Suppression of metal-to-insulator transition and appearance of superconductivity inCu1−xZnxIr2S4

The thiospinel system ${\mathrm{Cu}}_{1\ensuremath{-}x}{\mathrm{Zn}}_{x}{\mathrm{Ir}}_{2}{\mathrm{S}}_{4}$ $(0l~xl~0.9)$ has been studied by x-ray diffraction, electrical resistivity, and magnetic susceptibility measurements. The parent compound ${\mathrm{CuIr}}_{2}{\mathrm{S}}_{4},$ being metallic at room temperature, undergoes a structural phase transition towards lower symmetry around 230 K and becomes an insulator at the low temperature. The Zn substitution for Cu was found to drastically suppress the metal-to-insulator (MI) transition, resulting in the appearance of superconductivity. The MI transition temperature ${T}_{\mathrm{MI}}$ and the extent of the structural distortion both decrease with increasing x until the phase transition is completely suppressed at $x\ensuremath{\sim}0.4.$ In the region of $xl~0.4,$ the cubic spinel phase coexists with the low-symmetry phase below ${T}_{\mathrm{MI}}.$ For the metallic phase, the change of the Pauli paramagnetic susceptibility indicates the hole-filling mechanism due to an excess electron from the Zn substitution for Cu. The insulating state of the low-symmetry phase is tentatively explained in terms of charge ordering combined with the ${\mathrm{Ir}}^{4+}$ dimerization. Bulk type-II superconductivity below 3.4 K is observed for $0.25l~xl~0.8$ samples. The superconducting transition temperature decreases with increasing the Zn content. The abnormal behavior of the normal-state resistivity below 200 K for $0.3l~xl~0.5$ samples suggests modification of the electronic states, which might be related to the occurrence of superconductivity.