Strong Pinning Enhancement in MgB2 Using Very Small Dy2O3 Additions

0.5 to 5.0 wt.% Dy2O3 was in-situ reacted with Mg + B to form pinned MgB2. While Tc remained largely unchanged, Jc was strongly enhanced. The best sample (only 0.5 wt.% Dy2O3) had a Jc of 6.5 x 10^5 A/cm^2 at 6K, 1T and 3.5 x 10^5 A/cm^2 at 20K, 1T, around a factor of 4 higher compared to the pure sample, and equivalent to hot-pressed or nano-Si added MgB2 at below 1T. Even distributions of nano-scale precipitates of DyB4 and MgO were observed within the grains. The room temperature resistivity decreased with Dy2O3 indicative of improved grain connectivity.Comment: 13 pages, 4 figures and 1 tabl

Improved Current Densities in MgB2 By Liquid-Assisted Sintering

Polycrystalline MgB2 samples with GaN additions were prepared by reaction of Mg, B, and GaN powders. The presence of Ga leads to a low melting eutectic phase which allowed liquid phase sintering and produces plate-like grains. For low-level GaN additions (5% at. % or less), the critical transition temperature, Tc, remained unchanged and in 1T magnetic field, the critical current density, Jc was enhanced by a factor of 2 and 10, for temperatures of \~5K and 20K, respectively. The values obtained are approaching those of hot isostatically pressed samples.Comment: 12 pages, 1 table, 4 figures, accepted in Applied Physics Letter

Speculations about plessite

Metallographic examination and microprobe analysis of plessite areas in meteorites - iron and nickel concentration in meteorite

Structure and magnetism of self-organized Ge(1-x)Mn(x) nano-columns

We report on the structural and magnetic properties of thin Ge(1-x)Mn(x)films grown by molecular beam epitaxy (MBE) on Ge(001) substrates at temperatures (Tg) ranging from 80deg C to 200deg C, with average Mn contents between 1 % and 11 %. Their crystalline structure, morphology and composition have been investigated by transmission electron microscopy (TEM), electron energy loss spectroscopy and x-ray diffraction. In the whole range of growth temperatures and Mn concentrations, we observed the formation of manganese rich nanostructures embedded in a nearly pure germanium matrix. Growth temperature mostly determines the structural properties of Mn-rich nanostructures. For low growth temperatures (below 120deg C), we evidenced a two-dimensional spinodal decomposition resulting in the formation of vertical one-dimensional nanostructures (nanocolumns). Moreover we show in this paper the influence of growth parameters (Tg and Mn content) on this decomposition i.e. on nanocolumns size and density. For temperatures higher than 180deg C, we observed the formation of Ge3Mn5 clusters. For intermediate growth temperatures nanocolumns and nanoclusters coexist. Combining high resolution TEM and superconducting quantum interference device magnetometry, we could evidence at least four different magnetic phases in Ge(1-x)Mn(x) films: (i) paramagnetic diluted Mn atoms in the germanium matrix, (ii) superparamagnetic and ferromagnetic low-Tc nanocolumns (120 K 400 K) and (iv) Ge3Mn5 clusters.Comment: 10 pages 2 colonnes revTex formatte

Pt-induced nanowires on Ge(001): a DFT study

We study formation of the nanowires formed after deposition of Pt on a Ge(001) surface. The nanowires form spontaneously after high temperature annealing. They are thermodynamically stable, only one atom wide and up to a few hundred atoms long. Ab initio density functional theory calculations are performed to identify possible structures of the Pt-Ge (001) surface with nanowires on top. A large number of structures is studied. With nanowires that are formed out of Pt or Ge dimers or mixed Pt-Ge dimers. By comparing simulated scanning tunneling microscopy images with experimental ones we model the formation of the nanowires and identify the geometries of the different phases in the formation process. We find that the formation of nanowires on a Pt-Ge(001) surface is a complex process based on increasing the Pt density in the top layers of the Ge(001) surface. Most remarkably we find the nanowires to consist of germanium dimers placed in troughs lined by mixed Pt-Ge dimer rows.Comment: 22 pages, 24 figure

Ohmic contacts to n-type germanium with low specific contact resistivity

A low temperature nickel process has been developed that produces Ohmic contacts to n-type germanium with specific contact resistivities down to (2.3 ± 1.8) x10<sup>-7</sup> Ω-cm<sup>2</sup> for anneal temperatures of 340 degC. The low contact resistivity is attributed to the low resistivity NiGe phase which was identified using electron diffraction in a transmission electron microscope. Electrical results indicate that the linear Ohmic behaviour of the contact is attributed to quantum mechanical tunnelling through the Schottky barrier formed between the NiGe alloy and the heavily doped n-Ge.<p></p&gt

Structure and giant magnetoresistance of granular Co-Cu nanolayers prepared by cross-beam PLD

A series of Co_xCu_{100-x} (x = 0, 40...75, 100) layers with thicknesses in-between 13 nm and 55 nm were prepared on silicon substrates using cross-beam pulsed laser deposition. Wide-angle X-ray diffraction (WAXRD), transmission electron microscopy (TEM) and electrical transport measurements revealed a structure consisting of decomposed cobalt and copper grains with grain sizes of about 10 nm. The influence of cobalt content and layer thickness on the grain size is discussed. Electron diffraction (ED) indicates the presence of an intermetallic Co-Cu phase of Cu3Au structure-type. Thermal treatment at temperatures between 525 K and 750 K results in the progressive decomposition of Co and Cu, with an increase of the grain sizes up to about 100 nm. This is tunable by controlling the temperature and duration of the anneal, and is directly observable in WAXRD patterns and TEM images. A careful analysis of grain size and the coherence length of the radiation used allows for an accurate interpretation of the X-ray diffraction patterns, by taking into account coherent and non-coherent scattering. The alloy films show a giant magnetoresistance of 1...2.3 % with the maximum obtained after annealing at around 725 K.Comment: 9 pages, 9 figure

Lattice diffusion and surface segregation of B during growth of SiGe heterostructures by molecular beam epitaxy: effect of Ge concentration and biaxial stress

Si1-xGex/Si1-yGey/Si(100) heterostructures grown by Molecular Beam Epitaxy (MBE) were used in order to study B surface segregation during growth and B lattice diffusion. Ge concentration and stress effects were separated. Analysis of B segregation during growth shows that: i) for layers in epitaxy on (100)Si), B segregation decreases with increasing Ge concentration, i.e. with increased compressive stress, ii) for unstressed layers, B segregation increases with Ge concentration, iii) at constant Ge concentration, B segregation increases for layers in tension and decreases for layers in compression. The contrasting behaviors observed as a function of Ge concentration in compressively stressed and unstressed layers can be explained by an increase of the equilibrium segregation driving force induced by Ge additions and an increase of near-surface diffusion in compressively stressed layers. Analysis of lattice diffusion shows that: i) in unstressed layers, B lattice diffusion coefficient decreases with increasing Ge concentration, ii) at constant Ge concentration, the diffusion coefficient of B decreases with compressive biaxial stress and increases with tensile biaxial stress, iii) the volume of activation of B diffusion () is positive for biaxial stress while it is negative in the case of hydrostatic pressure. This confirms that under a biaxial stress the activation volume is reduced to the relaxation volume

Superconductivity mediated by a soft phonon mode: specific heat, resistivity, thermal expansion and magnetization of YB6

The superconductor YB6 has the second highest critical temperature Tc among the boride family MBn. We report measurements of the specific heat, resistivity, magnetic susceptibility and thermal expansion from 2 to 300 K, using a single crystal with Tc = 7.2 K. The superconducting gap is characteristic of medium-strong coupling. The specific heat, resistivity and expansivity curves are deconvolved to yield approximations of the phonon density of states, the spectral electron-phonon scattering function and the phonon density of states weighted by the frequency-dependent Grueneisen parameter respectively. Lattice vibrations extend to high frequencies >100 meV, but a dominant Einstein-like mode at ~8 meV, associated with the vibrations of yttrium ions in oversized boron cages, appears to provide most of the superconducting coupling and gives rise to an unusual temperature behavior of several observable quantities. A surface critical field Hc3 is also observed.Comment: 29 pages, 5 tables, 17 figures. Accepted for publication in Phys. Rev.