Cooperative atomic motion probed by ultrafast transmission electron diffraction

Abstract

In numerous solids exhibiting broken symmetry ground states, changes in electronic (spin) structure are accompanied by structural changes. Femtosecond time-resolved techniques recently contributed many important insights into the origin of their ground states by tracking dynamics of the electronic subsystem with femtosecond light pulses. Moreover, several studies of structural dynamics in systems with periodic lattice modulation (PLD) were performed. Since intensities of the super-lattice diffraction peaks are in the first approximation proportional to the square of the PLD amplitude, their temporal dynamics provides access to cooperative atomic motion. This process takes place on a fraction of a period of the corresponding lattice vibration (typically 100 fs timescale). However, since energy transfer from the excited electronic system to the lattice takes place on a comparable timescale, contribution of the incoherent lattice motion on diffraction intensities has to be taken into account. Furthermore we demonstrate an ultrafast transmission electron diffraction set-up, where relative changes in individual diffraction peaks of less than 1% can be studied. Here we show, that by simultaneously tracking the dynamics of intensities in super-lattice peaks, lattice peaks and in the incoherent background over multiple diffraction orders the two processes can be effectively disentangled.