Superconductivity in MgB2and Its Related Materials

Abstract

After the discovery of MgB 2 , 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 MgB 2 , 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 MgB 2 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 W 7 Re 13 X (X = B and C) (T c = 7.1 K and 7.3 K. respectively) and Y 2 C 3 (T c = 18 K), and we elucidated their superconducting properties. The crystal structure of W 7 Re 13 X (X = B and C) is β-Mn type (cubic, space group: P4 1 32), and Re atoms form a distorted octahedron. The superconductivity in W 7 Re 12 X system is recognized as strong coupling s-wave symmetry from the specific heat measurement. We discovered the superconductivity at T c == 18 K in Y 2 C 3 , although this material with a maximum T c of 11.5 K has already investigated by Krupka et al. The crystal structure of Y 2 C 3 is body centered cubic (Pu 2 C 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 A, and is smaller than those previously reported (8.21-8.25 A).