The often involved coupling between different electronic and structural degrees of freedom in complex materials shall be addressed in this project from their dynamics either in the time or in the frequency domain. The femtosecond-timeslicing facility at BESSY-II and the new RICXS station for resonant inelastic X-ray scattering allow for both kinds of experiments in a coherent and complementary way.
One central question in the field of strongly correlated electron systems concerns the coupling between electronic and structural degrees of freedom. Any local symmetry reduction, which leads to an energy gain by lifting electronic degeneracy, is necessarily connected with structural changes. In particular for mixed-valent systems this coupling to the lattice can be the essential mechanism for charge localization that leads to, e.g. a change of macroscopic transport properties. In a time-resolved X-ray diffraction experiment electronic and structural degrees of freedom can be selectively probed by choice of the momentum transfer. Resonant inelastic X-ray scattering experiments allow via the choice of the excitation energy and polarization to prepare a well-defined intermediate state and to probe the coupling between electronic and lattice degrees of freedom even in an orbital-selective way.
The focus shall be on prototype materials for a cooperative order of charge and orbital degrees of freedom. The work shall be complemented by reference experiments from model systems like transition-metal monoxides.
Probe the electron-phonon couplings in correlated materials via the response of electronic and structural degrees of freedom to an external perturbation and via their low-energy excitation spectrum.
Complementary to the time-resolved photoemission experiments (P9, electron-phonon coupling probed from band-structure renormalization effects), to studies with coherent X-rays (P5, P8, investigation of the relation between structural order and disorder and the stability of certain phases), to the work on model systems in P6 and it relates to theory on selective phonon pumping (P10); methodological synergy with P2, P3.