Dynamical properties of low dimensional CuGeO3 and NaV2O5 systems

Properties of low-dimensional spin-Peierls systems are described by using a one dimensional S=1/2 antiferromagnetic Heisenberg chain linearly coupled to a single phonon mode of wave vector pi (whose contribution is expected to be dominant). By exact diagonalizations of small rings with up to 24 sites supplemented by a finite size scaling analysis, static and dynamical properties are investigated. Numerical evidences are given for a spontaneous discrete symmetry breaking towards a spin gapped phase with a frozen lattice dimerization. Special emphasis is put on the comparative study of the two inorganic spin-Peierls compounds CuGeO3 and NaV2O5 and the model parameters are determined from a fit of the experimental spin gaps. We predict that the spin-phonon coupling is 2 or 3 times larger in NaV2O5 than in CuGeO3. Inelastic neutron scattering spectra are calculated and similar results are found in the single phonon mode approximation and in the model including a static dimerization. In particular, the magnon S=1 branch is clearly separated from the continuum of triplet excitations by a finite gap.Comment: 10 pages, RevTex, revised version submitted to Euro. Phys. Rev.

Coexistence of charge density wave and spin-Peierls orders in quarter-filled quasi-one dimensional correlated electron systems

Charge and spin-Peierls instabilities in quarter-filled (n=1/2) compounds consisting of coupled ladders and/or zig-zag chains are investigated. Hubbard and t-J models including local Holstein and/or Peierls couplings to the lattice are studied by numerical techniques. Next nearest neighbor hopping and magnetic exchange, and short-range Coulomb interactions are also considered. We show that, generically, these systems undergo instabilities towards the formation of Charge Density Waves, Bond Order Waves and (generalized) spin-Peierls modulated structures. Moderate electron-electron and electron-lattice couplings can lead to a coexistence of these three types of orders. In the ladder, a zig-zag pattern is stabilized by the Holstein coupling and the nearest-neighbor Coulomb repulsion. In the case of an isolated chain, bond-centered and site-centered 2k_F and 4k_F modulations are induced by the local Holstein coupling. In addition, we show that, in contrast to the ladders, a small charge ordering in the chains, strongly enhances the spin-Peierls instability. Our results are applied to the NaV_2O_5 compound (trellis lattice) and various phases with coexisting charge disproportionation and spin-Peierls order are proposed and discussed in the context of recent experiments. The role of the long-range Coulomb potential is also outlined.Comment: 10 pages, Revtex, 10 encapsulated figure

Mixing of magnetic and phononic excitations in incommensurate Spin-Peierls systems

We analyze the excitation spectra of a spin-phonon coupled chain in the presence of a soliton. This is taken as a microscopic model of a Spin-Peierls material placed in a high magnetic field. We show, by using a semiclassical approximation in the bosonized representation of the spins that a trapped magnetic state obtained in the adiabatic approximation is destroyed by dynamical phonons. Low energy states are phonons trapped by the soliton. When the magnetic gap is smaller than the phonon frequencies the only low energy state is a mixed magneto-phonon state with the energy of the gap. We emphasize that our results are relevant for the Raman spectra of the inorganic Spin-Peierls material CuGeO$_3$.Comment: 5 pages, latex, 2 figures embedded in the tex

Competencies, Capabilities and the Neoschumpeterian Tradition

In this paper, Mie Augier and David Teece outline the history and development of the ideas underlying an emerging approach within strategic management research: the dynamic capabili-ties framework. The framework was first outlined by Teece and Pisano (1994), and in the pre-sent paper elaborated further so the reader will be able to appreciate some of the most impor-tant intellectual resources underpinning it, such as the work of Schumpeter, Penrose, William-son, Cyert and March, Rummelt, Nelson and Winter. Although listed as intellectual resources by the authors, they also turn (some of) them into a topic for further discussion. For example, Augier and Teece identify not only the merits but also the limitations of transaction costs eco-nomics. In this way, the authors pave the way for a more dynamic framework while drawing upon organization theory and scholars like Cyert and March (a behavioral theory of the firm) and Nelson and Winter (an evolutionary theory of economic change). In the dynamic capability framework firms and markets co-evolve. Managers are now allowed to perform distinct strate-gic roles in shaping both firms and their markets, e.g. through asset- selection and orchestra-tion, including also the task of allocating resources between exploitation and exploration

Les terres agricoles face à l’urbanisation

La perte de terres agricoles liées à l’urbanisation constitue l’une des facettes de la consommation des terres. Commencé dans les années 1970, ce phénomène — essentiellement dû à l’étalement urbain — prend des proportions jusque-là inégalées. Les conséquences de ces processus d’artificialisation sont multiples et portent à la fois sur la production et sur la sécurité alimentaire ainsi que sur la perte de biodiversité. Ces processus interrogent aussi les formes de solidarité territoriale entre les villes et les espaces péri-urbains et ruraux. Issu d’une collaboration scientifique lancée au début des années 2010 entre l’Université de technologie de Sydney (University of Technology Sydney, UTS) et l’Institut national de recherche en sciences et technologies pour l’environnement et l’agriculture (Irstea), cet ouvrage aborde des points clés de la problématique de la consommation des terres en se focalisant sur les terres agricoles en France et en Australie. Plutôt que d’offrir une analyse comparative approfondie de la planification des terres agricoles périurbaines entre les deux pays, il propose une exploration des « boîtes à outils » de l’ingénierie territoriale développées et mobilisées pour faire face à l’enjeu de la perte de terres agricoles liée à l’urbanisation. Il offre également un « arrêt sur image » dans un panorama de champs de recherche en pleine évolution, autant du point de vue théorique que méthodologique

Trade, Standards, and Poverty: Evidence from Senegal

An emerging literature on standards, global supply chains, and development argues that enhanced quality and safety standards are major trade barriers for developing country exports and cause the marginalization of small businesses and poor households in developing countries. This paper is the first to quantify income and poverty effects of such high-standards trade and to integrate labor market effects, by using company and household survey data from the vegetable export chain in Senegal. First, horticultural exports from Senegal to the EU have grown sharply over the past decade, despite strongly increasing food standards in the EU. Second, these exports have strong positive effects on poor households?income. We estimate that these exports reduced regional poverty by around 12 percentage points and reduced extreme poverty by half. Third, tightening food standards induced structural changes in the supply chain including a shift from smallholder contract-based firming to large-scale integrated estate production. However, these changes mainly altered the mechanism through which poor households benefit: through labor markets instead of product markets. Moreover, the impact on poverty reduction is stronger as the poorest benefit relatively more from working on large-scale firms than from contract firming. These findings challenge several basic arguments in this research field

From the Guest Editors: the legitimacy and impact of business schools—key issues and a research agenda

It is an appropriate moment to review research into the legitimacy and impact of business schools. It is more than a decade now since Pfeffer and Fong's (2002) provocative paper challenging the perceived orthodoxy of business school success in the very first edition of the Academy of Management Learning & Education