Low-energy phonons in Bi2 Sr2 CaCu2 O8+δ and their possible interaction with electrons measured by inelastic neutron scattering

Electron-phonon interaction in copper oxide superconductors is still enigmatic. Strong coupling for certain optic phonons is now well established experimentally, but a 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 Bi2Sr2CaCu2O8+δ to show that low-energy LA phonons could couple to electronic density fluctuations only at small phonon wave vectors, which naturally limits any interaction to forward scattering. Such scattering should not be pair-breaking in the case of the d-wave gap. We also found that the previously 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.