Resonant Soft X-ray Diffraction

Schematics of a pump-probe resonant diffraction experiment
from a material with charge and orbital order. A pump pulse
excites the electronic degrees of freedom, the temporal evolution of the
pump effect is then probed selectively via x-ray diffraction. (N. Pontius).

Resonant soft x-ray diffraction (RSXD) is a particularly sensitive probe for materials in which the electronic state is spatially modulated. Well known examples are antiferromagnets, in which the spin direction alters from one magnetic site to the next. Especially in correlated materials, however, also other degrees of freedom are found to be modulated. In charge ordered systems the oxidation state of otherwise equivalent ions changes from one site to the other; in orbital ordered systems it is the symmetry of the occupied orbitals. Often different orders are combined like in the prototypical material La0.5Sr1.5MnO4, which exhibits spin order, charge order and orbital order. RSXD makes use of the high spectroscopic sensitivity of resonances in the soft x-ray range to the electronic character of the scatterers. One uses the contrast achieved in this way in a diffraction experiment to probe the spatial pattern. First applied to artificial magnetic systems like multilayers RSXD has proven very powerful for the study of correlated materials. More advanced techniques allow now to determine the local symmetry of the scattering ions with high sensitivity and to critically test theoretical models for correlated materials.

RSXD is highly selective, because different ordered degrees of freedom typically lead to different diffraction peaks. Its straightforward extension is hence to use the technique to probe the coupling between different degrees of freedom in a pump-probe experiment. The system is driven out of equilibrium by, e.g., an optical laser pulse. The way the laser excitation spreads to the different electronic and magnetic degrees of freedom is probed via the temporal evolution of the different diffraction peaks, which in turn reveals the fundamental coupling mechanisms.

G-I2 operates a dedicated scattering chamber for time-resolved soft x-ray diffraction experiments at the slicing facility of BESSY II. A second instrument for static and time-resolved experiments at PETRA III and FLASH is presently being commissioned.