Reversible structural changes of crystalline solids studied by ultrafast x-ray diffraction

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This project aims at understanding fundamental microscopic mechanisms which underlie ultrafast reversible structural changes of crystalline solids. The sensitivity of x-ray diffraction experiments in the ultrafast time domain will be improved by using a novel plasma source for hard x-ray pulses which works at a repetition rate of 1 kHz. In this way, small changes of stationary diffraction signals will become accessible for experiments. The following experiments are planned: (i) Coherent dynamics of acoustic phonons will be studied in GaAs/AlAs semiconductor superlattices. The changes of the phonon dispersion by the superlattice periodicity will allow for the impulsive femtosecond generation of coherent lattice excitations in a restricted range of k-vectors. Both single excitation pulses and pulse sequences will be applied for excitation. Atomic motions in the lattice will be monitored in real-time by diffraction of delayed hard x-ray pulses. The propagation properties of acoustic phonon wavepackets and the mechanisms of impulsive phonon generation will be analyzed. In addition, scenarios of coherent control of phonon excitations will be explored. (ii) Phase transitions in solids with a correlated electronic system, in particular high-transition-temperature superconductors, represent the second topic of this project. The dynamics and the mechanisms of the structural phase transition from an orthorhombic to a tetragonal symmetry will be investigated in La2-xBaxCuO4. Central problems are if this phase transition is of purely thermal nature, i.e. determined by the lattice temperature, and/or if nonequilibrium excitations of the correlated electronic system contribute to the structural change. Time-resolved diffraction experiments should allow for a separation of such contributions through their markedly different dynamics. At a later stage of the project, we plan to investigate systems in which reversible changes of the spin structure and the magnetic properties occur.