One-dimensional(1D)one-way waveguides based on topological edge states of two-dimensional(2D)gyromagnetic photonic crystals have been studied extensively.Here,we theoretically propose a three-dimensional(3D)all-dielec...One-dimensional(1D)one-way waveguides based on topological edge states of two-dimensional(2D)gyromagnetic photonic crystals have been studied extensively.Here,we theoretically propose a three-dimensional(3D)all-dielectric gyromagnetic photonic crystal with type-Ⅱ Weyl points.Based on the inclined band properties of the type-Ⅱ Weyl surface states,we design a 2D one-way interface transmission channel on the Weyl crystal.Light waves in such 2D waveguides can be transmitted robustly over metal obstacles almost without any back-reflection,topologically protected by the type-Ⅱ Weyl points.By manipulating the magnetic field or structural parameters,we also achieve the topological phase transition between Weyl phase and 3D Chern insulator phase and obtain the corresponding phase diagram,which provides more possibilities for topological regulation of the surface states.This work suggests a new way to construct unidirectional 2D waveguides with larger area of energy transmission in 3D space,which is a promising platform for developing 3D topological photonic devices.展开更多
In reinforced concrete structures,corrosion of the rebar produces 2–6 times more corrosion product than the original material,creating pressure on the surrounding concrete,leading to cracking.The study of corrosion a...In reinforced concrete structures,corrosion of the rebar produces 2–6 times more corrosion product than the original material,creating pressure on the surrounding concrete,leading to cracking.The study of corrosion and cracking in reinforced concrete structures is therefore of great importance for enhancing the durability of concrete.Unlike many previous studies,we used ribbed rebar similar to that used commercially and considered the mechanical behavior of the interface transition zone(ITZ)between the aggregate and mortar to simulate the processes of corrosion and cracking of reinforced concrete structures.We explored the failure mode of the interface layer under uniform corrosion and the influence of different factors on the corrosion expansion cracking and the shedding mode of a concrete cover.This was achieved by establishing a three-phase meso-scale model of concrete based on secondary development of ABAQUS,simulating the mechanical behavior of the ITZ using a cohesive element,and establishing a rust expansion cracking model for single and multiple rebars.The results showed that:(1)Under uniform rust expansion,concrete cracks are distributed in a cross pattern with a slightly shorter lower limb.(2)When the corrosion rate is low,the ITZ is not damaged.With an increase in the corrosion rate,the proportion of elements with tensile damage in the ITZ first increases and then decreases.(3)In the case of a single rebar,the larger the cover thickness,the higher the corrosion rate corresponding to ITZ failure,and the arrangement of the rebar has little influence on the ITZ failure mode.(4)In the case of multiple rebars,the concrete cover cracks when the rebar spacing is small,and wedge-shaped spalling occurs when the spacing is large.展开更多
Atomically thin transition metal dichalcogenides(TMDs)have distinct opto-electronic properties including enhanced luminescence and high on-off current ratios,which can be further modulated by making more complex TMD h...Atomically thin transition metal dichalcogenides(TMDs)have distinct opto-electronic properties including enhanced luminescence and high on-off current ratios,which can be further modulated by making more complex TMD heterostructures.However,resolution limits of conventional optical methods do not allow for direct nanoscale optical-structural correlation measurements in these materials,particularly of buried interfaces in TMD heterostructures.Here we use,for the first time,electron beam induced cathodoluminescence in a scanning transmission electron microscope(CL-STEM)to measure optical properties of monolayer TMDs(WS2,MoS2 and WSSe alloy)encapsulated between layers of hBN.We observe dark areas resulting from localized(~100 nm)imperfect interfaces and monolayer folding,which shows that the intimate contact between layers in this application-relevant heterostructure is required for proper inter layer coupling.We also realize a suitable imaging method that minimizes electron-beam induced changes and provides measurement of intrinsic properties.To overcome the limitation of small electron interaction volume in TMD monolayer(and hence low photon yield),we find that encapsulation of TMD monolayers with hBN and subsequent annealing is important.CL-STEM offers to be a powerful method to directly measure structure-optical correspondence in lateral or vertical heterostructures and alloys.展开更多
基金supported by the National Natural Science Foundation of China(Grant No.11974119)Science and Technology Project of Guangdong(Grant No.2020B010190001)+1 种基金Guangdong Innovative and Entrepreneurial Research Team Program(Grant No.2016ZT06C594)National Key R&D Program of China(Grant No.2018YFA0306200)。
文摘One-dimensional(1D)one-way waveguides based on topological edge states of two-dimensional(2D)gyromagnetic photonic crystals have been studied extensively.Here,we theoretically propose a three-dimensional(3D)all-dielectric gyromagnetic photonic crystal with type-Ⅱ Weyl points.Based on the inclined band properties of the type-Ⅱ Weyl surface states,we design a 2D one-way interface transmission channel on the Weyl crystal.Light waves in such 2D waveguides can be transmitted robustly over metal obstacles almost without any back-reflection,topologically protected by the type-Ⅱ Weyl points.By manipulating the magnetic field or structural parameters,we also achieve the topological phase transition between Weyl phase and 3D Chern insulator phase and obtain the corresponding phase diagram,which provides more possibilities for topological regulation of the surface states.This work suggests a new way to construct unidirectional 2D waveguides with larger area of energy transmission in 3D space,which is a promising platform for developing 3D topological photonic devices.
基金the National Natural Science Foundation of China(Nos.U1934213 and 51878572)。
文摘In reinforced concrete structures,corrosion of the rebar produces 2–6 times more corrosion product than the original material,creating pressure on the surrounding concrete,leading to cracking.The study of corrosion and cracking in reinforced concrete structures is therefore of great importance for enhancing the durability of concrete.Unlike many previous studies,we used ribbed rebar similar to that used commercially and considered the mechanical behavior of the interface transition zone(ITZ)between the aggregate and mortar to simulate the processes of corrosion and cracking of reinforced concrete structures.We explored the failure mode of the interface layer under uniform corrosion and the influence of different factors on the corrosion expansion cracking and the shedding mode of a concrete cover.This was achieved by establishing a three-phase meso-scale model of concrete based on secondary development of ABAQUS,simulating the mechanical behavior of the ITZ using a cohesive element,and establishing a rust expansion cracking model for single and multiple rebars.The results showed that:(1)Under uniform rust expansion,concrete cracks are distributed in a cross pattern with a slightly shorter lower limb.(2)When the corrosion rate is low,the ITZ is not damaged.With an increase in the corrosion rate,the proportion of elements with tensile damage in the ITZ first increases and then decreases.(3)In the case of a single rebar,the larger the cover thickness,the higher the corrosion rate corresponding to ITZ failure,and the arrangement of the rebar has little influence on the ITZ failure mode.(4)In the case of multiple rebars,the concrete cover cracks when the rebar spacing is small,and wedge-shaped spalling occurs when the spacing is large.
基金This work was supported by the Singapore-MIT Alliance for Research and Technology and the National Research Foundation,Prime Ministers Office,Singapore.The authors acknowledge helpful discussions with Yuan Cao and Kha Tran.This work made use of the MRSEC Shared Experimental Facilities at MIT,supported by the National Science Foundation under award number DMR-1419807.Any opinion,findings,and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.
文摘Atomically thin transition metal dichalcogenides(TMDs)have distinct opto-electronic properties including enhanced luminescence and high on-off current ratios,which can be further modulated by making more complex TMD heterostructures.However,resolution limits of conventional optical methods do not allow for direct nanoscale optical-structural correlation measurements in these materials,particularly of buried interfaces in TMD heterostructures.Here we use,for the first time,electron beam induced cathodoluminescence in a scanning transmission electron microscope(CL-STEM)to measure optical properties of monolayer TMDs(WS2,MoS2 and WSSe alloy)encapsulated between layers of hBN.We observe dark areas resulting from localized(~100 nm)imperfect interfaces and monolayer folding,which shows that the intimate contact between layers in this application-relevant heterostructure is required for proper inter layer coupling.We also realize a suitable imaging method that minimizes electron-beam induced changes and provides measurement of intrinsic properties.To overcome the limitation of small electron interaction volume in TMD monolayer(and hence low photon yield),we find that encapsulation of TMD monolayers with hBN and subsequent annealing is important.CL-STEM offers to be a powerful method to directly measure structure-optical correspondence in lateral or vertical heterostructures and alloys.
