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Surface Tamm states arise in one-dimensional lattices from some defects at the lattice edge and their energy generally falls in a gap of the crystal. The defects at the surface change rather generally the phase of propagative Bloch waves scattered off at the lattice edge, so that an observer, far from the surface, can detect the existence of edge defects from e.g. time-of-flight measurements as a delay or an advancement of a Bloch wave packet. Here we show that a special class of defects can sustain surface Tamm states which are invisible, in a sense that reflected waves acquire the same phase as in a fully homogeneous lattice with no surface state. Surface states have an energy embedded into the tight-binding lattice band and show a lower than exponential (algebraic) localization. Like most of bound states in the continuum of von Neumann-Wigner type, such states are fragile and decay into resonance surface states in presence of perturbations or lattice disorder. The impact of structural lattice imperfections and disorder on the invisibility of the defects is investigated by numerical simulations. 相似文献
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J. A. Van Vechten 《Czechoslovak Journal of Physics》1980,30(4):388-394
It is now generally agreed that almost all deep defects in as-grown III–V compounds, and in other semiconductors as well, are not simple, single, substitutional impurities or single vacancies or interstitial atoms. The deep defects observed at room temperature are complexes that may involve all of these. Complexes form from single defects and impurity atoms introduced during crystal growth and processing as the material cools to room temperature. The study of deep defects is therefore an inherently complex subject. In the author's opinion, it is no use to propose simple models with a minimal number of assumed defects and parameters which are adjusted so as to fit some limited set of data. Rather one ought to model the thermodynamic history of the material allowing all plausible defects in the calculations with the best theoretical estimates of their parameters to be adjusted only slightly to fit the widest possible range of data. This modeling must consider not only equilibrium defect concentrations, but also the kinetics of crystal growth, dissolution and oxidation as well as defect diffusion, trapping and complex formation both with and without the effects of photoionization, recombination and electromigration. We are almost to the point where such a model can be performed. 相似文献
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A fast energy transfer from the host crystal to impurity centres and charged lattice defects (e.g., vacancies in an ionic crystal) is due to the two-site exciton motion on high vibrational levels. At low temperatures this motion occurs in a coherent directional way with the sound velocity whereas a small separation between vibrational levels provides an easy exciton trapping by shallow potential wells near impurity centres. Charged defects have a much larger trapping section than isovalent impurity centres and manifest themselves in luminescence at a concentration smaller by two orders of magnitude. 相似文献
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一维光子晶体中多缺陷耦合导致的杂质带 总被引:17,自引:8,他引:9
利用传输矩阵方法研究了包含多个周期性分布的缺陷的一维光子晶体的透射谱.以33个周期的1/4波堆存在5个缺陷的光子晶体为例作了数值计算.结果表明,在周期性光子晶体中加入多个缺陷时,将在截止带中产生多个缺陷模.这些缺陷模构成的杂质能带依赖于缺陷在晶体中的分布.当缺陷密集时,缺陷模耦合较强,杂质带中透射峰相距较远,它们对应禁带中许多分立定域的杂质能级;当缺陷稀疏时,缺陷模耦合较弱,透射峰相距较近,分立的杂质能级趋于简并,由此形成一个很窄的通带. 相似文献
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V. V. Ponevchinsky A. I. Goncharuk V. I. Vasil’ev N. I. Lebovka M. S. Soskin 《JETP Letters》2010,91(5):241-244
The structural features, as well as the optical and electrophysical properties of a 5CB nematic liquid crystal with additions
of multilayer carbon nanotubes, have been investigated in the concentration range C = 0.0025–0.1 wt %. The self-aggregation of nanotubes into clusters with a fractal structure occurs in the liquid crystal.
At 0.025 wt %, the clusters are merged, initiating the percolation transition of the composite to a state with a high electric
conductivity. The strong interaction of 5CB molecules with the surface of nanotube clusters is responsible for the formation
of micron surface liquid crystal layers with an irregular field of elastic stresses and a complex structure of birefringence.
They are easily observed in a polarization microscope and visualize directly invisible submicron nanotube aggregates. Their
transverse size increases when an electric field is applied to the liquid crystal cell. Two mechanisms of the generation of
optical singularities in the passing laser beam have been revealed. Optical vortices appear in the speckle fields of laser
radiation scattered at the indented boundaries of the nanotube clusters, whereas the birefringence of the beam in surface
liquid-crystal layers is accompanied by the appearance of polarization C points. 相似文献
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本文采用第一性原理方法, 在190 GPa的压力范围内, 计算了BaLiF3理想晶体和含空位点缺陷晶体的光学性质. 吸收谱数据表明, 压力因素不会导致BaLiF3晶体在可见光区有光吸收的行为. 空位点缺陷的存在会使得BaLiF3的吸收边红移(其中氟空位点缺陷引起的红移最显著) , 但这些红移不会导致它在可见光区内出现光吸收的现象. 波长在532 nm处的折射率数据显示, BaLiF3的折射率将随压力升高而增大. 氟空位点缺陷将导致BaLiF3的折射率增大, 但钡空位点缺陷和锂空位点缺陷的存在对其基本没有影响. 本文预测, BaLiF3晶体有成为冲击光学窗口材料的可能. 相似文献
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Fine structure of the diffraction image of an edge dislocation in high-resolution section topography
E. V. Suvorov I. A. Smirnova 《Journal of Surface Investigation: X-ray, Synchrotron and Neutron Techniques》2008,2(5):777-782
The propagation of an x-ray wave field in an elastic field of an edge dislocation crossing a scattering triangle exactly along the bisector of the scattering angle has been considered. The scattering of the x-ray wave field by a complex elastic field of the edge dislocation has been analyzed using the methods of geometrical optics. It has been established that the fine structure of a diffraction image of defects in thick crystals is determined by the differences in scattering of the normal and anomalous modes of the x-ray wave field in the vicinity of the Bragg reflection. In the case of thick crystals, the x-ray diffraction image of defects can have a symmetry different from the symmetry of the function of local misorientations of the crystal lattice. X-ray wave scattering by local distortions of the crystal lattice can occur according to two different mechanisms depending on the gradient of space changes in the deformation field. In the crystal regions where the elastic field varies slowly with a change in the distance, the x-ray wave field has had time to adjust itself to follow the course of deviations of the crystal lattice from the exact Bragg condition. In the crystal region where the elastic field changes significantly at distances of the order of the extinction length, this region leaves the reflecting position and interference scattering occurs at the interface of the region. It is important that the form of the deformation field in this case is of no significance. 相似文献
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The mechanism of electrical breakdown in solid dielectrics is analyzed using the results of our investigations performed in this direction over a period of several decades. It is shown that the electrical breakdown in solid dielectrics involves interrelated prebreakdown processes, such as high-voltage polarization, defect formation, electron impact excitation and electron impact ionization of luminescence centers and ions in the host crystal lattice, etc. The electrical breakdown is initiated by electric-field and thermal generation of defects in the crystal. In turn, the generation of defects leads to the formation of defect regions and channels that provide an assisted transfer of charge carriers. Electron currents flow (and electrons are accelerated by the electric field to energies sufficient to induce impact ionization) in these regions of the crystal with a lattice distorted by defects. In this respect, the known approaches to the elaboration of the breakdown theory for alkali halide and other dielectric crystals on the basis of analyzing the motion and acceleration of electrons in an ideal crystal structure have appeared to be incorrect. 相似文献
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V. A. Kashurnikov I. A. Rudnev M. E. Gracheva O. A. Nikitenko 《Journal of Experimental and Theoretical Physics》2000,90(1):173-182
AbstractThe phase states and phase transitions in a system consisting of a two-dimensional vortex lattice with defects are studied by the Monte Carlo method. It is shown that a “rotating lattice” phase, which is an intermediate phase between the vortex crystal and vortex liquid phases, is present. The dependence of the temperature of the transition from the rotating lattice phase into a vortex liquid on the strength of the defect potential is determined. The current-voltage characteristics of the system are calculated at various temperatures for point, square, and linear defects. It is shown that the phase state of the system strongly affects its transport properties. 相似文献
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Igor Aharonovich Jonathan C. Lee Andrew P. Magyar David O. Bracher Evelyn L. Hu 《Laser \u0026amp; Photonics Reviews》2013,7(5):L61-L65
Engineering nanostructures from the bottom up enables the creation of carefully sculpted complex structures that are not accessible via top down fabrication techniques, in particular, complex periodic structures for applications in photonics and sensing. In this work a proof of principle that bottom up approach can be adopted and utilized for sculpting devices from diamond is proposed and demonstrated. A realization of periodic structures is achieved by growing nanoscale single crystal diamond through a defined pattern. Optical wave‐guiding of a narrow band emission attributed to the SiV defects in diamond is demonstrated by overgrowth on a thin diamond membrane. In addition, an array of hexagonal microdisks with diameter sizes ranging from 1 to 4 μm is demonstrated. The bottom up approach for diamond opens up new avenues for devices fabrication and sculpting three dimensional structures. 相似文献
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《光谱学与光谱分析》2021,41(11):3524-3531
擦除、密写、掩盖等隐性字迹的快速、无损显现与检验是法庭科学文件检验领域的研究难点。当前多采用切换多波段光源与滤光片的方法对隐性字迹进行显现,但对隐性字迹显现的光谱学机理分析较少,因此隐性字迹的显现效率与检验成功率均不高。为提高擦除、密写、掩盖三类隐性字迹的显现效率与检验精度,通过测量字迹的激发与荧光光谱、反射与透过光谱、微观形貌,对其显现机理与快速显现方法进行深入研究。并且基于液晶可调谐滤光器(LCTF)的高光谱成像技术与支持向量机(SVM)分类算法,提出一种对隐性字迹同时显现与分类的快速检验方法。晨光与百乐可擦笔字、荧光密写笔、柠檬汁均可在365 nm长波紫外光激发下发出较强荧光,其中可擦笔和柠檬汁的荧光波长为716 nm左右,荧光密写笔荧光波长为447 nm。此外,采用254或365 nm波长对柠檬汁隐性字迹进行紫外反射成像也可有效显现柠檬汁字迹。掩盖字迹的研究中发现,在700~2 500 nm红外波段,走珠笔、记号笔、可擦笔字迹透过率在60%以上,而中性笔字迹透过率在20%以下。因此,采用近红外波段850 nm成像有效显现了百乐走珠笔所覆盖的晨光中性笔字迹。同时,采用LCTF高光谱相机对三类隐性字迹在400~720 nm范围进行步长为5 nm的高光谱成像,并通过SVM分类算法对图像中不同笔迹成分进行同时显现和分类,分类总精度达99.284 4%,Kappa系数达0.959 1。以365 nm波长光源作为激发光进行光致发光成像可有效显现擦除与密写字迹。由于不同墨迹在近红外波段反射率差异较大,近红外成像可以有效显现掩盖字迹。基于LCTF高光谱成像的SVM分类技术可实现不同类别隐性字迹的同时显现与分类,并且有较高的显现效率与分类精度。 相似文献
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É. L. Aéro 《Physics of the Solid State》2003,45(8):1557-1565
Nonlinear theory of microscopic and macroscopic strains is developed for the case of large inhomogeneous relative displacements of two sublattices making up a complex crystal lattice; in this case, in addition to an acoustic mode, a pseudooptical, strongly nonlinear mode is excited. The equation of relative motion of the sublattices can be solved exactly for the specific case of a centrosymmetric crystal. The corresponding equilibrium equation is the sine-Helmholtz equation and has a doubly periodic solution. This solution describes fragmentation of the lattice, more specifically, the appearance of a domain superstructure with large periods, whose building blocks contain oppositely sensed rotons separated by topological defects that are opposite in sign. Purely elastic microscopic strains are followed by elastoplastic ones. Both types of strain arise as a result of bifurcation, which causes a change from the initially homogeneous strain field to an inhomogeneous one. The domain sizes take on optimal values when the external homogeneous macroscopic strains reach a certain threshold magnitude. 相似文献
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R. M. Sardarly O. A. Samedov I. Sh. Sadykhov A. I. Nadzhafov F. T. Salmanov 《Physics of the Solid State》2005,47(9):1729-1733
The density of states at the energy levels associated with radiation-induced defects, the localization length of a defect center, and the hopping distance of charge carriers are determined in a TlInS2 crystal. It is demonstrated that, by varying the dose of gamma irradiation, it is possible to control the dielectric properties of ferroelectrics and to attain a stable relaxor state. In the temperature range of existence of this state, charge carriers execute tunneling from electron levels in the band gap through potential barriers created by an incommensurate superstructure of the TlInS2 crystal. 相似文献