Phase transitions and dynamics in solids

The relevant degrees of freedom (left) and the axes of
the multidimensional parameter space, in which we are
probing their dynamics (right).

The project phase transition and dynamics in solids aims at the understanding and control of material properties. In the focus are materials, whose properties are strongly affected by the coupling between different degrees of freedoms. Even when the isolated interactions are strong (like Coulomb repulsion) their coupling can lead to an effective low-energy landscape, which may be reflected, e.g., in the formation of several almost degenerate phases. Close to these phase boundaries materials’ properties are sensitively affected by external stimuli, which render them particularly suited for functionality. The coupling mechanisms in these materials are probed via their dynamics in the time and in the frequency domain, and via their band structures and Fermi surfaces.

The main experimental tools are photoelectron spectroscopy (PES), time-resolved spectroscopy and time-resolved scattering and resonant inelastic x-ray scattering (RIXS). These latter two experimental approaches have been rapidly developing in the recent past because of the availability of pulsed x-rays and - for RIXS - because of tremendous improvements in energy resolution and theoretical understanding. Recent advances in PES are true bulk sensitivity in high-energy PES (HAXPES or HIKE) experiments and very recent gains of orders of magnitude in transmission for spectrometers and spin filters. The central scientific goal of the project is to obtain a deeper understanding of fundamental coupling mechanisms in functional materials by combining these different experimental approaches. The potential is huge: The energy scales and the dispersion of fundamental excitations probed by RIXS or quasi-particle dispersions in photoemission as well as the temporal response of different degrees of freedom to external stimuli both reflect relevant coupling mechanisms. Furthermore the inelastic losses define the energy scale on which by resonant optical pumping a certain well-defined excited state can be prepared. Its decay and the energy transfer to other degrees of freedom can then be followed again in the time domain. Another closely-related goal of this project is to identify and characterize novel transient phases that are formed far from equilibrium by, e.g., optical pumping.