Insulator-metal transition and crossover from negative to positive magnetoresistance in Cu2IrO3 under high pressure

Quantum spin liquid (QSL) state has attracted a great deal of attention as a precursor of high-temperature superconductor. The promising QSL candidates such as ??-RuCl3, Na2IrO3 and ??-Li2IrO3 with honeycomb structure are insulators or semiconductors. An insulator-metal transition (IMT) is necessary for hosting superconducting state. However, all of them have a robust insulating state, even under pressure. Here, an alternative candidate of QSL, Cu2IrO3 is investigated under high pressure and low temperature through x-ray diffraction and electric properties measurement. A dimerization occurs at 7.1 GPa which is similar to ??-RuCl3 and ??-Li2IrO3, accompanied with an anomaly of resistance at -5.0 GPa. The IMT appears under a pressure of -13.8 GPa, which is absent in ??-RuCl3, Na2IrO3 and ??-Li2IrO3 under pressure. It is related to the crystal structure transition at -11.1 GPa, which induces the decline of interlayer distance. Furthermore, a strange crossover from negative to positive magnetoresistance is observed under pressure of 21.8 to 24.2 GPa at 2 K. This may be due to the valence change of Cu+ to Cu2+, as a result of the decreasing of interlayer distance and the shortening of O???Cu???O bond under pressure.