KNOWING, CHARACTERIZING AND ASSESSING SYSTEMS OF ORGANIC CROP ROTATIONS

The choice of crop rotations in organic stockless cropping systems is the first leverage used to manage technical issues (to maintain soil fertility, to control pest and weeds) and economic issues (to insure income). The RotAB project (French Casdar funding 2008-2010) implemented complementary approaches to better knowing, characterizing and assessing arable crop rotations. Their conception depends on numerous factors such as the types of soil and climate (on which depend the types of crops, yield potential, possibility of mechanical weed control…) or the economic context (existence of outlets and continuity of markets). If nitrogen supply and weed control are the most important agronomic issues of organic farmers in stockless cropping systems, phosphorus availability appears to be the next important issue for soil fertility and system sustainability

Quantum localization and delocalization of charge carriers in organic semiconducting crystals

Charge carrier transport in organic semiconductors is at the heart of many revolutionary technologies ranging from organic transistors, light-emitting diodes, flexible displays and photovoltaic cells. Yet, the nature of charge carriers and their transport mechanism in these materials is still unclear. Here we show that by solving the time-dependent electronic Schrödinger equation coupled to nuclear motion for eight organic molecular crystals, the excess charge carrier forms a polaron delocalized over up to 10–20 molecules in the most conductive crystals. The polaron propagates through the crystal by diffusive jumps over several lattice spacings at a time during which it expands more than twice its size. Computed values for polaron size and charge mobility are in excellent agreement with experimental estimates and correlate very well with the recently proposed transient localization theor

Système de mesure du Doppler différentiel sur raies fluctuantes

In order to use differential Doppler (D.D) in target motion analysis, we present a measurement system providing accurate D.D estimation when the primary frequency lines are linearly modulated. Such low frequency variation, generally due to source motion, is frequently perturbating classical high resolution measurement techniques on real signals (Time delay-Doppler ambiguity, acceleration,..). The proposed system provides a non biaised estimation and avoids any perturbation induced by this phenomena. The algorithm, based on a maximum likehood estimator associated to a phase tracking loop, reaches theoretically the Cramer-Rao bound. Futhermore simulation results demonstrate the good practical behaviour of this system

Resultats experimentaux de l'intercorrelateur avec compensation de Doppler differentiel

Cet article aborde le problème de l'estimation conjointe d'écarts de temps et de dopplers différentiels, et dans ce contexte rend compte de résultats expérimentaux. Un réseau fixe de capteurs est utilisé pour localiser et trajectographier à partir d'estimations de retards et de dopplers différentiels une source se déplacant rapidement dans le champ proche du réseau. On rappelle ici la structure de l'estimateur optimal, de retard et de doppler différentiel, l'intercorrélateur avec compensation de doppler différentiel, ses performances , ainsi que son comportement en présence de plusieurs sources. On compare par simulation les performances théoriques et les performances effectivement atteintes par l'estimateur réellement implanté. Enfin des resultats expérimentaux sont fournis

Flickering Polarons Extending over Ten Nanometres Mediate Charge Transport in High‐Mobility Organic Crystals

Progress in the design of high‐mobility organic semiconductors has been hampered by an incomplete fundamental understanding of the elusive charge carrier dynamics mediating electrical current in these materials. To address this problem, a novel fully atomistic non‐adiabatic molecular dynamics approach termed fragment orbital‐based surface hopping (FOB‐SH) that propagates the electron‐nuclear motion has been further improved and, for the first time, used to calculate the full 2D charge mobility tensor for the conductive planes of six structurally well characterized organic single crystals, in good agreement with available experimental data. The nature of the charge carrier in these materials is best described as a flickering polaron constantly changing shape and extensions under the influence of thermal disorder. Thermal intra‐band excitations from modestly delocalized band edge states (up to 5 nm or 10–20 molecules) to highly delocalized tail states (up to 10 nm or 40–60 molecules in the most conductive materials) give rise to short, ≈ 10 fs‐long bursts of the charge carrier wavefunction that drives the spatial displacement of the polaron, resulting in carrier diffusion and mobility. This study implies that key to the design of high‐mobility materials is a high density of strongly delocalized and thermally accessible tail states