Identification of coherent lattice modulations coupled to charge and orbital order in a manganite

We apply grazing-incidence femtosecond x-ray diffraction to investigate the details of the atomic motion connected with a displacively excited coherent optical phonon. We concentrate on the low frequency phonon associated with the charge and orbital order in the mixed valence manganite La${}_{0.25}$Pr${}_{0.375}$Ca${}_{0.375}$MnO${}_3$ for $T\lesssim 210$ K. We measure the response of three superlattice reflections that feature different sensitivities to the motion of the unit cell constituents. The results support the assignment to a translational mode of the Mn${}^{4+}$ atoms together with the oxygen atoms connecting adjacent Mn${}^{4+}$ sites.

Figure 1

Temporal evolution of the normalized diffracted x-ray intensity for the three superlattice reflections ($2{-}^3/2_{}$ 0), ($2½0$), and ($1½0$) at an average excitation fluence $F_{\mathrm{avg}}\approx 1.6$ mJ/cm${}^2$. The solid lines are fits obtained by a simple displacive excitation model (blue) and an extended two-step displacive excitation model (red), respectively.

Figure 2

Calculated normalized diffraction intensity and their visualization for three different atomic displacements: (a) motion of the La/Pr/Ca sublattice, (b) motion of all the Mn${}^{4+}$ and oxygen atoms, and (c) the Jahn-Teller mode, that is solely motion of the oxygen occupying the distorted octahedra around the Mn${}^{3+}$ ions. The gray bar represents the estimated average excitation strength of $\eta =0.16$.

Figure 3

Visualization of the involved atomic motion for the two most likely scenarios.