Characteristics of oxygen isotope substitutions in the quasiparticle spectrum of Bi2Sr2CaCu2O8+δ

There is an ongoing debate about the nature of the bosonic excitations responsible for the quasiparticle self-energy in high-temperature superconductors -are they phonons or spin fluctuations? We present a careful analysis of the bosonic excitations as revealed by the kink feature at 70 meV in angle-resolved photoemission data using Eliashberg theory for a d-wave superconductor. Starting from the assumption that nodal quasiparticles are not coupled to the (pi, pi) magnetic resonance, the sharp structure at 70 meV can be assigned to phonons. We find that not only can we account for the shifts of the kink energy seen on oxygen isotope substitution but also get a quantitative estimate of the fraction of the area under the electron-boson spectral density which is due to phonons. We conclude that for optimally doped Bi2Sr2CaCu2O8+delta phonons contribute similar to 10% and non-phononic excitations similar to 90%. Copyright (C) EPLA, 2009