Group 5: Experiment

Task of WG 5: Sep-up new spectroscopy experiments

WG Leader: Maddalena Peddio, Deputy: Ana Violeta Girão 

The main objective of WG 5 is the design and realization of new experiments to test the theoretical models and their limitations. The ultimate goal will be to achieve a quantitative agreement between theory and experiment with the aim to provide a complete description of the geometric and electronic states at the atomic level either of the excited state or the ground state. This knowledge allows experimentalists to correlate this microscopic characterization with the macroscopic properties of different systems. The particular interest of the WG is the understanding of exotic properties, including the high-TC superconductors, the colossal magnetoresistive manganites and recently, the occurrence of multiferroicity, which strong correlated electron systems such as mixed-valence transition- metal compounds present. In these systems, charge, spin and orbital degrees of freedom are highly mixed and offer therefore a challenge to theory. In order to provide a unified description of the electronic ground state is then mandatory to carry out experiments using different electronic spectroscopies such as X-ray absorption spectroscopy, X-ray emission spectroscopy, resonant X-ray scattering in the soft and hard X-ray regimes and X-ray magnetic circular dichroism. One key point is to reach a common interpretation of K- and L2,3-edges absorption spectra for the transition-metal atoms that nowadays is made on the basis of different theoretical approaches, multiple scattering versus charge transfer multiplet approach, respectively. 

Thereby the WG 5 will follow two strategies for the experimental design: (1) Carrying out experiments for test systems different spectroscopies, as for example, hard X-ray coupled to soft X-ray absorption, photo emission and X-ray emission spectroscopy (XES). The simultaneous modelling gives a unified description and it serves as a check of the used approaches. (2) Performing experiments on systems whose macroscopic properties change abruptly as a function of external parameters as temperature (phase transitions), composition, magnetic field, pressure, etc. The theoretical description of these systems in the different phases will provide the geometrical and electronic changes responsible for their singular behaviour.

Finally, a series of spectroscopic investigations will be conducted in systems with an almost full 3d-electron shell (CuO, NiO, etc.) to study the effect of the Pauli exclusion principle. This is a challenging field for theoretical interpretations, since the treatment of exchange effects, as implemented up to now, is not adequate to treat these extreme cases.