Superconductivity in MgB₂ and its related materials

After the discovery of MgB₂, much attention has been paid to the boride and carbide systems. In this paper, we mainly review the following themes. First, we review the present situation of the superconductivity in MgB₂, which can be interpreted in terms of BCS-type phonon mediated pairing. In particular, we focus on the relationship between the superconductivity and the crystal structure in MgB₂ and two gap feature. Second, we review our present status for the new superconductors including the boron and carbon systems. Recently, we discovered new superconductivity in related compounds W₇Re₁₃X (X = B and C) (T_c = 7.1 K and 7.3 K, respectively) and Y₂C ₃ (T_c = 18 K), and we elucidated their superconducting properties. The crystal structure of W₇Re₁₃X (X = B and C) is β-Mn type (cubic, space group: P4₁32), and Re atoms form a distorted octahedron. The superconductivity in W₇Re₁₃X system is recognized as strong coupling s-wave symmetry from the specific heat measurement. We discovered the superconductivity at T_c = 18 K in Y₂C₃, although this material with a maximum T_c of 11.5 K has already investigated by Krupka et al. The crystal structure of Y₂C₃ is body centered cubic (Pu₂C ₃-type) structure, and Y atoms are one-dimensionally aligned along [111] axis. The lattice parameter, a, obtained in our work is between 8.18-8.23 Å, and is smaller than those previously reported (8.21-8.25 Å).