Semiclassical theory of spin-orbit interaction in the extended phase space

We consider the semiclassical theory in a joint phase space of spin and orbital degrees of freedom. The method is developed from the path integrals using the spin-coherent-state representation, and yields the trace formula for the density of states. We discuss special cases, such as weak and strong spin-orbit coupling, and relate the present theory to the earlier approaches.Comment: 36 pages, 8 figures. Version 2: revised Sec. 4.4 and Appendix B; minor corrections elsewher

Spin-orbit coupling and intrinsic spin mixing in quantum dots

Spin-orbit coupling effects are studied in quantum dots in InSb, a narrow-gap material. Competition between different Rashba and Dresselhaus terms is shown to produce wholesale changes in the spectrum. The large (and negative) $g$-factor and the Rashba field produce states where spin is no longer a good quantum number and intrinsic flips occur at moderate magnetic fields. For dots with two electrons, a singlet-triplet mixing occurs in the ground state, with observable signatures in intraband FIR absorption, and possible importance in quantum computation.Comment: REVTEX4 text with 3 figures (high resolution figs available by request). Submitted to PR

Semiclassical theory of spin-orbit interactions using spin coherent states

We formulate a semiclassical theory for systems with spin-orbit interactions. Using spin coherent states, we start from the path integral in an extended phase space, formulate the classical dynamics of the coupled orbital and spin degrees of freedom, and calculate the ingredients of Gutzwiller's trace formula for the density of states. For a two-dimensional quantum dot with a spin-orbit interaction of Rashba type, we obtain satisfactory agreement with fully quantum-mechanical calculations. The mode-conversion problem, which arose in an earlier semiclassical approach, has hereby been overcome.Comment: LaTeX (RevTeX), 4 pages, 2 figures, accepted for Physical Review Letters; final version (v2) for publication with minor editorial change

Spin-density-functional theory of circular and elliptical quantum dots

Using spin-density-functional theory, we study the electronic states of a two-dimensional parabolic quantum dot with up to N=58 electrons. We observe a shell structure for the filling of the dot with electrons. Hund's rule determines the spin configuration of the ground state, but only up to 22 electrons. At specific N, the ground state is degenerate, and a small elliptical deformation of the external potential induces a rotational charge-density-wave (CDW) state. Previously identified spin-density-wave (SDW) states are shown to be artifacts of broken spin symmetry in density-functional theory.Comment: 10 pages, 3 figure

Semiclassical trace formulae for systems with spin-orbit interactions: successes and limitations of present approaches

We discuss the semiclassical approaches for describing systems with spin-orbit interactions by Littlejohn and Flynn (1991, 1992), Frisk and Guhr (1993), and by Bolte and Keppeler (1998, 1999). We use these methods to derive trace formulae for several two- and three-dimensional model systems, and exhibit their successes and limitations. We discuss, in particular, also the mode conversion problem that arises in the strong-coupling limit.Comment: LaTeX2e, 25 pages incl. 9 figures, version 3: final version in print for J. Phys.

Diversity of experimentation by farmers engaged in agroecology

International audienceAbstractAgroecology questions the production of generic knowledge. Rather than searching for the best practices for large-scale transfer, it would be more efficient to help farmers find their own solutions. A promising activity for farmers is experimentation because it answers their needs and helps them learn. However, how agroecological practices are tested by farmers in their own experiments is still poorly known. In this study, we examined the short-term experimental activity, i.e., experiments carried out at a yearly scale in pre-defined fields. Seventeen farmers in south eastern France were surveyed. The farmers practiced conventional or organic farming and cultivated either arable or market garden crops. Experiments on agroecological practices were characterized, located along a timeline, and discussed with them. To conduct the interviews with the farmers, each experiment was described in three stages: (1) designing the experiment, (2) managing it in real time, and (3) evaluating the results of the experiment. The data collected in the interviews were first analyzed to build a descriptive framework of farmers’ experiments, after which hierarchical cluster analysis was used to analyze the diversity of the farmers’ experiments. Here, we propose for the first time a generic framework to describe farmers’ experiments at a short time scale based on the consistency between the Design, Management, and Evaluation stages. We used the framework to characterize the diversity of farmers’ experiments and identified four clusters. The originality of this work is both building a descriptive framework resulting from in-depth analyses of farmers’ discourse and using statistical tools to identify and interpret the groups of experiments. Our results provide a better understanding of farmers’ experiments and suggest tools and methods to help them experiment, a major challenge in the promotion of a large-scale agroecological transition

How organic farmers view their own practice: results from the Czech Republic

This paper addresses the development of organic agriculture in the Czech Republic, which is seen as a success story among post-communist countries. The relatively short history of organic farming and specific contextual factors raises questions about the nature and meaning of Czech organic farming. The goal of this study was to find out how farmers view their own practice, interpret its symbolic value, and construct its content. This empirical study uses Q methodology aimed at the identification of the collectively-shared perspectives belonging engaged actors. Data were gathered through semi-standardized interviews with Czech farmers registered in official organic scheme. The analysis emphasized three components, which are considered as three distinct perspectives possessed by organic farmers; that is, (1) organic farming as a way of life, (2) as an occupation, and (3) as a production of food of an alternative quality compared to conventional food. Each viewpoint entails a different understanding of what organic farming means; each then—when considered together—comprises the meaning of organic agriculture in the Czech Republic. The presented classification of the farmers holding the viewpoints contributes to the ongoing theoretical discussion regarding the nature of the current organic sector, its development and potential conventionalization

Adapting agricultural water use to climate change in a post-Soviet context: challenges and opportunities in southeast Kazakhstan

The convergence of climate change and post-Soviet socio-economic and institutional transformations has been underexplored so far, as have the consequences of such convergence on crop agriculture in Central Asia. This paper provides a place-based analysis of constraints and opportunities for adaptation to climate change, with a specific focus on water use, in two districts in southeast Kazakhstan. Data were collected by 2 multi-stakeholder participatory workshops, 21 semi-structured in-depth interviews, and secondary statistical data. The present-day agricultural system is characterised by enduring Soviet-era management structures, but without state inputs that previously sustained agricultural productivity. Low margins of profitability on many privatised farms mean that attempts to implement integrated water management have produced water users associations unable to maintain and upgrade a deteriorating irrigation infrastructure. Although actors engage in tactical adaptation measures, necessary structural adaptation of the irrigation system remains difficult without significant public or private investments. Market-based water management models have been translated ambiguously to this region, which fails to encourage efficient water use and hinders adaptation to water stress. In addition, a mutual interdependence of informal networks and formal institutions characterises both state governance and everyday life in Kazakhstan. Such interdependence simultaneously facilitates operational and tactical adaptation, but hinders structural adaptation, as informal networks exist as a parallel system that achieves substantive outcomes while perpetuating the inertia and incapacity of the state bureaucracy. This article has relevance for critical understanding of integrated water management in practice and adaptation to climate change in post-Soviet institutional settings more broadly