Andreev Reflection Like Enhancement Above Bulk $T_c$ in Electron Underdoped Iron Arsenides

Abstract

We use point contact spectroscopy (PCS) to probe the superconducting properties of electron doped $\mathrm{Ba}(\mathrm{F}{\mathrm{e}}_{1-\mathrm{x}}\mathrm{C}{\mathrm{o}}_{\mathrm{x}}{)}_2\mathrm{A}{\mathrm{s}}_2$ ($\mathrm{x}=0.05,0.055,0.07,0.08$) and hole doped $\mathrm{B}{\mathrm{a}}_{0.8}{\mathrm{K}}_{0.2}\mathrm{F}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$. PCS directly probes the low energy density of states via Andreev reflection, revealing two distinct superconducting gaps in both compound families. Apart from the electron underdoped $\mathrm{Ba}(\mathrm{F}{\mathrm{e}}_{1-\mathrm{x}}\mathrm{C}{\mathrm{o}}_{\mathrm{x}}{)}_2\mathrm{A}{\mathrm{s}}_2$, the excess current due to Andreev reflection for the compounds follows the typical BCS temperature dependence. For underdoped $\mathrm{Ba}(\mathrm{F}{\mathrm{e}}_{1-\mathrm{x}}\mathrm{C}{\mathrm{o}}_{\mathrm{x}}{)}_2\mathrm{A}{\mathrm{s}}_2$, the temperature dependence of the excess current deviates from that of BCS, developing a tail at higher temperatures and surviving above bulk $T_c$. Possible explanations for this anomalous behavior are explored.