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Notes: Electrical conductivities (σdc) of the as-quenched Bi3.5Pb0.5Sr3Ca3Cu4Ox+zAg2O (with z=1, 3, 5, and 10 wt %) glassy precursors for high Tc superconductors are found to be much higher (∼10−5−101 Ω−1cm−1) than those of the corresponding Ag2O free Bi3.5Pb0.5Sr3Ca3Cu4Ox (denoted by BPB) precursor glass (∼10−13−10−6 Ω−1cm−1). This unusually high conductivity is attributed to the increase of carrier concentrations caused by the addition of Ag2O (also observed from the Hall effect measurements). The experimentally observed high values of σdc do not follow Mott's variable range hopping model which is in sharp contrast to the behavior of the corresponding pure BPB and many other conventional transition metal oxide glasses having high resistivities. Moreover, the Seebeck coefficients (S) of these glassy precursors show nonlinear variations (from negative at lower temperature to positive at higher temperature) which cannot be clearly explained by phonon drag or electron-phonon interaction. This behavior of S which is also supported from Hall effect measurement is considered to be due to the nonlinear thermal variations of carrier concentrations (both hole and electron) present in the glassy samples. © 1997 American Institute of Physics.

Notes: Rotationally-resolved Beutler–Fano line shapes observed in the photoabsorption spectrum of the (2,0) band of the 3pπuf 1Σu+←X 3Σg− Rydberg system of O2 are interpreted using a coupled-channel Schrödinger equations model. It is found that the f 1Σu+ state is indirectly predissociated by the B 3Σu− continuum, and that the f←X transition borrows oscillator strength primarily from dipole-allowed transitions into the mixed Rydberg-valence states of 3Σu− symmetry. Both the predissociation linewidth and oscillator strength of the (2,0) resonance are controlled by the spin-orbit interaction between the 1Σu+ and 3Σu− components of the 3p-complex. There is some evidence for a destructive quantum interference between the transition amplitude borrowed from the 3pπuE 3Σu−←X 3Σg− transition and that borrowed weakly from the f 1Σu+←b 1Σg+ transition through spin-orbit mixing between the b 1Σg+ and X 3Σg− states. © 1995 American Institute of Physics.

Notes: Nanometer-sized iron particles with diameters in the range 5.5–11.1 nm were grown within a silica gel by an electrodeposition method. Electron diffraction measurements show that some of the iron particles were oxidized to Fe3O4. dc resistivity measurements over the temperature range 110–300 K show a T−1/4 variation indicating a variable range hopping transport. ac conductivity over the frequency range 100 Hz–2 MHz show an overlapping large polaron tunneling mechanism to be operative. The dielectric modulus spectra as a function of frequency were analyzed on the basis of a stretched exponential relaxation function. The values of the exponent β as extracted from this analysis were in the range 0.38–0.46. The activation energies corresponding to the maximum of the imaginary part of the dielectric modulus were in the range 0.13–0.20 eV. These are ascribed to an electron tunneling mechanism. © 1999 American Institute of Physics.

Notes: Nanoparticles of silver with diameters in the range 10.3–25.7 nm were grown within a silica gel medium by an electrodeposition technique. The dc resistivity of the nanocomposites was measured over the temperature range 100–300 K. The resistivity as a function of inverse temperature shows a maximum at around 175 K. This is explained as arising due to the presence of two conduction mechanisms, viz., an electron tunnelling between metal particles and conduction through a percolated metal structure which is fractal in nature. © 1999 American Institute of Physics.

Notes: High (2 eV) and low kinetic energy supersonic jets of disilane as well as ultrahigh vacuum chemical vapor deposition have been employed to grow epitaxial silicon thin films on Si(100) wafers at temperatures ranging from 500 to 650 °C. The growth properties and film uniformity are compared in order to characterize the high energy technique. High translational energy disilane supersonic jets increase the efficiency of deposition by increasing the disilane reaction probability. The growth profiles from the high energy jet are sharply peaked due to a focusing of the precursor along the jet centerline. © 1995 American Institute of Physics.

Notes: We report on the effect of annealing on a thin Fe film deposited on a Si(111) substrate, using x-ray reflectivity and secondary ion mass spectrometry (SIMS) techniques. Using Fourier transform of the x-ray reflectivity data, we have estimated the layer thickness of the film. From the estimated thickness and critical value of the scattering vector qc obtained from the reflectivity data, an initial guess model of the electron density profile of the film is made. Using an iterative inversion technique, based on the Born approximation, with the obtained initial guess model, we have extracted the actual electron density profile of the film as a function of depth from the specular x-ray reflectivity data. On annealing, we observe interdiffusion of Fe and Si resulting in an increase in the thickness of the film. We have also carried out a SIMS measurement on the annealed sample to support the result of the annealing effect observed from the analysis of x-ray reflectivity data. The SIMS analysis indicates that the top of the film is rich in Si which has diffused from the substrate to the surface of the Fe film on annealing. © 1999 American Institute of Physics.

Notes: This work focuses on subgrid-scale (SGS) modeling for finite-difference large-eddy simulations, employing filters in physical space. When a filter in physical space is used, an overlap is allowed between the unresolved and the resolved scales. For such a filter, all the three terms in the classical decomposition of the SGS stress tensor are present: the Leonard and cross-terms, due to the overlap between scales, and the true SGS Reynolds tensor, expressing the pure effect of the small scales. A dynamic subgrid-scale stress model is proposed, for finite-difference large-eddy simulation of incompressible and compressible flows in which the Leonard and cross-parts of the SGS stress tensor are assumed to be proportional to the resolved part (the "modified Leonard term''), which is computed explicity. The SGS Reynolds stress is modeled by the eddy-viscosity Smagorinsky model. The two unknown parameters in this model are computed dynamically, as in Germano et al. [Phys. Fluids A 3, 1790 (1991)], but using a least squares technique. The model is tested using direct numerical simulation data for fully developed turbulent incompressible flows in presence of solid boundaries and free surfaces, and for compressible homogeneous turbulence. A "box filter'' in physical space is used. Other SGS models are also tested, viz. the dynamic model of Germano et al. (DSM), and its compressible extension by Moin et al. [Phys. Fluids A 3, 2746 (1991)], and the dynamic mixed model in Zang et al. [Phys. Fluids A 5, 3186 (1993)] (DMM) and its compressible version developed here. Results on the behavior of the different models with regard to energy exchanges and correlation with the exact SGS stresses are presented for different filter widths. In particular high correlation is found between the modified Leonard and cross-terms thus justifying the basic assumption made in the model. © 1995 American Institute of Physics.

Notes: The structural features of turbulence at the free surface of a channel flow have been experimentally investigated. The experiments were conducted in a horizontal channel of large aspect ratio in the (depth based) Reynolds number range of 2800–8800. The results indicate that the persistent structures on the free surface can be classified as upwellings, downdrafts, and spiral eddies. Upwellings are shown to be related to the bursts originating in the sheared region at the channel bottom and the eddies are seen to be generated at the edges of the upwellings. The eddies often merge if rotating in the same direction, and form "pairs" if rotating in opposite directions—though there are occasional mergers of such counter-rotating ones. The spiral eddies decay slowly and are sometimes annihilated by fresh upwellings. The population densities and the persistence times of the various structures were measured for different flow conditions. The resulting data show that the physical parameters characterizing the structures at the interface, scale with a mix of inner (wall shear stress and viscosity) and outer variables. Measurement of the streamwise and spanwise velocities at the free-surface were made by particle imaging velocimetry (PIV) and the surface normal velocity near the free-surface estimated by continuity. The results indicate that the upwellings and spiral eddy regions would be expected to dominate scalar transport rates at high Prandtl/Schmidt numbers. The one-dimensional energy spectra of the flow field at the free-surface compare well with direct numerical simulations and show a region with −5/3 slope at low wave numbers. This experimentally confirms a previous result regarding the two-dimensionality of turbulence near the free surface, based on numerical simulations by Pan and Banerjee [Phys. Fluids 7, 1649 (1995)]. © 1998 American Institute of Physics.

Notes: A method for particle image velocimetry (PIV) is presented which improves upon the accuracy, computational efficiency and dynamic range (i.e., the difference between the largest and smallest resolvable particle displacement vectors) of conventional PIV techniques. The technique is applied to free-surface turbulence to resolve energy spectra for motions with a wide dynamic range. The methodology—based on multi-grid image processing algorithms for rigid body motion analysis, estimates the displacement vectors at discrete particle locations. The essence of this technique is to estimate large scale motions from image intensity patterns of low spatial frequencies and small scale motions from intensity patterns of high spatial frequencies. Cross-correlation between a pair of time separated particle images is implemented by the hierarchical computational scheme of Burt ["Fast filter transforms for image processing," Int. J. Comput. Vision 16, 20 (1981)]. Each image is convolved with a series of band-pass filters and subsampled to obtain a set of images progressively decreasing in resolution and size. A coarse estimate of the displacement field obtained from pairs of lower resolution images are used to obtain more accurate estimates at the next (finer) level. Processing starts at the level of lowest resolution and stops at the highest resolution level, which contains the original image pair. Due to subsampling of low resolution images, the match template size can be kept constant for all stages of computation, thus eliminating the dependence of the largest resolvable displacement on the size of match template. In the present work, the search area at each level is kept constant at 3×3 pixels and the match template size at 5×5 pixels for all levels of computation. The algorithm has been implemented using simple thresholding based on the confidence level of an estimated displacement vector, as suggested by Anandan ["A computational framework and an algorithm for measurement of visual motion," Int. J. Comput. Vision, 2, 283, (1987)]. However, the confidence-level-based smoothing technique for rigid body motions (continuous velocity fields) could not be applied to displacement estimates obtained at discrete points i.e., the particle locations. Instead, smoothing was performed over the area covered by each particle. The algorithm has been tested against direct numerical simulations of turbulent flows when the flow field is known and particle images have been generated from these with the addition of noise. Both the accuracy of motion estimation and the computation time are seen to improve as compared to conventional PIV methods. Finally, video images taken of particle motion on the free-surface of a channel flow have been used to determine the capabilities of the technique in an experimental study. The resulting spectra show a quasi-two-dimensional character of the free-surface turbulent flow field, which corresponds well with the direct numerical simulations. © 1998 American Institute of Physics.