RIXS study of spontaneously forming superstructures in the thiospinel CuIr2S4

The thiospinel CuIr2S4 is discussed as an outstanding example for the spontaneous formation of complex superstructures (octamers) below a metal-insulator transition. The mechanism is still controversial. Theories range from an itinerant picture of an orbitally induced Peierls transition, neglec¬ting spin-orbit coupling, to local models in which spin-orbit coupling and intersite Coulomb repulsion are decisive. In preliminary RIXS data, we found evidence for dimers in the metallic phase at 300 K, a very unexpected result. We propose a thorough RIXS study to clarify the origin of superstructures in this fascinating material, e.g. establishing whether the dimerisation is intrinsic or a remnant of the insulating phase.