Anharmonic phonon frequency shift in MgB2

We compute the anharmonic shift of the phonon frequencies in MgB2, using density functional theory. We explicitly take into account the scattering between different phonon modes at different q-points in the Brillouin zone. The shift of the E2g mode at the Gamma point is +5 % of the harmonic frequency. This result comes from the cancellation between the contributions of the four- and three-phonon scattering, respectively +10 % and -5 %. A similar shift is predicted at the A point, in agreement with inelastic X-ray scattering phonon-dispersion measurements. A smaller shift is observed at the M point.Comment: 4 pages, 1 figur

Anharmonic properties from a generalized third order ab~initio approach: theory and applications to graphite and graphene

We have implemented a generic method, based on the 2n+1 theorem within density functional perturbation theory, to calculate the anharmonic scattering coefficients among three phonons with arbitrary wavevectors. The method is used to study the phonon broadening in graphite and graphene mono- and bi-layer. The broadening of the high-energy optical branches is highly nonuniform and presents a series of sudden steps and spikes. At finite temperature, the two linearly dispersive acoustic branches TA and LA of graphene have nonzero broadening for small wavevectors. The broadening in graphite and bi-layer graphene is, overall, very similar to the graphene one, the most remarkable feature being the broadening of the quasi acoustical ZO' branch. Finally, we study the intrinsic anharmonic contribution to the thermal conductivity of the three systems, within the single mode relaxation time approximation. We find the conductance to be in good agreement with experimental data for the out-of-plane direction but to underestimate it by a factor 2 in-plane

Raman spectra of BN-nanotubes: Ab-initio and bond-polarizability model calculations

We present it ab-initio calculations of the non-resonant Raman spectra of zigzag and armchair BN nanotubes. In comparison, we implement a generalized bond-polarizability model where the parameters are extracted from first-principles calculations of the polarizability tensor of a BN sheet. For light-polarization along the tube-axis, the agreement between model and it ab-initio spectra is almost perfect. For perpendicular polarization, depolarization effects have to be included in the model in order to reproduce the it ab-initio Raman intensities.Comment: 4 pages, submitted to Phys. Rev. B rapid com

Dieta Mediterranea, una storia infinita

La dieta mediterranea attuale rappresenta il risultato di un lunghissimo percorso che, dalla preistoria fino alla seconda guerra mondiale, ha permesso l’interazione, tra i popoli, di aspetti sociali, culturali, religiosi e alimentari. Dopo 100 anni di ricerche biochimiche, chimiche e di studi prospettici, la Dieta Mediterranea è stata riconosciuta come la più idonea ad acquisire e mantenere un ottimale stato di salute. Verdura in grande quantità, frutta, cereali, legumi consumati con pesce, uova e un moderato consumo di carne, olio e vino sembrano essere la chiave del successo. Chi mangia mediterraneo vive meglio

Conflits de reconnaissance et mobilisation collective

Cet article s’intéressera au problème de la mobilisation collective pour entrer dans des conflits de reconnaissance en prenant en compte aussi bien la motivation que la participation à la mobilisation. Plus précisément, ce problème sera abordé en examinant la thèse classique défendue par Mancur Olson relative à la défection d’un nombre significatif d’agents rationnels pouvant affecter le succès d’une action collective et aux incitations sélectives nécessaires pour y prendre part. L’article présente un cadre théorique fondé sur le concept d’identité expressive destiné à surmonter le paradoxe d’Olson.This article will focus on the problem of collective mobilization for entering into conflicts of recognition taking into account both motivation and participation to mobilization. Specifically, this problem will be addressed by examining the classic argument supported by Mancur Olson relating to the defection of a significant number of rational agents that could affect the success of collective action and to the selective incentives needed to take part in it. The article presents a theoretical framework grounded on the concept of expressive identity that could overcome the Olson’s paradox

Non-adiabatic Kohn-anomaly in a doped graphene monolayer

We compute, from first-principles, the frequency of the E2g, Gamma phonon (Raman G-band) of graphene, as a function of the charge doping. Calculations are done using i) the adiabatic Born-Oppenheimer approximation and ii) time-dependent perturbation theory to explore dynamic effects beyond this approximation. The two approaches provide very different results. While, the adiabatic phonon frequency weakly depends on the doping, the dynamic one rapidly varies because of a Kohn anomaly. The adiabatic approximation is considered valid in most materials. Here, we show that doped graphene is a spectacular example where this approximation miserably fails.Comment: 5 pages, 3 figures, Accepted by Phys. Rev. Let

Zener tunneling in the electrical transport of quasi-metallic carbon nanotubes

We study theoretically the impact of Zener tunneling on the charge-transport properties of quasi-metallic (Qm) carbon nanotubes (characterized by forbidden band gaps of few tens of meV). We also analyze the interplay between Zener tunneling and elastic scattering on defects. To this purpose we use a model based on the master equation for the density matrix, that takes into account the inter-band Zener transitions induced by the electric field (a quantum mechanical effect), the electron-defect scattering and the electron-phonon scattering. In presence of Zener tunnelling the Qm tubes support an electrical current even when the Fermi energy lies in the forbidden band gap. In absence of elastic scattering (in high quality samples), the small size of the band gap of Qm tubes enables Zener tunnelling for realistic values of the the electric field (above $\sim$ 1 V/\mu m). The presence of a strong elastic scattering (in low quality samples) further decreases the values of the field required to observe Zener tunnelling. Indeed, for elastic-scattering lengths of the order of 50 nm, Zener tunnelling affects the current/voltage characteristic already in the linear regime. In other words, in quasi-metallic tubes, Zener tunneling is made more visible by defects.Comment: 10 pages, 8 figure

Modeling and development of a microwave heated pilot plant for the production of SiC-based ceramic matrix composites

This paper outlines the development of a microwave heated apparatus for the production of silicon carbide (SiC) based ceramic matrix composites via chemical vapor infiltration. An innovative pilot scale reactor was designed and built. A coupled thermal and electromagnetic model was developed in order to predict the temperature profile inside the reactor. The results obtained from the model demonstrated that the electric field inside the sample was constant. This fact is particularly important in order to prevent the thermal instabilities (run-aways) that are typical in the case of microwave heating. Therefore the heating was uniform with the aid of a mode stirrer that achieved a better distribution of the microwave power and then improved the process efficiency. The infiltration cycles were carried out on SiC fiber preforms and resulted in an excellent average weight increase with respect to the initial sample. By using microwave heating, the treatment times were considerably reduced with respect to the conventional process times reported in the literature. The microstructure of the SiC composites were observed by scanning the electron microscopy in order to evaluate the quality and the degree of densification which was achieved within the fiber tows. The SiC deposition inside of the sample was sufficiently homogeneous and compact, even if a certain degree of inter-tow porosity was still evident

An agronomic evaluation of new safflower (Carthamus tinctorius L.) germplasm for seed and oil yields under Mediterraean climate conditions

Interest in oilseed crops for agro-industrial research and development projects has increased in the Mediterranean area, in recent years. Saffloower (Carthamus tinctorius L.) is of potential interest for agriculture mainly due to fatty acid content variability in the seed oil. The aim of this study was to assess the agronomic performance of 16 new safflower accessions together with safflower variety Montola 2000, used as a reference, in a semi-arid environment. Research was carried out in Sicily (Italy) from 2013–2014. Hierarchical cluster analysis carried out on the fatty acid composition of safflower accessions resulted in their division into four main groups. Linoleic, oleic and palmitic acids were the main fatty acids present in the accessions. Seed yield was 1.11 t ha-1 on average and seed oil content was found to be approximately 35.01% of dry matter on average. Positive and significant relationships between seed/oil yield and other tested traits were found. The carbon, hydrogen and nitrogen content as a percentage of dry matter varied greatly both for the above- and belowground plant parts on average. This study confirms the interest of safflower for both food and non-food applications, offering interesting prospects in semi-arid regions