Search results for "Sciences de la Terre"

Enhancing $l_i 3 l_j$ with the $Z^0$-penguin

2012

OMC: An Optical Monitoring Camera for INTEGRAL - Instrument description and performance

2003

The Optical Monitoring Camera (OMC) will observe the optical emission from the prime targets of the gammaray instruments onboard the ESA mission INTEGRAL, with the support of the JEM-X monitor in the X-ray domain. This capability will provide invaluable diagnostic information on the nature and the physics of the sources over a broad wavelength range. Its main scientific objectives are: ( 1) to monitor the optical emission from the sources observed by the gamma- and X-ray instruments, measuring the time and intensity structure of the optical emission for comparison with variability at high energies, and ( 2) to provide the brightness and position of the optical counterpart of any gamma- or X…

Isotropic-nematic interfacial tension of hard and soft rods: Application of advanced grand canonical biased-sampling techniques

2005

Coexistence between the isotropic and the nematic phase in suspensions of rods is studied using grand canonical Monte Carlo simulations with a bias on the nematic order parameter. The biasing scheme makes it possible to estimate the interfacial tension gamma in systems of hard and soft rods. For hard rods with L/D=15, we obtain gamma ~ 1.4 kB T/L^2, with L the rod length, D the rod diameter, T the temperature, and kB the Boltzmann constant. This estimate is in good agreement with theoretical predictions, and the order of magnitude is consistent with experiments.

Simple monoclinic crystal phase in suspensions of hard ellipsoids

2006

We present a computer simulation study on the crystalline phases of hard ellipsoids of revolution. For aspect ratios $\ensuremath{\geqslant}3$ the previously suggested stretched-fcc phase [Frenkel and Mulder, Mol. Phys. 55, 1171 (1985)] is replaced by a different crystalline phase. Its unit cell contains two ellipsoids with unequal orientations. The lattice is simple monoclinic. The angle of inclination of the lattice, $\ensuremath{\beta}$, is a very soft degree of freedom, while the two right angles are stiff. For one particular value of $\ensuremath{\beta}$, the close-packed version of this crystal is a specimen of the family of superdense packings recently reported [Donev et al., Phys. R…

Computing absolute free energies of disordered structures by molecular simulation

2009

We present a Monte Carlo simulation technique by which the free energy of disordered systems can be computed directly. It is based on thermodynamic integration. The central idea is to construct an analytically solvable reference system from a configuration which is representative for the state of interest. The method can be applied to lattice models (e.g., the Ising model) as well as off-lattice molecular models. We focus mainly on the more challenging off-lattice case. We propose a Monte Carlo algorithm, by which the thermodynamic integration path can be sampled efficiently. At the examples of the hard sphere liquid and a hard disk solid with a defect, we discuss several properties of the …

Tuning the Pseudospin Polarization of Graphene by a Pseudomagnetic Field.

2016

One of the intriguing characteristics of honeycomb lattices is the appearance of a pseudo-magnetic field as a result of mechanical deformation. In the case of graphene, the Landau quantization resulting from this pseudo-magnetic field has been measured using scanning tunneling microscopy. Here we show that a signature of the pseudo-magnetic field is a local sublattice symmetry breaking observable as a redistribution of the local density of states. This can be interpreted as a polarization of graphene's pseudospin due to a strain induced pseudo-magnetic field, in analogy to the alignment of a real spin in a magnetic field. We reveal this sublattice symmetry breaking by tunably straining grap…

Importance des récentes variations pluviométriques à Djibouti et besoins de quantification de leurs impacts

2014

Ces dernières décennies, l’Afrique de l’Est souffre d’une longue et lente détérioration pluviométrique. De 2007 à 2011, la ville de Djibouti a enregistré un déficit de 73% des précipitations moyennes annuelles par rapport à la moyenne de 30 ans (1981-2010). Pour caractériser la sécheresse actuelle, nous analysons une série pluviométrique reconstruite pour Djibouti-Ville de 1901 à 2013, les données de terrain permettant d’illustrer les impacts de ces variations pluviométriques. Les précipitations enregistrées à Djibouti ne présentent aucune tendance sur le long terme (1901-2013). Par contre, ces dernières années montrent une très grande variabilité: le suivi de la moyenne pluviométrique sur …

Impacts of Integrated Weed management in cropping systems on N2O emissions from soils

2014

International audience

Excitons in few-layer hexagonal boron nitride: Davydov splitting and surface localization

2018

Hexagonal boron nitride (hBN) has been attracting great attention because of its strong excitonic effects. Taking into account few-layer systems, we investigate theoretically the effects of the number of layers on quasiparticle energies, absorption spectra, and excitonic states, placing particular focus on the Davydov splitting of the lowest bound excitons. We describe how the inter-layer interaction as well as the variation in electronic screening as a function of layer number $N$ affects the electronic and optical properties. Using both \textit{ab initio} simulations and a tight-binding model for an effective Hamiltonian describing the excitons, we characterize in detail the symmetry of t…

Time-dependent screening explains the ultrafast excitonic signal rise in 2D semiconductors

2020

We calculate the time evolution of the transient reflection signal in an MoS$_2$ monolayer on a SiO$_2$/Si substrate using first-principles out-of-equilibrium real-time methods. Our simulations provide a simple and intuitive physical picture for the delayed, yet ultrafast, evolution of the signal whose rise time depends on the excess energy of the pump laser: at laser energies above the A- and B-exciton, the pump pulse excites electrons and holes far away from the K valleys in the first Brillouin zone. Electron-phonon and hole-phonon scattering lead to a gradual relaxation of the carriers towards small $\textit{Active Excitonic Regions}$ around K, enhancing the dielectric screening. The acc…