# Low-energy phonons in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ and their possible interaction with electrons measured by inelastic neutron scattering

## TL;DR

This study uses inelastic neutron scattering to investigate low-energy phonons in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$, revealing limited electron-phonon coupling at small wavevectors and clarifying phonon spectral features.

## Contribution

It provides experimental evidence that low-energy LA phonons couple to electrons only at small wavevectors, suggesting forward scattering and clarifying phonon spectral features in cuprate superconductors.

## Key findings

- Low-energy LA phonons couple only at small wavevectors.
- Phonon spectral weight is consistent with conventional lattice dynamics.
- Forward scattering likely does not break pairs in d-wave superconductors.

## Abstract

Electron-phonon interaction in copper oxide superconductors is still enigmatic. Strong coupling for certain optic phonons is now well established experimentally, but theoretical understanding is challenging. Scattering of electrons near the Fermi surface by the longitudinal acoustic (LA) phonons is expected from basic theory because these phonons modulate electron density. We used inelastic neutron scattering on a large single crystal sample of optimally-doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ to show that low-energy LA phonons could couple to electronic density fluctuations only at small phonon wavevectors, which naturally limits any interaction to forward scattering. Such scattering should not be pairbreaking in the case of the d-wave gap. We also found that previously the reported low energy phonon spectral weight half-way to the zone boundary is consistent with conventional lattice dynamics and does not reflect an incipient charge density wave.