Metastable phase in the quantum Hall ferromagnet

Time-dependent capacitance measurements reveal an unstable phase of electrons in gallium arsenide quantum well that occurs when two Landau levels with opposite-spin are brought close to degeneracy by applying a gate voltage. This phase emerges below a critical temperature and displays a peculiar non-equilibrium dynamical evolution. The relaxation dynamics is found to follow a stretched-exponential behaviour and correlates with hysteresis loops observed by sweeping the magnetic field. These experiments indicate that metastable randomly distributed magnetic domains with peculiar excitations are involved in the relaxation process in a way that is equivalently tunable by a change in gate voltage or temperature.     

Growth and decay of the Pd(1 1 1)-Pd5O4 surface oxide: Pressure-dependent kinetics and structural aspects

Growth and decomposition of the Pd₅O₄ surface oxide on Pd(1 1 1) were studied at sample temperatures between 573 and 683 K and O₂ gas pressures between 10⁻⁷ and 6 × 10⁻⁵ mbar, by means of an effusive O₂ beam from a capillary array doser, scanning tunnelling microscopy (STM) and thermal desorption spectrometry (TDS). Exposures beyond the p(2 × 2)O adlayer (saturation coverage 0.25) at 683 K (near thermodynamic equilibrium with respect to Pd₅O₄ surface oxide formation) lead to incorporation of additional oxygen into the surface. To initiate the incorporation, a critical pressure beyond the thermodynamic stability limit of the surface oxide is required. This thermodynamic stability limit is near 8.9 × 10⁻⁶ mbar at 683 K, in good agreement with calculations by density functional theory. A controlled kinetic study was feasible by generating nuclei by only a short O₂ pressure pulse and then following further growth kinetics in the lower (10⁻⁶ mbar) pressure range.Growth of the surface oxide layer at a lower temperature (573 K) studied by STM is characterized by a high degree of heterogeneity. Among various metastable local structures, a seam of disordered oxide formed at the step edges is a common structural feature characteristic of initial oxide growth. Further oxide nucleation appears to be favoured along the interface between the p(2 × 2)O structure and these disordered seams. Among the intermediate phases one specifically stable phase was detected both during growth and decomposition of the Pd₅O₄ layer. It is hexagonal with a distance of about 0.62 nm between the protrusions. Its well-ordered form is a superstructure.Isothermal decay of the Pd₅O₄ oxide layer at 693 K involves at first a rearrangement into the structure, indicating its high-temperature stability. This structure can break up into small clusters of uniform size and leaves a free metal surface area covered by a p(2 × 2)O adlayer. The rate of desorption increases autocatalytically with increasing phase boundary metal-oxide. We propose that at close-to-equilibrium conditions (693 K) surface oxide growth and decay occur via this intermediate structure.     

In situ XPS study of Pd(1 1 1) oxidation. Part 1: 2D oxide formation in 10 mbar O2

The oxidation of the Pd(1 1 1) surface was studied by in situ XPS during heating and cooling in 3 × 10⁻³ mbar O₂. A number of adsorbed/dissolved oxygen species were identified by in situ XPS, such as the two dimensional surface oxide (Pd₅O₄), the supersaturated O_ads layer, dissolved oxygen and the R 12.2° surface structure.Exposure of the Pd(1 1 1) single crystal to 3 × 10⁻³ mbar O₂ at 425 K led to formation of the 2D oxide phase, which was in equilibrium with a supersaturated O_ads layer. The supersaturated O_ads layer was characterized by the O 1s core level peak at 530.37 eV. The 2D oxide, Pd₅O₄, was characterized by two O 1s components at 528.92 eV and 529.52 eV and by two oxygen-induced Pd 3d_5/2 components at 335.5 eV and 336.24 eV. During heating in 3 × 10⁻³ mbar O₂ the supersaturated O_ads layer disappeared whereas the fraction of the surface covered with the 2D oxide grew. The surface was completely covered with the 2D oxide between 600 K and 655 K. Depth profiling by photon energy variation confirmed the surface nature of the 2D oxide. The 2D oxide decomposed completely above 717 K. Diffusion of oxygen in the palladium bulk occurred at these temperatures. A substantial oxygen signal assigned to the dissolved species was detected even at 923 K. The dissolved oxygen was characterised by the O 1s core level peak at 528.98 eV. The “bulk” nature of the dissolved oxygen species was verified by depth profiling.During cooling in 3 × 10⁻³ mbar O₂, the oxidised Pd²⁺ species appeared at 788 K whereas the 2D oxide decomposed at 717 K during heating. The surface oxidised states exhibited an inverse hysteresis. The oxidised palladium state observed during cooling was assigned to a new oxide phase, probably the R 12.2° structure.     

Siliceous spicules in marine demosponges (example Suberites domuncula)

All metazoan animals comprise a body plan of different complexity. Since--especially based on molecular and cell biological data--it is well established that all metazoan phyla, including the Porifera (sponges), evolved from a common ancestor the search for common, basic principles of pattern formation (body plan) in all phyla began. Common to all metazoan body plans is the formation of at least one axis that runs from the apical to the basal region; examples for this type of organization are the Porifera and the Cnidaria (diploblastic animals). It seems conceivable that the basis for the formation of the Bauplan in sponges is the construction of their skeleton by spicules. In Demospongiae (we use the model species Suberites domuncula) and Hexactinellida, the spicules consist of silica. The formation of the spicules as the building blocks of the skeleton, starts with the expression of an enzyme which was termed silicatein. Spicule growth begins intracellularly around an axial filament composed of silicatein. When the first layer of silica is made, the spicules are extruded from the cells and completed extracellularly to reach their the final form and size. While the first steps of spicule formation within the cells are becoming increasingly clear, it remains to be studied how the extracellularly present silicatein strings are formed. The understanding of especially this morphogenetic process will allow an insight into the construction of the amazingly diverse skeleton of the siliceous sponges; animals which evolved between two periods of glaciations, the Sturtian glaciation (710-680 MYA) and the Varanger-Marinoan ice ages (605-585 MYA). Sponges are--as living fossils--witnesses of evolutionary trends which remained unique in the metazoan kingdom.     

Thermodynamic modeling of Fe–Ni pentlandite

Gibbs energy modeling of iron–nickel pentlandite has been performed using experimental data of ternary phase equilibria. A three-sublattice approach in the framework of the Compound Energy Formalism is developed to refine a two-sublattice model of pentlandite recently applied within a complete assessment of the Fe–Ni–S system. Experimental data about the iron site fraction on the octahedral sublattice at 523.15 K for the composition Fe₅Ni₄S₈ as well as the enthalpy of formation at 298.15 K for the composition Fe_4.5Ni_4.5S₈ are predicted satisfactorily by the novel model. New possibilities to interpret experimental phase equilibrium data on complex phase relations with pentlandite are discussed together on the basis of the recent extension of a second high-temperature heazlewoodite phase to a ternary solution phase.     

New insight into the role of the β3 subunit of the GABA<Subscript>A</Subscript>-R in development,behavior, body weight regulation,and anesthesia revealed by conditional gene knockout

Background  

The β3 subunit of the γ-aminobutyric acid type A receptor (GABA_A-R) has been reported to be important for palate formation, anesthetic action, and normal nervous system function. This subunit has also been implicated in the pathogenesis of Angelman syndrome and autism spectrum disorder. To further investigate involvement of this subunit, we previously produced mice with a global knockout of β3. However, developmental abnormalities, compensation, reduced viability, and numerous behavioral abnormalities limited the usefulness of that murine model. To overcome many of these limitations, a mouse line with a conditionally inactivated β3 gene was engineered.     

Two-step melting in two dimensions: first-order liquid-hexatic transition

Melting in two spatial dimensions, as realized in thin films or at interfaces, represents one of the most fascinating phase transitions in nature, but it remains poorly understood. Even for the fundamental hard-disk model, the melting mechanism has not been agreed upon after 50 years of studies. A recent Monte?Carlo algorithm allows us to thermalize systems large enough to access the thermodynamic regime. We show that melting in hard disks proceeds in two steps with a liquid phase, a hexatic phase, and a solid. The hexatic-solid transition is continuous while, surprisingly, the liquid-hexatic transition is of first order. This melting scenario solves one of the fundamental statistical-physics models, which is at the root of a large body of theoretical, computational, and experimental research.     

Inversion domain boundaries in ZnO with additions of Fe2O3 studied by high-resolution ADF imaging

Columns of metal atoms in the polytypoid compound Fe2O3(ZnO)15 could be resolved by high angle annular dark field imaging in a transmission electron microscopy (TEM)/STEM electron microscope--a result which could not be realized by high-resolution bright field imaging due to inherent strain from inversion domains and inversion domain boundaries (IDBs) in the crystals. The basal plane IDB was imaged in [11 00] yielding the spacing of the two adjacent ZnO domains, while imaging in [21 1 0] yields the position of single metal ions. The images allow the construction of the entire domain structure including the stacking sequence and positions of the oxygen ions. The IDB consists of a single layer of octahedrally co-ordinated Fe3+ ions, and the inverted ZnO domains are related by point symmetry at the iron position. The FeO6 octahedrons are compressed along the ZnO c-axis resulting in a FeO bond length of 0.208 nm which is in the range of FeO distances in iron containing oxides. The model of the basal plane boundary resembles that of the IDB in polytypoid ZnO-In2O3 compounds.     

A new polar nephelometer for measurement of atmospheric aerosols

The angular dependence of light scattered by aerosol particles in the atmosphere—the aerosol phase function—is one of the key properties of every radiative transfer and climate model. Side-scattering in the atmosphere is currently believed to be underestimated by most of the radiative transfer models in certain cases. The aerosol phase function can be measured with polar nephelometers.An innovative polar nephelometer able to measure the phase function of ambient aerosol directly is presented in this publication. The performance of the device was simulated completely using ray-tracing techniques. The results of these simulations are used to interpret the measurement data precisely. The measurements with the new polar nephelometer are fully automated. The quality of the measurement results was verified using different approaches. The values of the statistical and systematic error in measuring atmospheric aerosol are about 4% each. Scattering angles from 10° to 160° were measured and it is shown that with the same design this range can be extended to 3-177° in steps of 0.16° with an aperture of less than 1.8°.Measurements of the aerosol phase function are presented and compared with data taken with an integrating nephelometer and measurements of the particulate matter concentration. All data correlate very well. The described polar nephelometer can measure the phase function and the scattering coefficient of atmospheric aerosol with high accuracy and can be used for continuous monitoring measurements as well as in field campaigns.