Isotope Effect on Electron-Phonon Coupling in Multiband Superconductor MgB2

We systematically investigate the isotope effect of electron-phonon coupling in multi-band superconductor MgB$_2$ by laser based Angle Resolved Photoemission Spectroscopy. The kink structure around 70 meV on two $\sigma$ bands, which is caused by electron coupling to $E_{2g}$ phonon mode, is shifted to higher binding energy in Mg$^{10}$B$_2$ than that in Mg$^{11}$B$_2$. The measured shifting energy of 3.5 meV is consistent with theoretical calculation based on harmonic phonon in MgB$_2$. Our temperature dependent measurement also indicates the isotope effect of kink structure is not dependent on superconducting transition.