Point contact Andreev reflection spectroscopic (PCARS) studies on 122-type iron-based superconductors*

PCARS is applied to investigate the superconducting gap in iron pnictide single crystal superconductors of the AFe$_2$As$_2$ (A=Ba, Sr) family with two categories of G(V) curves observed [1]: one where Andreev reflection (AR) is present for (Ba$_{0.6}$K$_{0.4}$)Fe$_2$As$_2$ and Ba(Fe$_{0.9}$Co$_{0.1}$)$_2$As$_2$, and the other without AR but a V$^{2/3}$ shape for Sr$_{0.6}$Na$_{0.4}$Fe$_2$As$_2$ and Sr(Fe$_{0.9}$Co$_{0.1}$)$_2$As$_2$. The latter is also observed in the nonsuperconducting parent compound BaFe$_2$As$_2$. Mesoscopic phase-separated coexistence of magnetic and superconducting orders is considered to explain distinct behaviors. A gap size $\sim$3.0-4.0 meV with $2\Delta_0/k_BT_c \sim$2.0-2.6 is observed for PCARS on Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$. For the Ba(Fe$_{0.9}$Co$_{0.1}$)$_2$As$_2$, G(V) curves typically display a zero-bias conductance peak, sometimes with a V-shape background. [1] Xin Lu \textit{et al.,} arXiv:0910.4230