Coherent lattice motion in freestanding perovskite films probed by ultrafast electron diffraction*

Advances in femtosecond lasers make it possible to coherently generate specific phonon modes in crystals. These coherent lattice vibrations enable one to pinpoint specific atomic motion on the picometer scale, and this knowledge, in turn, helps one to realize ultrafast control of material property by laser pulses. In this experiment, we performed ultrafast electron diffraction on a 20-nm freestanding film of La_0.67Ca_0.33MnO_3. Following photoexcitation, we observed a 0.15 THz acoustic phonon extracted from Bragg peak intensity, position and width. By tracing the coherent oscillations in these three channels for more than 20 Bragg peaks, we are able to reconstruct in real space the unconventional atomic trajectory following the arrival of the laser pulse.