Evidence for two-dimensional nucleation of superconductivity in MgB2_{2}

According to the crystal structure of MgB$_{2}$ and band structure calculations quasi-two-dimensional (2D) boron planes are responsible for the superconductivity. We report on critical fields and resistance measurements of 30 nm thick MgB$_{2}$ films grown on MgO single crystalline substrate. A linear temperature dependence of the parallel and perpendicular upper critical fields indicate a 3D-like penetration of magnetic field into the sample. Resistivity measurements, in contrast, yield a temperature dependence of fluctuation conductivity above T$_{c}$ which agrees with the Aslamazov-Larkin theory of fluctuations in 2D superconductors. We consider this finding as an experimental evidence of two-dimensional nucleation of superconductivity in MgB$_{2}$.Comment: 5 RevTex pages, 3 PostScript Figures ZIPed in archive Sidoren.zip. Submitted to EuroPhys. Lett. December 3, 200

Evidence for two-dimensional nucleation of superconductivity in MgB2

According to the crystal structure of MgB2 and band structure calculations quasi-two-dimensional (2D) boron planes are responsible for the superconductivity. We report on critical fields and resistance measurements of 30 nm thick MgB2 films grown on MgO single crystalline substrate. A linear temperature dependence of the parallel and perpendicular upper critical fields indicates a 3D-like penetration of magnetic field into the sample. Resistivity measurements, in contrast, yield a temperature dependence of fluctuation conductivity above Tc which agrees with the Aslamazov-Larkin theory of fluctuations in 2D superconductors. We consider this finding as an experimental evidence of two-dimensional nucleation of superconductivity in MgB2

Numerical Analysis of Liquid Menisci in the EFG Technique

This chapter is devoted to the analysis of the behavior of the profile curves of the melt menisci for the sapphire crystal growth by edge-defined film-fed growth (EFG) technique. The menisci of the shaped crystals with capillary channels, fibers, and tubes (including cases of outer and inner circular menisci) are considered. Also, we investigated the profile curves of menisci both in the cases of the positive and negative angles between profile curve and the working edge of the die. The cases of outer and inner circular menisci of the tubular crystals and menisci at capillaries and fibers are considered

Interaction of 3D mesostructures composed of Pd-Ni alloy nanowires with low-temperature oxygen plasma

© 2017We report that volumetric mesoporous structures obtained via self-organization of Pd-Ni nanowires interact actively with low temperature non-equilibrium oxygen plasma. This interaction leads to strong red incandescence of the composite nanostrucutures. After the plasma-chemical treatment, the X-ray diffraction pattern reveals presence of nanocrystalline nickel (II) oxide phase on surface of the samples. Measurements of electromagnetic response indicate that the samples exposed to oxygen plasma retain metallic type conductivity which is similar to the pristine Pd-Ni samples. Thus our studies indicate that the plasma-chemical treatment yields novel composite 3D hierarchical structures with a highly conductive skeleton of Pd-Ni nanowires covered by a thin nickel (II) oxide semiconducting layer of large surface area. These structures are attractive for applications

Fluctuation conductivity in superconducting MgB2

According to the crystal structure of MgB2 and band structure calculations, quasi-two-dimensional (2D) boron planes are responsible for the superconductivity. We report on critical-field and resistance measurements of 5.6-μm-thick MgB2 films grown on a sapphire single-crystal substrate. Resistivity measurements yield a temperature dependence of the fluctuation conductivity above the critical temperature, which agrees with the Aslamazov-Larkin and Maki-Thompson theory of fluctuations in layered superconductors, indicating a quasi-two-dimensional nucleation of superconductivity in MgB2. © 2002 MAIK "Nauka/Interperiodica"

Simulation of non-axisymmetric floating zone crystal growth under microgravity

The possibility of growing crystals of non-circular cross-section by using the floating zone technique is considered, basing the analysis on the isothermal liquid bridge model, and restricting the study to a linear asymptotic analysis. An almost square initial seed is considered in more detail, although the analysis is general and can be applied to other initial shapes, not only of the seed but also of the supply rod, that can be of any cross-section. The influence of non-circular seed and/or feed cross-section shape, and of a possible axial acceleration typical of microgravity platforms (sounding rockets and space labs), on both the liquid interface shape and the solid grown shape, are analysed, including the stability limits by breaking of the molten bridge

Observation of the magnetic domain structures in Cu0,47_{0,47}Ni0,53_{0,53} thin films at low temperatures

We report on the first experimental visualization of domain structure in films of weakly ferromagnetic Cu$_{0,47}$Ni$_{0,53}$ alloy with different thickness at liquid helium temperatures. Improved high-resolution Bitter decoration technique was used to map the magnetic contrast on the top of the films well below the Curie temperature T$_{Curie}$ ($\sim$ 60 K). In contrast to magnetic force microscopy, this technique allowed visualization of the domain structure without its disturbance while the larger areas of the sample were probed. Maze-like domain patterns, typical for perpendicular magnetic anisotropy, were observed. The average domain width was found to be about 100 nm.Comment: 4 pages, 5 figures, will be published in JETP Let