École Polytechnique

About: École Polytechnique is a education organization based out in Palaiseau, France. It is known for research contribution in the topics: Laser & Plasma. The organization has 18995 authors who have published 39265 publications receiving 1225163 citations. The organization is also known as: Ecole Polytechnique & Polytechnique.

Rates of primary electron transfer in photosynthetic reaction centres and their mechanistic implications

Abstract: The conversion of light energy to chemical energy during photosyn-thesis involves the transfer of electrons between pigments embedded in a membrane protein. This process occurs with high quantum efficiency, the result of extremely fast electron transfer over a long distance preventing back transfer and energy loss. Recently the three-dimensional structures of the photosynthetic reaction centres of the bacteria Rhodopseudomonas viridis1 and Rhodobacter sphaeroides2 have been determined, allowing a molecular descrip-tion of the primary charge separation process. There are two symmetrically related branches of pigments in the structure (L and M), extending from the special pair of bacteriochlorophyll molecules (P) to the two bacteriopheophytins (HL and HM) via two bacteriochlorophylls (BLand BM). Many features of the electron transfer process are poorly understood, such as the nature of the excited states involved, the identity of the primary charge separation step and the roles of the protein and of B3–13. We have determined the rates of electron transfer in isolated reaction centre complexes of Rps. viridis and Rb. sphaeroides as a function of temperature. The rates increase as temperature is decreased, which may be due to either changes in electronic coupling of the pigments or changes in the population of coupled vibrational modes, or a combination of the two. We see no evidence of a B−L intermediate, which sets a lower limit on the rate of electron transfer from BL to HL. This is so high as to rule out transfer by two non-adiabatic steps.

Features of heavy physics in the CMB power spectrum

Abstract: The computation of the primordial power spectrum in multi-field inflation models requires us to correctly account for all relevant interactions between adiabatic and non-adiabatic modes around and after horizon crossing. One specific complication arises from derivative interactions induced by the curvilinear trajectory of the inflaton in a multi-dimensional field space. In this work we compute the power spectrum in general multi-field models and show that certain inflaton trajectories may lead to observationally significant imprints of `heavy' physics in the primordial power spectrum if the inflaton trajectory turns, that is, traverses a bend, sufficiently fast (without interrupting slow roll), even in cases where the modes normal to the trajectory have masses approaching the cutoff of our theory. We emphasize that turning is defined with respect to the geodesics of the sigma model metric, irrespective of whether this is canonical or non-trivial. The imprints generically take the form of damped superimposed oscillations on the power spectrum. In the particular case of two-field models, if one of the fields is sufficiently massive compared to the scale of inflation, we are able to compute an effective low energy theory for the adiabatic mode encapsulating certain relevant operators of the full multi-field dynamics. As expected, a particular characteristic of this effective theory is a modified speed of sound for the adiabatic mode which is a functional of the background inflaton trajectory and the turns traversed during inflation. Hence in addition, we expect non-Gaussian signatures directly related to the features imprinted in the power spectrum.

Gamma-Ray Flares from the Crab Nebula

Abstract: A young and energetic pulsar powers the well-known Crab Nebula. Here, we describe two separate gamma-ray (photon energy greater than 100 mega-electron volts) flares from this source detected by the Large Area Telescope on board the Fermi Gamma-ray Space Telescope. The first flare occurred in February 2009 and lasted approximately 16 days. The second flare was detected in September 2010 and lasted approximately 4 days. During these outbursts, the gamma-ray flux from the nebula increased by factors of four and six, respectively. The brevity of the flares implies that the gamma rays were emitted via synchrotron radiation from peta-electron-volt (10(15) electron volts) electrons in a region smaller than 1.4 × 10(-2) parsecs. These are the highest-energy particles that can be associated with a discrete astronomical source, and they pose challenges to particle acceleration theory.

Geometrical barriers in high-temperature superconductors.

Abstract: A theoretical description of vortex dynamics in thin flat samples is derived and is found to compare favorably with experimental results. In perpendicular applied magnetic field the vortex penetration is delayed significantly due to the presence of a potential barrier of geometrical origin. This novel geometrical barrier effect results in hysteretic magnetization and in the existence of an irreversibility line in the absence of bulk pinning. Among the unique characteristics of the barrier are a vortex concentration in the center of the sample and a zero-field peak in the magnetization loops.

ChemSage—A computer program for the calculation of complex chemical equilibria

Abstract: An extensive computer program called ChemSage, based upon the SOLGASMIX Gibbs energy minimizer, is presented together with several examples which illustrate its use. ChemSage was designed to perform three types of thermochemical calculations in complex systems involving phases exhibiting nonideal mixing properties. These are the calculation of thermodynamic functions, heterogeneous phase equilibria, and steady-state conditions for the simulation of simple multistage reactors. The thermodynamic functions module calculates specific heat, enthalpy, entropy, and Gibbs energy with respect to a chosen reference state for a given phase and, if this phase is a mixture, the partial properties of its components. Chemical equilibrium calculations can be made for a system which has been uniquely defined with respect to temperature, pressure (or volume), and composition. One of these quantities may also be replaced by an extensive property or phase target,e.g., for the calculation of adiabatic and liquidus temperatures, respectively.