How do the charges move?
Figure: The structure follows the electronic state in CuIr2S4. An insulating state at low temperature (left) is characterized by the formation of short Ir4+-Ir4+ dimers that order on a global lengthscale. Ir3+ ions form a sublattice of eight-membered rings (blue). The metallic state (right) has no differentiation between Ir3+ and Ir4+ sites on any length scale, and can be considered an Ir3.5+ hybrid. X-ray data (center) show the evolution of the Ir4+-Ir4+ dimers across the metal-insulator transition as a function of temperature.
The nature of the distinct metallic states of CuIr2S4 was understood by looking at nanometer length-scale structural fluctuations. In this material at low temperature, charges pair up and order into long one-dimensional chains extending through the structure. In this insulating state the charges cannot move. On warming, or under irradiation with x-rays or visible light, a metallic state is induced. The charges become mobile, but do they move in pairs or are the pairs destroyed? To study this, DOE-BES funded researchers at Brookhaven and Argonne National Laboratories used a modern x-ray probe of the local atomic structure that could sense the presence of nano-scale pairs even when they were moving around in the structure. Surprisingly, both behaviors were observed. On warming the sample the pairs broke up and the charges moved as individuals, but under irradiation the charges stayed paired but nonetheless became mobile. This finding brings crucial new insights to understand the important class of materials that can switch from insulator to metal in response to some external stimulus, a feature explored for many technological applications.