EFFECTS OF THE Al CONTENT IN MgB2: A RAMAN STUDY

The partial substitution of Al for Mg in the MgB2 superconductor produces both a progressive reduction of its superconducting transition temperature T-c and the occurrence of two distinct structural phases at low and high Al content, as shown by X-ray diffraction data. To achieve a deeper understanding of the effect of the Al content, we investigated the x dependence of the Raman phonon spectrum in Mg1-xAlxB2 compounds. We analyze the evolution of the E-2g Raman-active phonon mode observed on increasing x both in our measurements (0 less than or equal to x less than or equal to 0.5) and in recent experiments (0 less than or equal to x less than or equal to 1). Our analysis shows the connection between the evolution with x of the E-2g phonon and the T-c suppression, thus supporting the hypothesis that superconductivity in MgB2 is mediated by a strong e-ph coupling with the E-2g Mode. We then derive the electron-phonon coupling constant lambda in the 0-0.5 x range.