Charge Segregation in the Metal-Insulator Transition of the Thiospinel Cu_1-xZn_xIr₂S₄

Abstract

The metal-insulator transition (MIT) in the thiospinel Cu_1-xZn_xIr₂S₄ has been investigated by means of ⁶³Cu NMR. We find that the sample with x=0.3, having a uniform metallic phase at room temperature, exhibits a phase separation into the metallic and insulating regions below T_MI ∼ 70 K, where the first order MIT is observed in the resistivity data. The insulating regions have the same Cu Knight shift as that in the diamagnetic insulator at x=0. This indicates that the charge density in the low-T insulating phase is invariant, which causes the carriers introduced by the Zn substitution to be segregated from the insulating regions. Based on these observations, a possible mechanism of the MIT is discussed.