Vach H.,Ecole Polytechnique - Palaiseau
Physical Review Letters | Year: 2014
Based on first-principles calculations, we predict the use of pure silicon nanocrystals as nano-oscillators in the giga- and terahertz region. Small- and large-amplitude, one-dimensional vibrations are observed. The former are spontaneously excited thermally at frequencies around 3 THz. Large-amplitude vibrations originate from oscillations between the inversion geometries of the nanocrystal and can be caused either classically by an external excitation or by quantum tunneling. The latter causes a ground-state splitting of 4.2 GHz, suggesting the use of the proposed nanocrystals as laser elements in a configuration analogous to that of the ammonia maser. © 2014 American Physical Society.
Palti E.,Ecole Polytechnique - Palaiseau
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2013
We study the consequences of cancellation of cubic Abelian anomalies in F-theory grand unified theory (GUT) models that use hypercharge flux in the presence of additional U(1) symmetries. We show that a mixed anomaly between the hypercharge and two U(1) gauge fields is not automatically canceled in local models based on the spectral cover and, therefore, imposes additional constraints on local models in F-theory that have not been accounted for so far. The constraints imply that, unless the local U(1)s are globally geometrically massive, there are only two possible classes of models in F-theory GUTs, which have fields on matter curves in noncomplete GUT representations that are not vectorlike under all U(1)s: the ones based on a 3+2 and 2+2+1 split. We comment on some phenomenological implications of these results for realizing models from F-theory GUTs like the next to minimal supersymmetric Standard Model. © 2013 American Physical Society.
Schmid P.J.,Ecole Polytechnique - Palaiseau
Journal of Fluid Mechanics | Year: 2010
The description of coherent features of fluid flow is essential to our understanding of fluid-dynamical and transport processes. A method is introduced that is able to extract dynamic information from flow fields that are either generated by a (direct) numerical simulation or visualized/measured in a physical experiment. The extracted dynamic modes, which can be interpreted as a generalization of global stability modes, can be used to describe the underlying physical mechanisms captured in the data sequence or to project large-scale problems onto a dynamical system of significantly fewer degrees of freedom. The concentration on subdomains of the flow field where relevant dynamics is expected allows the dissection of a complex flow into regions of localized instability phenomena and further illustrates the flexibility of the method, as does the description of the dynamics within a spatial framework. Demonstrations of the method are presented consisting of a plane channel flow, flow over a two-dimensional cavity, wake flow behind a flexible membrane and a jet passing between two cylinders. © 2010 Cambridge University Press.
Morlon H.,Ecole Polytechnique - Palaiseau
Ecology Letters | Year: 2014
Estimating rates of speciation and extinction, and understanding how and why they vary over evolutionary time, geographical space and species groups, is a key to understanding how ecological and evolutionary processes generate biological diversity. Such inferences will increasingly benefit from phylogenetic approaches given the ever-accelerating rates of genetic sequencing. In the last few years, models designed to understand diversification from phylogenetic data have advanced significantly. Here, I review these approaches and what they have revealed about diversification in the natural world. I focus on key distinctions between different models, and I clarify the conclusions that can be drawn from each model. I identify promising areas for future research. A major challenge ahead is to develop models that more explicitly take into account ecology, in particular the interaction of species with each other and with their environment. This will not only improve our understanding of diversification; it will also present a new perspective to the use of phylogenies in community ecology, the science of interaction networks and conservation biology, and might shift the current focus in ecology on equilibrium biodiversity theories to non-equilibrium theories recognising the crucial role of history. © 2014 John Wiley & Sons Ltd/CNRS.
Bouchoux G.,Ecole Polytechnique - Palaiseau
Mass Spectrometry Reviews | Year: 2012
The present article is the third part of a general overview of the gas-phase protonation thermochemistry of polyfunctional molecules (first part: Mass Spectrom. Rev., 2007, 26:775-835, second part: Mass Spectrom. Rev., 2011, in press). This review is devoted to the 20 proteinogenic amino acids and is divided in two parts. In the first one, the experimental data obtained during the last 30 years using the equilibrium, thermokinetic and kinetic methods are presented. A general re-assignment of the values originating from these various experiments has been done on the basis of the commonly accepted Hunter & Lias 1998 gas-phase basicity scale in order to provide an homogeneous set of data. In the second part, theoretical investigations on gaseous neutral and protonated amino acids are reviewed. Conformational landscapes of both types of species were examined in order to provide theoretical protonation thermochemistry based on the truly identified most stable conformers. Proton affinities computed at the presently highest levels of theory (i.e. composite methods such as Gn procedures) are presented. Estimates of thermochemical parameters calculated using a Boltzmann distribution of conformers at 298K are also included. Finally, comparison between experiment and theory is discussed and a set of evaluated proton affinities, gas-phase basicities and protonation entropies is proposed. © 2011 Wiley Periodicals, Inc.