Dynamics and extreme conditions

Following the reorganisation of the Experiments Division group structure in April 2009, the two high pressure beamlines ID09A and ID27 as well the Large Volume Press project joined the former High Resolution and Resonance Scattering Group to constitute the Dynamics and Extreme Conditions Group. Without forgetting the other high-pressure programmes, a large portion of high-pressure science is now conducted within our group, thus further strengthening joint efforts, developments and collaborations. Beyond this, research on the beamlines covers all classical domains of natural sciences with a significant impact in condensed matter physics and focus on applied fields such as environmental and planetary science, catalysis, soft matter, material synthesis, and nanoscience.

The highlights selected for this chapter represent only a small sub-set of many exciting results which have been published over the course of 2009. They reflect the broad spectrum of scientific applications, and clearly confirm previously witnessed trends; (i) the combination of synchrotron-based techniques with additional probes such as Raman and infrared spectroscopy, and resistivity measurements, and (ii) the importance of theoretical calculations in support of the experimental results.

The first part of the chapter is dedicated to high pressure structural studies. The first two contributions feature studies on FeSe and SrFe₂As₂, combining X-ray diffraction at high pressures and low temperature with resistivity measurements. Here, pressure was used to effectively tune interatomic interactions and study their effect on the magnetic order and atomic structure in this class of novel superconductors. Using the same experimental techniques, the third contribution clarified the role of structural distortions and spin degrees of freedom in the metal-insulator transition in TbBaCo₂O_5.5. The structure determination of the oxygen - and -phase underlines the advantages of single crystal work, revealing an amazingly complex arrangement of atoms.

The second part of the chapter focuses on electronic and magnetic structure and dynamics. The first example exploits the isotope selectivity of nuclear resonance scattering to determine the chemical and magnetic state of an iron oxide layer grown in situ. The study reveals how the anti-ferromagnetic state of a layer only 2 nm thick can be stabilised by the surrounding ferromagnetic iron. The second contribution presents a combined X-ray absorption near edge structure (XANES) and X-ray emission spectroscopy (XES) study on sulphur-bearing compounds. It is highlighted that XES provides a powerful tool to precisely determine the valence state, and to quantify the amount of sulphur species in heterogeneous systems. The following two articles address fundamental issues. Using resonant inelastic X-ray scattering, the argon 1s3p double photoexcitation spectrum was studied in detail, and resonant contributions could be separated from other multielectron processes, thus providing valuable input for a detailed comparison with model calculations. The study of the dielectric response of potassium valence electrons by inelastic X-ray scattering (IXS) reveals large deviations from a free-electron behaviour which can be attributed to empty electronic bands of d symmetry.

The last part of the chapter is devoted to phonons and collective atom dynamics. The work on the high-frequency dynamics in glasses clearly underlines how important high energy resolution is. This holds true not only for the present example of an IXS study in which a resolution of 1.3 meV could be achieved, but also for nuclear inelastic scattering where already sub-meV resolution is being reached. A concept to push the energy resolution further down to 0.1 meV is discussed in the Enabling Technologies chapter.

The last two contributions are devoted to geophysical applications. The work on Fe₃C concludes a series of experiments to determine the density dependence of the sound velocity in iron and iron-bearing compounds with the aim to predict the amount of light elements present in Earth’s inner core. Finally, the IXS study on stishovite exploits a novel methodology to extract the complete lattice dynamics from polycrystalline materials, therefore allowing, for example, the determination of the full elasticity tensor.

Besides on-going research and developments on the beamlines, related activities kept the momentum high. The ID18 and ID22N nuclear resonance stations were reviewed in fall 2009. Furthermore, work on the technical design report for UPBL6 (IXS from electronic excitations) has started. Two meetings were held to discuss a possible partnership for extreme conditions science, which would allow users of the ESRF and ILL to access the best possible instrumentation and environment. The case has been presented to the ESRF Science Advisory committee and was well received. It is hoped that a Memorandum of Understanding will be signed in 2010.

M. Krisch