional sharp features are related
to monolayer fluctuations of the two dimensional InAs layer present during the
early stages of the quantum wire self-assembling process.Comment: 4 pages and 3 figures submitted to Applied Physics Letter
Improved constraints on primordial non-Gaussianity for the Wilkinson Microwave Anisotropy Probe 5-yr data
We present new constraints on the non-linear coupling parameter fnl with the
Wilkinson Microwave Anisotropy Probe (WMAP) data. We use an updated method
based on the spherical Mexican hat wavelet (SMHW) which provides improved
constraints on the fnl parameter. This paper is a continuation of a previous
work by Curto et al. where several third order statistics based on the SMHW
were considered. In this paper, we use all the possible third order statistics
computed from the wavelet coefficient maps evaluated at 12 angular scales. The
scales are logarithmically distributed from 6.9 arcmin to 500 arcmin. Our
analysis indicates that fnl is constrained to -18 < fnl < +80 at 95% confidence
level (CL) for the combined V+W WMAP map. This value has been corrected by the
presence of undetected point sources, which adds a positive contribution of
Delta_fnl = 6 +- 5. Our result excludes at ~99% CL the best-fitting value
fnl=87 reported by Yadav & Wandelt. We have also constrained fnl for the Q, V
and W frequency bands separately, finding compatibility with zero at 95 % CL
for the Q and V bands but not for the W band. We have performed some further
tests to understand the cause of this deviation which indicate that systematics
associated to the W radiometers could be responsible for this result. Finally
we have performed a Galactic North-South analysis for fnl. We have not found
any asymmetry, i.e. the best-fitting fnl for the northern pixels is compatible
with the best-fitting fnl for the southern pixels.Comment: 6 pages, 4 figures, 4 tables. Accepted for publication in Ap
Charge control in laterally coupled double quantum dots
We investigate the electronic and optical properties of InAs double quantum
dots grown on GaAs (001) and laterally aligned along the [110] crystal
direction. The emission spectrum has been investigated as a function of a
lateral electric field applied along the quantum dot pair mutual axis. The
number of confined electrons can be controlled with the external bias leading
to sharp energy shifts which we use to identify the emission from neutral and
charged exciton complexes. Quantum tunnelling of these electrons is proposed to
explain the reversed ordering of the trion emission lines as compared to that
of excitons in our system.Comment: 4 pages, 4 figures submitted to PRB Rapid Com
On the regularity of the covariance matrix of a discretized scalar field on the sphere
We present a comprehensive study of the regularity of the covariance matrix
of a discretized field on the sphere. In a particular situation, the rank of
the matrix depends on the number of pixels, the number of spherical harmonics,
the symmetries of the pixelization scheme and the presence of a mask. Taking
into account the above mentioned components, we provide analytical expressions
that constrain the rank of the matrix. They are obtained by expanding the
determinant of the covariance matrix as a sum of determinants of matrices made
up of spherical harmonics. We investigate these constraints for five different
pixelizations that have been used in the context of Cosmic Microwave Background
(CMB) data analysis: Cube, Icosahedron, Igloo, GLESP and HEALPix, finding that,
at least in the considered cases, the HEALPix pixelization tends to provide a
covariance matrix with a rank closer to the maximum expected theoretical value
than the other pixelizations. The effect of the propagation of numerical errors
in the regularity of the covariance matrix is also studied for different
computational precisions, as well as the effect of adding a certain level of
noise in order to regularize the matrix. In addition, we investigate the
application of the previous results to a particular example that requires the
inversion of the covariance matrix: the estimation of the CMB temperature power
spectrum through the Quadratic Maximum Likelihood algorithm. Finally, some
general considerations in order to achieve a regular covariance matrix are also
presented.Comment: 36 pages, 12 figures; minor changes in the text, matches published
versio
Multi-resolution internal template cleaning: An application to the Wilkinson Microwave Anisotropy Probe 7-yr polarization data
Cosmic microwave background (CMB) radiation data obtained by different
experiments contain, besides the desired signal, a superposition of microwave
sky contributions. We present a fast and robust method, using a wavelet
decomposition on the sphere, to recover the CMB signal from microwave maps. An
application to \textit{WMAP} polarization data is presented, showing its good
performance particularly in very polluted regions of the sky. The applied
wavelet has the advantages of requiring little computational time in its
calculations, being adapted to the \textit{HEALPix} pixelization scheme, and
offering the possibility of multi-resolution analysis. The decomposition is
implemented as part of a fully internal template fitting method, minimizing the
variance of the resulting map at each scale. Using a characterization
of the noise, we find that the residuals of the cleaned maps are compatible
with those expected from the instrumental noise. The maps are also comparable
to those obtained from the \textit{WMAP} team, but in our case we do not make
use of external data sets. In addition, at low resolution, our cleaned maps
present a lower level of noise. The E-mode power spectrum is
computed at high and low resolution; and a cross power spectrum
is also calculated from the foreground reduced maps of temperature given by
\textit{WMAP} and our cleaned maps of polarization at high resolution. These
spectra are consistent with the power spectra supplied by the \textit{WMAP}
team. We detect the E-mode acoustic peak at , as predicted by
the standard model. The B-mode power spectrum is
compatible with zero.Comment: 8 pages, 6 figures. Some changes have been done from the original
manuscript. This paper is accepted by MNRA
Exciton Gas Compression and Metallic Condensation in a Single Semiconductor Quantum Wire
We study the metal-insulator transition in individual self-assembled quantum
wires and report optical evidences of metallic liquid condensation at low
temperatures. Firstly, we observe that the temperature and power dependence of
the single nanowire photoluminescence follow the evolution expected for an
electron-hole liquid in one dimension. Secondly, we find novel spectral
features that suggest that in this situation the expanding liquid condensate
compresses the exciton gas in real space. Finally, we estimate the critical
density and critical temperature of the phase transition diagram at
cm and K, respectively.Comment: 4 pages, 5 figure
Exploring two-spin internal linear combinations for the recovery of the CMB polarization
We present a methodology to recover cosmic microwave background (CMB)
polarization in which the quantity is linearly combined at
different frequencies using complex coefficients. This is the most general
linear combination of the and Stokes parameters which preserves the
physical coherence of the residual contribution on the CMB estimation. The
approach is applied to the internal linear combination (ILC) and the internal
template fitting (ITF) methodologies. The variance of of the resulting map
is minimized to compute the coefficients of the linear combination. One of the
key aspects of this procedure is that it serves to account for a global
frequency-dependent shift of the polarization phase. Although in the standard
case, in which no global E-B transference depending on frequency is expected in
the foreground components, minimizing is
similar to minimizing and separately (as previous methodologies proceed), multiplying
and by different coefficients induces arbitrary changes in the
polarization angle and it does not preserve the coherence between the spinorial
components. The approach is tested on simulations, obtaining a similar residual
level with respect to the one obtained with other implementations of the ILC,
and perceiving the polarization rotation of a toy model with the frequency
dependence of the Faraday rotation.Comment: 14 pages, 8 figures, 2 tables. Accepted for publication in MNRA
Non-Gaussianity analysis on local morphological measures of WMAP data
The decomposition of a signal on the sphere with the steerable wavelet
constructed from the second Gaussian derivative gives access to the
orientation, signed-intensity, and elongation of the signal's local features.
In the present work, the non-Gaussianity of the WMAP temperature data of the
cosmic microwave background (CMB) is analyzed in terms of the first four
moments of the statistically isotropic random fields associated with these
local morphological measures, at wavelet scales corresponding to angular sizes
between 27.5 arcminutes and 30 degrees on the celestial sphere. While no
detection is made neither in the orientation analysis nor in the elongation
analysis, a strong detection is made in the excess kurtosis of the
signed-intensity of the WMAP data. The non-Gaussianity is observed with a
significance level below 0.5% at a wavelet scale corresponding to an angular
size around 10 degrees, and confirmed at neighbour scales. This supports a
previous detection of an excess of kurtosis in the wavelet coefficient of the
WMAP data with the axisymmetric Mexican hat wavelet (Vielva et al. 2004).
Instrumental noise and foreground emissions are not likely to be at the origin
of the excess of kurtosis. Large-scale modulations of the CMB related to some
unknown systematics are rejected as possible origins of the detection. The
observed non-Gaussianity may therefore probably be imputed to the CMB itself,
thereby questioning the basic inflationary scenario upon which the present
concordance cosmological model relies. Taking the CMB temperature angular power
spectrum of the concordance cosmological model at face value, further analysis
also suggests that this non-Gaussianity is not confined to the directions on
the celestial sphere with an anomalous signed-intensity.Comment: 10 pages, 3 figures. Version 2 includes minor changes to match
version accepted for publication in MNRA
Searching for a dipole modulation in the large-scale structure of the Universe
Several statistical anomalies in the CMB temperature anisotropies seem to
defy the assumption of a homogeneous and isotropic universe. In particular, a
dipole modulation has been detected both in WMAP and Planck data. We adapt the
methodology proposed by Eriksen et al. (2007) on CMB data to galaxy surveys,
tracing the large-scale structure. We analyse the NRAO VLA Sky Survey (NVSS)
data at a resolution of ~2 degrees for three different flux thresholds: 2.5,
5.0 and 10.0 mJy respectively. No evidence of a dipole modulation is found.
This result suggests that the origin of the dipole asymmetry found in the CMB
cannot be assigned to secondary anisotropies produced at redshifts around z =
1. However, it could still have been generated at redshifts higher or lower,
such as the integrated Sachs-Wolfe effect produced by the local structures.
Other all-sky surveys, like the infrared WISE catalogue, could help to explore
with a high sensitivity a redshift interval closer than the one probed with
NVSS.Comment: 6 pages, 2 figures. Some minor changes have been done from the
original manuscript. This paper is accepted by MNRA
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