Characteristics of oxygen isotope substitutions in the quasiparticle spectrum of Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$

TL;DR

This study analyzes the 70 meV kink in ARPES data of Bi2212, attributing it mainly to phonons with about 10% contribution, clarifying the nature of bosonic excitations in high-Tc superconductors.

Contribution

It provides a quantitative analysis of phonon versus non-phonon contributions to the quasiparticle self-energy in Bi2212 using Eliashberg theory.

Findings

Phonons contribute approximately 10% to the electron-boson spectral density.

The 70 meV kink can be explained by phonons, not magnetic resonance.

Isotope substitution shifts support phonon involvement.

Abstract

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 $meV can be assigned to phonons. We find that not only can we account for the shifts of the kink energy seen on oxygen isotope substitution but also get a quantitative estimate of the fraction of the area under the electron-boson spectral density which is due to phonons. We conclude that for optimally doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ phonons contribute $\sim 10$% and non-phononic excitations $\sim 90$%.