Isotope effect on the E 2 g phonon and mesoscopic phase separation near the electronic topological transition in Mg 1- x Al x B 2

We report boron isotope effect on the E2g phonon mode by micro-Raman spectroscopy on the ternary Mg1-xAlxB2 system, synthesized with pure isotopes 10B and 11B. The isotope coefficient on the E2g mode frequency is nearly 0.5 in the wide range of Al, with a tendency to decrease at MgB2 (x=0). The intraband electron-phonon (e-ph) coupling relative to the sigma band has been extracted from the E2g line-shape parameters. By tuning the Fermi energy near the electronic topological transition (ETT), where the sigma Fermi surface changes from two-dimensional to three-dimensional topology (in range 0<x<0.28), the E2g mode shows the Kohn anomaly accompanied with a splitting into a hard and a soft component. The results suggest that the intraband hardly plays any role to control the high Tc of Mg1-xAlxB2 The common physical features of diborides with the multigap FeAs-based superconductors and cuprates are discussed.