Resolving phonons in superconductor Bi2Sr2CaCu2O8+δ at sub-unit-cell resolution

The role of phonons in cuprates remains controversial, with their complex lattice structure complicating the investigation. Here, we identify phonon modes originating from charge reservoir and superconducting layers of Bi 2 Sr 2 CaCu 2 O 8+δ using sub-unit-cell resolved electron energy loss spectroscopy in a scanning transmission electron microscope. We reveal several phonon modes exhibiting layer-specific localization: ~78 meV in-plane modes localized in the CuO 2 planes, ~42 meV hybrid (in-plane and out-of-plane) modes in the CuO 2 planes, and ~38 meV hybrid modes in the BiO layers. We also observe a periodic modulation of phonon frequencies induced by the superstructure, spatially correlating with the superconducting gap variation. Our findings offer new insights into the lattice dynamics in cuprates, fostering deeper exploration of the microscopic linkage between structural modulation, electronic inhomogeneity, and electron-phonon coupling.