4,349 research outputs found
Quantum spin Hall effect induced by electric field in silicene
We investigate the transport properties in a zigzag silicene nanoribbon in
the presence of an external electric field. The staggered sublattice potential
and two kinds of Rashba spin-orbit couplings can be induced by the external
electric field due to the buckled structure of the silicene. A bulk gap is
opened by the staggered potential and gapless edge states appear in the gap by
tuning the two kinds of Rashba spin-orbit couplings properly. Furthermore, the
gapless edge states are spin-filtered and are insensitive to the non-magnetic
disorder. These results prove that the quantum spin Hall effect can be induced
by an external electric field in silicene, which may have certain practical
significance in applications for future spintronics device.Comment: 4 pages, 5 figure
Impact of shocks to economies on the efficiency and robustness of the international pesticide trade networks
Pesticides are important agricultural inputs to increase agricultural
productivity and improve food security. The availability of pesticides is
partially achieved through international trade. However, economies involved in
the international trade of pesticides are impacted by internal and external
shocks from time to time, which influence the redistribution efficiency of
pesticides all over the world. In this work, we adopt simulations to quantify
the efficiency and robustness of the international pesticide trade networks
under shocks to economies. Shocks are simulated based on nine node metrics, and
three strategies are utilized based on descending, random, and ascending node
removal. It is found that the efficiency and robustness of the international
trade networks of pesticides increased for all the node metrics except the
clustering coefficient. Moreover, the international pesticide trade networks
are more fragile when import-oriented economies are affected by shocks.Comment: 12 pages, 5 figure
Economy importance and structural robustness of the international pesticide trade networks
Pesticides are a kind of agricultural input, whose use can greatly reduce
yield loss, regulate plant growth, effectively liberate agricultural
productivity, and improve food security. The availability of pesticides in
economies all over the world is ensured by pesticide redistribution through
international trade and economies play different roles in this process. In this
work, we measure and rank the importance of economies using nine node metrics
in an evolutionary way. It is found that the clustering coefficient is
correlated negatively with the other eight node metrics, while the other eight
node metrics are positively correlated with each other and can be grouped into
three communities (betweenness; in-degree, PageRank, authority, and
in-closeness; out-degree, hub, and out-closeness). We further investigate the
structural robustness of the international pesticide trade networks proxied by
the giant component size under three types of shocks to economies (node removal
in descending order, randomly, and in ascending order). The results show that,
except for the clustering coefficient, the international pesticide trade
networks are relatively robust under shocks to economies in ascending orders
and randomly, but fragile under shocks to economies in descending order. In
contrast, removing nodes with the clustering coefficient in ascending and
descending orders gives similar robustness curves. Moreover, the structural
robustness related to the giant component size evolves over time and exhibits
an inverse U-shaped pattern.Comment: 17 pages, 14 figure
Evolving community structure in the international pesticide trade networks
The statistical properties including community structure of the international
trade networks of all commodities as a whole have been studied extensively.
However, the international trade networks of individual commodities often
behave differently. Due to the importance of pesticides in agricultural
production and food security, we investigate the evolving community structure
in the international pesticide trade networks (iPTNs) of five categories from
2007 to 2018. We unveil the community structures in the undirected and directed
iPTNs exhibits regional patterns. However, the regional patterns are very
different for undirected and directed networks and for different categories of
pesticide. Moreover, the community structure is stabler in the directed iPTNs
than in the undirected iPTNs. We also extract the intrinsic community blocks
for the directed international trade networks of each pesticide category. It is
found that the largest intrinsic community block is the stablest that appears
in every pesticide category and contains important economies (Belgium, Germany,
Spain, France, United Kingdom, Italy, Netherlands, and Portugal) in Europe.
Other important and stable intrinsic community blocks are Canada and the United
States in North America, Argentina and Brazil in South America, and Australia
and New Zealand in Oceania. These findings imply the importance of geographic
distance and the complementarity of important adjacent economies in the
international trade of pesticides.Comment: 31 pages including 22 figure
Pure spin current in a two-dimensional topological insulator
We predict a mechanism to generate a pure spin current in a two-dimensional
topological insulator. As the magnetic impurities exist on one of edges of the
two-dimensional topological insulator, a gap is opened in the corresponding
gapless edge states but another pair of gapless edge states with opposite spin
are still protected by the time-reversal symmetry. So the conductance plateaus
with the half-integer values can be obtained in the gap induced by
magnetic impurities, which means that the pure spin current can be induced in
the sample. We also find that the pure spin current is insensitive to weak
disorder. The mechanism to generate pure spin currents is generalized for
two-dimensional topological insulators.Comment: 5 pages, 6 figure
5,5′-(p-Phenylene)di-1H-tetrazole
Crystals of the title organic compound, C8H6N8, were generated in situ through the [2 + 3]-cycloaddition reaction involving the precursor 1,4-dicyanobenzene and azide in water with Zn2+ as Lewis acid. The asymmetric unit consists of one half-molecule, and a twofold axis of symmetry passes through the centre of the benzene ring. There is an intermolecular N—H⋯N hydrogen bond. The molecules are assembled into a three-dimensional supramolecular framework by π–π stacking interactions, with a perpendicular distance of 3.256 Å [centroid–centroid = 3.9731 (8) Å] between two tetrazole ring planes, and 3.382 Å between the benzene ring and tetrazole ring planes [centroid–centroid = 3.5010 (9) Å]
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