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 (TCOO=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 TCOO and provides direct experimental evidence for a link between electron-phonon coupling and charge order in manganites.