Response of acoustic phonons to charge and orbital order in the 50% doped bilayer manganite LaSr₂Mn₂O₇

We report an inelastic neutron scattering study of acoustic phonons in the charge and orbitally ordered bilayer manganite LaSr₂Mn ₂O₇. For excitation energies less than 15 meV, we observe an abrupt increase (decrease) of the phonon energies (linewidths) of a transverse acoustic phonon branch at q=(h,h,0), h≤0.3, upon entering the low temperature charge and orbital ordered state (T_COO=225K). This indicates a reduced electron-phonon coupling due to a decrease of electronic states at the Fermi level leading to a partial removal of the Fermi surface below T_COO and provides direct experimental evidence for a link between electron-phonon coupling and charge order in manganites.