Gate-defined superconducting nanostructures in bilayer graphene weak links

State-of-the-art edge-connected graphene/hexagonal boron nitride van der Waals heterostructures provide low contact resistivity, high charge carrier mobilities as well as a large mean free path. In combination with their high device geometry flexibility they appear thus to be predestined for realizing high-quality tunable weak links in Josephson junctions, which can be readily implemented into superconducting circuits for quantum technological applications. However, designing gate-controlled nanostructures in monolayer graphene remains a serious challenge due to its lack of a band gap which hinders the confinement of charge carriers. The present thesis aims to address this shortcoming by establishing bilayer graphene as a suitable alternative. Unlike the single-layer relative, bilayer graphene offers the opportunity to open an electronic band gap by breaking the layer symmetry which is possible with the ease of exposing electric displacement fields across the two layers. In this regard, employing the combination of locally defined back and top gate architectures allows to design electrostatically induced nanostructures based on spatial band structure engineering. In this thesis, at first the realization of a gate-tunable charge carrier confinement is presented. The formation of the constriction is demonstrated by means of superconducting magneto-interferometry measurements. Building on the successfully induced electrostatic confinement and in combination with a more sophisticated double top gate structure, a fully operable quantum point contact is implemented within the bilayer graphene weak link. When the junction is measured in the normal state, quantized conductance is observed due to the formation of one-dimensional subbands. Though, unlike in other material systems we here explore the complexity of the degeneracy of spin, valley and unusual mini-valley quantum degrees of freedom. In final measurements, the quantum point contact is probed in the superconducting state. The measured critical current through the junction displays a discrete variation directly correlated to the quantized steps in the normal state conductance. These results pave the way towards the study of individual Andreev bound levels through this superconducting quantum point contact. In conclusion, the presented work demonstrates the implementation of electrostatically tunable superconducting nanostructures in bilayer graphene weak links which serves as a platform for the design of more complex electronic circuits

BIOGEOGRAPHICAL STUDIES OF VENEZUELAN SPECIES OF STYLOSANTHES (LEGUMINOSAE)

Stylosanthes is a mainly Neotropical genus comprising ca. 25 species with Venezuela hosting high species richness. Despite of the economic importance of some species as forage legumes, the biogeography of Stylosanthes in Venezuela has hitherto been poorly studied. Information of herbarium specimens and germplasm accessions was collated in order to assess distribution, range size, geographic species richness and rarity in the country. Stylosanthes guianensis has the widest geographical distribution and the largest range size. The highest species richness occurs in the eastern plains (“Llanos”) (7-10° N, 62-65° W). The endemic S. sericeiceps and S. venezuelensis fall within the rarest category. This study contributes to better understand the pattern of biogeographical variation of Venezuelan Stylosanthes species, and can help to design conservation strategies

Clearing confusion in Stylosanthes taxonomy. 2. S. macrocephala MB Ferreira & Sousa Costa vs. S. capitata Vogel and S. bracteata Vogel

Stylosanthes macrocephala was described as a new species in 1977 and has become an economically important pasture legume. It has recently been claimed to be conspecific with S. capitata, also an economically important species. This paper refutes this synonymization and summarizes morphological descriptions as well as genetic studies and cytological evidence indicating that diploid S. macrocephala should be considered a separate species, even being a likely progenitor of tetraploid S. capitata. Early confusion with S. bracteata is also discussed

Lessons from Silage Adoption Studies in Honduras

Silage adoption has so far been low in the tropics, particularly under smallholder conditions. Innovation and adoption processes of silage technologies were promoted in drought-constrained areas of Honduras using a flexible, site-specific and participatory research and extension approach. A total of about 250 farmers participated in training workshops and field days conducted in 13 locations. Smallholders successfully ensiled maize, sorghum and/or Pennisetum spp. mainly in heap and earth siloswhereas little bag silage (LBS) adoption was low. LBS proved useful as a demonstration, experimentation and learning tool. A ‘silage boom’ occurred in five locations where favourable adoption conditions included the presence of demonstration farms and involvement of key innovators, lack of alternative dry season feeds, perceived benefits of silage feeding, a favourable milk market and both extension continuity and intensity. The lack of chopping equipment was the main reason for non-adoption by low-income smallholders. The study showed that when targeting production systems needs and farmer demands, silage promotion can lead to significant adoption, including at smallholder level, in the tropics. This experience could contribute to increase the effectiveness and sustainability of silage extension in similar situations elsewhere

Spin and valley degrees of freedom in a bilayer graphene quantum point contact: Zeeman splitting and interaction effects

We present a study on the lifting of degeneracy of the size-quantized energy levels in an electrostatically defined quantum point contact in bilayer graphene by the application of in-plane magnetic fields. We observe a Zeeman spin splitting of the first three subbands, characterized by effective Landé g factors that are enhanced by confinement and interactions. In the gate-voltage dependence of the conductance, a shoulderlike feature below the lowest subband appears, which we identify as a 0.7 anomaly stemming from the interaction-induced lifting of the band degeneracy. We employ a phenomenological model of the 0.7 anomaly to the gate-defined channel in bilayer graphene subject to in-plane magnetic field. Based on the qualitative theoretical predictions for the conductance evolution with increasing magnetic field, we conclude that the assumption of an effective spontaneous spin splitting is capable of describing our observations, while the valley degree of freedom remains degenerate

La coleccion de Centrosema de CIAT: Datos de pasaporte basicos y distribucion geografica del germoplasma en la coleccion de CIAT

Se presentan los datos de pasaporte basicos de la coleccion de Centrosema del CIAT: no. de acceso, fuente, institucion colaboradora, origen, fecha de coleccion y registro, y mapas de la distribucion geografica de las 29 especies que conforman la coleccion. (CIAT