Propagation characteristics of the laser-generated lower-order lamb waves in one-dimensional periodic thin plates

Using the finite element method (FEM), we study the thermoelastic generation and the transmitted power spectra (TPS) of Laser-generated lower-order Lamb wave mode in an one-dimensional thin aluminum plate engraved by a periodic rectangular grating. The calculation verified that the equally good Lamb waves band gaps can also exist in an one-component periodic structure and the width and location of the band gaps can be regulated by the parameters of the depth of the grooves (h), the ratio of the lattice period to the plate thickness (D/L) and the ratio of the width of the grooves to the lattice period (d/D). Specially, we suggest that Lamb wave band gaps can be substantially enlarged by using multiple periodic systems which consist of several pieces of periodic structure with different D/L.     

Influence of thickness on the optical properties of amorphous GeSe2 thin films: analysis using Raman spectra,Urbach energy and Tauc parameter

An analysis is reported of thickness-induced defects in amorphous GeSe₂ thin films deposited by the vacuum evaporation technique. X-ray diffraction studies confirmed the amorphous nature of the thin films. Optical absorption measurements revealed an indirect transition with an energy gap that increases with film thickness. A blue shift in optical transmittance edges was observed in annealed GeSe₂ thin films. The obtained lower values of Urbach energy (E _U) indicate that as thickness increases more ordered films can be produced. Raman spectra suggest that annealing promotes corner-sharing GeSe_4/2 tetrahedra and edge-sharing Ge₂Se_8/2 bi-tetrahedra bonding and leads to the reduction in disorder in bonding network, which is amply supported by the way of increase in band gap, increase in Tauc parameter (B ^1/2) and reduction in E _U from the analysis of transmittance spectra. Increasing the thickness promotes tetrahedral and bi-tetrahedral bonding through the reduction in bonding defects.     

Intense spontaneous amplified emission of Li2 diffuse violet bands in the 4400 Å region

We report observation of intense spontaneous amplified radiation of Li₂ diffuse violet band in the 4100–4900 Å region. The radiation is strongly enhanced when the lithium vapor in a heat pipe is optically pumped with a pulsed dye laser with the output wavelength tuned to near the Li 2s–4s two-photon resonance transition. The diffuse violet band can probably be assigned to a recently reported triplet bound-free (2³ _g a ³ _u ⁺ ) transition. It is found that the productions of the molecular diffuse band are contributed from Li atoms as well as Li₂ molecules. The excitation functions and their dependence on laser power density are presented and the mechanisms for producing the diffuse bands are discussed.Department of Physics, National Central University, Chung-Li, Taiwan, Republic of Chinabl]References     

Landau-Fermi liquid analysis of the 2D t-t' Hubbard model

We calculate the Landau interaction function f (k,k') for the two-dimensional t-t' Hubbard model on the square lattice using second and higher order perturbation theory. Within the Landau-Fermi liquid framework we discuss the behavior of spin and charge susceptibilities as function of the onsite interaction and band filling. In particular we analyze the role of elastic umklapp processes as driving force for the anisotropic reduction of the compressibility on parts of the Fermi surface. Received 18 March 2002 Published online 9 July 2002     

Detecting and avoiding the necking deformation along polypropylene fibre axis using the fringe pattern analysis of multiple-beam microinterferometry

A fibre-drawing device attached with the system for producing multiple-beam Fizeau fringes in transmission is used to optimize the optical properties during the cold drawing of polypropylene (PP) fibres. This system is automated for interference pattern analysis. Two drawing processes for the PP fibres are applied and investigated. The first one is fast drawing in which the necking deformation is predicted and the other is the slow (step) drawing in which the necking can be avoided. The refractive index profiles (n and n) of PP fibres are determined at different positions along the fibre axis during the fast and slow drawing processes. The fibre interference patterns are automatically digitized and stored in a computer storage media. The slow drawing technique for PP fibres is recommended to overcome the deformation difficulties along the fibre axis due to necking during the drawing processes. Microinterferograms in case of light vibrating parallel and perpendicular to the fibre axis are given for illustration.     

掺Cd氧化锌的电子结构及相结构稳定性的第一性原理研究

采用基于密度泛函理论结合投影缀加平面波方法的VASP软件包,在考虑所有掺杂原子构型的前提下,对Cd掺杂ZnO合金的晶格常数、禁带宽度、电子态密度和形成焓进行了计算,分析了Cd含量和掺杂原子构型对纤锌矿wz-Zn_1-xCd_xO合金的电子结构和结构稳定性的影响.计算结果表明:随着Cd含量的不断增加,纤锌矿ZnCdO合金的平均晶格常数a,c均线性增加,但c/a的比值不会发生显著的变化;纤锌矿ZnCd 关键词： 密度泛函理论 ZnCdO合金 电子结构 形成焓     

Band-gap bowing and p-type doping of (Zn,Mg, Be)O wide-gap semiconductor alloys: a first-principles study

Using a first-principles band-structure method and a special quasirandom structure (SQS) approach, we systematically calculate the band gap bowing parameters and p-type doping properties of (Zn, Mg, Be)O related random ternary and quaternary alloys. We show that the bowing parameters for ZnBeO and MgBeO alloys are large and dependent on composition. This is due to the size difference and chemical mismatch between Be and Zn(Mg) atoms. We also demonstrate that adding a small amount of Be into MgO reduces the band gap indicating that the bowing parameter is larger than the band-gap difference. We select an ideal N atom with lower p atomic energy level as dopant to perform p-type doping of ZnBeO and ZnMgBeO alloys. For N doped in ZnBeO alloy, we show that the acceptor transition energies become shallower as the number of the nearest neighbor Be atoms increases. This is thought to be because of the reduction of p-d repulsion. The N_O acceptor transition energies are deep in the ZnMgBeO quaternary alloy lattice-matched to GaN substrate due to the lower valence band maximum. These decrease slightly as there are more nearest neighbor Mg atoms surrounding the N dopant. The important natural valence band alignment between ZnO, MgO, BeO, ZnBeO, and ZnMgBeO quaternary alloy is also investigated.     

Conductance Spectra of Carbyne Transverse to Carbon Chains. Is It Related to the Soliton Lattice?

The influence of the thickness of an oriented carbyne film on the conductance of a special sample was investigated with an electric field of up to 1.8 ⋅ 10⁶ V/cm applied transverse to the carbon chains. A steplike dependence of the conductance on the thickness was observed. The general shape of the current-voltage characteristics, I ∝ U ^2.3, changes to I ∝ U ² when the thickness corresponding to a rapid change of conductance is attained. The characteristics presumably correspond to the space-charge limited current in the presence or absence of the trap band within the carbyne band gap. We propose the formation of a charge-topological soliton lattice at “magic” thicknesses which results in a layered structure of the film and in a bandlike conduction transverse to the chains.     

Analysis of plastic strain localization by a combination of the speckle interferometry with the bulge test

The process of localization of strains, diffuse and localized necking, up to fracture in equi-biaxial loading was analyzed through the images obtained by electronic speckle pattern interferometry (ESPI). The problem of localization is important in the sheet metal forming processes. The ESPI technique is used to have a better resolution on the measured strains (10⁻⁵) than other technique such as the image correlation (10⁻², 10⁻³). The bulge test is currently used to determine the mechanical properties of materials by measuring the deformation that occurs in response to the application of a controlled pressure. This test is used to determine the mechanical properties of sheet metals submitted to an equi-biaxial loading path, the strains at failure are used as data to determine forming limit diagrams (FLDs).The aim of this study is to detect the localization of plastic strain (diffuse and localized necking) during the bulge test combined with a common speckle interferometer. This paper describes an original technique to detect the localization using the strain rate at different positions on the sample.The main originality of this work is the application of an optical interferometer to determine the field of plastic strain increments and strain rates in the region of the top of the specimen, at various stages of global strains.     

Thickness and stacking geometry effects on high frequency overtone and combination Raman modes of graphene

We investigate two high frequency Raman overtone and combination modes of graphene named 2D' and 2D + G bands, and located at ~3240 and ~ 4260 cm^–1, respectively. The graphene thickness and stacking geometry effects for these two modes are systematically studied. The features of the 2D' band, which arises from intravalley double resonance, are not sensitive to the variation of thickness with single Lorentzian peak and fixed linewidth. We explain it theoretically by calculating the phonon dispersion mode in k‐space and find that the flat band region of longitudinal optical phonon near Γ point is the mechanism leading to the 2D' band nonsplit. With the thickness increasing, the band position exhibits blueshift and the linewidth increases for the 2D + G band. With changing thickness and stacking geometry of graphene, the intensities of these two high‐frequency bands show obvious different evolution compared with that of G band. Copyright © 2012 John Wiley & Sons, Ltd.     

X射线同轴轮廓成像中影响成像质量的若干因素研究

采用数字模拟方法较为系统地研究了光子能量、样品直径和散射强度对成像质量的影响，克服了已有实验结果的局限性.研究得到成像质量随光子能量的变化关系，模拟结果与已有实验结果相符.研究发现，当其他成像参数不变时，同一样品存在多个光子能量可实现相近的成像质量，且成像质量都较好，这可用于定量相衬成像中多图重构时图像的选择，也为对辐射剂量有要求的样品提供理论依据.得到了不同直径样品在成像质量最佳时所对应的样品到探测器距离，发现这一距离随样品直径的增加而增加.研究了样品厚度或折射率变化导致的散射X射线对成像质量的影响，发 关键词： X射线同轴轮廓成像 成像质量 X射线散射     

Superconductor–insulator transition induced by over-deposited Ge in the insulating ultrathin a-Nb film

We have observed the superconductor–insulator (S–I) transition induced by over-deposited Ge in the insulating ultrathin amorphous Nb (a-Nb) film. The experiments are performed by depositing Ge onto the insulating a-Nb film with a thickness of 1.04 nm. For , where d_Ge was a thickness for Ge film, the reduction of electrical sheet resistances was seen at low temperature region, suggesting superconductor. The normal sheet resistance at 8 K decreased monotonically with increasing d_Ge. The simplest explanation of this S–I transition is that the electron localization effect is weakened due to the addition of Ge film.     

立方氮化硼薄膜的光学带隙

用射频溅射法在p型Si(100)衬底上沉积立方氮化硼(c-BN)薄膜,薄膜的成分由傅里叶变换红外谱标识,用紫外-可见分光光度计测量了c-BN薄膜的反射光谱,利用K-K(Kramers-Kroning)关系从反射谱计算出c-BN薄膜的光吸收系数,进而确定c-BN薄膜的光学带隙.对于立方相含量为55.4%的c-BN薄膜,光学带隙为5.38eV. 关键词： 立方氮化硼薄膜 光学带隙 K-K关系     

Excitation mechanisms and spectral shape of the diffuse violet emission band in Na2

A dense sodium vapour (T=450°C) has been illuminated with a laser light tuned across the whole visible region. For any laser wavelength the diffuse violet band of Na₂ molecule (peaking at 436.5 nm) has been found in fluorescence spectrum. Several mechanisms are proposed for excitation of the diffuse band, including collisional energy transfer and ionization-recombination processes. New observations of a spectral shape of the diffuse band are reported.     

On the theory of layered high-temperature superconductors

Assuming that charge carriers form a Fermi liquid state, we study a model for layered high-temperature superconductors with unretarded intralayer and interlayer pairing. Guided by band structure calculations and inverse photoemission experiments, we adopt a tight binding band with nearest and next-nearest neighbors hopping within the sheets and weak interlayer hopping. The gap equations are solved numerically, without imposing a cutoff energy, characteristic to phonon mediated superconductivity. On this basis we calculate the gap parameters,T _c , the tunneling conductance, infrared absorption and the coherence length for various band fillings =1/2–x by introducing excess holes of concentrationx. Assuming the interlayer coupling strength to be smaller than the intralayer one, our main results are as follows:T _c is dominated by the intralayer properties, reaching a maximum atx0.3, where strong coupling features appear. In the presence of interlayer pairing, the gap becomes anisotropic perpendicular to the layers, and standard BCS-behavior is modified. In particular the BCS-square root singularity in the density of states and in the tunneling conductance is replaced by van Hove singularities characterizing the anisotropic gap. In particular, we investigate the anisotropy of the tunneling conductance for specular and diffuse tunneling for a junction parallel or perpendicular to the layers, infrared absorption, as well as the coherence length, parallel and perpendicular to the layers.     

Crystallization Kinetics of Lamellar Crystals Confined in Polymer Thin Films

We used dynamic Monte Carlo simulations to investigate the crystallization kinetics of flat-on lamellar polymer crystals in variable thickness films. We found that the growth rates linearly reduced with decreasing film thickness for the films thinner than a transition thickness d_t , while they were constant for the films thicker than d_t . Moreover, the mean stem lengths (crystal thickness) we calculated decreased with film thickness in a similar way to the growth rates, and the intramolecular crystallinities we calculated confirmed the film thickness dependence of the crytsal thickness. Besides, the crystal melting rates in thin films were calculated and increased with decreasing film thickness. We proposed a new interpretation on the film thickness dependence of the crystal growth rate in thin films, suggesting that it is dominated by the crystal thickness in terms of the driving force term (l–l _min) expressed by Sadler, rather than the chain mobility based on experiments. The crystal thickness can determine whether a crystal grows or melts in a thin film at a fixed temperature, indicating the reversibility between the crystal growth and melting.     

Electronic properties of orthorhombic LiGaS<Subscript>2</Subscript> and LiGaSe<Subscript>2</Subscript>

We report theoretical calculations of the band structure and density of states for orthorhombic LiGaS₂ (LGS) and LiGaSe₂ (LGSe). These calculations are based on the full potential linear augmented plane wave (FP-LAPW) method within a framework of density functional theory. Our calculations show that these crystals have similar band structures. The valence band maximum (VBM) and the conduction band minimum (CBM) are located at Γ, resulting in a direct energy band gap. The VBM is dominated by S/Se-p and Li-p states, while the CBM is dominated by Ga-s, S/Se-p and small contributions of Li-p and Ga-p. From the partial density of states we find that Li-p hybridizes with Li-s below the Fermi energy (E _F), while Li-s/p hybridizes with Ga-p below and above E _F. Also, we note that S/Se-p hybridizes with Ga-s below and above E _F.     

Dynamical mean-field study of the Mott transition in thin films

The correlation-driven transition from a paramagnetic metal to a paramagnetic Mott-Hubbard insulator is studied within the half-filled Hubbard model for a thin-film geometry. We consider simple-cubic films with different low-index surfaces and film thickness d ranging from d=1 (two-dimensional) up to d=8. Using the dynamical mean-field theory, the lattice (film) problem is self-consistently mapped onto a set of d single-impurity Anderson models which are indirectly coupled via the respective baths of conduction electrons. The impurity models are solved at zero temperature using the exact-diagonalization algorithm. We investigate the layer and thickness dependence of the electronic structure in the low-energy regime. Effects due to the finite film thickness are found to be the more pronounced the lower is the film-surface coordination number. For the comparatively open sc(111) geometry we find a strong layer dependence of the quasi-particle weight while it is much less pronounced for the sc(110) and the sc(100) film geometries. For a given geometry and thickness d there is a unique critical interaction strength U _c2 (d) at which all effective masses diverge and there is a unique strength U _c1 (d) where the insulating solution disappears. U _c2 (d) and U _c1 (d) gradually increase with increasing thickness eventually approaching their bulk values. A simple analytical argument explains the complete geometry and thickness dependence of U_c2. U_c1 is found to scale linearly with U_c2. Received 19 August 1998     

Constraining the size of the carrier of the λ5797.1 diffuse interstellar band

The diffuse interstellar band (DIB) at 5797.1 Å is simulated based on three premises: (1) The carrier of the DIB is polar as concluded by T. Oka et al. from the anomalous spectrum toward Herschel 36 (Her 36). (2) The sharp central feature observed by P.J. Sarre's group is the Q-branch of a parallel band of a prolate top. (3) The radiative temperature of the environment is T_r = 2.73 K. A ²Π ← ²Π transition of a linear radical is simulated. Results depend on 10 parameters, with the rotational constant B being the most critical. Comparisons of calculated spectra with observed data constrain B, which in turn constrains the number of heavy atoms to 5 ≤ n ≤ 7. Upper limits based on the Her 36 spectrum and lower limits based on stability against photodissociation are also discussed. The latter is based on the assumption that the DIB molecules are produced top-down from the breakdown of dust rather than bottom-up by chemical reactions. A difficulty with this limit is that J.P. Maier's laboratory has observed many molecules within this size range, some of which have been tested astronomically. Candidates are discussed in light of many prolate tops observed by the P. Thaddeus and M.C. McCarthy microwave group.