La$_{0.5-x}$Na$_{0.5+x}$Fe$_{2}$As$_{2}$: electron and hole doping in the spacing layer

The electron-hole asymmetry in the phase diagram of iron-based superconductors is well illustrated in doped BaFe$_{2}$As$_{2}$ by comparing hole-doped Ba$_{\mathrm{1-x}}$K$_{\mathrm{x}}$Fe$_{2}$As$_{2}$ and electron doped BaFe$_{\mathrm{2-x}}$Co$_{\mathrm{x}}$As$_{2}$, mainly due to the availability of high quality single crystals which enable systematic studies using various probes. In Ba$_{\mathrm{1-x}}$K$_{\mathrm{x}}$Fe$_{2}$As$_{2}$, K-doping takes place at the spacing layer while FeAs layers remain intact. In contrast, Co substitution in BaFe$_{\mathrm{2-x}}$Co$_{\mathrm{x}}$As$_{2}$ disturbs the contiguity of the [FeAs$_{4}$] tetrahedra and interferes with superconductivity in the FeAs layers. This effect coming from substitution at different crystallographic sites has been suggested to contribute to the electron-hole asymmetry. In this talk, I will present the magnetic and structural transitions of La$_{\mathrm{0.5-x}}$Na$_{\mathrm{0.5+x}}$Fe$_{2}$As$_{2}$. Our results show that La$_{\mathrm{0.5-x}}$Na$_{\mathrm{0.5+x}}$Fe$_{2}$As$_{2}$, or even compounds with other rare earth and alkali ions in the spacing layer, provides a new material platform for the study of iron-based superconductors. The material could be tuned from electron-doped to hole-doped by varying the ratio between the alkali metal and rare earth ions.