Effect of Isoelectronic Doping at the As Site in Iron-based Superconducting Systems

FeAs-based superconductivity can be induced with various doping strategies---either electron doping, hole doping or isoelectronic doping. In this talk, we will focus on the unique isoelectronic doping at the As site. By using different dopants such as P [1,2] or Sb [3], positive or negative chemical pressure can be generated onto the FeAs layers. The positive chemical pressure suppresses/destroys the spin-density-wave (SDW) ordering, and then superconductivity emerges around a quantum critical point. In contrast, the negative pressure tends to recover the suppressed/hidden SDW ordering. The isoelectronic doping also influences the electronic and magnetic state of 4f electrons in the rare-earth atomic layers and 3d electrons of the Fe planes, especially in the case of proximity between 4f and 3d energy levels. This was manifested by the observation of local-moment ferromagnetism of 4f electrons in EuFe$_{2}$(As$_{1-x}$P$_{x})_{2}$, CeFeAs$_{1-x}$P$_{x}$O and CeFeAs$_{1-x}$P$_{x}$O$_{0.95}$F$_{0.05}$ [4] systems. Our results demonstrate the intriguing interplay/competition of intersite RKKY coupling among 4f-moments, intrasite Kondo interaction between 4f- and 3d- electrons, and \textbf{k}-space Cooper pairing of 3d electrons. This work was done in collaboration with Zhu-An Xu and Jian-Hui Dai, and was supported by National Basic Research Program of China (Grant Nos. 2007CB925001 and 2010CB923003). \\[4pt] [1] Zhi Ren et al., \textit{Phys. Rev. Lett}. 102, 137002 (2009). \\[0pt] [2] C. Wang et al., \textit{EPL }86, 47002 (2009)~; S. Jiang et al., \textit{JOP-CM }21, 382203 (2009)~. \\[0pt] [3] C. Wang et al., \textit{Science China G }53, 1225 (2010). \\[0pt] [4] Y. K. Luo et al., \textit{Phys. Rev. B} 81, 134422 (2010)~; Y. K. Luo et al., to be published.