Types Of Modulated Structures Research Articles

Observation of diffraction-managed discrete solitons in curved waveguide arrays

We observe the formation of discrete diffraction-managed optical solitons in arrays of periodically curved coupled waveguides for two types of modulated structures: laser-written arrays in silica glass with self-focusing nonlinearity and lithium niobate waveguide arrays with self-defocusing photorefractive nonlinearity. Our results demonstrate that, for both types of nonlinear response, soliton formation occurs after transitional self-induced beam broadening, being fundamentally different from nonlinear self-focusing and defocusing in a bulk medium or discrete self-trapping in straight waveguides.

Local structure modulation of the low La-doped layered strontium manganite Sr4Mn3O10 studied by transmission electron microscopy and electron energy-loss spectroscopy

Transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS) have been used to characterize the structure and microstructure of the low La-doped layered manganite LaxSr4−xMn3O10 (x ≤ 0.15). The structure of the perfect phase has been identified as an n = 3 layered structure of the hexagonal series by electron diffraction (ED), high-resolution electron microscopy (HRTEM) together with numerical image simulations. The presence of local superstructure modulation associated with La ordering in the compounds was revealed by electron diffraction and high-resolution electron microscopy. Electron diffraction data indicate that this superstructure modulation is two-dimensional, and the modulation plane lies in the ac plane. The two primary modulation vectors are \( q_{1} = (1/4)a^{*} \) and \( q_{2} = (1/2)c^{*} . \) In addition, another type of modulated structure associated with periodic lattice distortion was occasionally visible in the La0.15Sr3.85Mn3O10 phase. Energy-loss spectroscopy data indicate that the periodic lattice distortion is led by ordered oxygen deficiency.

Double “Mn3+” Stripes in Bi1‐xSrxMnO3: New Type of Charge Ordering at Room Temperature.

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Double “Mn3+” Stripes in Bi1-xSrxMnO3: New Type of Charge Ordering at Room Temperature

The electron microscopy study of the manganites Bi1-xSrxMnO3, especially for x = 1/2 and x = 2/3, has allowed a new type of modulated structure, correlated to charge ordering (CO) phenomena, to be evidenced. The Bi1-xSrxMnO3 manganites are characterized by a modulated structure, the modulation vector being qa* (q ≈ 1 − x), and the I-type symmetry of the perovskite subcell. A model is proposed: the structure can be described by double Mn3+ stripes alternating with double (x = 1/2) or with quadruple (x = 2/3) Mn4+ stripes. This type of ordering, observed at room temperature (RT), is compared to that observed in the manganites Ln1-xCaxMnO3 at low temperature and in Bi1-xCaxMnO3 at RT. The absence of magnetoresistance up to 7T suggests that the CO state of Bi0.5Sr0.5MnO3 is exceptionally stable, with a very high TCO close to 500 K. The electron diffraction study carried out versus T shows that the superstructure disappears at 500 K and that the phenomenon is reversible.

Structural modulation and phase transition in a Na-rich alkali feldspar

Transmission electron microscopy (TEM) revealed a commensurate modulated structure in a Na-rich alkali feldspar (Ab660r29Ans) from a basalt from Anhui Province, China. High-resolution TEM images with wavelike (001) lattice fringes directly reveal one-dimensional modulation waves in the crystal. The average modulation period along the b axis is l4dolO (;;;; 180 A), although both larger and smaller spacings have been observed. The modulated structure can be described as periodic stacking of (010) layer domains following the albite twin law. The relationship between the extended cell (supercell) of the modulated structure and the normal alkali feldspar unit cell (subcell) is as follows: a,up ;;;; a,ub, b,up ;;;; l4boub,c,uP ;;;; C,ub,{3,up ;;;; {3,ub (subscripts sup and sub indicate the supercell and subcell, respectively). A possible space group for the modulated structure is Pm. In situ heating and cooling experiments suggestthat the phase transition in this feldspar thin foil might be a weakly first-order and displacive transformation. The transition temperature is close to 500 DCfor the crystal examined. The modulated structure formed during the phase transition of the feldspar from C2/m symmetry to Cl at a rapid cooling rate. A heuristic nonlinear dynamics model for the arrangement of triclinic domains in the feldspar at the phase transition from monoclinic to triclinic symmetry was developed on the basis of a Landau potential, a damping term accounting for the structural differences among neighboring units, and a periodic force in the crystal along the b axis. Numerical solutions indicate that periodic modulation arises during the monoclinic to triclinic transition only under special conditions relatively far from equilibrium, and a nonperiodic arrangement of triclinic domains along the b axis is the more general case. Therefore, according to this model, the modulated structure is a metastable and intermediate structure between monoclinic (C2/m) and triclinic (Cl) phases. This type of modulated structure in Na-rich alkali feldspars may indicate high-temperature growth as a monoclinic crystal, followed by very rapid cooling into the triclinic stability field.

Modulated structures in the Cu-Au and (CuAu)-Ag alloys with antiphase structures of CuAu(II) type

Cu-Au alloys and ternary alloys of (CuAu)-Ag containing 5–15 at% Ag, which have antiphase domain structures of the CuAu(II) type, have been studied by electron diffraction and high-resolution electron microscopy. Existence of the antiphase boundary steps proposed previously on the basis of an anomaly in the splitting of superlattice reflections, has been confirmed in all the alloys examined on an atomic scale. In the (CuAu)-Ag alloys, a new type of modulated structure has been observed, and its nature is discussed on the basis of the diffraction patterns and high-resolution images.

Modulated Structures in Dielectric Liquid Crystals

Abstract Review of theoretical and experimental results concerned to formation of space-modulated orientational structures in liquid crystals is given. Properties of ideal liquid crystal dielectrics are considered. Origion conditions and properties of flexoelectric and flexoelectromagnetic structures in nematics, dielectric instabilities in cholesterics and smectics, ferroelectric structures in chiral smectics are discussed. It is well known that an electric field induces a modulation of the orientational structure of liquid crystals. There are two types of such modulated structures (MS) having different physical natures. The dissipative one is connected with the transport of electric charges and mass of the substance. The thermodynamic MS are not accompanied by the irreversible transport processes. Such MS can exist in ideal liquid crystal dielectrics. In fact there are always some ion impurities and as a result two types of MS are possible in liquid crystals. However it is possible to change the thresho...

ゆらぎと変調構造

Various types of modulated structures are reconsidered from a unified point of view in which the structures are regarded as the wave-structures which are generated as a result of divergent fluctuation of some entity in prototype structures. K2SeO4, CuAu II and TiO2-X are considered as examples.