Extracting correlation effects from Momentum-Resolved Electron Energy Loss Spectroscopy (M-EELS): Synergistic origin of the dispersion kink in Bi$_{2.1}$Sr$_{1.9}$CaCu$_2$O$_{8+x}$

TL;DR

This study uses M-EELS and GW approximation to analyze the origin of dispersion kinks in cuprate superconductors, revealing that both electron-phonon and electron-electron interactions contribute to these features.

Contribution

It introduces a method combining M-EELS with GW calculations to disentangle phononic and electronic effects on dispersion kinks in cuprates, highlighting the role of local repulsive interactions.

Findings

Phonon contributions cause kink features at 40-80 meV, regardless of doping.

A repulsive interaction in momentum space can produce phonon-related kinks in ARPES.

Electron-phonon and electron-electron interactions jointly influence the kink strength.

Abstract

We employ Momentum-Resolved Electron Energy Loss Spectroscopy (M-EELS) on Bi2.1Sr1.9CaCu2O8+x to resolve the issue of the kink feature in the electron dispersion widely observed in the cuprates. To this end, we utilize the GW approximation to relate the density response function measured in in M-EELS to the self-energy, isolating contributions from phonons, electrons, and the momentum dependence of the effective interaction to the decay rates. The phononic contributions, present in the M-EELS spectra due to electron-phonon coupling, lead to kink features in the corresponding single-particle spectra at energies between 40 meV and 80 meV, independent of the doping level. We find that a repulsive interaction constant in momentum space is able to yield the kink attributed to phonons in ARPES. Hence, our analysis of the M-EELS spectra points to local repulsive interactions as a factor that enhances the spectroscopic signatures of electron-phonon coupling in cuprates. We conclude that the strength of the kink feature in cuprates is determined by the combined action of electron-phonon coupling and electron-electron interactions.