A team has resolved lattice vibrations, or phonons, in the high-temperature superconductor Bi₂Sr₂CaCu₂O₈+δ with spatial resolution finer than a single crystallographic unit cell. It is the first time phonon excitations have been visualised at this scale in a cuprate superconductor, a class of materials whose high-temperature superconductivity remains one of the biggest open problems in condensed-matter physics.

Cuprates such as Bi₂Sr₂CaCu₂O₈+δ superconduct above the temperature of liquid nitrogen, but the role phonons play in the electron-pairing mechanism is still debated. While electron-phonon interaction neatly explains superconductivity in conventional superconductors, in cuprates the phonon energies and their coupling to charge carriers have been difficult to measure directly on the atomic scale. Resolving these excitations below the unit cell makes it possible for the first time to map how the vibrational modes are distributed around individual atoms in the structure.

The advance rests on a microscopy technique able to probe the solid's collective excitations with sub-nanometre precision, combining extreme spatial resolution with enough energy sensitivity to distinguish individual phonon modes. By reaching resolution finer than the lattice parameter, the researchers could separate vibrational contributions that appear averaged at coarser scales, and observe how the electron-phonon coupling varies from one atomic site to another within the same CuO₂ layer.

The results provide a quantitative map of phonon behaviour in a doped cuprate, an input previously available only through macroscopic averaging measurements such as inelastic neutron scattering or Raman spectroscopy. The observation opens the door to directly comparing the vibrational structure with the regions of the sample where superconductivity is most robust, a line of work that could help discriminate between the different theoretical mechanisms proposed to explain high-temperature superconductivity.