Response of acoustic phonons to charge and orbital order in the 50\% doped bilayer manganite LaSr${}_2$Mn${}_2$O${}_7$

Abstract

We report an inelastic neutron scattering study of acoustic phonons in the charge and orbitally ordered bilayer manganite LaSr${}_2$Mn${}_2$O${}_7$. For excitation energies less than $15\mathrm{meV}$, we observe an abrupt increase (decrease) of the phonon energies (linewidths) of a transverse acoustic phonon branch at $\mathbf{q}=(h,h,0)$, $h\le 0.3$, upon entering the low temperature charge and orbital ordered state ($T_{COO}=225\mathrm{K}$). 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.

accepted in Physical Review Letters