Abstract: The electron transport properties in delta-doped semiconductor systems-are studied. The subband electronic structure of the delta-doped system is obtained by solving the coupled Schrodinger and Poisson equations. The screening of the quasi-two-dimensional electron gas is taken into account for the ionized impurity scattering through the matrix dielectric function within the random-phase approximation. The quantum and transport mobilities are calculated numerically as a function of the total electron density and the width of the doped layer at zero temperature. The intersubband scattering and the effect of empty subbands above the Fermi level on the electron mobilities are investigated. The calculated mobilities are in reasonable agreement with the available experimental results.

Abstract: In the present paper, the theory of the large bipolaron is reviewed and the possibility of bipolaron formation in the high-T(c) superconductors is indicated. Operator and path-integral formulations of the large bipolaron problem are compared. In the strong-coupling limit, the effect of non-optimal upper-bounds to the single-polaron groundstate energy is emphasized. The fact that the interaction with multiple phonon branches enhances the electron-phonon interaction and might result in a larger stability region for bipolaron formation is indicated. Experimental values for the static and high-frequency dielectric constants are used to discuss the relevance of bipolaron formation as a pair-forming mechanism in the high-T(c) superconductors.

Abstract: Electrical transport properties of a two-dimensional electron gas axe studied in the presence of a perpendicular magnetic field B modulated weakly and periodically along one direction, B = (B + B0 cos Kx)z, with B0 much less than B, K = 2pi/a, and a being the period of the modulation. B0 is taken constant or proportional to B. The Landau levels broaden into bands and their width, proportional to the modulation strength B0, oscillates with B and gives rise to oscillations in the magnetoresistance at low B. These oscillations reflect the commensurability between the cyclotron diameter at the Fermi level and the period a and consequently hey are distinctly different from the Shubnikov-de Ha.as ones, at higher B, in period and temperature dependence. The bandwidth at the Fermi energy can be one order of magnitude larger, at low B, than that of the electric case for equal modulation strengths. The resulting magnetoresistance oscillations have a much higher amplitude than those of the electric case with which they are out of phase. Explicit asymptotic expressions are derived for the temperature dependence of the transport coefficients. The case when both electric and magnetic modulations are present is also considered. The position of the resulting oscillations depends on the ratio delta between the two modulation strengths. When the modulations are out of phase there is no shift in the position of the oscillations when delta varies and for a particular value of delta the oscillations are suppressed.

Abstract: We investigate the self-assembly of vortices in a type-II superconducting disk subjected to highly nonuniform confining potentials produced by inhomogeneous magnetic textures. Using a series of numerical experiments performed within the Ginzburg–Landau theory, we show that vortices can arrange spontaneously in highly nonuniform, defect-free crystals, reminiscent of conformal lattices, even though the strict conditions for the conformal crystal are not fulfilled. These results contradict continuum-limit theory, which predicts that the order of a nonuniform crystal is unavoidably frustrated by the presence of topological defects. By testing different cooling routes of the superconductor, we observed several different self-assembled configurations, each of which corresponding to one in a set of allowed conformal transformations, which depends on the magnetic and thermal histories of the system.

Abstract: A theoretical and experimental study of the work function of few-layer graphene is reported. The influence of the number of layers on the work function is investigated in the presence of a substrate, a molecular dipole layer, and combinations of the two. The work function of few-layer graphene is almost independent of the number of layers with only a difference between monolayer and multilayer graphene of about 60 meV. In the presence of a charge-donating substrate the charge distribution is found to decay exponentially away from the substrate and this is directly reflected in the work function of few-layer graphene. A dipole layer changes the work function only when placed in between the substrate and few-layer graphene through a change of the charge transfer between the two.

Abstract: Homogenized Ni39.6Mn47.5T12.9 material was investigated by different electron microscopy techniques. Apart from the martensite precursor distortions typical for B2 phase alloys undergoing a thermoelastic martensitic transformation upon cooling, coherent dodecahedron-shaped precipitates with sizes between 20 and 100 nm and faceted by lozenge shapes of {110}-type planes are observed. Selected-area and microdiffraction patterns reveal an overall unit cell with a size of 3 x 3 x 3 units of the bcc lattice of the matrix and a body-centred symmetry without screw axes. Finally a ternary gamma-brass-type atomic structure of space group 14(3) over bar m is suggested for these precipitates in accordance with the obtained symmetry constraints, the energy-dispersive X-ray measurements and high-resolution transmission electron microscopy images. This is the first time this type of structure is found in an alloy completely consisting of transition-metal elements.

Abstract: 'Natural' platelets are planar defects in {001} planes found in natural type-IaA/B diamonds. 'Man-made' platelets are platelets formed in the laboratory by annealing type-IaA diamonds at temperatures over 2500 degrees C. Careful study shows that the infrared (IR) spectra of the 'man-made' platelets are different from the IR spectra of 'natural' platelets. High-temperature (T greater than or equal to 2000 degrees C) annealing of platelets containing type-IaA/B diamonds modifies the IR absorption spectrum owing to the 'natural' platelets and makes it similar to the IR spectrum of the 'man-made' platelets. It is suggested that such high-temperature annealing changes the structure of the 'natural' platelets. The changes are too subtle to be detected by electron microscopy techniques. Topographic electron-energy-loss spectroscopy shows that platelets contain nitrogen at an average density of 0.7 atoms per a(0)(2); however, high-temperature annealing does not seem to affect the concentration of the nitrogen in the platelets.