Superconducting Properties of Carbonaceous Chemical Doped MgB2

Abstract

The discovery of superconductivity in magnesium diboride (MgB2: 39 K, in January 2001) (Nagamatsu et al., 2001) has generated enormous interest and excitement in the superconductivity community and the world in general, but especially among researchers into superconductivity in non-oxide and boron related compounds. MgB2 possesses an AlB2type hexagonal structure (space group P6/mmm) with alternating boron honeycomb planes and magnesium triangular planes, as shown in Fig. 1. Each Mg atom is located at the center of a hexagon formed by boron, and it donates its electrons to the boron planes; hence, the BB bonding is strongly anisotropic. The unit cell parameters are a = 0.3086 nm and c = 0.3524 nm at room temperature. These values of lattice parameters for MgB2 are in the middle of the values of lattice parameters of AlB2-type compounds. Owing to the simple hexagonal structure with space group P6/mmm, four optical modes at the Г point of the Brillouin zone are predicted for MgB2 (An & Pickett, 2001): a silent B1g mode (at 87.1 meV, ~700 cm-1), the E2g Raman mode (at 74.5 meV, ~600 cm-1), and the infrared active E1u (at 40.7 meV, ~330 cm1) and A2u (at 49.8 meV, ~400 cm-1) modes. The E2g mode is responsible for the high transition temperature, Tc, in MgB2.