Negative quantum capacitance in graphene nanoribbons with lateral gates

We present numerical simulations of the capacitive coupling between graphene nanoribbons of various widths and gate electrodes in different configurations. We compare the influence of lateral metallic or graphene side gate structures on the overall back gate capacitive coupling. Most interestingly, we find a complex interplay between quantum capacitance effects in the graphene nanoribbon and the lateral graphene side gates, giving rise to an unconventional negative quantum capacitance. The emerging non-linear capacitive couplings are investigated in detail. The experimentally relevant relative lever arm, the ratio between the coupling of the different gate structures, is discussed.Comment: 8 pages, 6 figure

Food Security in Crop, Livestock and Mixed Farming Systems in Mali

This study evaluates the food security status, its determinants and the coping strategies in crop, livestock and mixed crop-livestock systems in southern Mali. Interviews were conducted with 258 households that were categorized into the three farming systems based on the revenue from livestock and crop production. A linear mixed model was used to analyse the effects of household characteristics on food security using the food consumption score (FCS), household dietary diversity score (HDDS), a modified household food insecurity access scale (mHFIAS) and coping strategies as indicators. Food consumption score was significantly influenced by the farming system with highest FCS for the livestock system (88) followed by the mixed system (77) and the crop system (69). Moreover, FCS was positively influenced by the number of crops cultivated, total farm milk production, off-farm income and number of raised chickens (p \u3c 0.1). The main difference in food intake between systems was for milk with average daily consumption of 201 ml, 110 ml and 60 ml in the livestock, mixed and crop systems, respectively. HDDS was also significantly influenced by the farming system with highest HDDS values for the livestock system (8.9), followed by the mixed system (8.2) and the crop system (8.0). Further, HDDS was positively influenced by crop diversity and number of chickens, and negatively influenced by the number of family dependents. During the food shortage period of August, households from the crop system were more food insecure than households from the mixed and livestock systems with mHFIAS values of 0.9, 1.5 and 4.3, respectively. Livestock and mixed systems were more resilient to food insecurity situations than the crop system. This study confirms the direct effect of milk production on milk consumption and the importance of livestock for enhancing food security and livelihood resilience

Planetary-geometric constraints on isopycnal slope in the Southern Ocean

On planetary scales, surface wind stress and differential buoyancy forcing act together to produce isopycnal surfaces that are relatively flat in the tropics/subtropics and steep near the poles, where they tend to outcrop. Tilted isopycnals in a rapidly rotating fluid are subject to baroclinic instability. The turbulent, mesoscale eddies generated by this instability have a tendency to homogenize potential vorticity (PV) along density surfaces. In the Southern Ocean (SO), the tilt of isopycnals is largely maintained by competition between the steepening effect of surface forcing and the flattening effect of turbulent, spatially inhomogeneous eddy fluxes of PV. Here we use quasi-geostrophic theory to investigate the influence of a planetary-geometric constraint on the equilibrium slope of tilted density/buoyancy surfaces in the SO. If the meridional gradients of relative vorticity and PV are small relative to β, then quasi-geostrophic theory predicts ds/dz = β/ f0 = cot(ϕ0)/a, or equivalently r ≡ |∂zs/(β/ f0)| = 1, where s is the isopycnal slope, ϕ0 is a reference latitude, a is the planetary radius, and r is the depth-averaged criticality parameter. We find that the strict r = 1 condition holds over specific averaging volumes in a large-scale climatology. A weaker r = O(1) condition for depth-averaged quantities is generally satisfied away from large bathymetric features. We employ the r = O(1) constraint to derive a depth scale to characterize large-scale interior stratification, and we use an idealized sector model to test the sensitivity of this relationship to surface wind forcing. Finally, we discuss the possible implications for eddy flux parameterization and for the sensitivity of SO circulation/stratification to changes in forcing

Similar millennial climate variability on the Iberian margin during two early Pleistocene glacials and MIS 3

Although millennial-scale climate variability (<10 ka) has been well studied during the last glacial cycles, little is known about this important aspect of climate in the early Pleistocene, prior to the Middle Pleistocene Transition. Here we present an early Pleistocene climate record at centennial resolution for two representative glacials (marine isotope stages (MIS) 37–41 from approximately 1235 to 1320 ka) during the “41 ka world” at Integrated Ocean Drilling Program Site U1385 (the “Shackleton Site”) on the southwest Iberian margin. Millennial-scale climate variability was suppressed during interglacial periods (MIS 37, MIS 39, and MIS 41) and activated during glacial inceptions when benthic δ18O exceeded 3.2‰. Millennial variability during glacials MIS 38 and MIS 40 closely resembled Dansgaard-Oeschger events from the last glacial (MIS 3) in amplitude, shape, and pacing. The phasing of oxygen and carbon isotope variability is consistent with an active oceanic thermal bipolar see-saw between the Northern and Southern Hemispheres during most of the prominent stadials. Surface cooling was associated with systematic decreases in benthic carbon isotopes, indicating concomitant changes in the meridional overturning circulation. A comparison to other North Atlantic records of ice rafting during the early Pleistocene suggests that freshwater forcing, as proposed for the late Pleistocene, was involved in triggering or amplifying perturbations of the North Atlantic circulation that elicited a bipolar see-saw response. Our findings support similarities in the operation of the climate system occurring on millennial time scales before and after the Middle Pleistocene Transition despite the increases in global ice volume and duration of the glacial cycles

Single-valley high-mobility (110) AlAs quantum wells with anisotropic mass

We studied a doping series of (110)-oriented AlAs quantum wells (QWs) and observed transport evidence of single anisotropic-mass valley occupancy for the electrons in a 150 \AA wide QW. Our calculations of strain and quantum confinement for these samples predict single anisotropic-mass valley occupancy for well widths $W$ greater than 53 \AA. Below this, double-valley occupation is predicted such that the longitudinal mass axes are collinear. We observed mobility anisotropy in the electronic transport along the crystallographic directions in the ratio of 2.8, attributed to the mass anisotropy as well as anisotropic scattering of the electrons in the X-valley of AlAs

Valley degeneracy in biaxially strained aluminum arsenide quantum wells

This paper details a complete formalism for calculating electron subband energy and degeneracy in strained multi-valley quantum wells grown along any orientation with explicit results for the AlAs quantum well case. A standardized rotation matrix is defined to transform from the conventional- cubic-cell basis to the quantum-well-transport basis whereby effective mass tensors, valley vectors, strain matrices, anisotropic strain ratios, and scattering vectors are all defined in their respective bases. The specific cases of (001)-, (110)-, and (111)-oriented aluminum arsenide (AlAs) quantum wells are examined, as is the unconventional (411) facet, which is of particular importance in AlAs literature. Calculations of electron confinement and strain in the (001), (110), and (411) facets determine the critical well width for crossover from double- to single-valley degeneracy in each system. The notation is generalized to include miscut angles, and can be adapted to other multi-valley systems. To help classify anisotropic inter-valley scattering events, a new primitive unit cell is defined in momentum space which allows one to distinguish purely in-plane inter-valley scattering events from those that requires an out-of-plane momentum scattering component.Comment: 17 pages, 4 figures, 2 table

The “efficient boundaries” of international agricultural research: A conceptual framework with empirical illustrations

The international agricultural research centers known as CGIAR have played an important role with regard to global food security. Yet, their mandate remains debated: Should they concentrate on producing global public goods, for which they arguably have a comparative advantage, or should they engage in “downstream” activities of the research-development continuum and promote technology adoption on the ground, so as to increase their impact? This paper contributes to resolving this debate by developing a new conceptual framework, which is based on transaction costs economics and makes it possible to identify a range of factors that determine the comparative advantage of international versus national organizations. The different transactions involved in the development and uptake of products from international agricultural research are illustrated by an empirical case study of the legume improvement program of the International Crops Research Institute for the Semi-Arid Tropics (ICRISAT). Data collection involved a participatory mapping technique (Net-Map) as well as key informant interviews. The paper draws attention to the governance problems involved in downstream activities, which influence the comparative advantage of international versus national organizations in the research-development continuum. Policy implications are derived for the ongoing reform of the CGIAR and for future research on this topic

The "Efficient Boundaries" of International Agricultural Research: A Conceptual Framework with Empirical Illustrations

The role of international agricultural research centers (IARCs) has long been a subject of discussion, often with emphasis that they should conduct research that produces international public goods (IPGs). However, centers still face a dilemma on how to balance between IPGs and location-specific work. This paper contributes to the development of principles by which they should position themselves. Transaction cost economics was applied to develop a framework, which is then illustrated with an empirical case study of legume research at the International Crops Research Institute for the Semi-Arid Tropics. A participatory mapping technique (Net-Map) was combined with key informant interviews in India, Malawi and Ethiopia. We find that IARCS play an important role in germplasm improvement, the field in which they have a comparative advantage. However, due to insufficient capacity of national systems, they also engage in downstream activities. This reduces incentives for governments and donors to overcome governance challenges