De geschiedenis van de Buckfastbij

De afkomst en ontwikkeling van de Buckfastbij, ontstaan uit het kruisings- en selectiewerk van broeder Adam op Buckfast Abbey in Zuidwest-Engeland. De uitgangspunten van de bedrijfsmethode en de selectiecriteria voor de bijen worden besproken. De bedrijfsmethode gaat uit van vruchtbare koninginnen met grote broedkamers en minimaal ingrijpen van de imker. Voor het kruisen en selecteren werden koninginnen verzameld in geheel Europ

Quantitative Study of Magnetotransport through a (Ga,Mn)As Single Ferromagnetic Domain

We have performed a systematic investigation of the longitudinal and transverse magnetoresistance of a single ferromagnetic domain in (Ga,Mn)As. We find that, by taking into account the intrinsic dependence of the resistivity on the magnetic induction, an excellent agreement between experimental results and theoretical expectations is obtained. Our findings provide a detailed and fully quantitative validation of the theoretical description of magnetotransport through a single ferromagnetic domain. Our analysis furthermore indicates the relevance of magneto-impurity scattering as a mechanism for magnetoresistance in (Ga,Mn)As.Comment: 5 pages, 4 figures; v2: missing references included, figures recompressed to improve readabilit

Study of a high spatial resolution 10B-based thermal neutron detector for application in neutron reflectometry: the Multi-Blade prototype

Although for large area detectors it is crucial to find an alternative to detect thermal neutrons because of the 3He shortage, this is not the case for small area detectors. Neutron scattering science is still growing its instruments' power and the neutron flux a detector must tolerate is increasing. For small area detectors the main effort is to expand the detectors' performances. At Institut Laue-Langevin (ILL) we developed the Multi-Blade detector which wants to increase the spatial resolution of 3He-based detectors for high flux applications. We developed a high spatial resolution prototype suitable for neutron reflectometry instruments. It exploits solid 10B-films employed in a proportional gas chamber. Two prototypes have been constructed at ILL and the results obtained on our monochromatic test beam line are presented here

Large tunable image-charge effects in single-molecule junctions

The characteristics of molecular electronic devices are critically determined by metal-organic interfaces, which influence the arrangement of the orbital levels that participate in charge transport. Studies on self-assembled monolayers (SAMs) show (molecule-dependent) level shifts as well as transport-gap renormalization, suggesting that polarization effects in the metal substrate play a key role in the level alignment with respect to the metal's Fermi energy. Here, we provide direct evidence for an electrode-induced gap renormalization in single-molecule junctions. We study charge transport in single porphyrin-type molecules using electrically gateable break junctions. In this set-up, the position of the occupied and unoccupied levels can be followed in situ and with simultaneous mechanical control. When increasing the electrode separation, we observe a substantial increase in the transport gap with level shifts as high as several hundreds of meV for displacements of a few \aa ngstroms. Analysis of this large and tunable gap renormalization with image-charge calculations based on atomic charges obtained from density functional theory confirms and clarifies the dominant role of image-charge effects in single-molecule junctions

Effect of annealing on the depth profile of hole concentration in (Ga,Mn)As

The effect of annealing at 250 C on the carrier depth profile, Mn distribution, electrical conductivity, and Curie temperature of (Ga,Mn)As layers with thicknesses > 200 nm, grown by molecular-beam epitaxy at low temperatures, is studied by a variety of analytical methods. The vertical gradient in hole concentration, revealed by electrochemical capacitance-voltage profiling, is shown to play a key role in the understanding of conductivity and magnetization data. The gradient, basically already present in as-grown samples, is strongly influenced by post-growth annealing. From secondary ion mass spectroscopy it can be concluded that, at least in thick layers, the change in carrier depth profile and thus in conductivity is not primarily due to out-diffusion of Mn interstitials during annealing. Two alternative possible models are discussed.Comment: 8 pages, 8 figures, to appear in Phys. Rev.

Optical Conductivity of Ferromagnetic Semiconductors

The dynamical mean field method is used to calculate the frequency and temperature dependent conductivity of dilute magnetic semiconductors. Characteristic qualitative features are found distinguishing weak, intermediate, and strong carrier-spin coupling and allowing quantitative determination of important parameters defining the underlying ferromagnetic mechanism