266 research outputs found
Localization-delocalization wavepacket transition in Pythagorean aperiodic potentials
We introduce a composite optical lattice created by two mutually rotated square patterns and
allowing observation of continuous transformation between incommensurate and completely
periodic structures upon variation of the rotation angle θ. Such lattices acquire periodicity only for
rotation angles cosθ=a/c, sinθ=b/c, set by Pythagorean triples of natural numbers (a, b, c). While
linear eigenmodes supported by lattices associated with Pythagorean triples are always extended,
composite patterns generated for intermediate rotation angles allow observation of the localizationdelocalization
transition of eigenmodes upon modification of the relative strength of two sublattices
forming the composite pattern. Sharp delocalization of supported modes for certain θ values can be
used for visualization of Pythagorean triples. The effects predicted here are general and also take place
in composite structures generated by two rotated hexagonal latticesPeer ReviewedPostprint (published version
Wetland mapping in the Balqash Lake Basin Using Multi-source Remote Sensing Data and Topographic features Synergic Retrieval
AbstractWetland plays a major role in the hydrological cycle, the carbon sink (carbon sequestration), nitrogen absorption, geochemical cycle, water conservation, biological diversity. Traditional field surveys for mapping wetlands distribution in large areas are very difficult to undertake. Remote sensing techniques offer promising solutions to this problem. But spectral confusion with other land cover classes and different types of wetlands, it is difficult to extract wetland information automatically. The overarching goal of this study was to develop a hybrid method for lake wetlands automated delineation by integrated using multi-source remote sensing data and DEM data. Firstly, it is to do radiance correction and convert image DN value to reflectance or radiance. Secondly, spectral index calculation and topographic indices derive, such as NDVI, NDWI, TVDI, slope and others topographic feature indices and etc. Thirdly, water bodies extraction through the NDWI iterative computation. Finally, it is to retrieve marsh land from image via comprehensive information of soil moisture character, topographic factors and spatial analysis. By the above steps, we got the ultimate wetlands distribution information. The methodology was evaluated by the balqash lake basin wetland extraction in Kazakhstan. Experiments result shows that the hybrid method performs well in lake wetlands delineation. The overall accuracies of wetland classes exceed 85%, which can meet the application requirements
Semiconducting nonperovskite ferroelectric oxynitride designed ab initio
Recent discovery of HfO2-based and nitride-based ferroelectrics that are
compatible to the semiconductor manufacturing process have revitalized the
field of ferroelectric-based nanoelectronics. Guided by a simple design
principle of charge compensation and density functional theory calculations, we
discover HfO2-like mixed-anion materials, TaON and NbON, can crystallize in the
polar Pca21 phase with a strong thermodynamic driving force to adopt anion
ordering spontaneously. Both oxynitrides possess large remnant polarization,
low switching barriers, and unconventional negative piezoelectric effect,
making them promising piezoelectrics and ferroelectrics. Distinct from HfO2
that has a wide band gap, both TaON and NbON can absorb visible light and have
high charge carrier mobilities, suitable for ferroelectric photovoltaic and
photocatalytic applications. This new class of multifunctional nonperovskite
oxynitride containing economical and environmentally benign elements offer a
platform to design and optimize high-performing ferroelectric semiconductors
for integrated systems
Topologically enhanced nonlinear optical response of graphene nanoribbon heterojunctions
We study the nonlinear optical properties of heterojunctions made of graphene
nanoribbons (GNRs) consisting of two segments with either the same or different
topological properties. By utilizing a quantum mechanical approach that
incorporates distant-neighbor interactions, we demonstrate that the presence of
topological interface states significantly enhances the second- and third-order
nonlinear optical response of GNR heterojunctions that are created by merging
two topologically inequivalent GNRs. Specifically, GNR heterojunctions with
topological interface states display third-order harmonic hyperpolarizabilities
that are more than two orders of magnitude larger than those of their similarly
sized counterparts without topological interface states, whereas the
secondorder harmonic hyperpolarizabilities exhibit a more than ten-fold
contrast between heterojunctions with and without topological interface states.
Additionally, we find that the topological state at the interface between two
topologically distinct GNRs can induce a noticeable red-shift of the quantum
plasmon frequency of the heterojunctions. Our results reveal a general and
profound connection between the existence of topological states and an enhanced
nonlinear optical response of graphene nanostructures and possible other
photonic systems.Comment: 7 pages,5 figure
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