Evolution of the short-range Ir4+—Ir4+ dimer structure across the metal-insulator transition in CuIr2S4 measured on thermal cycling by x-ray PDF at the Advanced Photon Source. The insets show the evolution of the dimer PDF peak on cooling (bottom left) into the insulating state and warming (upper right) into the metallic phase. The main panel shows the deviation of this peak height with respect to its that at 230 K, a temperature lying just within the thermally-accessed metallic regime.
Strong coupling between the atomic lattice and electronic carriers is a defining characteristic of highly correlated materials such as high temperature superconductors, magnetoresistive oxides, or multiferroic compounds. Electron-lattice coupling in these and related systems controls key properties of the material, and understanding and directing this coupling will be essential to harnessing these properties for functionality. Often it is the local structural response to electron-lattice coupling, manifested as deviations at the nanoscale from the average atomic arrangement, that crucially influences the global materials electronic behavior. The key result of this highlight is our demonstration that in a mixed valence sulfide spinel, CuIr2S4, Ir4+—Ir4+ dimer pairs found in an insulating phase vanish completely on all length scales during heating through an insulator-metal transition but persist locally during an x-ray induced transition. Our finding implies that the electronic state of the high conductivity phase must be fundamentally and qualitatively different in these two scenarios, despite an identical average crystal structure.
We performed x-ray Pair Distribution Function (PDF) at the Advanced Photon Source as a function of temperature and x-ray fluence. PDF measures local atomic distances and their evolution with electronic structure and shows definitively that all short Ir—Ir contacts vanish in the metallic phase at high temperature. This thermally accessed metal thus has no long- or short-range memory of the insulator, and the uniform Ir—Ir distance is consistent with a delocalized, band-like metal. In contrast, as a function of x-ray exposure, local Ir—Ir dimers remain extant, even as long-range ordering of these dimers disappears and electrical conductivity increases. This argues that x-ray irradiation has depinned localized dimer units from the ordered lattice, freeing them to hop through the disordered structure rather than forming band-like states. More generally, the local structure of CuIr2S4 may provide insights into the factors that select between band-like and hopping conductivity in the presence of strong electron-lattice coupling.
Detailed Mapping of the Local Ir4+ Dimers through the Metal-Insulator Transitions of CuIr2S4 Thiospinel by X-Ray Atomic Pair Distribution Function Measurements
E.S. Bozin, A. S. Masadeh, Y. S. Hor, J. F. Mitchell, and S. J. L. Billinge
Phys. Rev. Lett 106, 045501 (2011)
Research performed by J.F. Mitchell and Y.S. Hor, Materials Science Division, Argonne National Laboratory; Emil Bozin and Simon Billinge, Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory; and A.S. Masadeh, Department of Physics and Astronomy, Michigan State University.