Micro-Raman scattering investigation of MgB2 and RB2 (R=Al, Mn, Nb and Ti)

Phonon spectra have been investigated by micro-Raman scattering from 290 to 20K. New peaks at 500 and 674 cm-1 appear below 250K in MgB2. These peaks are explained by the Fermi resonance, where Raman-active E2g intereacts with the overtone of E1u due to anhamonicity. Raman spectra of the isostructural RB2 (R=Al, Mn, Nb and Ti) have been also measured at 290K. The line width of E2g phonon of the superconductor MgB2 and NbB2 shows the broader than that of the non-superconductors. It is found that the anharmonicity of phonons is important for the superconductivity for MgB2.