Dynamique de la structuration de l'espace agricole : l'exemple de l'Est du Québec des années trente.

Une description de la distribution spatiale des activités agricoles, de l'intégration des cultivateurs dans les circuits commerciaux et de l'organisation productive de la ferme, dans l'Est du Québec, au cours des années précédant la Deuxième Guerre mondiale, nous fournit la base pour dégager le principe structurant l'espace agroforestier de cette région. Deux dimensions apparaissent pertinentes dans cette perspective. Tout d'abord, une propriété structurelle de la petite production, à savoir la ''pénibilité'' élevée de l'accumulation : elle fonde une propension à la multi-activité caractérisant certains espaces périphériques. Ensuite, les rapports entre l'agriculture et les activités forestières : ils se nourrissent de cette multi-activité mais aussi, à la faveur de certaines circonstances, ils permettent un "décollage" du développement agricole. Appliquée à la situation de l'Est du Québec, l'analyse de ces dimensions permet de rendre compte des variations du déploiement spatial du système agroforestier ainsi que de sa stabilité séculaire. Through a description of the spatial distribution of agricultural activities in Eastern Quebec in the pre-Second World War era, and of the integration of farmers into the market network, this article provides us with the background required to identify the principle governing this region's agroforestal space. Within this perspective, two dimensions seem particularly relevant. The first one is a structure of small production : i.e., the high effort factor involved in accumulation since it engenders a propensity towards the multiple activities which characterize peripheral spaces. The second is the relationship between agriculture and forestry activities. This relationship feeds upon the varied activities characteristic of peripherics but also, given certain circumstances, allows a "take off" of agricultural development. When applied to the prevailing situation in Eastern Quebec, the analysis of these dimensions enables one to account for the variations in the spatial distribution of the agroforestal system as well as its traditional stability

Self-consistent spin-wave theory for a frustrated Heisenberg model with biquadratic exchange in the columnar phase and its application to iron pnictides

Recent neutron scattering studies revealed the three dimensional character of the magnetism in the iron pnictides and a strong anisotropy between the exchange perpendicular and parallel to the spin stripes. We extend studies of the J1-J2-Jc Heisenberg model with S = 1 using self-consistent spin-wave theory. A discussion of two scenarios for the instability of the columnar phase is provided. The relevance of a biquadratic exchange term between in-plane nearest neighbors is discussed. We introduce mean-field decouplings for biquadratic terms using the Dyson-Maleev and the Schwinger boson representation. Remarkably their respective mean-field theories do not lead to the same results, even at zero temperature. They are gauged in the N'eel phase in comparison to exact diagonalization and series expansion. The J1-J2-Jc model is analyzed under the influence of the biquadratic exchange Jbq and a detailed description of the staggered magnetization and of the magnetic excitations is given. The biquadratic exchange increases the renormalization of the in-plane exchange constants which enhances the anisotropy between the exchange parallel and perpendicular to the spin stripes. Applying the model to iron pnictides, it is possible to reproduce the spin-wave dispersion for CaFe2As2 in the direction perpendicular to the spin stripes and perpendicular to the planes. Discrepancies remain in the direction parallel to the spin stripes which can be resolved by passing from S = 1 to S = 2. In addition, results for the dynamical structure factor within the self-consistent spin-wave theory are provided.Comment: 18 pages, 12 figures. Updated version, several references adde

Three dimensional generalization of the J1J_1-J2J_2 Heisenberg model on a square lattice and role of the interlayer coupling JcJ_c

A possibility to describe magnetism in the iron pnictide parent compounds in terms of the two-dimensional frustrated Heisenberg $J_1$-$J_2$ model has been actively discussed recently. However, recent neutron scattering data has shown that the pnictides have a relatively large spin wave dispersion in the direction perpendicular to the planes. This indicates that the third dimension is very important. Motivated by this observation we study the $J_1$-$J_2$-$J_c$ model that is the three dimensional generalization of the $J_1$-$J_2$ Heisenberg model for $S = 1/2$ and S = 1. Using self-consistent spin wave theory we present a detailed description of the staggered magnetization and magnetic excitations in the collinear state. We find that the introduction of the interlayer coupling $J_c$ suppresses the quantum fluctuations and strengthens the long range ordering. In the $J_1$-$J_2$-$J_c$ model, we find two qualitatively distinct scenarios for how the collinear phase becomes unstable upon increasing $J_1$. Either the magnetization or one of the spin wave velocities vanishes. For $S = 1/2$ renormalization due to quantum fluctuations is significantly stronger than for S=1, in particular close to the quantum phase transition. Our findings for the $J_1$-$J_2$-$J_c$ model are of general theoretical interest, however, the results show that it is unlikely that the model is relevant to undoped pnictides.Comment: 11 pages, 10 figures. Updated version, several references adde

Revue d'histoire du Bas-Saint-Laurent, vol. 14 (2)

Éditorial -- Enseignement -- Archéologie -- Mont Commis ou Mont Camille? -- Joachim Vautour, pêcheur résidant à Rimouski au XVIIIe siècle -- Les guerres du bois -- L'incorporation de Trois-Pistoles a donné lieu à une série de conflits (1916-1924) -- Chroniques rimouskoises -- Histoire orale -- Patrimoine -- Archives -- Des livres à lire! -- Photos ancienne

Searching for dark matter in X-rays: how to check the dark matter origin of a spectral feature

A signal from decaying dark matter (DM) can be unambiguously distinguished from spectral features of astrophysical or instrumental origin by studying its spatial distribution. We demonstrate this approach by examining the recent claim of 0912.0552 regarding the possible DM origin of the 2.5 keV line in Chandra observations of the Milky Way satellite known as Willman 1. Our conservative strategy is to adopt a relatively large dark mass for Willman 1 and relatively small dark masses for the comparison objects. We analyze archival observations by XMM-Newton of M31 and Fornax dwarf spheroidal galaxy (dSph) and Chandra observations of Sculptor dSph. By performing a conservative analysis of X-ray spectra, we show the absence of a DM decay line with parameters consistent with those of 0912.0552. For M31, the observations of the regions between 10 and 20 kpc from the center, where the uncertainties in the DM distribution are minimal, make a strong exclusion at the level above 10sigma. The minimal estimate for the amount of DM in the central 40 kpc of M31 is provided by the model of 0912.4133, assuming the stellar disk's mass to light ratio ~8 and almost constant DM density within a core of 28 kpc. Even in this case one gets an exclusion at 5.7sigma from central region of M31 whereas modeling all processed data from M31 and Fornax produces more than 14sigma exclusion. Therefore, despite possible systematic uncertainties, we exclude the possibility that the spectral feature at ~2.5 keV found in 0912.0552 is a DM decay line. We conclude, however, that the search for DM decay line, although demanding prolonged observations of well-studied dSphs, M31 outskirts and other similar objects, is rather promising, as the nature of a possible signal can be checked. An (expected) non-observation of a DM decay signal in the planned observations of Willman 1 should not discourage further dedicated observations.Comment: 16 pages, 8 figures; journal version; analysis of additional data from M31 outskirts and comments on arXiv:1001.4055 are adde