3,631 research outputs found
Minimal graphene thickness for wear protection of diamond
We show by means of molecular dynamics simulations that graphene is an
excellent coating for diamond. The transformation of diamond to amorphous
carbon while sliding under pressure can be prevented by having at least two
graphene layers between the diamond slabs, making this combination of materials
suitable for new coatings and micro- and nanoelectromechanical devices. Grain
boundaries, vacancies and adatoms on the diamond surface do not change this
picture whereas reactive adsorbates between the graphene layers may have
detrimental effects. Our findings can be explained by the properties of layered
materials where the weak interlayer bonding evolves to a strong interlayer
repulsion under pressure
Towards understanding tree root profiles: simulating hydrologically optimal strategies for root distribution
In this modelling study differences in vertical root distributions measured in four contrasting forest locations in the Netherlands were investigated. Root distributions are seen as a reflection of the plant’s optimisation strategy, based on hydrological grounds. The 'optimal' root distribution is defined as the one that maximises the water uptake from the root zone over a period of ten years. The optimal root distributions of four forest locations with completely different soil physical characteristics are calculated using the soil hydrological model SWIF. Two different model configurations for root interactions were tested: the standard model configuration in which one single root profile was used (SWIF-NC), and a model configuration in which two root profiles compete for the same available water (SWIF-C). The root profiles were parameterised with genetic algorithms. The fitness of a certain root profile was defined as the amount of water uptake over a simulation period of ten years. The root profiles of SWIF-C were optimised using an evolutionary game. The results showed clear differences in optimal root distributions between the various sites and also between the two model configurations. Optimisation with SWIF-C resulted in root profiles that were easier to interpret in terms of feasible biological strategies. Preferential water uptake in wetter soil regions was an important factor for interpretation of the simulated root distributions. As the optimised root profiles still showed differences with measured profiles, this analysis is presented, not as the final solution for explaining differences in root profiles of vegetation but as a first step using an optimisation theory to increase understanding of the root profiles of trees.</p> <p style='line-height: 20px;'><b>Keywords:</b> forest hydrology, optimisation, root
Moir{\'e} patterns as a probe of interplanar interactions: graphene on h-BN
By atomistic modeling of moir{\'e} patterns of graphene on a substrate with a
small lattice mismatch, we find qualitatively different strain distributions
for small and large misorientation angles, corresponding to the
commensurate-incommensurate transition recently observed in graphene on
hexagonal BN. We find that the ratio of C-N and C-B interactions is the main
parameter determining the different bond lengths in the center and edges of the
moir{\'e} pattern. Agreement with experimental data is obtained only by
assuming that the C-B interactions are at least twice weaker than the C-N
interactions. The correspondence between the strain distribution in the
nanoscale moir{\'e} pattern and the potential energy surface at the atomic
scale found in our calculations, makes the moir{\'e} pattern a tool to study
details of dispersive forces in van der Waals heterostructures.Comment: 5 pages, 3 figure
Towards understanding tree root profiles: simulating hydrologically optimal strategies for root distribution
International audienceIn this modelling study differences in vertical root distributions measured in four contrasting forest locations in the Netherlands were investigated. Root distributions are seen as a reflection of the plant's optimisation strategy, based on hydrological grounds. The "optimal" root distribution is defined as the one that maximises the water uptake from the root zone over a period of ten years. The optimal root distributions of four forest locations with completely different soil physical characteristics are calculated using the soil hydrological model SWIF. Two different model configurations for root interactions were tested: the standard model configuration in which one single root profile was used (SWIF-NC), and a model configuration in which two root profiles compete for the same available water (SWIF-C). The root profiles were parameterised with genetic algorithms. The fitness of a certain root profile was defined as the amount of water uptake over a simulation period of ten years. The root profiles of SWIF-C were optimised using an evolutionary game. The results showed clear differences in optimal root distributions between the various sites and also between the two model configurations. Optimisation with SWIF-C resulted in root profiles that were easier to interpret in terms of feasible biological strategies. Preferential water uptake in wetter soil regions was an important factor for interpretation of the simulated root distributions. As the optimised root profiles still showed differences with measured profiles, this analysis is presented, not as the final solution for explaining differences in root profiles of vegetation but as a first step using an optimisation theory to increase understanding of the root profiles of trees. Keywords: forest hydrology, optimisation, root
Managing soil fertility diversity to enhance resource use efficiencies in smallholder farming systems: a case from Murewa District, Zimbabwe
Smallholder farms in sub-Saharan African exhibit substantial heterogeneity in soil fertility, and nutrient resource allocation strategies that address this variability are required to increase nutrient use efficiencies. We applied the Field-scale resource Interactions, use Efficiencies and Long-term soil fertility Development (FIELD) model to explore consequences of various manure and fertilizer application strategies on crop productivity and soil organic carbon (SOC) dynamics on farms varying in resource endowment in a case study village in Murewa District, Zimbabwe. FIELD simulated a rapid decline in SOC and maize yields when native woodlands were cleared for maize cultivation without fertilizer inputs coupled with removal of crop residues. Applications of 10 t manure ha-1 year-1 for 10 years were required to restore maize productivity to the yields attainable under native woodland. Long-term application of manure at 5 and 3 t ha-1 resulted in SOC contents comparable to zones of high and medium soil fertility observed on farms of wealthy cattle owners. Targeting manure application to restore SOC to 50–60% of contents under native woodlands was sufficient to increase productivity to 90% of attainable yields. Short-term increases in crop productivity achieved by reallocating manure to less fertile fields were short-lived on sandy soils. Preventing degradation of the soils under intensive cultivation is difficult, particularly in low input farming systems, and attention should be paid to judicious use of the limited nutrient resources to maintain a degree of soil fertility that supports good crop response to fertilizer applicatio
Effect of structural relaxation on the electronic structure of graphene on hexagonal boron nitride
We performed calculations of electronic, optical and transport properties of
graphene on hBN with realistic moir\'e patterns. The latter are produced by
structural relaxation using a fully atomistic model. This relaxation turns out
to be crucially important for electronic properties. We describe experimentally
observed features such as additional Dirac points and the "Hofstadter
butterfly" structure of energy levels in a magnetic field. We find that the
electronic structure is sensitive to many-body renormalization of the local
energy gap.Comment: 5 pages, 6 figures. Supplementary material is available at
http://www.theorphys.science.ru.nl/people/yuan/attachments/sm_hbn.pd
Effect of structural relaxation on the electronic structure of graphene on hexagonal boron nitride
We performed calculations of electronic, optical and transport properties of
graphene on hBN with realistic moir\'e patterns. The latter are produced by
structural relaxation using a fully atomistic model. This relaxation turns out
to be crucially important for electronic properties. We describe experimentally
observed features such as additional Dirac points and the "Hofstadter
butterfly" structure of energy levels in a magnetic field. We find that the
electronic structure is sensitive to many-body renormalization of the local
energy gap.Comment: 5 pages, 6 figures. Supplementary material is available at
http://www.theorphys.science.ru.nl/people/yuan/attachments/sm_hbn.pd
An Alternative Approach to the Calculation and Analysis of Connectivity in the World City Network
Empirical research on world cities often draws on Taylor's (2001) notion of
an 'interlocking network model', in which office networks of globalized service
firms are assumed to shape the spatialities of urban networks. In spite of its
many merits, this approach is limited because the resultant adjacency matrices
are not really fit for network-analytic calculations. We therefore propose a
fresh analytical approach using a primary linkage algorithm that produces a
one-mode directed graph based on Taylor's two-mode city/firm network data. The
procedure has the advantage of creating less dense networks when compared to
the interlocking network model, while nonetheless retaining the network
structure apparent in the initial dataset. We randomize the empirical network
with a bootstrapping simulation approach, and compare the simulated parameters
of this null-model with our empirical network parameter (i.e. betweenness
centrality). We find that our approach produces results that are comparable to
those of the standard interlocking network model. However, because our approach
is based on an actual graph representation and network analysis, we are able to
assess cities' position in the network at large. For instance, we find that
cities such as Tokyo, Sydney, Melbourne, Almaty and Karachi hold more strategic
and valuable positions than suggested in the interlocking networks as they play
a bridging role in connecting cities across regions. In general, we argue that
our graph representation allows for further and deeper analysis of the original
data, further extending world city network research into a theory-based
empirical research approach.Comment: 18 pages, 9 figures, 2 table
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