Micro - Raman scattering investigation of MgB 2 and RB 2 ( 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 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 nonsuperconductors. It is found that the anharmonicity of phonons is important for the superconductivity for MgB2. 78.30.-j, 74.25.Kc, 63.20.Kr, 63.20.Ry Soon after the discovery of high-TC superconductivity of MgB2 by Akimitsu and his coworkers, it was revealed by the isotope effect by the substitution of B and the observation of a coherent peak in the NMR experiment that MgB2 is a BCS-type supercondoctor with the electron-phonon coupling and its symmetry is a s-wave. With increasing the applied pressure, the transition temperature (TC) decreases , but, this pressure dependence is not explained by the simple BCS theory. Loa and Syassen explained the pressure depen-