Thermopower of Interacting GaAs Bilayer Hole Systems in the Reentrant Insulating Phase near ν=1\nu=1

We report thermopower measurements of interacting GaAs bilayer hole systems. When the carrier densities in the two layers are equal, these systems exhibit a reentrant insulating phase near the quantum Hall state at total filling factor $\nu=1$. Our data show that as the temperature is decreased, the thermopower diverges in the insulating phase. This behavior indicates the opening of an energy gap at low temperature, consistent with the formation of a pinned Wigner solid. We extract an energy gap and a Wigner solid melting phase diagram.Comment: to be published in Phys. Rev. Let

Professional training and participatory research: Combined actions for developing organic rice farming in the Camargue region of France

In 2006 and 2007, INRA’s Joint Research Unit, Innovation, was a partner in a European professional training project within the framework of the Leonardo da Vinci programme. The objective of this project was to help develop organic rice farming in the major European rice-growing regions where rice is mainly cultivated in ecologically-sensitive areas. In France, the rate of conversion to organic production is much lower that what would be expected, since organic rice farming presents particular technical problems. The availability of expert support is critical to successful conversion and no structured training was available in the past. This is the reason why we developed a participatory training method that helps rice growers and stakeholders to convert to organic farming and to improve their organic rice production. Different training sessions were organised. The participants shared their thoughts about technical problems encountered and identified possible solutions. Some of the topics developed were weeds, soils and fertility, and varieties. At the end of these sessions, a motivated workgroup was set up. Some of its members even proposed to assess the efficiency of some of the techniques that were discussed during the work sessions in fields on their own farms. Furthermore, field visits were organised in the Camargue region of France and in Spain. Scientists and group members hope to be able to continue to work together after the O.R.P.E.S.A. project is over. In order to make this possible, we are now planning to initiate new research and development actions using the same approach

Production de références pour optimiser la fertilisation organique en riziculture biologique camarguaise (France)

La vitesse de minéralisation d’un engrais organique commercial a été évaluée durant trois années dans une rizière biologique en Camargue (France). L’effet de différentes doses d’engrais apportées à différentes périodes a été testé. En conditions inondées, l’engrais organique minéralise rapidement. Sur la base de ce résultat, nous montrons qu’une adaptation des pratiques de fertilisation organique, inspirée de celles appliquées pour des engrais minéraux, permet une meilleure valorisation des engrais organiques, découlant sur une meilleure rentabilité

Heat Capacity Evidence for the Suppression of Skyrmions at Large Zeeman Energy

Measurements on a multilayer two-dimensional electron system (2DES) near Landau level filling $\nu$=1 reveal the disappearance of the nuclear spin contribution to the heat capacity as the ratio $\tilde{g}$ between the Zeeman and Coulomb energies exceeds a critical value $\tilde{g}_c \approx$0.04. This disappearance suggests the vanishing of the Skyrmion-mediated coupling between the lattice and the nuclear spins as the spin excitations of the 2DES make a transition from Skyrmions to single spin-flips above $\tilde{g}_c$. Our experimental $\tilde{g}_c$ is smaller than the calculated $\tilde{g}_c$=0.054 for an ideal 2DES; we discuss possible origins of this discrepancy.Comment: Experimental paper, 6 figure

Formation of quantum dots in the potential fluctuations of InGaAs heterostructures probed by scanning gate microscopy

The disordered potential landscape in an InGaAs/InAlAs two-dimensional electron gas patterned into narrow wires is investigated by means of scanning gate microscopy. It is found that scanning a negatively charged tip above particular sites of the wires produces conductance oscillations that are periodic in the tip voltage. These oscillations take the shape of concentric circles whose number and diameter increase for more negative tip voltages until full depletion occurs in the probed region. These observations cannot be explained by charging events in material traps, but are consistent with Coulomb blockade in quantum dots forming when the potential fluctuations are raised locally at the Fermi level by the gating action of the tip. This interpretation is supported by simple electrostatic simulations in the case of a disorder potential induced by ionized dopants. This work represents a local investigation of the mechanisms responsible for the disorder-induced metal-to-insulator transition observed in macroscopic two-dimensional electron systems at low enough density

Scanning Gate Spectroscopy of transport across a Quantum Hall Nano-Island

We explore transport across an ultra-small Quantum Hall Island (QHI) formed by closed quan- tum Hall edge states and connected to propagating edge channels through tunnel barriers. Scanning gate microscopy and scanning gate spectroscopy are used to first localize and then study a single QHI near a quantum point contact. The presence of Coulomb diamonds in the spectroscopy con- firms that Coulomb blockade governs transport across the QHI. Varying the microscope tip bias as well as current bias across the device, we uncover the QHI discrete energy spectrum arising from electronic confinement and we extract estimates of the gradient of the confining potential and of the edge state velocity.Comment: 13 pages, 3 figure

Thermodynamic Phase Diagram of the Quantum Hall Skyrmion System

We numerically study the interacting quantum Hall skyrmion system based on the Chern-Simons action. By noticing that the action is invariant under global spin rotations in the spin space with respect to the magnetic field direction, we obtain the low-energy effective action for a many skyrmion system. Performing extensive molecular dynamics simulations, we establish the thermodynamic phase diagram for a many skyrmion system.Comment: 4 pages, RevTex, 2 postscript figure

Scanning-gate microscopy of semiconductor nanostructures: an overview

This paper presents an overview of scanning-gate microscopy applied to the imaging of electron transport through buried semiconductor nanostructures. After a brief description of the technique and of its possible artifacts, we give a summary of some of its most instructive achievements found in the literature and we present an updated review of our own research. It focuses on the imaging of GaInAs-based quantum rings both in the low magnetic field Aharonov-Bohm regime and in the high-field quantum Hall regime. In all of the given examples, we emphasize how a local-probe approach is able to shed new, or complementary, light on transport phenomena which are usually studied by means of macroscopic conductance measurements.Comment: Invited talk by SH at 39th "Jaszowiec" International School and Conference on the Physics of Semiconductors, Krynica-Zdroj, Poland, June 201

French lag in scientific research on organic farming: a scientometric approach

France suffers from a large research deficit in most areas related to sustainable development and public health. The case of research on organic farming, within the framework of the broader sector of agronomic research, constitutes a largely under-investigated field. Even if the demand for organic products is rapidly growing in Europe, research in this field still suffers from a lack of funding and researchers. A bibliometric analysis based on key words from scientific publications (in English only) taken from Thomson Scientific’s ISI Web of Science reference database made it possible to determine sufficiently relevant indicators for a comparison of national research efforts from 2000-2006, and to therefore assess actual research priorities in the area of organic farming. Beyond traditional specialisation analyses, the ratio between the world share of publications of a given country in organic farming and its world share of publications in specific disciplines and sub-disciplines (according to the nomenclature of the French Observatory for Sciences and Techniques) allowed us to obtain these prioritisation indexes that act as policy and priority (or prioritisation activity) indicators for research institutions involved in the concerned area. An index above 1 indicates an over-specialisation, whereas an index below 1 indicates an under-specialisation. For the period 2000-2006, the European Union obtained a specialisation index of 1.52 in the area of organic farming, compared to 0.68 for the US, 0.98 for Brazil and 0.18 for China. However, this seemingly satisfactory average for Europe as a whole hides important disparities between European countries. Moreover, France is at the very end of the classification for all indexes with only 0.47 specialisation, compared to Germany with 1.19, Italy with 1.39, Austria with 2.78, Sweden with 3.99, Finland with 4.46 and Denmark with 12.19. The prioritisation index for organic farming in comparison to the discipline, “Applied biology and ecology”, is 1.65 for the EU27, 0.69 for the US, 1.7 for Germany, 3.98 for Austria, 3.84 for Finland, 9.45 for Denmark and 0.49 for France. In comparison to the subdiscipline, “Agriculture, plant biology” the index is 1.5 for the EU27, 0.98 for the US, 1.61 for Germany, 3.28 for Austria, 1.52 for Finland, 9.79 for Denmark, and 0.41 for France. Finally, for the sub-discipline, “Agro-food”, it is 1.86 for the EU27, 0.73 for the US, 1.98 for Germany, 6.92 for Austria, 7.39 for Finland, 9.39 for Denmark and 0.58 for France. These results confirm that research on organic farming is largely under-prioritised in France. The French national research effort is therefore far from meeting the ecological and economic challenges facing agriculture in the 21st century