Evolution of magnetic, transport, and thermal properties in Na_(4-x)Ir₃O₈

The hyper-kagome material Na$_4$Ir$_3$O$_8$ is a three-dimensional spin-liquid candidate proximate to a quantum critical point (QCP). We present a comprehensive study of the structure, magnetic susceptibility $\chi$, heat capacity $C$, and electrical transport on polycrystalline samples of the doped hyper-kagome material Na$_{4-x}$Ir$_3$O$_8$ ($x \approx 0, 0.1, 0.3, 0.7)$. Materials with $x\leq 0.3$ are found to be Mott insulators with strong antiferromagnetic interactions and no magnetic ordering down to $T = 2$ K\@. All samples show irreversibility below $T \approx 6$ K between the zero-field-cooled and field-cooled magnetization measured in low fields ($H = 0.050$ T) suggesting a frozen low temperature state although no corresponding anomaly is seen in the heat capacity. The $x = 0.7$ sample shows $\rho(T)$ which weakly increases with decreasing temperature $T$, nearly $T$ independent $\chi$, a linear in $T$ contribution to the low temperature $C$, and a Wilson ratio $R_W \approx 7$ suggesting anomalous semi-metallic behavior.