Quantitative planar Raman imaging through a spectrograph: visualisation of a supersonic wedge flow

Planar Raman imaging through a spectrograph is demonstrated as a diagnostic tool for quantitative flow visualisation of internal supersonic wedge flow. A dedicated Bayesian deconvolution filter is used to remove the spectral structure that is introduced by the spectrograph. The 2D density field is determined with ca. 10% precision using average images over 6,000 laser pulses, down to 0.5 mm from the surface of the wedge. Direct interpretations of Raman intensities provide more precise density data than indirect interpretations based on shock geometry in 2D inviscid flow.

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Capillary Stokes drift: a new driving mechanism for mixing in AC-electrowetting

We studied the flow fields generated inside sessile drops that oscillate periodically between states of high and low contact angle under the influence of alternating electric fields of variable frequency and amplitude. Following the motion of dye patches, we show that the number of oscillation cycles required to achieve mixing scales logarithmically with the Péclet number as expected for chaotic mixing. High speed movies reveal an asymmetry of the drop shape between the spreading and receding phase of the oscillations. This results in net internal flow fields that we characterize by tracing the motion of colloidal seed particles. The strength and frequency dependence of the flow are explained in terms of Stokes drift driven by capillary waves that emanate from the oscillating contact line.     

Structure models for β-MnGa2S4 as derived from electron diffraction and high-resolution electron microscopy

A study by high-resolution electron microscopy and electron diffraction on the tetrahedrally coordinated compound β-MnGa₂S₄ is reported. The structure-imaging technique enabled a new structure model to be proposed for the accommodation of deviations from stoichiometry in compounds of the type $\text{x}\text{MnS}\text{y}\text{Ga}{}_2\text{S}{}_3$. The model is based on the Ga₂S₃ structure wherein by the substitution of MnS layers chemical twin planes are formed. Depending on the spacing of the periodic twin planes a homologous series of compounds is expected and a number of members of this series was identified mainly by electron diffraction. In the β-MnGa₂S₄ compound, two structure modifications are observed mainly differing in the orientation of the twin planes (coherent or incoherent). This conclusion is substantiated by structure image calculations for the interpretation of the high-resolution results.     

Hydrodynamic description of femtosecond transient thermoreflectivity

Femtosecond transient reflectivity curves are computed using hydrodynamic approximation for describing the behaviour of the material in the collisionless free-electron domain. Two cases, the dominant electron-impurity scatterings and the dominant electron-phonon relaxations are considered to describe the heat diffusion in non-equilibrated lattice and electronic subsystems.     

The (twin) composition plane as an extended defect and structure-building entity in crystals

The burden of this paper is the recognition of the composition plane as an extended (planar) “defect” in twinned crystals, together with the notion that new structure types may be generated from simple prototype structures by regular and frequent repetition of a twinning operation, usually on a very fine scale. Less regular application of the operation leads to disorder and variation in stoichiometry, without point defects being necessary.

The range of compounds whose structures may be described in these terms is very wide, from metal alloys through metal carbides and borides etc. and inorganic materials, such as metal oxides and sulphides, to minerals, including silicates and the “sulphosalts” of the heavy metals (Pb, Bi etc.). Isostructural compounds frequently span this whole gamut of substances.

The main reasons for the twinning appear to be () the generation of new types of interstices (not present in the prototype) of appropriate size for some of the constituent atoms, and/or (b) a variation in the concentration of appropriate interstices, i.e. in the stoichiometric ratio anion/cation.

After a general introduction to the relevant aspects of twinning, attention is successively concentrated on reflection-twinned h.c.p., then c.c.p. and finally mixed c.p. arrays: first with “interstitial” atoms only in the composition planes; and then with other interstices also occupied. In each case lamellar twinning is first considered, and then cyclic twinning. Glide-reflection-twinning of similar arrays is then considered.

Attention is also drawn to small topological distortions that, for example, convert the octahedra in a c.c.p. array (twinned or even un-twinned) to arrays of capped trigonal prisms (or to cubes). In this way the apparently complex structures of a number of compounds are derived from those of much simpler twinned c.c.p. arrays.

Finally, the not-very-large amount of available, relevant, experimental evidence is reviewed. As far as it goes, this confirms that the notion of the composition plane as an extended defect is well founded. But more experimental data are needed.

[In most cases specific literature references are provided for each structure. For a few, rather common, structures they are not. In such cases reference may be made to standard texts, e.g. for alloys, to Schubert⁹⁹ or Pearson¹³³ and, for more ionic structures, to Wyckoff¹³⁴ or Povarennykh¹³⁵. Diagrams are to scale: 1 cm = 4.0 Å.]     

Measuring charge transport from transient photovoltage rise times. A new tool to investigate electron transport in nanoparticle films

Charge transport rate at open-circuit potential (V(oc)) is proposed as a new characterization method for dye-sensitized (DS) and other nanostructured solar cells. At V(oc), charge density is flat and measurable, which simplifies quantitative comparison of transport and charge density. Transport measured at V(oc) also allows meaningful comparison of charge transport rates between different treatments, temperatures, and types of cells. However, in typical DS cells, charge transport rates at V(oc) often cannot be measured by photocurrent transients or modulation techniques due to RC limitations and/or recombination losses. To circumvent this limitation, we show that charge transport at V(oc) can be determined directly from the transient photovoltage rise time using a simple, zero-free-parameter model. This method is not sensitive to RC limitation or recombination losses. In trap limited devices, such as DS cells, the comparison of transport rates between different devices or conditions is only valid when the Fermi level in the limiting conductor is at the same distance from the band edge. We show how to perform such comparisons, correcting for conduction band shifts using the density of states (DOS) distribution determined from the same photovoltage transients. Last we show that the relationship between measured transport rate and measured charge density is consistent with the trap limited transport model.     

Interplay of Peltier and Seebeck effects in nanoscale nonlocal spin valves

We have experimentally studied the role of thermoelectric effects in nanoscale nonlocal spin valve devices. A finite element thermoelectric model is developed to calculate the generated Seebeck voltages due to Peltier and Joule heating in the devices. By measuring the first, second, and third harmonic voltage response nonlocally, the model is experimentally examined. The results indicate that the combination of Peltier and Seebeck effects contributes significantly to the nonlocal baseline resistance. Moreover, we found that the second and third harmonic response signals can be attributed to Joule heating and temperature dependencies of both the Seebeck coefficient and resistivity.