The charge, spin, orbital and lattice degrees of freedom of correlated electron materials lead to the inhomogeneous and dynamic coexistence of material phases with novel orderings. Prominent examples, the dynamic polarons in manganites and the charge-spin stripes in cuprates, have characteristic time and length scales (ps and nm) which are well-suited to be studied with the SwissFEL.
Hard X-ray diffraction and the resonant soft- X-ray techniques of elastic diffraction and inelastic scattering provide high sensitivity to charge, spin, orbital and lattice degrees of freedom, in wavelength ranges covered by the SwissFEL. Fur thermore, the SwissFEL will provide excellent access to the sub-ps dynamics of these degrees of freedom, either using laser-pump/X-ray probe experiments or by sampling equilibrium fluctuations via the intermediate scattering function S(Q,t).
Photoemission spectroscopy, a preferred technique for static studies of correlated electron materials in the laboratory and at synchrotrons, is poorly suited to timeresolved measurements at the SwissFEL, due to resolution degradation by electron space-charge effects. However, the sensitivity to electronic structure provided by the photon-in/photon-out X-ray absorption and resonant scattering techniques make them highly promising alternatives, par ticularly in single-shot mode.
The nature of metal-insulator transitions in correlated-electron materials can be elegantly determined by time-resolved pump-probe experiments. In this way, the metal-insulator transition in 1T-TaS2 is shown, by its ultrafast character (i.e., much faster than typical lattice vibratons), to be due to the electronic Mott-Hubbard transition, and not to the lattice-related Peierls instability.
The electron-phonon interaction can be directly studied in time-resolved pumpprobe experiments. Examples are the creation via hot electrons of coherent phonons in bismuth and the triggering of a dynamic metal-insulator transition by the IR-excitation of a particular phonon mode in Pr0.7Ca0.3MnO3.
In many respects, correlated-electron materials exhibit electronic complexity, characterized by glassy dynamics and giant responses to small external perturbations. This complexity is reminiscent of that of molecular systems, such as liquid crystals and even biological macromolecules. The ability of the SwissFEL to provide novel information over a large range of time and length scales makes it an ideal tool to establish an experimental foundation for a unification in the theory of correlated electrons at the micro- and macro-scales
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