Response of Acoustic Phonons to Charge and Orbital Order in the 50% Doped Bilayer Manganite ${\mathrm{LaSr}}_2{\mathrm{Mn}}_2{\mathrm{O}}_7$

We report an inelastic neutron scattering study of acoustic phonons in the charge and orbitally ordered bilayer manganite ${\mathrm{LaSr}}_2{\mathrm{Mn}}_2{\mathrm{O}}_7$. 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 $\mathbf{q}=(h,h,0)$, $h\le 0.3$, upon entering the low temperature charge and orbital ordered state ($T_{\mathrm{COO}}=225\mathrm{K}$). 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 $T_{\mathrm{COO}}$ and provides direct experimental evidence for a link between electron-phonon coupling and charge order in manganites.

Figure 1

Elastic $\mathbf{Q}$ scans (heating cycles) at (a) $\mathbf{Q}=(1.25,0.75,0)$ and (c) $(2.3,0,1)$. Solid lines are Gaussian fits for the given temperatures shown on estimated experimental backgrounds. (b),(d) Integrated intensities of the $\mathbf{Q}$ scans around $\mathbf{Q}=(1.25,0.75,0)$ and $(2.3,0,1)$, respectively, as a function of temperature. The inset in (b) shows the temperature hysteresis near the phase transition temperature $T_{\mathrm{COO}}\approx 225\mathrm{K}$.

Figure 2

Constant $\mathbf{Q}$ scans of the TA phonon at $\mathbf{Q}=(2+h,2-h,0)$, $h=0.1–0.5$, at $T=5\mathrm{K}$. Solid lines are Lorentzian fits convoluted with the instrumental resolution on an experimental background. Dashed horizontal lines indicate the respective scan base line. The insets show energies measured at $\mathbf{Q}=(2+h,2-h,0)$ (squares) and $(3-h,3+h,0)$ (circles) (upper panel) and phonon linewidths (FWHM) at $T=300\mathrm{K}$ (squares) and $T=5\mathrm{K}$ (triangles) of the TA phonon (lower panel). The black line is the calculated resolution.

Figure 3

Temperature-dependent (a) background subtracted data, (b) energy, and (d) Lorentzian linewidth (full width at half maximum) of the transverse acoustic phonon at $\mathbf{Q}=(2+h,2-h,0)$, $h=0.25$. Energies and linewidths were extracted from Lorentzian fits (solid lines) to the data convoluted with a Gaussian energy resolution ($\approx 2\mathrm{meV}$). The dashed vertical lines in (b) and (d) mark the onset of orbital order at $T_{\mathrm{COO}}=225\mathrm{K}$. (c) Energy shift of TA phonons at $\mathbf{Q}=(2+h,2-h,0)$, $0.1\le h\le 0.35$, between $T=5$ and 300 K plotted versus phonon energy at $T=5\mathrm{K}$.