Electron-boson coupling and oxygen-isotope effect in the optical conductivity of high-Tc superconductors

We have investigated electron-boson coupling in the optical conductivity of high-Tc superconductors through the optical self-energy. The real part of the self-energy (ReΣop(ω)) of YBa2Cu3Oy (YBCO) shows a characteristic doping dependence. In the optimally doped YBCO, ReΣop(ω) has a single peak around 65 meV, which corresponds to the kink structure of the band dispersion. On the other hand, in the under-doped YBCO, the peak structure of ReΣop(ω) splits into two parts. To evaluate contribution from the phonons in electron-boson coupling, we have measured oxygen-isotope effects by substituting 16O→18O for the optimally doped and under-doped YBCO.