12,256 research outputs found
Maxbias curves of robust location estimators based on subranges.
A maxbias curve is a powerful tool to describe the robustness of an estimator. It tells us how much an estimator can change due to a given fraction of contamination. In this paper, maxbias curves are computed for some univariate location estimators based on subranges: midranges, trimmed means and the univariate Minimum Volume Ellipsoid (MVE) location estimators. These estimators are intuitively appealing and easy to calculate.Breakdown value; Location estimator; Maxbias curve; Robustness;
Assessment of the notions of band offsets, wells and barriers at nanoscale semiconductor heterojunctions
Epitaxially-grown semiconductor heterostructures give the possibility to
tailor the potential landscape for the carriers in a very controlled way. In
planar lattice-matched heterostructures, the potential has indeed a very simple
and easily predictable behavior: it is constant everywhere except at the
interfaces where there is a step (discontinuity) which only depends on the
composition of the semiconductors in contact. In this paper, we show that this
universally accepted picture can be invalid in nanoscale heterostructures
(e.g., quantum dots, rods, nanowires) which can be presently fabricated in a
large variety of forms. Self-consistent tight-binding calculations applied to
systems containing up to 75 000 atoms indeed demonstrate that the potential may
have a more complex behavior in axial hetero-nanostructures: The band edges can
show significant variations far from the interfaces if the nanostructures are
not capped with a homogeneous shell. These results suggest new strategies to
engineer the electronic properties of nanoscale objects, e.g. for sensors and
photovoltaics.Comment: Accepted for publication in Phys. Rev.
Adsorption behavior of conjugated {C}3-oligomers on Si(100) and HOPG surfaces
A pi-conjugated {C}3h-oligomer involving three dithienylethylene branches
bridged at the meta positions of a central benzenic core has been synthesized
and deposited either on the Si(100) surface or on the HOPG surface. On the
silicon surface, scanning tunneling microscopy allows the observation of
isolated molecules. Conversely, by substituting the thiophene rings of the
oligomers with alkyl chains, a spontaneous ordered film is observed on the HOPG
surface. As the interaction of the oligomers is different with both surfaces,
the utility of the Si(100) surface to characterize individual oligomers prior
to their use into a 2D layer is discussed
Quasar Host Environments: The view from Planck
We measure the far-infrared emission of the general quasar (QSO) population
using Planck observations of the Baryon Oscillation Spectroscopic Survey QSO
sample. By applying multi-component matched multi-filters to the seven highest
Planck frequencies, we extract the amplitudes of dust, synchrotron and thermal
Sunyaev-Zeldovich (SZ) signals for nearly 300,000 QSOs over the redshift range
. We bin these individually low signal-to-noise measurements to obtain
the mean emission properties of the QSO population as a function of redshift.
The emission is dominated by dust at all redshifts, with a peak at ,
the same location as the peak in the general cosmic star formation rate.
Restricting analysis to radio-loud QSOs, we find synchrotron emission with a
monochromatic luminosity at (rest-frame) rising from
to between
and 3. The radio-quiet subsample does not show any synchrotron emission,
but we detect thermal SZ between and 4; no significant SZ emission is
seen at lower redshifts. Depending on the supposed mass for the halos hosting
the QSOs, this may or may not leave room for heating of the halo gas by
feedback from the QSO.Comment: 14 pages, 11 figures, accepted by A&
Ground state of a tightly bound composite dimer immersed in a Fermi Sea
In this paper we present a theoretical investigation for the ground state of
an impurity immersed in a Fermi sea. The molecular regime is considered where a
two-body bound state between the impurity and one of the fermions is formed.
Both interaction and exchange of the bound fermion take place between the dimer
and the Fermi sea. We develop a formalism based on a two channel model allowing
us to expand systematically the ground state energy of this immersed dimer with
the scattering length . Working up to order , associated to the
creation of two particle-hole pairs, reveals the first signature of the
composite nature of the bosonic dimer. Finally, a complementary variational
study provides an accurate estimate of the dimer energy even at large
scattering length.Comment: 11 pages; 3 figure
Collision-induced galaxy formation: semi-analytical model and multi-wavelength predictions
A semi-analytic model is proposed that couples the Press-Schechter formalism
for the number of galaxies with a prescription for galaxy-galaxy interactions
that enables to follow the evolution of galaxy morphologies along the Hubble
sequence. Within this framework, we calculate the chemo-spectrophotometric
evolution of galaxies to obtain spectral energy distributions. We find that
such an approach is very successful in reproducing the statistical properties
of galaxies as well as their time evolution. We are able to make predictions as
a function of galaxy type: for clarity, we restrict ourselves to two categories
of galaxies: early and late types that are identified with ellipticals and
disks. In our model, irregulars are simply an early stage of galaxy formation.
In particular, we obtain good matches for the galaxy counts and redshift
distributions of sources from UV to submm wavelengths. We also reproduce the
observed cosmic star formation history and the diffuse background radiation,
and make predictions as to the epoch and wavelength at which the dust-shrouded
star formation of spheroids begins to dominate over the star formation that
occurs more quiescently in disks. A new prediction of our model is a rise in
the FIR luminosity density with increasing redshift, peaking at about , and with a ratio to the local luminosity density about 10 times higher than that in the blue
(B-band) which peaks near .Comment: Minor changes, replaced to match accepted MNRAS versio
Free-energy model for fluid helium at high density
We present a semi-analytical free-energy model aimed at characterizing the
thermodynamic properties of dense fluid helium, from the low-density atomic
phase to the high-density fully ionized regime. The model is based on a
free-energy minimization method and includes various different contributions
representative of the correlations between atomic and ionic species and
electrons. This model allows the computation of the thermodynamic properties of
dense helium over an extended range of density and temperature and leads to the
computation of the phase diagram of dense fluid helium, with its various
temperature and pressure ionization contours. One of the predictions of the
model is that pressure ionization occurs abruptly at \rho \simgr 10 g
cm, {\it i.e.} P\simgr 20 Mbar, from atomic helium He to fully ionized
helium He, or at least to a strongly ionized state, without He
stage, except at high enough temperature for temperature ionization to become
dominant. These predictions and this phase diagram provide a guide for future
dynamical experiments or numerical first-principle calculations aimed at
studying the properties of helium at very high density, in particular its
metallization. Indeed, the characterization of the helium phase diagram bears
important consequences for the thermodynamic, magnetic and transport properties
of cool and dense astrophysical objects, among which the solar and the numerous
recently discovered extrasolar giant planets.Comment: Accepted for publication in Phys. Rev.
The breakdown behavior of the maximum likelihood estimator in the logistic regression model.
In this note we discuss the breakdown behavior of the maximum likelihood (ML) estimator in the logistic regression model. We formally prove that the ML-estimator never explodes to infinity, but rather breaks down to zero when adding severe outliers to a data set. An example confirms this behavior. (C) 2002 Published by Elsevier Science B.V.breakdown point; logistic regression; maximum likelihood; robust estimation; generalized linear-models; robustness; existence; fits;
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