Point contact spectroscopy in the superconducting and normal state of $\mathrm{NaFe_{1-\textit{x}}Co_\textit{x}As}$

Abstract

We use point contact spectroscopy to probe the superconducting and normal state properties of the iron-based superconductor $\rm{NaFe_{1-\textit{x}}Co_{\textit{x}}As}$ with $\rm{\textit{x} = 0, 0.02, 0.06}$. Andreev spectra corresponding to multiple superconducting gaps are detected in the superconducting phase. For $\rm{\textit{x} = 0.02}$, a broad conductance enhancement around zero bias voltage is detected in both the normal and the superconducting phase. Such a feature is not present in the $\rm{\textit{x} = 0.06}$ samples. We suspect that this enhancement is caused by orbital fluctuations, as previously detected in underdoped $\rm{Ba(Fe_{1-\textit{x}}Co_\textit{x})_2As_2}$ (Phys. Rev. B 85, 214515 (2012)). Occasionally, the superconducting phase shows a distinct asymmetric conductance feature instead of Andreev reflection. We discuss the possible origins of this feature. NaFeAs (the parent compound) grown by two different techniques is probed. Melt-grown NaFeAs shows a normal state conductance enhancement. On the other hand, at low temperatures, flux-grown NaFeAs shows a sharp dip in the conductance at zero bias voltage. The compounds are very reactive in air and the different spectra are likely a reflection of their different oxidation and purity levels.