Dichotomy in ultrafast atomic dynamics as direct evidence of polaron formation in manganites

Polaron transport, in which electron motion is strongly coupled to the underlying lattice deformation or phonons, is crucial for understanding electrical and optical conductivities in many solids. However, little is known experimentally about the dynamics of individual phonon modes during polaron motion. It remains elusive whether polarons have a key role in materials with strong electronic correlations. Here we report the use of a new experimental technique, ultrafast MeV-electron diffraction, to quantify the dynamics of both electronic and atomic motions in the correlated LaSr2Mn2O7. Using photoexcitation to set the electronic system in motion, we find that Jahn-Teller-like O, Mn4+ and La/Sr displacements dominate the lattice response and exhibit a dichotomy in behaviour-overshoot-and-recovery for one sublattice versus normal behaviour for the other. This dichotomy, attributed to slow electronic relaxation, proves that polaron transport is a key process in doped manganites. Our technique promises to be applicable for specifying the nature of electron-phonon coupling in complex materials.