H. Takeya

An interface diffusion process approach for the fabrication of MgB2 wire

We propose a new approach for the fabrication of MgB2 tape utilizing an interface diffusion reaction between an Fe?Mg alloy substrate and a boron (B) layer. The Fe?Mg alloy has enough workability to be deformable into a tape form, and the Fe?Mg/(B(+ SiC)) composites were prepared by a lamination method. During the heat treatment at 700??C, the Mg in the Fe?Mg alloy diffuses towards the interface, forming a thin Mg rich layer along the interface. The Mg rich thin layer acts as a source of Mg for MgB2 formation, which proceeds to the inside of the B layer. Such a formation mechanism produces a dense structure of MgB2 with much less MgO impurity, which is difficult to achieve by the conventional powder-in-tube (PIT) process. SiC addition to the B layer is effective for improving the critical current density Jc in applied magnetic fields as in the case of the PIT process, and a Jc value of 3 ? 104?A?cm?2 at 5?K and 7?T has been obtained, which is comparable to that of PIT tape with SiC addition. This approach is expected to be a breakthrough in the development of MgB2 wire with a high quality superconducting layer in the composite.

Origin of the difference between the high and low-Tc phases in the yttrium sesquicarbide system

Abstract We report on an investigation of superconductivity in n-type InN. There is an optimum carrier density for the occurrence of the superconductivity. The lowest carrier density is limited by the Mott transition of ne~ 2 1017 cm 3 and the highest density is limited by the superconductor to insulator transition of ne~ 5 1020 cm 3. We propose a mechanism where the occurrence of the superconductivity is related to the presence of In–In chains of finite length in the ab plane. The In–In chains, which originate from the inversion domains of InN grown on sapphire (0 0 0 1) and elongate along , are coupled toform micro Josephson-junctions.

Physical Properties of Li2Pd3B and Li2Pt3B Superconductors

Superconductivity in two Li-containing compounds of Li2Pd3B and Li2Pt3B was recently discovered. These materials, showing the superconducting transition at 7.2 K and 2.6 K, respectively, have the same cubic structure (P4332) composed of distorted octahedrons without mirror or inversion symmetry along any directions. This is a very interesting feature of those materials in relation to the symmetry of superconductivity. Resistivity measurements in magnetic fields gave their upper critical fields, Hc2(0) = 45 kOe and 19 kOe, respectively. Their specific heat was measured using a heat-pulse relaxation method. The Sommerfeld coefficient (γ) and Debye temperature (θD) terms of Li2Pd3B were given as γ=9.5 mJmol-1K-2 and θD=228 K . The value of C/γT at Tc was calculated to be 1.7. In the same manner, those parameters were described for Li2Pt3B as γ=9.6 mJmol-1K-2, θD=240 K, and C/γTc =0.75, respectively. Since C/γTC in the weakcoupling limit by the BCS theory is 1.43, the value of 1.7 for Li2Pd3B is slightly higher. The electronic specific heat of Li2Pd3B at a zero magnetic field follows the typical exponetial behavior discribed in the BCS theory, while that of Li2Pt3B shows quadratic behavior. This result suggests the line nodes exist in the superconducting gap of Li2Pt3B driven by the spin-orbit interaction.

Heat Capacity Measurement on Li2Pd3B and Li2Pt3B

Superconductivity has been recently found in two Li containing compounds, Li2Pd3B and Li2Pt3B. They show superconducting transition at the temperatures, 7.5 K and 2.17 K respectively. The structure analysis has been reported by Eibenstein et. al. in 1997 and they take the same cubic structure with the symmetry of P4332. Heat capacity of these compounds was measured for confirming their bulk superconductivity and investigating superconducting properties. □C/Tc are evaluated to be around 18 mJ/mol K2 for Li2Pd3B and 9.7 mJ/mol K2 for Li2Pt3B. Electronic heat capacity (γ) and Debye temperature (θD) are derived from the normal state data and lead to 9.0 mJ/mol K2 and 221 K for Li2Pd3B, 7.0 mJ/mol K2 and 228 K for Li2Pt3B. Those physical parameters of the two compounds are discussed.