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

TL;DR

This study uses point contact spectroscopy to investigate superconducting gaps in doped BaFe2As2 compounds, revealing anomalous Andreev reflection enhancement above the bulk critical temperature in underdoped samples.

Contribution

It demonstrates the presence of Andreev reflection-like features above $T_c$ in underdoped iron arsenides, indicating unconventional superconducting phenomena.

Findings

Two distinct superconducting gaps identified.

Excess current follows BCS behavior in most samples.

Anomalous excess current persists above $T_c$ in underdoped samples.

Abstract

We use point contact spectroscopy (PCS) to probe the superconducting properties of electron doped $\rm{Ba(Fe_{1-x}Co_x)_2As_2}$ ($\rm{x = 0.05, 0.055, 0.07, 0.08}$) and hole doped $\rm{Ba_{0.8}K_{0.2}Fe_2As_2}$. 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 $\rm{Ba(Fe_{1-x}Co_{x})_2As_2}$, the excess current due to Andreev reflection for the compounds follows the typical BCS temperature dependence. For underdoped $\rm{Ba(Fe_{1-x}Co_{x})_2As_2}$, 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.