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Download Simon Billinge - The 3rd Super-PIRE REIMEI Workshop on Frontiers
Transcript
Local dimers and the metal insulator transition in Cu(Ir,Cr)2S4 from PDF and SR studies Simon Billinge Department of Applied Physics and Applied Mathematics, Columbia University Department of Condensed Matter Physics and Materials Science, Brookhaven National Laboratory. Hidden broken local symmetries may be behind some of the most important phenomena in condensed matter physics such as high temperature superconductivity, though this is still an open question. One reason for the lack of clarity is that they are notoriously difficult to study, requiring advanced experimental (and theoretical) tools that are only now being developed. In this talk I will show evidence from studies of the local structure of 3d, 4d and 5d transition metal compounds that these effects are much more ubiquitous than we had previously assumed, and that they couple closely to the material properties. I will take as an example a model system, CuIr2S4 doped with chromium, where new insights into the physics of the material emerge from combined PDF and MuSR experiments sensitive to local structural and magnetic structure. CuIr2S4 thiospinel exhibits a first order phase transition from a high temperature cubic Pauli paramagnetic metallic state to a low temperature triclinic orbital and charge ordered non-magnetic (Ir4+-dimerized spin singlet) insulating phase at around 230 K. A small amount of isovalent chemical substitution on any site of the CuIr2S4 spinel structure quickly suppresses the insulating phase, promoting metallic behavior and, in some instances, superconductivity. Cr-doping on Ir-site of the pyrochlore sublattice quickly suppresses the long range transition and stabilizes the metallic state and the cubic average crystal structure. However, superconductivity does not occur and the system displays semimetallic behaviour with the development of ferromagnetism for Cr-concentrations above 20 percent. Detailed tracking of the short range structural signature of Ir4+ dimer state has been performed across the Cu(Ir1-xCrx)2S4 thiospinel phase diagram (0 < x < 0.6). An atomic pair distribution function (PDF) survey reveals that although the low temperature long range dimer order is swiftly suppressed by Crsubstitution, fluctuating dimers exist at low temperature within the entire composition range studied, coexisting with Cr-ferromagnetism at higher Cr-doping. Upon heating local dimers disappear. The PDF analysis provides an (x, T) map of the short range dimer order, and estimates corresponding correlation length and dimerized fraction. The results demonstrate the robustness of the Ir4+ dimer state which shows reentrant behavior on increased doping at intermediate temperatures. We discuss the origin for this behavior.
