Square dance of the atoms: Shedding light on ultrafast phase transitions

Here we investigate a perovskite-type manganite, a prototypical example of a strongly correlated electron system which exhibits properties such as colossal magnetoresistance and insulator-to-metal transitions that are intrinsically related to symmetry changes of the atomic lattice and to intriguing ordering patterns of the spins, orbitals and charges. The application of an ultrashort optical pulse melts the electronic order and launches a structural phase transition. The long-range electronic and atomic order during the transition is probed directly with time-resolved resonant x-ray diffraction. Although the actual change in crystal symmetry associated with this transition occurs over different time scales characteristic of the many electronic and vibrational coordinates of the system, we find that the dynamics of the phase transformation can be well described using a single time-dependent order parameter that drives the electronic phase transition as well as the coherent motion of the atomic lattice. We hope that this concept is applicable to the general case.