Similarities between the t−Jt-J and Hubbard models in weakly correlated regimes

We present a comparative study of the Hubbard and $t-J$ models far away from half-filling. We show that, at such fillings the $t-J$ Hamiltonian can be seen as an effective model of the repulsive Hubbard Hamiltonian over the whole range of correlation strength. Indeed, the $|t/U| \in [0,+\infty [$ range of the Hubbard model can be mapped onto the finite range $|J/t| \in [1, 0 ]$ of the $t-J$ model, provided that the effective exchange parameter $J$ is defined variationally as the local singlet-triplet excitation energy. In this picture the uncorrelated limit U=0 is associated with the super-symmetric point $J=-2|t|$ and the infinitely correlated $U=+\infty$ limit with the usual J=0 limit. A numerical comparison between the two models is presented using different macroscopic and microscopic properties such as energies, charge gaps and bond orders on a quarter-filled infinite chain. The usage of the $t-J$ Hamiltonian in low-filled systems can therefore be a good alternative to the Hubbard model in large time-consuming calculations.Comment: To be published in EPJB. 6 pages. 5 figure

Fractionnally charged excitations in the charge density wave state of a quarter-filled t-J chain with quantum phonons

Elementary excitations of the 4k$_F$ charge density wave state of a quarter-filled strongly correlated electronic one-dimensional chain are investigated in the presence of dispersionless quantum optical phonons using Density Matrix Renormalization Group techniques. Such excitations are shown to be topological unbound solitons carrying charge $e/2$. Relevance to the 4k$_F$ charge density wave instability in $\rm (DI-DCNQI)_2Ag$ or recently discovered in (TMTTF)$_2$X (X=PF$_6$, AsF$_6$) is discussed.Comment: 4 pages, 4 figure

Local Development Stakeholders and the European Model: Learning the LEADER Approach in the New Member States

The introduction of a new model of territorial development based on a bottom-up approach appears to be an entirely new process in the formerly communist countries of Central Europe. However, the ongoing process of Europeanisation in these countries is acting as a catalyst for the transmission of this model of endogenous development, a model whose principles are to highlight the specific advantages of each territory and to foster the initiatives taken by decentralised actors. 'Territorial governance' is a notion that sums up these new patterns of public policies that are in keeping with a multi-level environment. At the crossroads of European and national policies with bottom-up initiatives, taken by local stakeholders (local representatives, project managers, etc.), new patterns in territorial resetting are also appearing as part of the change in territorial governance. How does such a transfer of model take place and what effects does it imply? This paper is aimed at analysing the reception of this European model of local development by local actors and their ability for social learning

Retour sur les tournants agraires au XXe siècle en Europe centrale

Review of the agrarian turning-points in Central Europe during the 20th century In less than half a century, agriculture in Central Europe experienced three major watersheds - land reform, collectivisation, and decollectivisation - which gave rise to transformations in agrarian structures. This article intends to examine the most recent episode, the decollectivisation of the 2010s, in order to show that this most recent change cannot be interpreted independently of the previous agrarian turning-points. We will put forward the idea that the direction taken by post-collectivist history should be seen in a longer historical context and is the result of choices made in the past of ways to get out of the previous collectivist system. This interpretation is based on an analysis of the mechanisms of institutional change and the principles they refer to, and of the practical details of their implementation. This analysis is based on the direction taken by the transformation of the social forms of production (carried out by examining monograph studies) and thus enables us to clarify the complexities of the processes under observation. It is organised in three parts: 1- the agrarian turning-points seen as times of reversal of social relationships; 2- decollectivisation understood as the process of dismantling a system involving the conjunction of a double series of agrarian legacies; 3- the post-collectivist social forms as the product of lines of socio-historical developmentEn moins d'un demi-siècle, les agricultures d'Europe centrale ont traversé trois ruptures majeures : réformes agraires, collectivisation, décollectivisation, qui ont déclenché une mutation des structures agraires. Cet article entend faire retour sur l'épisode le plus récent, la décollectivisation de la décennie 1990, pour montrer que cette dernière mutation ne peut être interprétée indépendamment des tournants agraires antérieurs. Nous avançons l'idée que les trajectoires post-collectivistes s'inscrivent dans le temps long de l'histoire et qu'elles dépendent des choix du passé comme des itinéraires de sortie du système collectiviste antérieur. L'interprétation s'appuie sur une analyse des dispositifs de changement institutionnel et des principes auxquels ils se réfèrent ainsi que des modalités de leur mise en œuvre. Cette analyse s'appuie sur les trajectoires de transformation des formes sociales de production (conduites à partir d'études monographiques) et permet ainsi d'éclairer la complexité des processus observés. Elle s'organise en trois parties : 1- les tournants agraires lus comme des temps d'inversion des rapports sociaux, 2- la décollectivisation appréhendée comme processus de démontage d'un système à l'interférence d'une double série d'héritages agraires, 3- les formes sociales post-collectivistes comme produit de trajectoires socio-historiques

Central European geography and the post-socialist transformation: a western point of view

Central European geography before 1989; Central European geography after the 1989 turnaround; a progressive renewal of topics

The evolution of catalytic function

It is very likely that the main driving force of enzyme evolution is the requirement to improve catalytic and regulatory efficiency which results from the intrinsic performance as well as from the spatial and functional organization of enzymes in living cells. Kinetic co-operativity may occur in simple monomeric proteins if they display “slow” conformational transitions, at the cost of catalytic efficiency. Oligomeric enzymes on the other hand can be both efficient and co-operative. We speculate that the main reason for the emergence of co-operative oligomeric enzymes is the need for catalysts that are both cooperative and efficient. As it is not useful for an enzyme to respond to a change of substrate concentration in a complex kinetic way, the emergence of symmetry has its probable origin in a requirement for “functional simplicity”. In a living cell, enzyme are associated with other macromolecules and membranes. The fine tuning of their activity may also be reached through mutations of the microenvironment. Our hypothesis is that these mutations are related to the vectorial transport of molecules, to achieve the hysteresis loops of enzyme reactions generated by the coupling of reaction and diffusion, through the co-operativity brought about by electric interactions between a charged substrate and a membrane, and last but not least, through oscillations. As the physical origins of these effects are very simple and do not require complex molecular devices, it is very likely that the functional advantage generated by the spatial and functional organization of enzyme molecules within the cell have appeared in prebiotic catalysis or very early during the primeval stages of biological evolution. We shall begin this paper by presenting the nature of the probable earliest catalysts in the RNA world

Domestic competition and export performance of manufacturing firms in Côte d’Ivoire

Because of transportation costs, African manufacturing firms benefit from some market power on their domestic market, where they can charge a higher price than the export price, net of transportation cost. We present a simple theoretical model of an exporting firm that discriminates between the export and the domestic markets, where firms engage in Cournot competition. It is then shown that the impact of increased competition on export performance by the firms is ambiguous, and may be negative for a non trivial range of parameter values. Using survey data on Ivoirian firms, our empirical analysis gives some support to this prediction, showing that the probability of a firm exporting decreases with increased competition.

The RNA world: hypotheses, facts and experimental results.

A biochemical world that would have existed before the contemporary DNA-RNA-Protein world, and baptized in 1986 "The RNA World" by Walter Gilbert, such a world had already been proposed during the preceding decades by Carl Woese, Francis Crick and Leslie Orgel. By demonstrating the remarkable diversity of the RNA molecule, Molecular Biology proved these predictions. RNA present in all living cells, performs structural and metabolic functions many of which were unsuspected only a few years ago. A truly modern "RNA world" exists in each cell; it contains RNAs in various forms, short and long fragments, single and double-stranded, endowed with multiple roles (informational, catalytic, that can serve as templates, guides, defense), certain molecules being even capable of carrying out several of these functions