Substitution effects on the thermoelectric properties of Sr$_{2}$IrO$_{4}$

Layered cobalt oxides with the low-spin (LS) state, such as Na$_{x}$CoO$_{2}$ and Ca$_{3}$Co$_{4}$O$_{9}$, have shown a good potential as p-type materials for thermoelectric applications [1, 2]. Layered rhodium oxides are also characterized by a high Seebeck coefficient and a low metallic resistivity [3, 4]. The LS configuration of the transition metal is considered to be at the origin of this unusual property. We have investigated the thermoelectric properties of Sr$_{2}$IrO$_{4}$, which is isostructural to the superconductors La$_{2}$CuO$_{4}$ and Sr$_{2}$RuO$_{4}$. Contrary to the later ones, Sr$_{2}$IrO$_{4}$ is a semiconductor with a narrow gap and a weak ferromagnetic ground state (T$_{c}\approx $ 240K, M$_{s}\approx $ 0.14$\mu _{B}$/Ir) [5]. For polycrystalline samples, the resistivity is of the order of 10$\Omega $.cm and the thermoelectric power shows a broad maximum around 110K (S $\approx $ 300$\mu $V.K$^{-1})$ denoting the activation of minority carriers. In order to increase the carrier concentration, we tried many substitutions for Sr and Ir. Results of resistivity, thermoelectric power and susceptibility will be discussed. \newline [1] I. Terasaki\textit{ et al.}, Phys. Rev. B \textbf{56}, R12685 (1997). [2] A. C. Masset \textit{et al.}, Phys. Rev. B \textbf{62}, 166 (2000). [3] Y. Klein \textit{et al.}, Phys. Rev. B \textbf{73} 165121 (2006). [4] S. Shibasaki \textit{et al.}, Phys. Rev. B \textbf{74}, 235110 (2006). [5] G. Cao \textit{et al}, Phys. Rev. B \textbf{57}, R11039 (1998).