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A correlation between the change in magnetic susceptibility ({\Delta}\c{hi}exp) upon crystallization of Cu-Zr, Hf metallic glasses (MG) with their glass forming ability (GFA) observed recently is found to apply to Cu-Ti and Zr-Ni alloys, too. In particular, a small {\Delta}\c{hi}exp , which reflects similar electronic structures, ES, of glassy and corresponding crystalline alloys, corresponds to high GFA. Here, we studied {\Delta}\c{hi}exp in five Cu-Ti and four Cu-Zr and Ni-Zr MGs. The fully crystalline final state of all alloys was verified from X-ray diffraction patterns. The variation of GFA with composition in Cu-Ti, Cu-Zr and Cu-Hf MGs was established from the variation of the corresponding critical casting thickness, dc. Due to the absence of data for dc in Ni-Zr MGs their GFA was described by using empirical criteria, such as the reduced glass transition temperature. A very good correlation between {\Delta}\c{hi}exp and dc (and/or other criteria for GFA) was observed for all alloys studied. The correlation between the ES and GFA showed up best for Cu-Zr and NiZr2 alloys where direct data for the change in ES ({\Delta}ES) upon crystallization are available. The applicability of the {\Delta}\c{hi}exp ({\Delta}ES) criterion for high GFA (which provides a simple way to select the compositions with high GFA) to other metal-metal MGs (including ternary and multicomponent bulk MGs) is briefly discussed., Comment: 29 pages, 6 figures

A comprehensive study of the effects of carbohydrate doping on the superconductivity of MgB2 has been conducted. In accordance with the dual reaction model, more carbon substitution is achieved at lower sintering temperature. As the sintering temperature is lowered, lattice disorder is increased. Disorder is an important factor determining the transition temperature for the samples studied in this work, as evidenced from the correlations among the lattice strain, the resistivity and the transition temperature. It is further shown that the increased critical current density in the high field region can be understood by a recently-proposed percolation model [Phys. Rev. Lett. 90 (2003) 247002]. For the critical current density analysis, the upper critical field is estimated from a correlation that has been reported in a recent review article [Supercond. Sci. Technol. 20 (2007) R47], where a sharp increase in the upper critical field by doping is mainly due to an increase in lattice disorder or impurity scattering. On the other hand, it is shown that the observed reduction in self-field critical current density is related to the reduction in the pinning force density by carbohydrate doping., Comment: 5 pages, 3 figures

A comparative study of pure, SiC, and C doped MgB2 wires has revealed that the SiC doping allowed C substitution and MgB2 formation to take place simultaneously at low temperatures. C substitution enhances Hc2, while the defects, small grain size and nanoinclusions induced by C incorporation and low temperature processing are responsible for the improvement in Jc. The irreversibility field (Hirr) for the SiC doped sample reached the benchmarking value of 10 T at 20 K, exceeding that of NbTi at 4.2 K. This dual reaction model also enables us to predict desirable dopants for enhancing the performance properties of MgB2., Comment: 15 pages, 4 figures, 1 table. accepted for publication in Physical Review Letter

The magnetoresistivity and critical current density of well characterized Si-nanoparticle doped and undoped Cu-sheathed MgB$_{2}$ tapes have been measured at temperatures $T\geq 28$ K in magnetic fields $B\leq 0.9$ T. The irreversibility line $B_{irr}(T)$ for doped tape shows a stepwise variation with a kink around 0.3 T. Such $B_{irr}(T)$ variation is typical for high-temperature superconductors with columnar defects (a kink occurs near the matching field $% B_{\phi}$) and is very different from a smooth $B_{irr}(T)$ variation in undoped MgB$_{2}$ samples. The microstructure studies of nanoparticle doped MgB$_{2}$ samples show uniformly dispersed nanoprecipitates, which probably act as a correlated disorder. The observed difference between the field variations of the critical current density and pinning force density of the doped and undoped tape supports the above findings., Comment: 4 pages, 4 figures

m-H loops for virgin and neutron irradiated bulk and powder samples of MgB_{2} were measured in the temperature range 5-30 K in magnetic field B<= 1 T. The irradiation at thermal neutron fluences 9*10^{13} and 4.5*10^{14} cm^{-2} caused very small enhancement of m-H loops at lower temperatures (T<20 K), whereas the effect at high temperatures was unclear due to difficulty in achieving exactly the same measurement temperature prior and after irradiation. However, the irradiation at 4.5*10^{15} cm^{-2} produced clear enhancement of m-H loops (hence J_{c}) at all investigated temperatures, which provides the evidence for the enhancement of flux pinning in MgB_{2} due to ion tracks resulting from n+^{10}B reaction. The potential of this technique for the enhancement of flux pinning in high temperature superconductors is briefly discussed., Comment: 5 pages, 4 figures

Fe-clad MgB2 long tapes have been fabricated using a powder-in-tube technique. An Mg + 2B mixture was used as the central conductor core and reacted in-situ to form MgB2. The tapes were sintered in pure Ar at 800 ^(o) C for 1 h at ambient pressure. SEM shows a highly dense core with a large grain size of 100 micron. The Fe clad tape shows a sharp transition with transition width of 0.2 K and Tc0 at 37.5 K. We have achieved the highest transport critical current reported so far at 1.6 times 10^(4) A/cm^2 for both 29.5 K in 1 Tesla and 33 K in null field. R-T and critical current were also measured for fields perpendicular and parallel to the tape plane. The iron cladding shielded on the core from the applied external field, with the shielding being less effective for the field in the tape plane. Fe cladding may be advantageous for some applications as it could reduce the effects of both the self-field and external fields., Comment: 14 pages, 5 figures, submitted to Physica C on May 7, 2001