We report the realization of superconductivity by $5d$ element Ir doping in LaFeAsO, a prototype parent compound of high-temperature iron-based superconductors. X-ray diffraction patterns indicate that the material has formed the ZrCuSiAs-type structure with a space group $P4/\text{nmm}$. The systematic evolution of the lattice constants demonstrated that the Fe ions were successfully replaced by Ir. Both electrical resistance and magnetization measurements show superconductivity up to 11.8 K in ${\text{LaFe}}_{1-x}{\text{Ir}}_x\text{AsO}$. The superconducting transitions at different magnetic fields were also measured yielding a slope of $-d{H}_{c2}/dT=6.7\text{T}/\text{K}$ near $T_c$. Using the Werthamer-Helfand-Hohenberg formula $H_{c2}=-0.69$ $\left(d{H}_{c2}/dT\right){\mid }_{T_c}{T}_c$, the upper critical field at zero K is found to be about 54 T. Hall-effect measurements indicate that the conduction in this material is dominated by electronlike charge carriers; the charge-carrier density determined at 100 K is about $3.71\times {10}^{20}/{\text{cm}}^3$, which is close to the ${\text{LaFeAsO}}_{1-x}{\text{F}}_x$ system. This is the example of bulk superconductivity induced by replacing the Fe sites with higher $d$-orbital electrons in FeAs-1111 family, which should add more ingredients to the underlying physics of the iron-based superconductors.

• Received 11 May 2009

DOI:https://doi.org/10.1103/PhysRevB.80.054502