Band gaps in periodically magnetized homogeneous anisotropic media

In [A. M. Merzlikin, A. P. Vinogradov, A. V. Dorofeenko, M. Inoue, M. Levy, A. B. Granovsky, Physica B 394 (2007) 277] it is shown that in anisotropic magnetophotonic crystal made of anisotropic dielectric layers and isotropic magneto-optical layers the magnetization leads to formation of additional band gaps (BG) inside the Brillouin zones. Due to the weakness of the magneto-optical effects the width of these BG is much smaller than that of usual BG forming on the boundaries of Brillouin zones. In the present communication we show that though the anisotropy suppresses magneto-optical effects. An anisotropic magnetophotonic crystal made of anisotropic dielectric layers and anisotropic magneto-optical; the width of additional BG may be much greater than the width of the usual Brillouin BG. Anisotropy tends to suppress Brillouin zone boundary band gap formation because the anisotropy suppresses magneto-optical properties, while degenerate band gap formation occurs around points of effective isotropy and is not suppressed.     

Band gaps and nonlinear defect modes in one-dimensional photonic crystals with anisotropic single-negative metamaterials

We propose a one-dimensional photonic crystal (1DPC) with multilayered periodic structures containing dispersive anisotropic single-negative (ASNG) (permittivity- or permeability-negative) metamaterials. The influences of the ratio of d_A/d_B and their scale on ASNG gaps are discussed. Moreover, the effect of a nonlinear defect layer introduced into the proposed structure is also analyzed, and the results show that the defect mode has two peaks of electrical fields in the defect layer and its bistability property is also different from a defect mode in PCs with isotropic SNG gaps.     

Instabilities in diamond under high shear stress

We investigate, through first-principles calculations, lattice instabilities induced in diamond by the application of high shear stresses. For shear stresses as low as 95 GPa a lattice instability will occur, leading to graphitelike layered structures. This effect is highly anisotropic. The reversal of the direction of the applied shear forces may cause a change of 80 GPa in the shear stress value at which the instability develops. The same reversal also causes different bonds to be broken, resulting in a drastic change in the orientation of the resulting graphitelike structures. We also find that an additional compressive stress of 50 GPa along the (111) direction does not eliminate the shear-induced instability.     

Bistability of bilayer islands under anisotropic misfit stress

The equilibrium shapes of bilayer islands under anisotropic misfit stress have been theoretically characterized from a thermodynamics point of view. Based on an energy variation calculation, it is found that when the ratio σ between the components of the misfit stress is such that σ = ? 1, both square and rectangular shaped bilayer islands may coexist depending on the island volume and stress level. When σ ≠ ? 1 and σ < 0, the possibility of formation rectangular islands along both perpendicular directions is also discussed.     

Strongly anisotropic s-wave gaps in exotic superconductors

The exotic superconductors, defined as those that follow the phenomenological trend reported by Uemura (T _c approximately proportional to λ_L ^?2), at present constitute the most broad and general class of superconductors which can reasonably be considered 'similar to the high-T _c cuprates'. It is therefore of much interest to determine the forms of their pairing gap functions. We examine evidence for the gap forms in non-cuprate exotics and demonstrate some general features. The cubic materials often have highly anisotropic gaps. The planar materials tend to have an even stronger gap anisotropy, to the extent that they often have gap nodes, but nevertheless they usually have an s-wave-like gap symmetry. There is good evidence for the latter even in the controversial cases of planar organics and nickel borocarbides. Exceptions to these generalizations are also pointed out and discussed.     

复合局域共振梁结构带隙研究

该文将周期局域共振梁结构中的基体结构由单一材料发展成多相材料,建立了周期复合局域共振梁结构。由共振单元在单胞中的排列位置,考虑了三种构型,并利用传递矩阵方法计算了在弹性基底作用下这些结构产生的局域共振带隙性质。结果表明,与局域共振梁结构相比,复合局域共振梁结构产生的局域共振带隙对应的频率范围更宽,禁带范围内弹性波的衰减峰值和衰减均值更高。除此之外,该文还分析了局域共振结构、单胞尺寸、基体材料和局域共振结构中的阻尼对弹性波在空间中传播和衰减特性的影响,为满足特定的隔声隔振要求的结构设计提供了依据。     

Band gaps and defect modes in periodically structured waveguides

This work examines a simple one-dimensional acoustic band gap system made from a diameter-modulated waveguide. Experimental and theoretical results are presented on perfectly periodic waveguide arrays showing the presence of band gaps--frequency intervals in which the transmission of sound is forbidden. The introduction of defects in the perfect periodicity leads to narrow frequency transmission bands--defect states--within the forbidden band gaps. The circular cross-section waveguide system is straightforward to simulate theoretically and experimental results demonstrate good agreement with theory. The experimental transmission of the periodic waveguide arrays is measured using an impulse response technique.     

Coupled polaritonic band gaps in the anisotropic piezoelectric superlattices

Anisotropic piezoelectric superlattices (APSs) with the periodic arrangement of polarized anisotropic piezoelectric domains in a certain direction are presented, in which the coupled polaritonic band gaps (CPBGs) can be obtained in the whole Brillouin Zone and the maximum relative bandwidth (band-gap sizes divided by their midgap frequencies) of $5.1\text{\%}$ can be achieved. The general characteristics of the APSs are similar to those of the phononic crystals composed of two types of materials, with the main difference being the formation mechanism of the CPBGs, which originate from the couplings between lattice vibrations along two different directions and electromagnetic waves rather than from the periodical modulation of density and elastic constants. In addition, there are no lattice mismatches because the APSs are made of the same material. Thus, the APSs can also be extended to the construction of novel acousto-optic devices.     

Unconventional anisotropic s-wave superconducting gaps of the LiFeAs iron-pnictide superconductor

We have performed high-resolution angle-resolved photoemission spectroscopy on Fe-based superconductor LiFeAs (T(c)=18 K). We reveal multiple nodeless superconducting (SC) gaps with 2Δ/k(B)T(c) ratios varying from 2.8 to 6.4, depending on the Fermi surface (FS). We also succeeded in directly observing a gap anisotropy along the FS with magnitude up to ~30%. The anisotropy is fourfold symmetric with an antiphase between the hole and electron FSs, suggesting complex anisotropic interactions for the SC pairing. The observed momentum dependence of the SC gap offers an excellent opportunity to investigate the underlying pairing mechanism.     

Band gaps structure and semi-Dirac point of two-dimensional function photonic crystals

Two-dimensional function photonic crystals, in which the dielectric constants of medium columns are the functions of space coordinates , are proposed and studied numerically. The band gaps structures of the photonic crystals for TE and TM waves are different from the two-dimensional conventional photonic crystals. Some absolute band gaps and semiDirac points are found. When the medium column radius and the function form of the dielectric constant are modulated, the numbers, width, and position of band gaps are changed, and the semi-Dirac point can either occur or disappear. Therefore,the special band gaps structures and semi-Dirac points can be achieved through the modulation on the two-dimensional function photonic crystals. The results will provide a new design method of optical devices based on the two-dimensional function photonic crystals.     

含磁电弹夹层的压电/压磁声子晶体带隙特性研究

将具有力电磁耦合性能的夹层引入到压电/压磁声子晶体中,在保持单胞长度为固定值的情况下,分别改变磁电弹夹层的厚度、磁电弹夹层中压电材料的体积分数和磁电弹夹层中压电材料的种类;并利用传递矩阵法和Bloch定理,得到波数k与频率ω的色散关系;通过色散关系图分析不同的磁电弹夹层对压电/压磁声子晶体带隙特性的影响.研究发现:当磁电弹夹层厚度增加时,带隙的中心频率上升,带隙宽度变宽;当磁电弹夹层中压电材料体积分数增加时,带隙中心频率下降,第一带隙宽度变窄,第二带隙宽度增加,第三带隙宽度保持不变;当磁电弹夹层中的压电材料种类不同时,带隙的中心频率和带隙宽度有明显的改变;磁电弹夹层对压电/压磁声子晶体带隙中心频率的影响在高频区比低频区更显著.     

A new application of the diamond anvil cell: Measurements under uniaxial stress

We present optical absorption measurements on thin films of CuCl and CuBr at 100K under uniaxial stress up to ～3.0 GPa. The thin films were prepared by evaporating onto the inner face of one diamond of a diamond anvil cell. From the splitting of the excitons we were able to determine the shear deformation potential d = -0.69 ± 0.03 eV for CuBr and d = +0.45 ± 0.08 eV for CuCl. Our measurements indicate also that the spin-orbit splitting increases in both materials with increasing pressure. A new shear-induced phase is found for CuCl above 4.8 GPa.     

LCR分流电路下压电声子晶体智能材料的带隙

将带有LCR分流电路的压电陶瓷片对贴在铝和环氧树脂组成的声子晶体结构中.使智能材料的机械振动与压电陶瓷的压电效应耦合起来,推导出机械振动在压电陶瓷片上的等效附加应力;使LCR分流电路中的电磁振荡效应和声子晶体的能带特性有机结合,计算了在分流电路作用下智能材料扭转和弯曲振动的带隙特性,研究了电阻、电感、电容元件的改变对压电声子晶体智能材料带隙的影响.研究结果表明:在合理尺寸下,随着分流电路中电阻值的增大,带隙的频率范围变宽,但衰减幅值有所降低;电感和电容值的增大都可以使带隙向低频移动,带隙的衰减幅值随着电感值的增大而升高,但随着电容值的增大而降低.从而给压电声子晶体智能材料减震降噪的控制提供了一种新思路.     

Disorder and optical gaps in strained dense amorphous carbon and diamond nanocomposites

We employ empirical tight-binding simulations on strained tetrahedral amorphous carbon and diamond nanocomposite networks. For each applied strain, the optoelectronic properties are monitored through the absorption coefficient and the dielectric function. These lead to the optical gaps and are able to quantify the amount of disorder in the structures. We compare our results to those of unstrained nanostructured diamond and amorphous carbon (a-C) phases and link the degree of disorder in these materials to their structural details as a function of the external load. The atomic rearrangements and distortions imposed by the external strain in these structures are directly observable in their optoelectronic properties. We thoroughly discuss the interplay between increased external strain, structural and topological disorder, atomic rearrangements and their effect on the calculated optoelectronic properties.     

Band gaps in the terahertz frequency range in quasiperiodic one-dimensional magnonic crystals

In this work we investigate magnonic band gaps, in the terahertz (THz) frequency range, in periodic and quasiperiodic (Fibonacci sequence) magnonic crystals formed by layers of Cobalt (Co) and Permalloy (Py). Our theoretical model is based on a magnetic Heisenberg Hamiltonian in the exchange regime, together with a transfer-matrix treatment within the random-phase approximation (RPA). For periodic arrangements the bulk band structure is analogous to those found in photonic crystals, while for quasiperiodic multilayers it presents additional pass bands similar to those found in doped electronic materials.     

Band gaps in the spectra of terahertz surface plasmons on metallic diffraction gratings

The effect of the appearance of band gaps in the energy spectra of terahertz surface plasmons has been experimentally observed and investigated. The band gaps are formed due to the interference interaction of the surface plasmons excited by pulsed terahertz radiation on metallic diffraction gratings. It has been shown that the experimental dispersion curves of terahertz plasmons are in good agreement with the dispersion curves obtained in the numerical simulation.     

Molecular dynamic simulations of elastomer structure and its influence on anisotropic stress under time-varying strain

We investigate the evolution of polymer structure and its influence on uniaxial anisotropic stress under time-varying uniaxial strain, and the role of external control variables such as temperature, strain rate, chain length, and density, using molecular dynamics simulation. At temperatures higher than glass transition, stress anisotropy in the system is reduced even though the bond stretch is greater at higher temperatures. There is a significant increase in the stress level with increasing density. At higher densities, the uncoiling of the chains is suppressed and the major contribution to the deformation is by internal deformation of the chains. At faster rates of loading stress anisotropy increases. The deformation mechanism is mostly due to bond stretch and bond bending rather than overall shape and size. Stress levels increase with longer chain length. There is a critical value of the functionality of the cross-linkers beyond which the uniaxial stress developed increases caused primarily by bond stretching due to increased constraint on the motion of the monomers. Stacking of the chains in the system also plays a dominant role in the behaviour in terms of excluded volume interactions. Low density, high temperature, low values of functionality of cross-linkers, and short chain length facilitate chain uncoiling and chain slipping in cross-linked polymers.