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 $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{RuCl}}_{3}, {\mathrm{Na}}_{2}{\mathrm{IrO}}_{3}$ and $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Li}}_{2}{\mathrm{IrO}}_{3}$ 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, ${\mathrm{Cu}}_{2}{\mathrm{IrO}}_{3}$ 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 $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{RuCl}}_{3}$ and $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Li}}_{2}{\mathrm{IrO}}_{3}$, accompanied with an anomaly of resistance at $\ensuremath{\sim}5.0\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. The IMT appears under a pressure of $\ensuremath{\sim}13.8\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$, which is absent in $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{RuCl}}_{3}, {\mathrm{Na}}_{2}{\mathrm{IrO}}_{3}$ and $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Li}}_{2}{\mathrm{IrO}}_{3}$ under pressure. It is related to the crystal structure transition at $\ensuremath{\sim}11.1\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$, 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 ${\mathrm{Cu}}^{+}$ to ${\mathrm{Cu}}^{2+}$, as a result of the decreasing of interlayer distance and the shortening of O--Cu--O bond under pressure.