Structural and photoluminescent properties of ternary Zn1-xCdxO crystal films grown on Si(111) substrates

The ternary Zn1-xCdxO (0less than or equal toxless than or equal to0.6) alloying films with highly c-axis orientation have been deposited on Si(111) substrates by direct current reactive magnetron sputtering method. X-ray diffraction measurement indicates that the wurtzite-type structure of ZnO can be stabilized up to nominal Cd content x similar to 0.6 without cubic CdO phase separation. The lattice parameter c of Zn1-xCdxO increases almost linearly from 5.229 Angstrom (x = 0) to 5.247 Angstrom (x = 0.6), indicating that Cd substitution takes place on the Zn lattice sites. The photoluminescence spectra of the Zn1-xCdxO thin films measured at 12 K display a substantial red shift (similar to0.3 eV) in the near-band-edges (NBEs) emission of ZnO: from 3.39 eV of ZnO to 3.00 eV of Zn0.4Cd0.6O. The direct modulation of band gap caused by Zn/Cd substitution is responsible for the red shift effect in NBE emission of ZnO. (C) 2003 Elsevier Science B.V. All rights reserved.

Structural and optical properties of InAlGaN films grown directly on low-temperature buffer layer with (0001)sapphire substrate

Quaternary InAlGaN film has been grown directly on top of low-temperature-deposited GaN buffer layer by low-pressure metalorganic vapor phase epitaxy. High-resolution X-ray diffraction and photoluminescence (PL) results show that the film has good crystal quality and optical property. Temperature-dependent PL and time-resolved PL (TRPL) have been employed to study the carriers recombination dynamics in the film. The TRPL signals can be well fitted as a stretched exponential function exp[-(t/tau)(beta)] from 14 to 250 K, indicating that the emission is attributed to the radiative recombination of excitons localized in disorder quantum nanostructures such as quantum disks originating from indium (In) clusters or In composition fluctuation. The cross-sectional high-resolution electron microscopy measurement further proves that there exist the disorder quantum nanostructures in the quaternary. By investigating the dependence of the exponential parameter beta on the temperature, it is shown that the multiple trapping-detrapping mechanism dominates the diffusion among the localized states. The localized states are considered to have two-dimensional density of states (DOS) at 250 K, since radiative recombination lifetime tau(r) increases linearly with increasing temperature. (C) 2002 Elsevier Science B.V. All rights reserved.

Structural characterization of cubic GaN grown on GaAs(001) substrates

Structural characteristics of cubic GaN epilayers grown on GaAs(001) were studied using X-ray double-crystal diffraction technique. The structure factors of cubic GaN(002) and (004) components are approximately identical. However, the integrated intensities of the rocking curve for cubic (002) components are over five times as those of (004) components. The discrepancy has been interpreted in detail considering other factors. In the conventional double crystal rocking curve, the peak broadening includes such information caused by the orientation distribution (mosaicity) and the distribution of lattice spacing. These two kinds of distributions can be distinguished by the triple-axis diffraction in which an analyser crystal is placed in front of the detector. Moreover, the peak broadening was analysed by reciprocal lattice construction and Eward sphere. By using triple-axis diffraction of cubic (002) and (113) components, domain size and dislocation density were estimated. The fully relaxed lattice parameter of cubic GaN was determined to be about 0.451 +/- 0.001nm.

Controlling parameter for wave types of long flexible riser undergoing vortex-induced vibration

Submerged floating tunnel (SFT) is a popular concept of crossing waterways. The failure of the cable may occur due to vortex-induced-vibration (VIV), and the stability of the cable is crucial to the safety of the entire tunnel. Investigation results in recent years show that the vortex-induced vibration of the flexible cables with large aspect ratio reveals some new phenomena, for example, the vortex-induced wave, multi-mode competition, wide band random vibration, which have brought new challenges to the study of vortex-induced vibration of long flexible cables. In this paper, the dimensionless parameter controlling the wave types of dynamic response of slender cables undergoing vortex-induced vibration is investigated by means of dimensional analysis and finite element numerical simulations. Our results indicate that there are three types of response for a slender cable, i.e. standing wave vibration, traveling wave vibration and intermediate state. Based on dimensional analysis the controlling parameter is found to be related to the system damping including fluid damping and structural damping, order number of the locked-in modes and the aspect ratio of cable. Furthermore through numerical simulations and parameter regression, the expression and the critical value of controlling parameter is presented. At last the physical meaning of the parameter is analyzed and discussed.

Crystal growth and structural analysis of PrMnO3 and TbMnO3

Single crystals of PrMnO3 and TbMnO3 were grown by floating zone method and the crystal structure was determined by single crystal X-ray diffractometry. The structure of these compounds belongs to the orthorhombic system (space group is Pnma, No. 62) with the lattice parameters alpha approximate to root (.) - a(p), b approximate to 2 (.) a(p) , c approximate to root 2.a(p) and Z = 4, where a(p) is ideal cubic perovskite cell parameter.

Analysis of structural relaxation in a Li(2)O2SiO(2) glass using rate heating approach

The structural relaxation process of an inorganic glass (Li(2)O2SiO(2)) at different cooling rates has been studied by differential scanning calorimetry. A four-parameter model-Tool-Narayanaswamy-Moynihan (TNM) model was applied to simulate the normalized specific heat curve measured. Four parameters, Delta h*/R, beta, In A, and x were obtained and compared with the values obtained from the isothermal approach. (C) 1999 Kluwer Academic Publishers.

A phenomenological study of the structural relaxation of an inorganic glass (Li(2)O2SiO(2))

The structural relaxation process of an inorganic glass (Li(2)O2SiO(2)) has been studied by differential scanning calorimetry. The sample is subjected to different thermal ageing histories with isothermal stages at an ageing temperature of T-g - 30 degrees C for different ageing times and at an ageing time of 16 h for different ageing temperatures. A four-parameter Tool-Narayanaswamy-Moynihan (TNM) model, is applied to simulate the normalized specific-heat curves measured. The ageing-temperature and ageing-time dependence of the structural relaxation parameters in the TNM model is obtained. (C) 1998 Elsevier Science S.A. All rights reserved.

The effect of structural parameters on the enthalpy relaxation of an inorganic glass (Li2O center dot 2SiO(2))

The structural relaxation process of an inorganic glass (Li2O . 2SiO(2)) at an ageing temperature of 703 K for an ageing time of 1 h has been studied by differential scanning calorimetry. A four-parameter model-the Tool-Narayanaswamy-Moynihan (TNM)-model was applied to simulate the normalized specific heat curve measured. A set of optimized parameters, Delta h*/R,beta,InA, and x was obtained. Then the effects of variation of each adjustable parameter on the calculated specific heat were summarized. (C) 1997 Elsevier Science S.A.

On how structural model variability influence transonic aeroelasticity stability.

This paper considers the ways in which structural model parameter variability can in?uence aeroelastic stability. Previous work on formulating the stability calculation (with the Euler equations providing the aerodynamic predictions) is exploited to use Monte Carlo, Interval and Perturbation calculations to allow this question to be investigated. Three routes are identi?ed. The ?rst involves variable normal mode frequencies only. The second involves normal mode frequencies and mode shapes. Finally, the third, in addition to normal mode frequencies and mode shapes, also includes their in?uence on the static equilibrium. Previous work has suggested only considering route 1, which allows signi?cant gains in computational e?ciency if reduced order models can be built for the aerodynamics. However, results in the current paper show that neglecting route 2 can give misleading results for the ?utter onset prediction.

Structural change of vortex patterns in anisotropic Bose-Einstein condensates

We study the changes in the spatial distribution of vortices in a rotating Bose-Einstein condensate due to an increasing eccentricity of the trapping potential. By breaking the rotational symmetry, the vortex system undergoes a rich variety of structural changes, including the formation of zigzag and linear configurations. These spatial rearrangements are well signaled by the change in the behavior of the vortex-pattern eigenmodes against the eccentricity parameter. This behavior allows to actively control the distribution of vorticity in many-body systems and opens the possibility of studying interactions between quantum vortices over a large range of parameters.

Spontaneous nucleation of structural defects in inhomogeneous ion chains

Structural defects in ion crystals can be formed during a linear quench of the transverse trapping frequency across the mechanical instability from a linear chain to a zigzag structure. The density of defects after the sweep can be conveniently described by the Kibble-Zurek mechanism (KZM). In particular, the number of kinks in the zigzag ordering can be derived from a time-dependent Ginzburg-Landau equation for the order parameter, here the zigzag transverse size, under the assumption that the ions are continuously laser cooled. In a linear Paul trap, the transition becomes inhomogeneous, since the charge density is larger in the center and more rarefied at the edges. During the linear quench, the mechanical instability is first crossed in the center of the chain, and a front, at which the mechanical instability is crossed during the quench, is identified that propagates along the chain from the center to the edges. If the velocity of this front is smaller than the sound velocity, the dynamics become adiabatic even in the thermodynamic limit and no defect is produced. Otherwise, the nucleation of kinks is reduced with respect to the case in which the charges are homogeneously distributed, leading to a new scaling of the density of kinks with the quenching rate. The analytical predictions are verified numerically by integrating the Langevin equations of motion of the ions, in the presence of a time-dependent transverse confinement. We argue that the non-equilibrium dynamics of an ion chain in a Paul trap constitutes an ideal scenario to test the inhomogeneous extension of the KZM, which lacks experimental evidence to date.

Structural Defects in Ion Chains by Quenching the External Potential: The Inhomogeneous Kibble-Zurek Mechanism

The nonequilibrium dynamics of an ion chain in a highly anisotropic trap is studied when the transverse trap frequency is quenched across the value at which the chain undergoes a continuous phase transition from a linear to a zigzag structure. Within Landau theory, an equation for the order parameter, corresponding to the transverse size of the zigzag structure, is determined when the vibrational motion is damped via laser cooling. The number of structural defects produced during a linear quench of the transverse trapping frequency is predicted and verified numerically. It is shown to obey the scaling predicted by the Kibble-Zurek mechanism, when extended to take into account the spatial inhomogeneities of the ion chain in a linear Paul trap.

Structural phase transitions in low-dimensional ion crystals

A chain of singly charged particles, confined by a harmonic potential, exhibits a sudden transition to a zigzag configuration when the radial potential reaches a critical value, depending on the particle number. This structural change is a phase transition of second order, whose order parameter is the crystal displacement from the chain axis. We study analytically the transition using Landau theory and find full agreement with numerical predictions by Schiffer [Phys. Rev. Lett. 70, 818 (1993)] and Piacente [Phys. Rev. B 69, 045324 (2004)]. Our theory allows us to determine analytically the system's behavior at the transition point.

How Structural Model Variability Influences Transonic Aeroelastic Stability

This paper considers the ways in which structural model parameter variability can influence aeroelastic stability. Previous work on formulating the stability calculation (with the Euler equations providing the aerodynamic predictions) is exploited to use Monte Carlo, interval, and perturbation calculations to allow this question to be investigated. Three routes are identified. The first involves variable normal-mode frequencies only. The second involves normal-mode frequencies and shapes. Finally, the third, in addition to normal-mode frequencies and shapes, also includes their influence on the static equilibrium. Previous work has suggested only considering the first route, which allows significant gains in computational efficiency if reduced-order models can be built for the aerodynamics. However, results in the current paper show that neglecting the mode-shape variation can give misleading results for the flutter-onset prediction, complicating the development of reduced aerodynamic models for variability analysis.

Structural and electrical anisotropy of (001)-, (116)-, and (103)-oriented epitaxial SrBi2Ta2O9 thin films on SrTiO3 substrates grown by pulsed laser deposition

Epitaxial SrBi2Ta2O9 (SBT) thin films with well-defined (001), (116), and (103) orientations have been grown by pulsed laser deposition on (001)-, (011)-, and (111)-oriented Nb-doped SrTiO3 substrates. X-ray diffraction pole figure and phi -scan measurements revealed that the three-dimensional epitaxial orientation relation SBT(001)parallel to SrTiO3(001), and SBT[1(1) over bar 0]parallel to SrTiO3[100] is valid for all cases of SET thin films on SrTiO3 substrates, irrespective of their orientations. Atomic force microscopy images of the c-axis-oriented SET revealed polyhedron-shaped grains showing spiral growth around screw dislocations. The terrace steps of the c-axis-oriented SET films were integral multiples of a quarter of the lattice parameter c of SBT (similar to 0.6 nm). The grains of (103)-oriented SET films were arranged in a triple-domain configuration consistent with the symmetry of the SrTiO3(111) substrate. The measured remanent polarization (2P(r)) and coercive field (2E(c)) of (116)-oriented SBT films were 9.6 muC/cm(2) and 168 kV/cm, respectively, for a maximum applied electric field of 320 kV/cm. Higher remanent polarization (2P(r)=10.4 muC/cm(2)) and lower coercive field (2E(c)=104 kV/cm) than those of SBT(116) films were observed in (103)-oriented SET thin films, and (001)-oriented SET revealed no ferroelectricity along the [001] axis. The dielectric constants of (001)-, (116)-, and (103)-oriented SBT were 133, 155, and 189, respectively. (C) 2000 American Institute of Physics.