Evidence for forward scattering and coupling to acoustic phonon modes in high-T$_c$ cuprate superconductors

TL;DR

This paper investigates how electron-phonon interactions, especially forward scattering with acoustic phonons, cause low-energy kinks in the electronic dispersion of high-Tc cuprate superconductors, influencing their superconducting properties.

Contribution

It demonstrates that coupling to in-plane acoustic phonons with forward scattering explains the observed low-energy kinks and their doping dependence in high-Tc cuprates.

Findings

Strong forward scattering peak in electron-phonon coupling to acoustic phonons.

Kink energy scale shifts with local superconducting gap.

Doping reduces screening, affecting the kink behavior.

Abstract

Recent laser angle-resolved photoemission spectroscopy studies have established the presence of a new kink in the low-energy nodal dispersion of Bi$_2$Sr$_2$CaCu$_2$O$+{8+\delta}$ (Bi-2212). The energy scale (~8-15 meV) of this kink appears below the maximum of the superconducting gap $\delta_0$. Therefore it is difficult to interpret this feature in terms of the usual coupling to a sharp dispersionless mode. In this paper we examine electron-phonon coupling to the in-plane acoustic phonon branch arising from the modulation of the screened Coulomb potential. We demonstrate that such a coupling has a strong forward scattering peak, and as a consequence, a kink occurs in the dispersion at an energy scale shifted by the local gap $\delta(k)$. In addition, considerations for the reduction of screening with underdoping naturally explains the observed doping dependence of the low-energy kink. These results point to a strong coupling to the acoustic branch which is peaked in the forward scattering direction and has important implications for transport and pairing in the high-T$_c$ cuprates.