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Cr doped CuIr2S4: a model system for studying nanoscale phase competition in the presence of disorder at the metal-insulator transition

Doping charges via chemical substitution is an effective channel frequently yielding competing phases resulting in desirable but incompletely understood colossal responses, such as high temperature superconductivity in copper and iron based materials, or colossal magnetoresistance in manganites. Doping induced disorder on the other hand is often treated as a passive bystander and frequently ignored. CuIr 2 S 4 thiospinel exhibits exotic first order phase transition from a high temperature cubic Pauli paramagnetic metallic state to a low temperature triclinic orbital and charge ordered non-magnetic (Ir 4+ -dimerized spin singlet) insulating phase at around 230 K. Small amount of chemical substitution on any one site of the CuIr 2 S 4 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, stabilizing the metallic state and cubic average crystal structure. However, superconductivity does not occur and the system displays semimetallic behavor with the development of ferromagnetism for Cr-concentrations above 20%. Detailed tracking of the short range structural signature of Ir 4+ dimer state has been performed across the Cu(Ir 1-x Cr x ) 2 S 4 thiospinel phase diagram (0 < x < 0.6). An atomic pair distribution function (PDF) survey of total scattering X-ray data reveals that although the low temperature long range dimer order is swiftly suppressed by Cr-substitution, 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 (x, T) map of the short range dimer order, and estimates corresponding correlation length and dimerized fraction. Results demonstrate the robustness of the Ir 4+ dimer state. Notably however, careful consideration of the PDF results in combination with transport measurements reveals that the competition between the metallic and the insulating ground states is critically affected by the presence of the doping-induced disorder, and that the observed local dimer state at higher Cr-concentration in fact represents a strain-enabled re-entrant behavior.