Metal-Insulator Transition in Ca$_{1-x}$Na$_{x}$IrO$_{3}$ with Post-Perovskite Structure

We developed a novel solid solution Ca$_{1-x}$Na$_{x}$IrO$_{3}$ (0 $< x \quad <$ 0.37) with the post-perovskite structure [1, 2]. Upon carrier doping into the $S$ = 1/2 antiferromagnetic Mott insulator CaIrO$_{3}$, the magnetic long-range order is gradually destabilized, culminating in a paramagnetic state at $x \quad >$ 0.30, with simultaneous change from the insulating to metallic behavior. The temperature dependence of the resistivity for metallic samples exhibits several characteristic features: (1) the $T^{\alpha }$ dependence with \textit{$\alpha $} $\sim $ 1.2 in the metallic range, (2) the ln$T$ dependence in the weak-localization regime, and (3) the positive magnetoresistance violating the Kohler's rule. These results indicate the anomalous metallic state caused by the strong electron correlation effect is realized on the verge of the Mott transition. [1] Nobuyoshi Miyajima, Kenya Ohgushi, Masaki Ichihara, and Takehiko Yagi, Geophys. Res. Lett. \textbf{33}, L12302 (2006). [2] K. Ohgushi, H. Gotou, T. Yagi, Y. Kiuchi, F. Sakai, and Y. Ueda, submitted.