Superconductivity in MgB_2 and Its Related Materials(Spin, Orbital, Charge and Lattice Coupled Systems, Field-Induced Phase Transitions and Dynamics in Quantum Spin Systems)

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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_7Re_<13>X (X=B and C) (T_c=7.1K and 7.3K, respectively) and Y_2C_3 (T_c=18K), and we elucidated their superconducting properties. The crystal structure of W_7Re_<13>X (X=B and C) is β-Mn type (cubic, space group : P4_132), and Re atoms form a distorted octahedron. The superconductivity in W_7Re_<13>X system is recognized as strong coupling s-wave symmetry from the specific heat measurement. We discovered the superconductivity at T_c=18K in Y_2C_3, although this material with a maximum T_c of 11.5K has already investigated by Krupka et al. The crystal structure of Y_2C_3 is body centered cubic (Pu_2C_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 (8.21-8.25Å).
理論物理学刊行会の論文
2005-10-31