Since the discovery of iron arsenide-based superconductor LaFeOAs, a series of '1111', '122' or '111' type pnictide superconductors have been synthesized. As high Tc cuprates, the superconductivity of iron pnictides are developed from the magnetically ordered parent state. The high Tc makes iron-based superconductors the second family to previously discovered cuprate superconductors, attracting world wide attention either for their unconventional superconducting mechanism or the prospects for applications.

Among iron-based superconductors, the '122' system was extensively investigated since the compounds are relatively stable while large size single crystals can be grown, which is very important for mechanism studies. The alkaline earth based AFe2As2 (where A is Ca, Sr or Ba) are the typical '122' parent compounds. Superconductivity can be generated by introducing carriers to the iron layer through hetorovalence substitution of monovalence alkaline metal to bivalence alkaline earth metal. The (Ba, K)Fe2As2 is the first reported '122' superconductor with Tc up to 38 K.

Similar results were obtained for (Sr, K)Fe2As2 with Tc ~35 K. However, superconducting transition for sodium doped CaFe2As2 is around Tc ~26 K surprisingly lower than those for BaFeAs2 or SrFeAs2. Using a sodium instead of potassium dopant for CaFe2As2 provides the advantage that the ionic radius of Na1+(1.18 Å) matches that of Ca2+(1.12 Å). Via increasing the doping level and hence the carrier density, the authors can substantially enhance the superconducting transition temperature of Ca122 with Tc above 33 K.

They address whether using CaAs or Na3As precursors instead of Ca or Na pure elements helps much to achieve high sodium doping content since the alkaline or alkaline earth metals are very volatile at high temperatures. Using these strategies the authors are able to grow single crystals of (Ca, Na)Fe2As2 with Tc above 33 K. The measurements on both the Meissner effect or the specific heat of single crystals indicated the bulk nature of the superconducting transition. The enhanced superconductivity leads the Tc of Ca122 comparable to those for Ba122 or Sr122.

The authors are planning to perform further systematic studies of the correlation between the superconducting transition temperature and the doping level for the (Ca, Na)Fe2As2 compounds.

Reference

arXiv:0806.4688 (2008) Solid State Commun. 148 538–40 (when the group found the '111' system)