The Study of Solids with X-Ray Lasers: Concepts, Opportunities and Challenges

Joachim Stöhr, Linac Coherent Light Source SLAC, Stanford, California, USA

Abstract
Synchrotron radiation research is well known for its contributions to the understanding of the structure of materials in well prepared equilibrium states, with impact in key areas such as energy, heath, environment and technology. With the advent of a new class of x-ray sources, x-ray free electron lasers, we are now ready to tackle more difficult and arguably more important problems dealing not only with “structure” but also with “function”. Function involves evolution or action, down to the intrinsic femtosecond time scale of atomic motion or changes in charge and spin configurations. Owing to their combined ability to both determine the structure of a material with a single shot and to monitor its evolution through excites states present in functional processes, x-ray lasers open the door for complete understanding and eventually control of material function.

My talk will illustrate concepts and opportunities underlying the use of these revolutionary light sources for the study of materials in space and time. I will also discuss potential impediments associated with the very nature of the x-ray interaction with the sample for such sources, namely the modification of the sample while it is being probed.

Ordering of Electronic Degrees of Freedom in Strongly Correlated Electron Systems - Observation by Resonant X-ray Scattering

Youichi Murakami High Energy Accelerator Research Organization, Institute of Materials Structure Science, Japan

Abstract
The strong correlation between electrons in solids causes a wide diversity of electronic states. In particular, transition metal oxides show various orders of electronic degrees of freedom (charge, spin, and orbital). These orders play important roles in the novel quantum phenomena such as superconductivity, colossal magnetoresistance, and multiferroics. The observation of the static and dynamic orders of these degrees of freedom is important to understand the electronic and magnetic properties. The resonant x-ray scattering technique using synchrotron radiation is very powerful to detect these orders. We have developed, especially, the technique for the detection of orbital ordering using the energy tunability and the polarization property of synchrotron x-rays. This technique has been applied to the detection of orbital ordering in 3d transition metal compounds (manganites, titanates, vanadates, copper fluorides and so on). These observations of the orbital ordering will be mentioned. Moreover, in some cobalt oxides, the spin-states (low-spin, high-spin, and intermediate spin states) come in the electronic degrees of freedom. The ordering of spin states will be discussed in connection with the orbital ordering.