Moiré flat bands of twisted few-layer graphite

We report that the twisted few layer graphite (tFL-graphite) is a new family of moiré heterostructures (MHSs), which has richer and highly tunable moiré flat band structures entirely distinct from all the known MHSs. A tFL-graphite is composed of two few-layer graphite (Bernal stacked multilayer graphene), which are stacked on each other with a small twisted angle. The moiré band structure of the tFL-graphite strongly depends on the layer number of its composed two van der Waals layers. Near the magic angle, a tFL-graphite always has two nearly flat bands coexisting with a few pairs of narrowed dispersive (parabolic or linear) bands at the Fermi level, thus, enhances the DOS at E_F . This coexistence property may also enhance the possible superconductivity as been demonstrated in other multiband superconductivity systems. Therefore, we expect strong multiband correlation effects in tFL-graphite. Meanwhile, a proper perpendicular electric field can induce several isolated nearly flat bands with nonzero valley Chern number in some simple tFL-graphites, indicating that tFL-graphite is also a novel topological flat band system.     

Raman scattering enhancement by dielectric spheres

Raman scattering experiments were performed on Si(60 nm)/metal/substrate structures with and without silica microspheres (with a diameter between 0.5 and 5 µm) on top. Raman scattering from the thin Si layer exhibits enhancements (~20) due to the dielectric spheres, where the enhancement factors depend on the diameter of the spheres. The interaction between light and dielectric spheres has been simulated by finite difference time domain calculations (FDTD), wherein particularly the electric energy density (ED) distribution in the thin Si layer was of concern. For microspheres with a diameter less than ~3 µm, the transverse ED distribution (perpendicular to the incident light direction) within the Si layer is characterised by a single peak centered on the optical axis. For larger diameters, a multimodal transverse ED distribution develops where the maximum is not centered on the optical axis. Using an ad‐hoc approach for surface enhanced Raman scattering in combination with the FDTD calculations, the experimental Raman observations are well accounted for. Copyright © 2013 John Wiley & Sons, Ltd.     

Structure and control of Coulomb crystals in a Penning trap

We apply rotating electric fields to ion plasmas in a Penning trap to obtain phase-locked rotation about the magnetic field axis. These plasmas, containing up to 10^{6 9}Be⁺ ions, are laser-cooled to millikelvin temperatures so that they freeze into solids. Single body-centered cubic (bcc) crystals have been observed by Bragg scattering in nearly spherical plasmas with ≳ 2 × 10⁵ ions. The detection of the Bragg patterns is synchronized with the plasma rotation, so individual peaks are observed. With phase-locked rotation, the crystal lattice and its orientation can be stable for longer than 30 min or ∼10⁸ rotations. This revised version was published online in July 2006 with corrections to the Cover Date.     

石墨烯纳米片电子结构和量子输运特性第一原理研究

用基于密度泛函理论的原子紧束缚方法计算研究单层石墨烯纳米圆片和纳米带的电子结构,并结合第一原理和非平衡函数法计算量子输运特性.通过电子能态和轨道密度分布研究纳米碳原子层的电子成键状态,结合电子透射谱、电导和电子势分布分析电子散射与输运机制.石墨烯纳米带和纳米圆片分别呈现金属和半导体的能带特征,片层边缘上电极化分别沿垂直和切向方向,电子电导出现较大的差异,来源于石墨烯纳米圆片边缘的突出碳原子环对电子的强散射.石墨烯纳米带的电子透射谱表现为近似台阶式变化并在费米能级处存在弹道电导峰,而石墨烯纳米圆片的电子能带和透射谱在费米能级处开口并且因量子限制作用呈现更加离散的多条高态密度窄能带和尖锐谱峰.     

Bragg spectroscopy of discrete axial quasiparticle modes in a cigar-shaped degenerate Bose gas

We propose an experiment in which long wavelength discrete axial quasiparticle modes can be imprinted in a 3D cigar-shaped Bose-Einstein condensate by using two-photon Bragg scattering experiments, similar to the experiment at the Weizmann Institute [J. Steinhauer et al., Phys. Rev. Lett. 90, 060404 (2003)] where short wavelength axial phonons with different number of radial modes have been observed. We provide values of the momentum, energy and time duration of the two-photon Bragg pulse and also the two-body interaction strength which are needed in the Bragg scattering experiments in order to observe the long wavelength discrete axial modes. These discrete axial modes can be observed when the system is dilute and the time duration of the Bragg pulse is long enough.     

New effects in low energy scattering of pµ atoms

Strong solid state effects in low energy scattering of pμ atoms in solid hydrogen are reported and analyzed. Such effects have been observed in TRIUMF experiment E742 where muons are stopped in thin frozen (3 K) layers of hydrogen. Emission of low energy pμ atoms from the hydrogen layer into adjacent vacuum was much higher than expected, based on calculations which ignored the solid nature of hydrogen. Monte Carlo simulations, performed using the scattering cross-sections with solid state effects taken into account, show the important role of the coherent elastic Bragg scattering in the diffusion of pμ atoms. For pμ energies lower than the Bragg cut-off limit (∼2 meV) the total scattering cross-section falls by several orders of magnitude, the hydrogen target becomes transparent and the emission of cold pμ atoms takes place. This revised version was published online in August 2006 with corrections to the Cover Date.     

The structure of crystallized suspensions of polystyrene spheres

We have determined the crystal structure of an ordered aqueous suspension of polystyrene spheres using Bragg scattering of laser light. At low concentrations a body-centered cubic structure is found. At higher concentrations this is found together with face-centered cubic. We believe that the ordering is due to Coulomb repulsion as in the Wigner crystal.     

Phase and structure state of titanium loaded by spherically converging shock waves

The phase and structural states of titanium spheres loaded by spherical converging shock waves of various intensities were studied layer by layer by means of X-ray diffraction, optical, and transmission electron microscopy. It was established that defects of different types (twins, dislocations, and adiabatic shear bands) are produced during high-rate deformation occurring in materials under such method of pulsed loading. The amount and distribution of the defects depend on the loading intensity. The presence of the ω-phase is revealed only in the layers near the external surface of the titanium sphere after low-intensity loading.     

Cancellation of acoustic scattering from an elastic sphere

Recent research has suggested the possibility of creating acoustic cloaks using metamaterial layers to eliminate the acoustic field scattered from an elastic object. This paper explores the possibility of applying the scattering cancellation cloaking technique to acoustic waves and the use of this method to investigate its effectiveness in cloaking elastic and fluid spheres using only a single isotropic elastic layer. Parametric studies showing the influence of cloak stiffness and geometry on the frequency dependent scattering cross-section of spheres have been developed to explore the design space of the cloaking layer. This analysis shows that an appropriately designed single isotropic elastic cloaking layer can provide up to 30 dB of scattering reduction for ka values up to 1.6. This work also illustrates the importance of accounting for the elasticity of the object and the relevant limitations of simplistic quasi-static analyses proposed in recent papers.     

Characterization of porous materials by small-angle scattering

Characterization of porous materials by small-angle scattering has been extensively pursued for several years now as the pores are often of mesoscopic size and compatible with the length scale accessible by the technique using both neutrons and X-rays as probing radiation. With the availability of ultra small-angle scattering instruments, one can investigate porous materials in the sub-micron length scale. Because of the increased accessible length scale vis-a-vis the multiple scattering effect, conventional data analysis procedures based on single scattering approximation quite often fail. The limitation of conventional data analysis procedures is also pronounced in the case of thick samples and long wavelength of the probing radiation. Effect of multiple scattering is manifested by broadening the scattering profile. Sample thickness for some technologically important materials is often significantly high, as the experimental samples have to replicate all its essential properties in the bulk material. Larger wavelength of the probing radiation is used in some cases to access large length scale and also to minimize the effect of double Bragg reflections.     

用于气体痕量检测的中红外空心布拉格光纤

布拉格光纤是一种一维光子晶体带隙导引光纤.针对布拉格光纤在气体痕量检测领域的应用,设计了传输波段中心波长位于中红外波段的半导体玻璃/有机聚合物基空心布拉格光纤.通过预制棒熔拉法,制备出了中红外空心布拉格光纤的样品.传输谱和弯曲特性测试表明布拉格光纤样品具有两个明显的传输波段,体现了带隙导光的特征.低阶传输波段的中心波长为4.4μm.     

Nonlinear BLOCH-wave interaction and bragg scattering in optically induced lattices

We study, both theoretically and experimentally, the Bragg scattering of light in optically induced photonic lattices and reveal the key physical mechanisms which govern the nonlinear self-action of narrow beams under the combined effects of Bragg scattering and wave diffraction, allowing for selecting bands with different effective dispersion.     

Analysis of Melting Particle Model and Microwave and MM-Wave Scatterings Due to a Melting Layer of Precipitation

In this paper, the water-coated snow sphere model of melting particles has been given. Based on this model, the scattering properties and albedo w₀ of melting particles have been calculated by use of the coated sphere BHCOAT method. It is shown that the albedo of melting particles may be more than the raindrops and snow spheres at 5 (C-band) and 14GHz (Ku-band). The albedo of melting particles are nearly less than the equivalent mass raindrops albedo at the Ka and mm-band of f > 20GHz. These results are in agreement with the conclusion radar observed and literature given^[1,2]. It demonstrates that the water-coated snow sphere model of melting particles is appropriate and practicable. The study is significant for the utilization of the low fade margin system in satellite communication.     

原子吸收限附近Bragg情况的衍射波和透射波的观测

在Ｇａ的Ｋ吸收限附近的强反常散射条件下，测定了ＧａＡｓ（６００）对称Ｂｒａｇｇ情况的衍射波和透射波的摇摆曲线，并将其和Ｘ射线衍射动力学理论的相应计算结果作了比较，指出了原子吸收限附近Ｂｒａｇｇ情况的透射波在研究反常散射中的应用价值 关键词：     

Magnetic response of split ring resonator metamaterials: From effective medium dispersion to photonic band gaps

On systematically investigating the electromagnetic response of periodic split-ring resonator (SRR) metamaterials as a function of the size-to-wavelength (a/λ) ratio, we find that the stop bands due to the geometric resonances of the SRR weaken with increasing (a/λ) ratio, and are eventually replaced by stop bands due to Bragg scattering. Our study traces the behaviour of SRR-based metamaterials as the resonance frequency increases and the wavelength of the radiation finally becomes comparable to the size of the unit cell of the metamaterial. In the intermediate stages, the dispersion of the SRR metamaterial can still be described as due to a localized magnetic resonances while Bragg scattering finally becomes the dominant phenomenon as a/λ∼1/2.     

Suppression of High-Reflection Fifth-, Sixth-, and Seventh-Order Bands in Wide-Band Interference Filters

We have investigated the spectral properties of multilayer interference systems formed by multiple repetition of the symmetric three-component period ABCBA. The problem of synthesizing structures, in whose spectral characteristics suppression of adjacent fifth-, sixth-, and seventh-order high-reflection bands is observed, has been solved. It has been shown that there exist only nine options of layer thicknesses in a period. It should be noted that only in six cases upon suppression of fifth-, sixth-, and seventh-order high-reflection bands is the first-order reflection band preserved. The values of the refractive indices of layers at which the problem has a solution have been found. The refractive indices of layers in all the nine cases are described by different relations defining the dependence of the refractive index of one of the components on the randomly chosen refractive indices of the other components.     

Matryoshka locally resonant sonic crystal

The results of numerical modeling of sonic crystals with resonant array elements are reported. The investigated resonant elements include plain slotted cylinders as well as their various combinations, in particular, Russian doll or Matryoshka configurations. The acoustic band structure and transmission characteristics of such systems have been computed with the use of finite element methods. The general concept of a locally resonant sonic crystal is proposed that utilizes acoustic resonances to form additional band gaps that are decoupled from Bragg gaps. An existence of a separate attenuation mechanism associated with the resonant elements that increases performance in the lower frequency regime has been identified. The results show a formation of broad band gaps positioned significantly below the first Bragg frequency. For low frequency broadband attenuation, a most optimal configuration is the Matryoshka sonic crystal, where each scattering unit is composed of multiple concentric slotted cylinders. This system forms numerous gaps in the lower frequency regime, below Bragg bands, while maintaining a reduced crystal size viable for noise barrier technology. The finding opens alternative perspectives for the construction of sound barriers in the low frequency range usually inaccessible by traditional means including conventional sonic crystals.     

用Monte Carlo方法模拟闪锌矿相ZnS电子的输运特性

用Monte Carlo方法模拟闪锌矿相(zinc blende)ZnS电子的输运特性.实验采用的是非抛物线模型计算电子的能带结构,模拟包含了声学声子散射,极性光学声子散射,压电散射,电离杂质散射,能谷间散射以及自散射等散射机理.通过模拟得到了ZnS材料的平均漂移速度、平均电子能量随电场强度变化的曲线图,以及总散射率随电子能量变化图,并将结果与文献报道的模拟结果[1]进行比较得出:本实验方法具有模型简单,计算速度快,获得结果比较准确的优点.     

Coherent scattering phenomena in apertureless scanning near-field fluorescence microscopy

We study near-field fluorescence images of samples composed of aggregates of 100 nm dye-doped latex spheres. These images have been performed by a reflection Apertureless Scanning Near-field Optical Microscope (A-SNOM). We show that the near-field distribution in fluorescence A-SNOM images arises from coherent scattering phenomena between all spheres. This is a consequence of the coherent nature of the fluorescence emission of each single sphere. The results shown here are significant for all fluorescent samples characterised by a nonnegligible topography.     

Comparison of short-range ion-ion correlations in the α, β and δ phases of Bi2O3

Neutron total scattering data for α-Bi₂O₃ and δ-Bi₂O₃, considering both the Bragg and diffuse components, have been analysed by the reverse Monte Carlo (RMC) method to extract experimental information concerning the short-range correlations between pairs of ions. These results complement the average structural information provided by conventional Rietveld refinement of the Bragg scattering alone. The radial and angular distribution functions in the stable ambient temperature α phase and superionic δ phase of Bi₂O₃ are presented and compared with the corresponding data for the metastable β phase reported previously. These data provide an important insight into the nature of the anion disorder within the highly conducting δ phase and the relationship between the δ phase and its ordered α and β counterparts.