On a moving liquid film and its instability on textured surfaces

Recently, two regimes of viscous friction on textured surfaces were proposed in the context of penetration of liquid film into the texture (EPL 79, 56005 (2007)): the Poiseuille and Stokes regimes. With this idea on viscous friction, we theoretically discuss instabilities on a liquid film on textured surfaces when the film is forced to move with external forces. When a film recedes due to a pressure drop, we find scaling laws for instabilities to be checked in future experiments. When a circular film expands due to centrifugal force we find that the expanding film is stable against rim fluctuations (within the linear stability analysis) with its radius determined by a simple equation. Our discussion sheds light on the curvature of the front of the moving liquid film on textured surfaces and how the film thickness is kept fixed to the texture height on textured surfaces, aspects which have not been discussed in previous studies.     

Generation and evolution of partial misfit dislocations and stacking faults in thin-film heterostructures

An analysis is made of the specific features in the generation and evolution of partial misfit dislocations at the vertices of V-shaped configurations of stacking fault bands, which terminate in the bulk of the growing film at 90° partial Shockley dislocations. The critical thicknesses h _c of an epitaxial film, at which generation of such defect configurations becomes energetically favorable, are calculated. It is shown that at small misfits, the first to be generated are perfect misfit dislocations and at large misfits, partial ones, which are located at the vertices of V-shaped stacking-fault band configurations emerging onto the film surface. Possible further evolution of stacking-fault band configurations with increasing film thickness are studied.     

Mechanical instabilities of bubble clusters between parallel walls

We have carried out a systematic study of buckling-like mechanical instabilities in simple two- (2D) and three-dimensional (3D) symmetric foam clusters sandwiched between parallel planar walls. These instabilities occur when the wall separation w is reduced below a critical value, w^*, for which the foam surface energy E reaches its minimum, E^*. The clusters under investigation consist of either a single bubble, or of twin bubbles of fixed equal sizes (areas A in 2D or volumes V in 3D), which are either free to slide or pinned at the confining walls. We have numerically obtained w^* for both free and pinned 2D and 3D clusters. Furthermore, we have calculated the buckled configurations of 2D twin bubbles, either free or pinned, and of 3D free twin bubbles, whose energy is independent of w and equal to the minimum energy E^* of the unbuckled state. Finally, we have also predicted the critical w_t^* at which the terminal configurations under extension of 2D and 3D single and twin bubbles are realised. Experimental illustrations of these transitions under compression and extension are presented. Our results, together with others from the literature, suggest that a bubble cluster bounded by two parallel walls is stable only if the normal force it exerts on the walls is attractive, i.e., if dE/dw > 0; clusters that cause repulsion between the walls are unstable. We correlate this with the distribution of film orientations: films in a stable cluster cannot be too parallel to the confining walls; rather, their average tilt must be larger than for a random distribution of film orientations.     

Sudden Jumps,Hysteresis, and Negative Resistance in an Argon Plasma Discharge. II. Discharges in Magnetic Fields

Experimental observations on the behavior of low-pressure, thermionic argon discharges in magnetic cusp and mirror configurations are presented. I–B characteristics showing jump and hysteresis behavior were found. In some cases, the sudden sharp decreases in discharge current that were observed were preceded by the onset of coherent, low-frequency plasma oscillations, suggesting the possible role of instabilities.     

Gravity driven instabilities in miscible non-Newtonian fluid displacements in porous media

Gravity driven instabilities in model porous packings of 1 mm diameter spheres are studied by comparing the broadening of the displacement front between fluids of slightly different densities in stable and unstable configurations. Water, water–glycerol and water–polymer solutions are used to vary independently viscosity and molecular diffusion and study the influence of shear-thinning properties. Both injected and displaced solutions are identical but for a different concentration of NaNO₃ salt used as an ionic tracer and to introduce the density contrast. Dispersivity in stable configuration increases with polymer concentration – as already reported for double porosity packings of porous grains. Gravity-induced instabilities are shown to develop below a same threshold Péclet number Pe for water and water–glycerol solutions of different viscosities and result in considerable increases of the dispersivity. Measured threshold Pe values decrease markedly on the contrary with polymer concentration. The quantitative analysis demonstrates that the development of the instabilities is controlled by viscosity through a characteristic gravity number G (ratio between hydrostatic and viscous pressure gradients). A single threshold value of G accounts for results obtained on Newtonian and non-Newtonian solutions.     

Propagation and spreading of unstable perturbations in electron-hole plasmas subject to crossed fields

A model is developed for electrostatic drift instabilities which arise in inhomogeneous electron-hole plasmas subject to crossed fieldsE ₀B ₀. The instabilities are initiated by gradients in the equilibrium plasma densityn ₀. Using two-fluid magnetohydrodynamics and linear perturbation theory the dispersion relation of local density oscillations is calculated for arbitrary inhomogeneous equilibrium distributions and plasma densities. For cases wheren ₀B ₀ andn ₀ ⁺ n ₀ ^- it is found that the propagation direction of maximal gain, , is the bisectrix of the angle between (–n ₀) and (E ₀ ×B ₀) and that stable and unstable configurations are distinguished by the angle between ₀ andn ₀. A local density perturbation built by superpositions of the plane waves, and initially chosen radially symmetric, broadens unisotropically. In the direction transverse to the broadening is anomaleously enhanced, as compared to the broadening by diffusion in the stable case. The results are referred to experimental observations of low-frequency instabilities reported in [1].     

Fractal flux jumps in an organic superconducting crystal

Angle dependant torque magnetization measurements have been carried out on the organic superconductor, κ-(ET)₂Cu(NCS)₂ at extremely low temperatures (25-300 mK). Magneto-thermal instabilities are observed in the form of abrupt magnetization (flux) jumps for magnetic field sweeps of 0-20 T. A fractal analysis of the flux jumps indicate that the instabilities do show a self similar structure with a fractal dimension of varying between 1.15 and 1.6. The fractal structure of the flux jumps in our sample shows a striking similarity to that of MgB₂ thin film samples, in which magneto-optical experiments have recently shown that the small flux jumps are due to the formation of dendritic flux structures. These smaller instabilities act to suppress the critical current density of the thin films. The similarity of the flux jump structure of our samples suggests that we may also observing the dendritic instability, but in a bulk sample rather than a thin film. If true, this is the first observation of the dendritic instability in a bulk superconducting sample, and is likely due to the layered nature of κ-(ET)₂Cu(NCS)₂, which results in a quasi-two dimensional flux structure over the majority it's mixed state phase diagram.     

High-resolution identification of ½⟨110⟩ stacking faults in epitaxial Ba0.3Sr0.7TiO3 thin films

The near-interface region of an epitaxial Ba_0.3Sr_0.7TiO₃ thin film grown on LaAlO₃ (001) was found to consist of a high density of ½?110? stacking faults bounded by partial dislocations. The stacking faults can extend over large distances (greater than 50 nm). Various possible atomic configurations of the faults were considered. The atomic structures of the faults were identified using high-resolution electron microscopy and simulation as well as energy-filtered imaging. The ½[101] and faults (where [001] is normal to the film plane) were found to lie predominately on the {100} and {110} planes. The ½lsqb;101] faults on (010), (110) or (1&1tilde;0) have never been observed before in perovskites. The stacking faults on [100] have a structure consisting of a double layer of edge-sharing TiO₆ octahedra. The excess of Ti was detected by energy-filtered imaging. The formation of the extended stacking faults is probably related to a small amount of excess Ti during the film deposition, which may originate from the non-stoichiometry of the ceramic targets BaTiO₃ and SrTiO₃. It is also enhanced by the misfit-induced compressive strain in the early stages of the film growth.     

Possible phases of the two-dimensional Hubbard model

We present a stability analysis of the 2D t - t' Hubbard model on a square lattice for various values of the next-nearest-neighbor hopping t' and electron concentration. Using the free energy expression, derived by means of the flow equations method, we have performed numerical calculation for the various representations under the point group C _4ν in order to determine at which temperature symmetry broken phases become more favorable than the symmetric phase. A surprisingly large number of phases has been observed. Some of them have an order parameter with many nodes in -space. Commonly discussed types of order found by us are antiferromagnetism, d _{x2 - y2} -wave singlet superconductivity, d-wave Pomeranchuk instability and flux phase. A few instabilities newly observed are a triplet analog of the flux phase, a particle-hole instability of p-type symmetry in the triplet channel which gives rise to a phase of magnetic currents, an s^*-magnetic phase, a g-wave Pomeranchuk instability and the band splitting phase with p-wave character. Other weaker instabilities are found also. A comparison with experiments is made. Received 25 July 2002 / Received in final form 28 November 2002 Published online 14 February 2003 RID="a" ID="a"Current address: Département de physique and Centre de recherche sur les propriétés électroniques de matériaux avancés, Université de Sherbrooke, Sherbrooke, Québec, Canada J1K 2R1 e-mail: vaha@physique.usherb.ca     

Epitaxial growth and magnetic and electric properties of Co-doped thin films

In this research, c-axis oriented epitaxial anatase TiO ₂ thin films were grown on SrTiO ₃ (100) substrates using a ceramic Ti _0.95 Co _0.05 O ₂ target by Pulsed Laser Deposition (PLD). The film growth processes were monitored by reflective high energy electronic diffraction (RHEED). Microstructure, conductivity, and magnetism of these doped films are found strongly affected by the oxygen pressure and substrate temperature T_s. Grown at a T_s around 750 ^° C in an oxygen pressure of 0.2 mbar, the dopants are found existing as oxide inclusions. The doped film thus behaves as an insulator and shows diamagnetism in a magnetic field parallel to the film surface. However, in the doped film grown at a reduced temperature of 630 ^° C in a vacuum, no impurity phase can be identified. The film shows a saturated magnetic moment of 0.16μ _B / Co and a fairly good conductivity at room temperature. It is then concluded that nonequilibrium growth at lower temperatures in vacuum is essential for a high solubility of Co in the TiO ₂ lattice and thus the ferromagnetism.     

States of hydrogen in crystalline semiconductors

Abstract

The stable configurations of hydrogen in undoped, and in n- and p-semiconductors are reviewed. We compare the experimentally determined configurations with theoretical predictions made by a variety of calculational methods. In undoped Si, as an example, hydrogen appears to occupy a energy occurring at a tetrahedral interstitial (T) site. In B-doped, p-type Si, hydrogen at a near BC site accounts for acceptor passivation. By contrast in n-type Si, hydrogen at the antibonding (AB) position appears to be responsible for donor passivation. The possible configurations in other semiconductors (Ge, GaAs and AlGaAs) are also summarized.     

Dense Packing on Uniform Lattices

We study the Hard Core Model on the graphs G obtained from Archimedean tilings i.e. configurations in {0,1} ^G with the nearest neighbor 1’s forbidden. Our particular aim in choosing these graphs is to obtain insight to the geometry of the densest packings in a uniform discrete set-up. We establish density bounds, optimal configurations reaching them in all cases, and introduce a probabilistic cellular automaton that generates the legal configurations. Its rule involves a parameter which can be naturally characterized as packing pressure. It can have a critical value but from packing point of view just as interesting are the noncritical cases. These phenomena are related to the exponential size of the set of densest packings and more specifically whether these packings are maximally symmetric, simple laminated or essentially random packings. Research partially supported by The Finnish Academy of Science and Letters.     

Formation of two-dimensional lattices of magnetic domains in a harmonic magnetic field

New configurations of two-dimensional lattices of magnetic domains with symmetry described by the P2 and Cmm2 space groups are revealed in studying the formation of domain structures in a harmonic magnetic field. Two-dimensional lattices belonging to five space groups of the orthorhombic and hexagonal systems are observed in a single iron garnet film. Changes in the lattice configurations and in the lattice elements occur upon the variation of only one parameter of the applied magnetic field, namely, its frequency.     

Landau level quantization and possible superconducting instabilities in highly oriented pyrolitic graphite

Measurements of the basal-plane resistivity ρ _a(T,H) performed on highly oriented pyrolitic graphite, with magnetic field H∥c-axis in the temperature interval 2–300K and fields up to 8 T, provide evidence for the occurrence of both field-induced and zero-field superconducting instabilities. Additionally, magnetization M(T,H) measurements suggest the occurrence of Fermi surface instabilities which compete with the superconducting correlations. Fiz. Tverd. Tela (St. Petersburg) 41, 2135–2138 (December 1999)     

Jet engine electrification as a phenomenon reflecting the evolution of charge instability in an outflowing combustion product mixture

It is hypothesized that charge instabilities developing in a mixture of outflowing combustion products make some contribution to the mechanism of engine electrification in jet engines. Investigations are carried out which support this hypothesis. Zh. Tekh. Fiz. 67, 21–26 (August 1997)     

Computer simulation of hexapole aberration correctors

The use of hexapole electron-optical elements to correct the spherical aberrations of the objective lenses of a low-voltage scanning electron microscope is investigated. Compared with the conventional quadrupole-octupole correctors, hexapole systems are simpler in design, easier to tune, and less sensitive to manufacturing imperfections and power supply instabilities. Two configurations of hexapole correctors, RHRHR and HRRH (where R and H stand for round lens and hexapole component, respectively), are considered. Both configurations considerably suppress the spherical aberration of the electron microscope objective lens but cannot correct chromatic aberrations. The second configuration possesses important advantages over the first one: it is mechanically and electrically simpler and also is easier to tune. In addition, as follows from our investigation, the hexapole electrode voltages in the second configuration are lower, the correction accuracy is higher, and the sensitivity to mechanical defects is lower. However, the chromatic aberration in the second configuration is somewhat larger.     

Optical multilayer post growth instabilities: Analyses of Gd2O3/SiO2 system in combination with scanning probe force spectroscopy

Post growth multilayer instabilities of a certain periodic Gd₂O₃/SiO₂ multilayer systems have been investigated using scanning probe force-distance spectroscopy and optical spectrophotometric techniques. In the present work, we have noticed a strong correlation between the force spectroscopic results and the spectral properties of multilayer thin films, although measurement techniques and operating principles are quite different. From the experimental analysis, it was quite evident that the instability process, which starts during the nucleation and growth stage in thin films, continues to persist at a much longer time scale under post growth conditions. During this study it has been noticed that the elastic properties of the constituent thin films, the layer geometry and the bilayer thickness have strong correlation in trickling the multilayer instabilities. Such aspects also have strong interconnections with the morphological and viscoelastic changes. It is also noticed that most of the instabilities results cannot only be explained through elastic nature of the material alone. Instead, total number of layers, the layer structures, morphological changes, corresponding stiffness and the adhesion properties of the multilayer contribute substantially to these phenomena.     

Wide bandpass optical filters with TiO2 and Ta2O5

Complex multilayer thin film filters for optical applications have been designed, prepared and characterized in this work. E-beam reactive evaporation technique has been used as a deposition process. In the first stage, optimized individual film layers of TiO₂, Ta₂O₅, and SiO₂ are deposited and characterized optically and structurally before the deposition of multilayered structures. The filter designs are based upon 33 layered SiO₂/TiO₂ and SiO₂/Ta₂O₅ configurations on glass substrate. These designs are optimized to achieve wideband transmission in the visible spectrum. After deposition, the two filter configurations are characterized optically and structurally using spectrophotometery, atomic force microscopy (AFM), X-ray diffraction (XRD) and scanning electron microscopy (SEM). SiO₂/Ta₂O₅/glass filter has been found sensitive to deposition conditions since high absorption is observed in multilayered configuration for the as-deposited samples. Post-deposition annealing of the filter in the temperature range 150 to 250°C was also performed in order to study the effect of temperature on absorption and spectral characteristics of the filter. Comparison of the two filter configurations was also performed to analyze their suitability for optical applications. Adhesion of the two filters was found to be very good by means of tape-peel test.      

Linear stability analysis of electrohydrodynamic instabilities at fluid interfaces in the “small feature” limit

The endeavour to effectively harness interfacial electrohydrodynamic instabilities, to create small patterns, involves reducing the wavelength of the instability. This can be accomplished by decreasing the separation between the electrodes which may not always be possible. One may therefore have to reduce the surface tension or increase the applied voltage at a fixed electrode spacing. This can result in the wavelength of the pattern becoming of the same order as the electrode separation. Pease and Russel (J. Chem. Phys. 118, 3790 (2003)) were the first to argue that the commonly used Thin-Film Approximation (TFA) that involves an asymptotic expansion in the small parameter δ = (ε ₀ φ ₀²/(γh ₀))^1/2 (where ε ₀ is the permittivity of vacuum, φ ₀ is the root mean square value of the applied potential, γ is the surface tension and h ₀ is the thickness of the thin film) need not always be valid and γ may not be small in experiments. Higher-order corrections to the TFA might therefore be necessary. We extend the Direct Current (DC) field analysis of Pease and Russel to an Alternating Current (AC) field. AC field has been suggested as an effective way of controlling the wavelength of electrohydrodynamic instabilities at fluid-fluid interfaces. Infact, the perfect and leaky dielectric limits can be realised in the same fluid at very high and very low electric field frequencies, respectively. Recently, Roberts and Kumar (J. Fluid Mech. 631, 255 (2009)) carried out an analysis using TFA to investigate AC-field-induced instabilities at air-polymer interfaces. We propose a Generalized Model (GM), without the lubrication approximation, and carry out detailed comparison with the TFA. We consider the top fluid to be air, a perfect dielectric, and the bottom fluid to be a perfect or a leaky dielectric. The analysis is carried out for both DC and AC fields, and the deviation from TFA is expressed in terms of the parameter B = γh ₀/(ε ₀ φ ₀²) = δ ^{− 2}. We discuss variation of the wavelength of the fastest growing mode with frequency of the applied field for any arbitrary value of B, unlike the analysis of Roberts and Kumar which is restricted to B ≫ 1(δ ≪ 1) . We also revisit the analysis of Pease and Russel for instabilities under DC field and present the results in terms of the single parameter, B.