Towards a Casimir force measurement between micromachined parallel plate structures

Ever since its prediction, experimental investigation of the Casimir force has been of great scientific interest. Many research groups have successfully attempted quantifying the force with different device geometries; however, measurement of the Casimir force between parallel plates with sub-micron separation distance is still a challenging task, since it becomes extremely difficult to maintain sufficient parallelism between the plates. The Casimir force can significantly influence the operation of micro devices and to realize reliable and reproducible devices it is necessary to understand and experimentally verify the influence of the Casimir force at sub-micron scale. In this paper, we present the design principle, fabrication and characterization of micromachined parallel plate structures that could allow the measurement of the Casimir force with tunable separation distance in the range of 100 to 1000 nm. Initially, a gold coated parallel plate structure is explored to measure the Casimir force, but also other material combinations could be investigated. Using gold-silicon eutectic bonding, a reliable approach to bond chips with integrated suspended plates together with a well-defined separation distance in the order of 1–2 μm is developed

Anisotropic magnetic behavior in Dy/Y films and superlattices

By neutron diffraction we show that superlattices of Dy and Y grown by molecular-beam epitaxy along the hcp b axis exhibit little magnetic coupling between successive Dy layers, even for Y spacers as thin as 9 atomic planes (26). Previous studies of Dy/Y superlattices grown along the hcp c axis established that long-range three-dimensional helimagnetic ordering takes place even through Y spacer layers as thick as 120. This highly anisotropic coupling behavior is shown to have its origin in nearly-two-dimensional nesting features of the Y and Dy Fermi surfaces. Nesting along the c axis gives rise to sharp peaks along c in the wave-vector-dependent magnetic susceptibility, and causes the exchange coupling to exhibit long-range oscillations in real space. The lack of nesting features along the b axis leaves a rapid exponential decay of the exchange interaction with spin separation. From magnetic measurements by superconducting-quantum-interference-device magnetometry on b-axis superlattices and films, we deduce that the first-order ferromagnetic transition of Dy is suppressed, and that the critical field required to produce the ferromagnetic alignment is much higher than the c-axis counterpart. This difference arises from anisotropy of the energy balance of the system. The magnetic coherence in b-axis superlattices and films is anisotropic and exhibits an unusual temperature dependence

Stomatal conductance and xylem-sap abscisic acid (ABA) in adult oak trees during a gradually imposed drought

Thirty- to 40-year-old oak trees (Quercus petraea Matt Liebl and Q robur L) growing in a forest stand near Champenoux (Nancy, France) were submitted to an imposed drought in a lysimeter during two successive summers. Xylem sap was extracted from leafy twigs of two trees per species at regular intervals during the onset of drought, and of two controls in parallel. Predawn leaf water potential, soil water potential at different depths and midday stomatal conductance of sun-exposed leaves were recorded at the same pace. Concentrations of abscisic acid (ABA) and of the glucose ester of ABA (ABA-GE) were measured with an ELISA technique, after purification of the sap samples using high performance liquid chromatography (HPLC). Mean concentrations of ABA recorded in the absence of drought constraint were around 30 μmol m-,3 independently from year and species, and 45 μmol m-3 for ABA-GE. No significant drought-related increase in either of these concentrations could be recorded while predawn leaf water potential dropped below-1.5 MPa, and stomatal conductance down to 10% of the values of controls. A good correlation between stomatal conductance and predawn leaf water potential was detected, and even a better one between stomatal conductance and soil water potential at 25 cm. It may be concluded from these results that, even if a close correlation exists between stomatal aperture and soil moisture, the existence of a root signalling process based solely on increased ABA (MPa). or ABA-GE transport in the xylem sap may be questioned. Stomatal conductance under natural conditions is probably controlled by a rather complex chain of processes which has still to be elucidated.Des chênes âgés de 30 à 40 ans appartenant aux deux espèces pédonculé et sessile (Quercus robur L et Q petraea Matt Liebl) ont été sélectionnés dans un haut perchis de la forêt domaniale de Champenoux (Nancy, France), et soumis à une sécheresse contrôlée dans une cuve lysimétrique au cours de deux étés successifs. De la sève brute a été extraite du xylème de rameaux feuillés à intervalles de temps réguliers pendant l’installation de la sécheresse, et sur des arbres témoins. Le potentiel hydrique de base, le potentiel hydrique du sol à différentes profondeurs et la conductance stomatique en milieu de journée ont été relevés sur les mêmes arbres. Les concentrations en acide abscissique (ABA) et en glucose-ester d’ABA (ABA-GE) ont été déterminées dans ces échantillons de sève à l’aide d’un dosage Elisa, après purification par HPLC. En l’absence de sécheresse, les concentrations moyennes en ABA étaient de l’ordre de 30 μmol m-,3 et de 45 μmol m- 3 pour l’ABA-GE. Ces concentrations étaient indépendantes de l’espèce et de l’année de mesure ; seules les concentrations en ABA-GE ont fortement augmenté d’une année sur l’autre. Aucune augmentation significative due à la sécheresse n’a pu être détectée, bien que les potentiels hydriques de base aient chuté jusqu’à des valeurs inférieures à -1,5 MPa, et que la conductance stomatique ait été réduite à 10 % des valeurs initiales dans certains cas. Une bonne corrélation entre conductance stomatique et potentiel hydrique de base a pu être mise en évidence. Cette corrélation a été améliorée en utilisant les valeurs de potentiel hydrique de sol à 25 cm de profondeur. Nous concluons que, même si une bonne corrélation existe entre conductance stomatique et humidité du sol, l’existence d’un signal racinaire sous la forme exclusive d’un transport d’ABA ou d’ABA-GE des racines vers les couronnes est peu probable. La conductance stomatique de chênes de grande taille poussant en conditions naturelles est vraisemblablement contrôlée par un processus complexe qui reste à préciser