On Monopolistic Competition and Optimal Product Diversity: a Comment on Cost Structure and Workers' Rents

In the Dixit-Stiglitz model of monopolistic competition, entry of firms is socially too small. Other authors have shown that excess entry is also a possibility with other preferences for diversity. We show that the cost structure and workers's rents can also explain excess entry.monopolistic competition; product diversity

Jahn-Teller, polarity and insulator-to-metal transition in BiMnO3 at high pressure

The interaction of coexisting structural instabilities in multiferroic materials gives rise to intriguing coupling phenomena and extraordinarily rich phase diagrams, both in bulk materials and strained thin films. Here we investigate the multiferroic BiMnO3 with its peculiar 6s2 electrons and four interacting mechanisms: electric polarity, octahedra tilts, magnetism, and cooperative Jahn-Teller distortion. We have probed structural transitions under high pressure by synchrotron x-ray diffraction and Raman spectroscopy up to 60 GPa. We show that BiMnO3 displays under pressure a rich sequence of five phases with a great variety of structures and properties, including a metallic phase above 53 GPa and, between 37 and 53 GPa, a strongly elongated monoclinic phase that allows ferroelectricity, which contradicts the traditional expectation that ferroelectricity vanishes under pressure. Between 7 and 37 GPa, the Pnma structure remains remarkably stable but shows a reduction of the Jahn-Teller distortion in a way that differs from the behavior observed in the archetypal orthorhombic Jahn-Teller distorted perovskite LaMnO3.Comment: 5 pages, 3 figures + supplemental material included (3 pages, 1 figure, 3 tables

Quantum oscillations and upper critical magnetic field of the iron-based superconductor FeSe

Shubnikov-de Haas (SdH) oscillations and upper critical magnetic field ($H\_{c2}$) of the iron-based superconductor FeSe ($T\_c$ = 8.6 K) have been studied by tunnel diode oscillator-based measurements in magnetic fields of up to 55 T and temperatures down to 1.6 K. Several Fourier components enter the SdH oscillations spectrum with frequencies definitely smaller than predicted by band structure calculations indicating band renormalization and reconstruction of the Fermi surface at low temperature, in line with previous ARPES data. The Werthamer-Helfand-Hohenberg model accounts for the temperature dependence of $H\_{c2}$ for magnetic field applied both parallel (\textbf{H} $\|$ $ab$) and perpendicular (\textbf{H} $\|$ $c$) to the iron conducting plane, suggesting that one band mainly controls the superconducting properties in magnetic fields despite the multiband nature of the Fermi surface. Whereas Pauli pair breaking is negligible for \textbf{H} $\|$ $c$, a Pauli paramagnetic contribution is evidenced for \textbf{H} $\|$ $ab$ with Maki parameter $\alpha$ = 2.1, corresponding to Pauli field $H\_{P}$ = 36.5

Taxation and Labor Markets

We exploit the common features of models such as union-firm wage bargaining, search and efficiency wage models to develop a framework that can be used for analyzing the effects of any budget-neutral tax reform on employment in these models. We show that taxes paid by workers are not equivalent to taxes paid by firms when taxes are non linear. Moreover, increasing progressivity is good for employment in these models.

Vitamin D and Liver

Le rôle de la vitamine D sur l’organisme en général et sur le foie en particulier, dépend d’une part, d’un héritage génétique et d’autre part, de facteurs environnementaux, notamment l’exposition solaire et dans une moindre mesure, des apports alimentaires. Le principal rôle de la vitamine D concerne le métabolisme phosphocalcique et osseux mais, agissant sur l’activité de plus de 200 gènes, elle intervient également dans l’immunité non spécifique et acquise, dans le système cardiovasculaire, le foie et les infections. Concernant le foie, la carence est fréquente dans les hépatopathies chroniques et un déficit sévère pourrait être impliqué dans l’aggravation de l’état hépatique. Des essais thérapeutiques sont néanmoins nécessaires pour préciser l’intérêt, l’efficacité et le mode optimal d’une supplémentation. La Haute autorité de santé (HAS) a rappelé en octobre 2013 les règles de bonnes pratiques et limite le dosage de la vitamine D à certains cas bien définis : diagnostics de rachitisme ou d'ostéomalacie, prescription de certains médicaments contre l'ostéoporose, adultes transplantés rénaux ou après une chirurgie de l'obésité, personnes âgées victimes de chutes répétées et, éventuellement, présentant un risque élevé de fractures. Dans toutes les autres situations, les données actuellement disponibles ne permettent pas de déterminer l’utilité clinique du dosage de la vitamine D. Enfin, compte tenu de la fréquence du déficit en population générale, il serait peut- être intéressant de redéfinir dans un avenir proche des seuils de normalité et de revoir les apports journaliers recommandés.The role of vitamin D in the organism in general and on the liver particularly, firstly depends on the genetic heritage and also on environmental factors, especially sun exposure and in lesser measure on dietary intakes. The main part of vitamin D is related to phosphocalcic and bone metabolism but, as it affects the activity of more than 200 genes, it is also involved in nonspecific acquired immunity in the cardiovascular system, liver and infections. Regarding the liver, deficiency is frequently observed in chronic hepatic diseases and severe deficiency might be involved in the worsening of liver condition. Clinical trials are nevertheless necessary to assess the interest, the efficiency and the optimal mode of supplementation. The HAS reminded in October 2013 the rules of good practice and limits the dosage of vitamin D to well-defined cases: diagnosis of rickets and osteomalacia, some drug prescription against osteoporosis, adult kidney transplant or after obesity surgery, elderly who are victims of repeated falls and eventually at high risk of fractures. In all other situations, currently available data is inadequate to determine the clinical usefulness of vitamin D dosage. Finally, taking into account the frequency of the deficit, it might be interesting to redefine, in a near future, the thresholds of normality and review the recommended daily intake

Collapse of critical nematic fluctuations in FeSe under pressure

We report the evolution of the electronic nematic susceptibility in FeSe via Raman scattering as a function of hydrostatic pressure up to 5.8 GPa where the superconducting transition temperature $T_{c}$ reaches its maximum. The critical nematic fluctuations observed at low pressure vanish above 1.6 GPa, indicating they play a marginal role in the four-fold enhancement of $T_{c}$ at higher pressures. The collapse of nematic fluctuations appears to be linked to a suppression of low energy electronic excitations which manifests itself by optical phonon anomalies at around 2 GPa, in agreement with lattice dynamical and electronic structure calculations using local density approximation combined with dynamical mean field theory. Our results reveal two different regimes of nematicity in the phase diagram of FeSe under pressure: a d-wave Pomeranchuk instability of the Fermi surface at low pressure and a magnetic driven orthorhombic distortion at higher pressure.Comment: 7 pages, 4 figures. Supplementary Material available upon reques

Dependence of the structural, transport and magnetic properties of Tl1-yFe2-z(Se1-xSx)2 with isovalent substitution of Se by S

International audienceThe effect of selenium substitution by sulfur in the Tl$_{1-y}$Fe$_{2-z}$Se$_{2}$ antiferromagnet was studied by x-ray and electron diffraction, magnetization and transport measurements. Tl$_{0.8}$Fe$_{1.5}$(Se$_{1-x}$S$_{x}$)$_{2}$ (nominal composition) solid solution was synthesized in the full x range (0~$\le x(S)\le$~1) using the sealed tube technique. No superconductivity was found down to 4.2K in the series despite the fact that the optimal crystallographic parameters, determined by Rietveld refinements, are reached in the series (i.e. the Fe-(Se,S) interplane height and (Se,S)-Fe-(Se,S) angle for which the critical superconducting transition T$_{c}$ is usually maximal in pnictides). Quasi~full Tl site ($y\sim0.05$) compared to significant alkaline deficiency ($y=0.2-0.3$) in analogous A$_{1-y}$Fe$_{2-z}$Se$_{2}$ (A~=~K,~Rb,~Cs), and the resulting differences in iron valency, density of states and doping, are suggested to explain this absence of superconductivity. Transmission electron microscopy confirmed the existence of ordered iron vacancies network in the samples of the Tl$_{0.8}$Fe$_{1.5}$(Se$_{1-x}$S$_{x}$)$_{2}$ series in the form of the tetragonal $\sqrt{5}$ a $\times \sqrt{5}$ a $\times$ c superstructure (\textit{I4/m}). The N\'{e}el temperature (T$_{N}$) indicating the onset of antiferromagnetism order in this $\sqrt{5}$ a $\times \sqrt{5}$ a $\times$ c supercell is found to decrease from 450K in the selenide (x=0) to 330K in the sulfide (x=1). We finally demonstrate a direct linear relationship between $T_{N}$ and the Fe-(Se,S) bond length (or Fe-(Se,S) height)

High pressure x-ray diffraction study of the volume collapse in Ba24Si100 clathrate

International audienceThe high pressure stability of the silicon type-III clathrate Ba24Si100 has been studied by x-ray diffraction (XRD) up to a maximum pressure of 37.4 GPa. The high pressure behavior of this Si type-III clathrate appears to be analogous to the structural type-I parent Ba8Si46. An isostructural volume collapse is observed at ~23 GPa, a value higher than for Ba8Si46 (13-15 GPa). The crystallinity of the structure is preserved up to the maximum attained pressure without amorphization, which appears to be in contradiction with the interpretation given in a Raman spectroscopy study [Shimizu et al., Phys. Rev. B 71, 094108 (2005)]. Nevertheless, the XRD analysis shows the appearance of a type-III disordered nanocaged-based crystalline structure after the volume collapse. Moreover, we find that the volume collapse transformation is (quasi)reversible after pressure release. Additionally, a low pressure transition first evidenced by Raman spectroscopy is also observed in our XRD study at 5 GPa: The variation of the isotropic thermal factors of Ba atoms shows a clear discontinuity at this pressure while the average positions of Ba atoms remain identical

The biological determinism of John Steinbeck as seen in Sea of Cortez and as exemplified in his novels

Thesis (M.A.)--Boston UniversityBy thus searching for evidences of Steinbeck's philosophy as he bas outlined it in Sea of Cortez, and illustrated it in his fiction, it can be seen that a coherent philosophy dominates all of Steinbeck's writings. By analyzing this philosophy in terms of the accepted philosophical schools ot thought it can be seen that the theory of biological determinism is consistent with those concepts which are Steinbeck's and its application thereby justified