The superconducting phase diagrams have been established by the measurements of electrical resistivity, magnetic susceptibility, and thermopower in cobalt-doped $L\text{FeAsO}$ ($L=\text{La}$ and Sm) systems. It is shown that the antiferromagnetic spin-density-wave order in the parent compounds is rapidly suppressed by Co doping, and superconductivity emerges at $x=0.025$ and 0.05 in ${\text{LaFe}}_{1-x}{\text{Co}}_x\text{AsO}$ and ${\text{SmFe}}_{1-x}{\text{Co}}_x\text{AsO}$, respectively. The $T_c\left(x\right)$ curves of both systems are domelike, with a maximum $T_c$ of 13 K at $x=0.075$ in ${\text{LaFe}}_{1-x}{\text{Co}}_x\text{AsO}$ and 17.2 K at $x=0.1$ in ${\text{SmFe}}_{1-x}{\text{Co}}_x\text{AsO}$. Thermopower measurement shows dominant electron-type transport for the Co-doped samples, in accordance with itinerant character of $\text{Co}3d$ electrons. We found a close correlation between $T_c$ and the abnormally enhanced part of normal-state thermopower. The occurrence of superconductivity via the Fe-site doping in the iron-based oxyarsenide contrasts sharply with the destruction of superconductivity by the Cu-site (within ${\text{CuO}}_2$ planes) doping in high-temperature superconducting cuprates.

• Received 31 August 2008

DOI:https://doi.org/10.1103/PhysRevB.79.054521