Photonic analogs of the moirésuperlattices mediated by interlayer electromagnetic coupling are expected to give rise to rich phenomena,such as nontrivial flatband topology.Here,we propose and demonstrate a scheme...Photonic analogs of the moirésuperlattices mediated by interlayer electromagnetic coupling are expected to give rise to rich phenomena,such as nontrivial flatband topology.Here,we propose and demonstrate a scheme to tune the flatbands in a bilayer moirésuperlattice by employing a band offset.The band offset is changed by fixing the bands of one slab while shifting those of the other slab,which is accomplished by modifying the thickness of the latter slab.Our results show that the band-offset tuning not only makes some flatbands emerge and disappear but also leads to two sets of flatbands that are robustly formed even with the change of band offset over a broad range.These robust flatbands form either at the AA-stack site or at the AB-stack site,and as a result,a single-cell superlattice can support a pair of high-quality localized modes with tunable frequencies.Moreover,we develop a diagrammatic model to provide an intuitive insight into the formation of the robust flatbands.Our work demonstrates a simple yet efficient way to design and control complex moiréflatbands,providing new opportunities to utilize photonic moirésuperlattices for advanced light–matter interaction,including lasing and nonlinear harmonic generation.展开更多
Flatbands are of significant interest due to their potential for strong energy confinement and their ability to facilitate strongly correlated physics such as unconventional superconductivity and fractional quantum Ha...Flatbands are of significant interest due to their potential for strong energy confinement and their ability to facilitate strongly correlated physics such as unconventional superconductivity and fractional quantum Hall states.When topology is incorporated into flatband systems,it further enhances flatband mode robustness against perturbations.We present the first realization of doubly degenerate topological flatbands of edge states in chiral-symmetric strained graphene.The flatband degeneracy stems from the merging of Dirac points,achieved by tuning the coupling ratios in a honeycomb lattice with newly discovered twig boundary conditions.The topology of these modes is characterized by the nontrivial winding number,which ensures their robustness against disorder.Experimentally,two types of topological edge states are observed in a strained photonic graphene lattice,consistent with numerical simulations.Moreover,the degeneracy of the topological flatbands doubles the density of states for zero-energy modes,facilitating the formation of compact edge states and providing greater control over edge states and light confinement.Our findings underscore the interplay among lattice geometry,symmetry,and topology in shaping doubly degenerate topological flatbands.This work opens new possibilities for advancements in correlated effects,nonlinear optical phenomena,and efficient energy transfer in materials science,photonic crystals,and quantum devices.展开更多
In this paper,under two different electromagnetic modes,the photonic band gaps（PBGs） in the two-dimensional plasma photonic crystals（PPCs） are theoretically investigated based on the plane wave expansion method.Th...In this paper,under two different electromagnetic modes,the photonic band gaps（PBGs） in the two-dimensional plasma photonic crystals（PPCs） are theoretically investigated based on the plane wave expansion method.The proposed PPCs are arranged in rhombus lattices,in which the homogeneous unmagnetized plasma rods are immersed in the isotropic dielectric background.The computed results showed that PBGs can be easily tuned by the angle of rhombus lattices,and a cutoff frequency and a flatbands region can be observed under the TM and TE polarized waves,respectively.The relationships between the relative bandwidths of first PBGs and the parameters of PPCs in two such cases also are discussed.The numerical simulations showed that the PBGs can be manipulated obviously by the parameters as mentioned above.The proposed results can be used to design the waveguide and filter based on the PPCs.展开更多
The effect of substrate doping on the flatband and threshold voltages of a strained-Si/SiGe p metal-oxide semiconductor field-effect transistor（pMOSFET） has been studied.By physically deriving the models of the flat...The effect of substrate doping on the flatband and threshold voltages of a strained-Si/SiGe p metal-oxide semiconductor field-effect transistor（pMOSFET） has been studied.By physically deriving the models of the flatband and threshold voltages,which have been validated by numerical simulation and experimental data,the shift in the plateau from the inversion region to the accumulation region as the substrate doping increases has been explained.The proposed model can provide a valuable reference to the designers of strained-Si devices and has been implemented in software for extracting the parameters of a strained-Si MOSFET.展开更多
Gd-doped HfO2 has drawn worldwide interest for its interesting features. It is considered to be a suitable material for N-type metal-oxide-semiconductor （MOS） devices due to a negative flatband voltage （Vfb） shift...Gd-doped HfO2 has drawn worldwide interest for its interesting features. It is considered to be a suitable material for N-type metal-oxide-semiconductor （MOS） devices due to a negative flatband voltage （Vfb） shift caused by the Gd doping. In this work, an anomalous positive shift was observed when Gd was doped into HfO2. The cause for such a phenomenon was systematically investigated by distinguishing the effects of different factors, such as Fermi level pinning （FLP）, a dipole at the dielectric/SiO2 interface, fixed interracial charge, and bulk charge, on Vfb. It was found that the FLP and interfacial dipole could make Vfb negatively shifted, which is in agreement with the conventional dipole theory. The increase in interfacial fixed charge resulting from Gd doping plays a major role in positive Vfb shift.展开更多
For Ti-doped hematite photoanodes, high temperature annealing drastically increases the water oxidation plateau photocurrent, but also induces an anodic shift of onset potential by about 100 m V, thus hindering the pe...For Ti-doped hematite photoanodes, high temperature annealing drastically increases the water oxidation plateau photocurrent, but also induces an anodic shift of onset potential by about 100 m V, thus hindering the performance under low applied bias. To the best of our knowledge, the effects of high temperature annealing on the onset potential have been rarely studied. Herein, both X-ray photoelectron spectroscopy（XPS） measurements and theoretical calculations indicated that the increase of surface Ti/Fe atomic ratio after high temperature annealing decreased the adsorption capacity of hydroxide ions on the hematite surface. Subsequently, the flatband potential（i.e., the theoretical onset potential） of Ti doped hematite photoanodes positively shifted, which was supported by the Mott-Schottky measurements.展开更多
The preparation and study of supported TiO2 for photocatalytic application in solar cell devices is a relevant research field. Thin films of TiO2 prepared on Ti by thermal oxidation in a wide range of temperatures (45...The preparation and study of supported TiO2 for photocatalytic application in solar cell devices is a relevant research field. Thin films of TiO2 prepared on Ti by thermal oxidation in a wide range of temperatures (450°C - 900°C) were characterized by electrochemical impedance spectroscopy, potentiometry and amperometry. This material presents photoelectrochemical activity, which depends dramatically of the oxidation temperature and the exposition time at the studied temperatures. The flatband potential as well as the donor density and the resistance to the charge transfer were measured. All these parameters are temperature dependent, and the optimal values are observed on the photoelectrodes prepared at 750°C. This result is consistent with the photochemical response reported in the literature for thin films of Ti/TiO2 prepared under similar conditions.展开更多
Due to their reduced dimensionality and strong quantumconfinement, two-dimensional (2D) materials have emerged as aversatile platform for exploring quantum phenomena absent intheir bulk counterparts. Stacking 2D mater...Due to their reduced dimensionality and strong quantumconfinement, two-dimensional (2D) materials have emerged as aversatile platform for exploring quantum phenomena absent intheir bulk counterparts. Stacking 2D materials to form bilayerheterostructures or homostructures introduces additionaldegrees of freedom to tune or even create novel quantumproperties [1]. A prominent example is the discovery of flatbands and superconductivity in twisted bilayer graphene at asmall “magic” twist angle, where suppressed electron kineticsamplifies electron correlation, giving rise to emergent quantumphenomena [2]. Another notable case is the formation of quasicrystalsin bilayer graphene at the large twist angle, whichfurther expands the possibilities for tailoring electronic structures[3].展开更多
Simultaneous localization of light to extreme spatial and spectral scales is of high importance for testing fundamental physics and various applications.However,there is a longstanding trade-off between localizing a l...Simultaneous localization of light to extreme spatial and spectral scales is of high importance for testing fundamental physics and various applications.However,there is a longstanding trade-off between localizing a light field in space and in frequency.Here we discover a new class of twisted lattice nanocavities based on mode locking in momentum space.The twisted lattice nanocavity hosts a strongly localized light field in a 0.048𝜆3 mode volume with a quality factor exceeding 2.9×1011(∼250𝜇s photon lifetime),which presents a record high figure of merit of light localization among all reported optical cavities.Based on the discovery,we have demonstrated silicon-based twisted lattice nanocavities with quality factor over 1 million.Our result provides a powerful platform to study light-matter interaction in extreme conditions for tests of fundamental physics and applications in nanolasing,ultrasensing,nonlinear optics,optomechanics and quantum-optical devices.展开更多
A state-of-the-art overview of N-dopant characterizations in nano-TiO_(2)second-generation photocatalysts is provided.The related literature is very rich and sometimes offers contrasting interpretations.Here we critic...A state-of-the-art overview of N-dopant characterizations in nano-TiO_(2)second-generation photocatalysts is provided.The related literature is very rich and sometimes offers contrasting interpretations.Here we critically discuss up-to-date literature results and our own findings,as retrieved by several experimental(BET,HR-TEM,XPS,DRS,HR-XRPD,EXAFS,electrochemical tools)and theoretical(periodic DFT)techniques.Our intent is to pull together outcomes from very different and complementary sources to make an as much as possible coherent picture of the morphological,electrochemical and electronic properties of N-TiO_(2)materials.It is commonly accepted that critical issues to be considered in the design of high-performing N-TiO_(2)photocatalysts are synthetic strategy,defect concentration and chemical nature of the lattice point defects.We focus on the latter two issues,with emphasis on sol-gel prepared materials,according to the specific area of expertise of our group.The problem of the chemical nature of vip N species into the lattice is crucial,as substitutional(N_(s))or interstitial(N_(i))nitrogen place their valence states just above the valence band or deeper into the band gap.Overall,we show how synergism among experimental and theoretical techniques is decisive to disentangle structural,electronic and morphological effects in complex N-doped TiO_(2)matrices.展开更多
The high tunability of two-dimensional(2D)materials makes them an optimal platform for simulating,achieving,and manipulating novel quantum states in condensed matter physics.Moire flatband systems formed through latti...The high tunability of two-dimensional(2D)materials makes them an optimal platform for simulating,achieving,and manipulating novel quantum states in condensed matter physics.Moire flatband systems formed through lattice mismatch or twisting between atomic layers have recently been discovered,allowing for bridging 2D materials with strongly correlated and topological physics.In this review,we briefly discuss band folding in graphene/hexagonal boron nitride(hBN)moire superlattices and related experimental results as well as introduce a general approach for developing 2D correlated systems and applying them in ABC trilayer graphene on hBN(ABC-TLG/hBN)moire superlattices.We also compare the experimental results of the tunable correlated and topological phenomenon of ABC-TLG/hBN with those of other related moire systems.展开更多
基金supported by Sichuan Science and Technology Program(2023NSFSC0460)the Fundamental Research Funds for the Central Universities(Grant No.ZYGX2020J010)+3 种基金the Open Project Funding of the Ministry of Education Key Laboratory of Weak-Light Nonlinear Photonics(Grant No.OS22-1)the Guangxi Natural Science Foundation(Grant No.2020GXNSFAA297041)the National Key R&D Program of China(Grant No.2022YFA1404800)the National Natural Science Foundation of China(Grant No.12134006).
文摘Photonic analogs of the moirésuperlattices mediated by interlayer electromagnetic coupling are expected to give rise to rich phenomena,such as nontrivial flatband topology.Here,we propose and demonstrate a scheme to tune the flatbands in a bilayer moirésuperlattice by employing a band offset.The band offset is changed by fixing the bands of one slab while shifting those of the other slab,which is accomplished by modifying the thickness of the latter slab.Our results show that the band-offset tuning not only makes some flatbands emerge and disappear but also leads to two sets of flatbands that are robustly formed even with the change of band offset over a broad range.These robust flatbands form either at the AA-stack site or at the AB-stack site,and as a result,a single-cell superlattice can support a pair of high-quality localized modes with tunable frequencies.Moreover,we develop a diagrammatic model to provide an intuitive insight into the formation of the robust flatbands.Our work demonstrates a simple yet efficient way to design and control complex moiréflatbands,providing new opportunities to utilize photonic moirésuperlattices for advanced light–matter interaction,including lasing and nonlinear harmonic generation.
基金supported by the National Key R&D Program of China(Grant No.2022YFA1404800)the National Natural Science Foundation of China(Grant Nos.12134006,12274242+2 种基金12474387)the Natural Science Foundation of Tianjin(Grant No.21JCJQJC00050)the 111 Project in China(Grant No.B23045)。
文摘Flatbands are of significant interest due to their potential for strong energy confinement and their ability to facilitate strongly correlated physics such as unconventional superconductivity and fractional quantum Hall states.When topology is incorporated into flatband systems,it further enhances flatband mode robustness against perturbations.We present the first realization of doubly degenerate topological flatbands of edge states in chiral-symmetric strained graphene.The flatband degeneracy stems from the merging of Dirac points,achieved by tuning the coupling ratios in a honeycomb lattice with newly discovered twig boundary conditions.The topology of these modes is characterized by the nontrivial winding number,which ensures their robustness against disorder.Experimentally,two types of topological edge states are observed in a strained photonic graphene lattice,consistent with numerical simulations.Moreover,the degeneracy of the topological flatbands doubles the density of states for zero-energy modes,facilitating the formation of compact edge states and providing greater control over edge states and light confinement.Our findings underscore the interplay among lattice geometry,symmetry,and topology in shaping doubly degenerate topological flatbands.This work opens new possibilities for advancements in correlated effects,nonlinear optical phenomena,and efficient energy transfer in materials science,photonic crystals,and quantum devices.
文摘In this paper,under two different electromagnetic modes,the photonic band gaps（PBGs） in the two-dimensional plasma photonic crystals（PPCs） are theoretically investigated based on the plane wave expansion method.The proposed PPCs are arranged in rhombus lattices,in which the homogeneous unmagnetized plasma rods are immersed in the isotropic dielectric background.The computed results showed that PBGs can be easily tuned by the angle of rhombus lattices,and a cutoff frequency and a flatbands region can be observed under the TM and TE polarized waves,respectively.The relationships between the relative bandwidths of first PBGs and the parameters of PPCs in two such cases also are discussed.The numerical simulations showed that the PBGs can be manipulated obviously by the parameters as mentioned above.The proposed results can be used to design the waveguide and filter based on the PPCs.
基金Project supported by the Funds from the National Ministries and Commissions (Grant Nos. 51308040203 and 6139801)the Fundamental Research Funds for the Central Universities (Grant Nos. 72105499 and 72104089)the Natural Science Basic Research Plan in Shaanxi Province of China (Grant No. 2010JQ8008)
文摘The effect of substrate doping on the flatband and threshold voltages of a strained-Si/SiGe p metal-oxide semiconductor field-effect transistor（pMOSFET） has been studied.By physically deriving the models of the flatband and threshold voltages,which have been validated by numerical simulation and experimental data,the shift in the plateau from the inversion region to the accumulation region as the substrate doping increases has been explained.The proposed model can provide a valuable reference to the designers of strained-Si devices and has been implemented in software for extracting the parameters of a strained-Si MOSFET.
基金Project supported by the National Science and Technology Major Project of the Ministry of Science and Technology of China(Grant No.2009ZX02035)the National Natural Science of China(Grant Nos.61176091 and 50932001)
文摘Gd-doped HfO2 has drawn worldwide interest for its interesting features. It is considered to be a suitable material for N-type metal-oxide-semiconductor （MOS） devices due to a negative flatband voltage （Vfb） shift caused by the Gd doping. In this work, an anomalous positive shift was observed when Gd was doped into HfO2. The cause for such a phenomenon was systematically investigated by distinguishing the effects of different factors, such as Fermi level pinning （FLP）, a dipole at the dielectric/SiO2 interface, fixed interracial charge, and bulk charge, on Vfb. It was found that the FLP and interfacial dipole could make Vfb negatively shifted, which is in agreement with the conventional dipole theory. The increase in interfacial fixed charge resulting from Gd doping plays a major role in positive Vfb shift.
基金supported by the National Natural Science Foundation of China (Nos. 21473090, U1663228)
文摘For Ti-doped hematite photoanodes, high temperature annealing drastically increases the water oxidation plateau photocurrent, but also induces an anodic shift of onset potential by about 100 m V, thus hindering the performance under low applied bias. To the best of our knowledge, the effects of high temperature annealing on the onset potential have been rarely studied. Herein, both X-ray photoelectron spectroscopy（XPS） measurements and theoretical calculations indicated that the increase of surface Ti/Fe atomic ratio after high temperature annealing decreased the adsorption capacity of hydroxide ions on the hematite surface. Subsequently, the flatband potential（i.e., the theoretical onset potential） of Ti doped hematite photoanodes positively shifted, which was supported by the Mott-Schottky measurements.
文摘The preparation and study of supported TiO2 for photocatalytic application in solar cell devices is a relevant research field. Thin films of TiO2 prepared on Ti by thermal oxidation in a wide range of temperatures (450°C - 900°C) were characterized by electrochemical impedance spectroscopy, potentiometry and amperometry. This material presents photoelectrochemical activity, which depends dramatically of the oxidation temperature and the exposition time at the studied temperatures. The flatband potential as well as the donor density and the resistance to the charge transfer were measured. All these parameters are temperature dependent, and the optimal values are observed on the photoelectrodes prepared at 750°C. This result is consistent with the photochemical response reported in the literature for thin films of Ti/TiO2 prepared under similar conditions.
文摘Due to their reduced dimensionality and strong quantumconfinement, two-dimensional (2D) materials have emerged as aversatile platform for exploring quantum phenomena absent intheir bulk counterparts. Stacking 2D materials to form bilayerheterostructures or homostructures introduces additionaldegrees of freedom to tune or even create novel quantumproperties [1]. A prominent example is the discovery of flatbands and superconductivity in twisted bilayer graphene at asmall “magic” twist angle, where suppressed electron kineticsamplifies electron correlation, giving rise to emergent quantumphenomena [2]. Another notable case is the formation of quasicrystalsin bilayer graphene at the large twist angle, whichfurther expands the possibilities for tailoring electronic structures[3].
基金This work is supported by the National Key R&D Program of China(2018YFA0704401)the Beijing Natural Science Foundation(Z180011)+1 种基金the National Natural Science Foundation of China(12225402,91950115,11774014,61521004 and 62175003)the Tencent Foundation.
文摘Simultaneous localization of light to extreme spatial and spectral scales is of high importance for testing fundamental physics and various applications.However,there is a longstanding trade-off between localizing a light field in space and in frequency.Here we discover a new class of twisted lattice nanocavities based on mode locking in momentum space.The twisted lattice nanocavity hosts a strongly localized light field in a 0.048𝜆3 mode volume with a quality factor exceeding 2.9×1011(∼250𝜇s photon lifetime),which presents a record high figure of merit of light localization among all reported optical cavities.Based on the discovery,we have demonstrated silicon-based twisted lattice nanocavities with quality factor over 1 million.Our result provides a powerful platform to study light-matter interaction in extreme conditions for tests of fundamental physics and applications in nanolasing,ultrasensing,nonlinear optics,optomechanics and quantum-optical devices.
文摘A state-of-the-art overview of N-dopant characterizations in nano-TiO_(2)second-generation photocatalysts is provided.The related literature is very rich and sometimes offers contrasting interpretations.Here we critically discuss up-to-date literature results and our own findings,as retrieved by several experimental(BET,HR-TEM,XPS,DRS,HR-XRPD,EXAFS,electrochemical tools)and theoretical(periodic DFT)techniques.Our intent is to pull together outcomes from very different and complementary sources to make an as much as possible coherent picture of the morphological,electrochemical and electronic properties of N-TiO_(2)materials.It is commonly accepted that critical issues to be considered in the design of high-performing N-TiO_(2)photocatalysts are synthetic strategy,defect concentration and chemical nature of the lattice point defects.We focus on the latter two issues,with emphasis on sol-gel prepared materials,according to the specific area of expertise of our group.The problem of the chemical nature of vip N species into the lattice is crucial,as substitutional(N_(s))or interstitial(N_(i))nitrogen place their valence states just above the valence band or deeper into the band gap.Overall,we show how synergism among experimental and theoretical techniques is decisive to disentangle structural,electronic and morphological effects in complex N-doped TiO_(2)matrices.
基金supported by National Key Research Program of China(grant nos.2020YFA0309000,2021YFA1400100)NSF of China(grant no.12174248)SJTU NO.21X010200846。
文摘The high tunability of two-dimensional(2D)materials makes them an optimal platform for simulating,achieving,and manipulating novel quantum states in condensed matter physics.Moire flatband systems formed through lattice mismatch or twisting between atomic layers have recently been discovered,allowing for bridging 2D materials with strongly correlated and topological physics.In this review,we briefly discuss band folding in graphene/hexagonal boron nitride(hBN)moire superlattices and related experimental results as well as introduce a general approach for developing 2D correlated systems and applying them in ABC trilayer graphene on hBN(ABC-TLG/hBN)moire superlattices.We also compare the experimental results of the tunable correlated and topological phenomenon of ABC-TLG/hBN with those of other related moire systems.
