Raman study of element doping effects on the superconductivity of MgB2

The influences of phonon frequency and unit cell volume on the superconductivity of element-doped MgB2 are discussed with reference to a Raman study on SiC, C, Mn, and Al-Ag -doped Mg-B materials. A phenomenon has been found in the doped samples, in that the phonon frequency changes to counteract the crystal lattice variation to keep the system stable within a Grüneisen parameter of 2.0-4.0. The chemical doping effects on phonon frequency and unit cell volume can be explained by the harmonicity-anharmonicity competition in the compounds. A decreased electronic density of states is responsible for the depression of superconductivity that is seen in doped MgB2. The possibility of a high critical temperature, Tc, in the Mg-B system exists if the material can possess both a high phonon frequency and a big unit cell volume at the same time, as indicated by the isotope effect and hydrogenation experiments.