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 $0.1<z<5$. 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 $z \sim 2$, 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 $100\,\rm{GHz}$ (rest-frame) rising from $\overline{L_{\rm synch}}=0$ to $0.2 \, {\rm L_\odot} {\rm Hz}^{-1}$ between $z=0$ and 3. The radio-quiet subsample does not show any synchrotron emission, but we detect thermal SZ between $z=2.5$ 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 $a$. Working up to order $a^3$, 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 $z\sim 3$, and with a ratio to the local luminosity density $\rho_{L,\nu} (z = z_{peak})/ \rho_{L,\nu} (z = 0)$ about 10 times higher than that in the blue (B-band) which peaks near $z\sim 2$.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$^{-3}$, {\it i.e.} P\simgr 20 Mbar, from atomic helium He to fully ionized helium He$^{2+}$, 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;